INSTRUMENTAL ANALYSIS CHEM 4811

CHAPTER 3

DR AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciences

Clayton state university

CHAPTER 3

NUCLEAR MAGNETIC RESONANCE

SPECTROSCOPY

(NMR)

NMR SPECTROSCOPY

- Technique is used for studying shape and structure of molecules

- For studying 1H 13C 31P 19F 29Si polymers and solids

- For studying reaction kinetics and molecular attractions

- Magnetic Resonance Imaging (MRI) is used for diagnosis of cancer and other medical problems

- Can be considered as absorption spectroscopy

NMR SPECTROSCOPY

- Technique involves absorption of radiowaves by the nuclei of atoms in a molecule located in a magnetic field

- Frquency of radiowaves is on the order of 107 Hz (low energy)

- Radiofrequency (RF) radiation energy = hν

h = Planckrsquos constant = 6626 x 10-34 Jsν = frequency (between 4 and 1000 MHz)

- E is high enough to affect the nuclear spin of atoms in a molecule but too small to vibrate rotate or electronically excite atoms

PROPERTIES OF NUCLEI

- It is assumed that nuclei rotate about an axis

- Spinning of nuclei of atoms in a molecule in a magnetic field absorbs RF radiation

- The direction of the spinning axis changes

- Different atoms in a molecule have unique absorption frequencies (or resonances) if the nuclei possess magnetic moment

PROPERTIES OF NUCLEI

- Nuclei have a nuclear spin represented by I

- I is referred to as the spin quantum number

- Nuclei are also charged so produce magnetic moment along the axis of rotation upon spinning

- Nuclei must have a nonzero spin quantum number and a magnetic dipole moment to produce an NMR signal

FORMS OF ENERGY

Two forms of energy are displayed by nuclei

Mechanical Energy

and

Magnetic Energy

FORMS OF ENERGY

Mechanical Energy

- Nuclear energy due to mass and spin of the nuclei

Mechanical energy of the hydrogen atom

1II2π

hmomentumangularSpin

I = spin quantum number

I = 12 for the proton (1H)

FORMS OF ENERGY

Mechanical Energy

Rules for Predicting I

Mass Number(P+N)

OddOddEvenEven

Atomic Number(Charge P)

OddEvenEvenOdd

Spin QuantumNumber (I)

12 32 52 hellip12 32 52 hellip

01 2 3

FORMS OF ENERGY

Mechanical Energy

- Mass number of oxygen = 16 - Atomic number of oxygen = 8

- Number of neutrons = 16 ndash 8 = 8- Both even numbers so I = 0 and no spin

- Oxygen does not possess a magnetic moment

- Nuclei with I = 0 do not absorb RF radiation and do not give an NMR signal

12C 16O 32S 28Si

FORMS OF ENERGY

Mechanical Energy

- The actual spin number such as 32 52 2 or 3 aredetermined experimentally

- The two most important nuclei in organic chemistry and biochemistry are 13C and 1H (I = 12 for both)

- Elements in the first six rows of the periodic table have at least one stable isotope with a nonzero I

(exceptions Ar Tc Ce Pm Bi Po)

FORMS OF ENERGYMagnetic Energy

- Nuclear energy attributed to the electrical charge of the nucleus

- Electrical charge in motion sets up a magnetic field

- Magnetogyric (gyromagnetic ratio) = γ = microI

- micro = nuclear magnetic moment

- The value of γ is different for each type of nucleus

- NMR is based on the response of magnetically active nuclei to an external applied magnetic field

QUANTIZATION OF NUCLEI

- Given a strong and uniform external magnetic field Bo

- Nucleus lines up in a definite direction relative to the direction of the external field

- Each relative direction is associated with an energy level

- Only certain well-defined energy levels are permitted(energy levels are quantized)

- The number of magnetic quantum states = 2I + 1(number of orientations)

QUANTIZATION OF NUCLEI

Consider 1H

I = 12

- Number of orientations = 2(12) + 1 = 2

- Implies only two energy levels are permitted

- Magnetic quantum number (m) = 12 and -12

Zeeman Splitting- The splitting of the energy levels in a magnetic field

QUANTIZATION OF NUCLEI

- A nucleus with I = 12 can exist in one of two discrete energy levels in an external magnetic field (Bo)

- Magnetic moment of the lower energy level (m = 12) is aligned with the magnetic field

- Magnetic moment of the higher energy level (m = -12) is aligned against the magnetic field

- The two energy levels are separated by ΔE (ΔE is proportional to Bo)

- The lower energy level is magnetically more favored

QUANTIZATION OF NUCLEI

Precession

- Spinning of nucleus causing axis of rotation to move in a circular path about the external magnetic field

- Direction of precession is either with or against the applied magnetic field

- The absorption of radiation causes a transition between these two energy states

- Radiation frequency equals ΔE for transition to occur

QUANTIZATION OF NUCLEI

- ΔE also depends on the magnetic moment micro of the nucleus

The absorption frequency that can result in a transition of ΔE

equation)(LarmorγBωor2π

Bγν o

o

- ω = frequency in radianssecond

- Frequency of RF radiation is directly proportional to the external magnetic field

QUANTIZATION OF NUCLEI

- Energy of precessing nucleus (E) = microBocosθ

- θ = angle between axis of rotation and Bo

- θ changes when a nucleus absorbs energy in the form of RF radiation

Larmor Frequency- The frequency of radiation that can be absorbed by a spinning

charged nucleus in a magnetic field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- Absorption involves flipping of the magnetic moment

- Magnetic moment aligned with the field becomes aligned against the field

- Absorption of RF radiation occurs when the rate of precession becomes equal to the frequency of RF radiation applied

- Nucleus is therefore in an excited state when it becomes aligned against the field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

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CHAPTER 3

NUCLEAR MAGNETIC RESONANCE

SPECTROSCOPY

(NMR)

NMR SPECTROSCOPY

- Technique is used for studying shape and structure of molecules

- For studying 1H 13C 31P 19F 29Si polymers and solids

- For studying reaction kinetics and molecular attractions

- Magnetic Resonance Imaging (MRI) is used for diagnosis of cancer and other medical problems

- Can be considered as absorption spectroscopy

NMR SPECTROSCOPY

- Technique involves absorption of radiowaves by the nuclei of atoms in a molecule located in a magnetic field

- Frquency of radiowaves is on the order of 107 Hz (low energy)

- Radiofrequency (RF) radiation energy = hν

h = Planckrsquos constant = 6626 x 10-34 Jsν = frequency (between 4 and 1000 MHz)

- E is high enough to affect the nuclear spin of atoms in a molecule but too small to vibrate rotate or electronically excite atoms

PROPERTIES OF NUCLEI

- It is assumed that nuclei rotate about an axis

- Spinning of nuclei of atoms in a molecule in a magnetic field absorbs RF radiation

- The direction of the spinning axis changes

- Different atoms in a molecule have unique absorption frequencies (or resonances) if the nuclei possess magnetic moment

PROPERTIES OF NUCLEI

- Nuclei have a nuclear spin represented by I

- I is referred to as the spin quantum number

- Nuclei are also charged so produce magnetic moment along the axis of rotation upon spinning

- Nuclei must have a nonzero spin quantum number and a magnetic dipole moment to produce an NMR signal

FORMS OF ENERGY

Two forms of energy are displayed by nuclei

Mechanical Energy

and

Magnetic Energy

FORMS OF ENERGY

Mechanical Energy

- Nuclear energy due to mass and spin of the nuclei

Mechanical energy of the hydrogen atom

1II2π

hmomentumangularSpin

I = spin quantum number

I = 12 for the proton (1H)

FORMS OF ENERGY

Mechanical Energy

Rules for Predicting I

Mass Number(P+N)

OddOddEvenEven

Atomic Number(Charge P)

OddEvenEvenOdd

Spin QuantumNumber (I)

12 32 52 hellip12 32 52 hellip

01 2 3

FORMS OF ENERGY

Mechanical Energy

- Mass number of oxygen = 16 - Atomic number of oxygen = 8

- Number of neutrons = 16 ndash 8 = 8- Both even numbers so I = 0 and no spin

- Oxygen does not possess a magnetic moment

- Nuclei with I = 0 do not absorb RF radiation and do not give an NMR signal

12C 16O 32S 28Si

FORMS OF ENERGY

Mechanical Energy

- The actual spin number such as 32 52 2 or 3 aredetermined experimentally

- The two most important nuclei in organic chemistry and biochemistry are 13C and 1H (I = 12 for both)

- Elements in the first six rows of the periodic table have at least one stable isotope with a nonzero I

(exceptions Ar Tc Ce Pm Bi Po)

FORMS OF ENERGYMagnetic Energy

- Nuclear energy attributed to the electrical charge of the nucleus

- Electrical charge in motion sets up a magnetic field

- Magnetogyric (gyromagnetic ratio) = γ = microI

- micro = nuclear magnetic moment

- The value of γ is different for each type of nucleus

- NMR is based on the response of magnetically active nuclei to an external applied magnetic field

QUANTIZATION OF NUCLEI

- Given a strong and uniform external magnetic field Bo

- Nucleus lines up in a definite direction relative to the direction of the external field

- Each relative direction is associated with an energy level

- Only certain well-defined energy levels are permitted(energy levels are quantized)

- The number of magnetic quantum states = 2I + 1(number of orientations)

QUANTIZATION OF NUCLEI

Consider 1H

I = 12

- Number of orientations = 2(12) + 1 = 2

- Implies only two energy levels are permitted

- Magnetic quantum number (m) = 12 and -12

Zeeman Splitting- The splitting of the energy levels in a magnetic field

QUANTIZATION OF NUCLEI

- A nucleus with I = 12 can exist in one of two discrete energy levels in an external magnetic field (Bo)

- Magnetic moment of the lower energy level (m = 12) is aligned with the magnetic field

- Magnetic moment of the higher energy level (m = -12) is aligned against the magnetic field

- The two energy levels are separated by ΔE (ΔE is proportional to Bo)

- The lower energy level is magnetically more favored

QUANTIZATION OF NUCLEI

Precession

- Spinning of nucleus causing axis of rotation to move in a circular path about the external magnetic field

- Direction of precession is either with or against the applied magnetic field

- The absorption of radiation causes a transition between these two energy states

- Radiation frequency equals ΔE for transition to occur

QUANTIZATION OF NUCLEI

- ΔE also depends on the magnetic moment micro of the nucleus

The absorption frequency that can result in a transition of ΔE

equation)(LarmorγBωor2π

Bγν o

o

- ω = frequency in radianssecond

- Frequency of RF radiation is directly proportional to the external magnetic field

QUANTIZATION OF NUCLEI

- Energy of precessing nucleus (E) = microBocosθ

- θ = angle between axis of rotation and Bo

- θ changes when a nucleus absorbs energy in the form of RF radiation

Larmor Frequency- The frequency of radiation that can be absorbed by a spinning

charged nucleus in a magnetic field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- Absorption involves flipping of the magnetic moment

- Magnetic moment aligned with the field becomes aligned against the field

- Absorption of RF radiation occurs when the rate of precession becomes equal to the frequency of RF radiation applied

- Nucleus is therefore in an excited state when it becomes aligned against the field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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NMR SPECTROSCOPY

- Technique is used for studying shape and structure of molecules

- For studying 1H 13C 31P 19F 29Si polymers and solids

- For studying reaction kinetics and molecular attractions

- Magnetic Resonance Imaging (MRI) is used for diagnosis of cancer and other medical problems

- Can be considered as absorption spectroscopy

NMR SPECTROSCOPY

- Technique involves absorption of radiowaves by the nuclei of atoms in a molecule located in a magnetic field

- Frquency of radiowaves is on the order of 107 Hz (low energy)

- Radiofrequency (RF) radiation energy = hν

h = Planckrsquos constant = 6626 x 10-34 Jsν = frequency (between 4 and 1000 MHz)

- E is high enough to affect the nuclear spin of atoms in a molecule but too small to vibrate rotate or electronically excite atoms

PROPERTIES OF NUCLEI

- It is assumed that nuclei rotate about an axis

- Spinning of nuclei of atoms in a molecule in a magnetic field absorbs RF radiation

- The direction of the spinning axis changes

- Different atoms in a molecule have unique absorption frequencies (or resonances) if the nuclei possess magnetic moment

PROPERTIES OF NUCLEI

- Nuclei have a nuclear spin represented by I

- I is referred to as the spin quantum number

- Nuclei are also charged so produce magnetic moment along the axis of rotation upon spinning

- Nuclei must have a nonzero spin quantum number and a magnetic dipole moment to produce an NMR signal

FORMS OF ENERGY

Two forms of energy are displayed by nuclei

Mechanical Energy

and

Magnetic Energy

FORMS OF ENERGY

Mechanical Energy

- Nuclear energy due to mass and spin of the nuclei

Mechanical energy of the hydrogen atom

1II2π

hmomentumangularSpin

I = spin quantum number

I = 12 for the proton (1H)

FORMS OF ENERGY

Mechanical Energy

Rules for Predicting I

Mass Number(P+N)

OddOddEvenEven

Atomic Number(Charge P)

OddEvenEvenOdd

Spin QuantumNumber (I)

12 32 52 hellip12 32 52 hellip

01 2 3

FORMS OF ENERGY

Mechanical Energy

- Mass number of oxygen = 16 - Atomic number of oxygen = 8

- Number of neutrons = 16 ndash 8 = 8- Both even numbers so I = 0 and no spin

- Oxygen does not possess a magnetic moment

- Nuclei with I = 0 do not absorb RF radiation and do not give an NMR signal

12C 16O 32S 28Si

FORMS OF ENERGY

Mechanical Energy

- The actual spin number such as 32 52 2 or 3 aredetermined experimentally

- The two most important nuclei in organic chemistry and biochemistry are 13C and 1H (I = 12 for both)

- Elements in the first six rows of the periodic table have at least one stable isotope with a nonzero I

(exceptions Ar Tc Ce Pm Bi Po)

FORMS OF ENERGYMagnetic Energy

- Nuclear energy attributed to the electrical charge of the nucleus

- Electrical charge in motion sets up a magnetic field

- Magnetogyric (gyromagnetic ratio) = γ = microI

- micro = nuclear magnetic moment

- The value of γ is different for each type of nucleus

- NMR is based on the response of magnetically active nuclei to an external applied magnetic field

QUANTIZATION OF NUCLEI

- Given a strong and uniform external magnetic field Bo

- Nucleus lines up in a definite direction relative to the direction of the external field

- Each relative direction is associated with an energy level

- Only certain well-defined energy levels are permitted(energy levels are quantized)

- The number of magnetic quantum states = 2I + 1(number of orientations)

QUANTIZATION OF NUCLEI

Consider 1H

I = 12

- Number of orientations = 2(12) + 1 = 2

- Implies only two energy levels are permitted

- Magnetic quantum number (m) = 12 and -12

Zeeman Splitting- The splitting of the energy levels in a magnetic field

QUANTIZATION OF NUCLEI

- A nucleus with I = 12 can exist in one of two discrete energy levels in an external magnetic field (Bo)

- Magnetic moment of the lower energy level (m = 12) is aligned with the magnetic field

- Magnetic moment of the higher energy level (m = -12) is aligned against the magnetic field

- The two energy levels are separated by ΔE (ΔE is proportional to Bo)

- The lower energy level is magnetically more favored

QUANTIZATION OF NUCLEI

Precession

- Spinning of nucleus causing axis of rotation to move in a circular path about the external magnetic field

- Direction of precession is either with or against the applied magnetic field

- The absorption of radiation causes a transition between these two energy states

- Radiation frequency equals ΔE for transition to occur

QUANTIZATION OF NUCLEI

- ΔE also depends on the magnetic moment micro of the nucleus

The absorption frequency that can result in a transition of ΔE

equation)(LarmorγBωor2π

Bγν o

o

- ω = frequency in radianssecond

- Frequency of RF radiation is directly proportional to the external magnetic field

QUANTIZATION OF NUCLEI

- Energy of precessing nucleus (E) = microBocosθ

- θ = angle between axis of rotation and Bo

- θ changes when a nucleus absorbs energy in the form of RF radiation

Larmor Frequency- The frequency of radiation that can be absorbed by a spinning

charged nucleus in a magnetic field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- Absorption involves flipping of the magnetic moment

- Magnetic moment aligned with the field becomes aligned against the field

- Absorption of RF radiation occurs when the rate of precession becomes equal to the frequency of RF radiation applied

- Nucleus is therefore in an excited state when it becomes aligned against the field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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NMR SPECTROSCOPY

- Technique involves absorption of radiowaves by the nuclei of atoms in a molecule located in a magnetic field

- Frquency of radiowaves is on the order of 107 Hz (low energy)

- Radiofrequency (RF) radiation energy = hν

h = Planckrsquos constant = 6626 x 10-34 Jsν = frequency (between 4 and 1000 MHz)

- E is high enough to affect the nuclear spin of atoms in a molecule but too small to vibrate rotate or electronically excite atoms

PROPERTIES OF NUCLEI

- It is assumed that nuclei rotate about an axis

- Spinning of nuclei of atoms in a molecule in a magnetic field absorbs RF radiation

- The direction of the spinning axis changes

- Different atoms in a molecule have unique absorption frequencies (or resonances) if the nuclei possess magnetic moment

PROPERTIES OF NUCLEI

- Nuclei have a nuclear spin represented by I

- I is referred to as the spin quantum number

- Nuclei are also charged so produce magnetic moment along the axis of rotation upon spinning

- Nuclei must have a nonzero spin quantum number and a magnetic dipole moment to produce an NMR signal

FORMS OF ENERGY

Two forms of energy are displayed by nuclei

Mechanical Energy

and

Magnetic Energy

FORMS OF ENERGY

Mechanical Energy

- Nuclear energy due to mass and spin of the nuclei

Mechanical energy of the hydrogen atom

1II2π

hmomentumangularSpin

I = spin quantum number

I = 12 for the proton (1H)

FORMS OF ENERGY

Mechanical Energy

Rules for Predicting I

Mass Number(P+N)

OddOddEvenEven

Atomic Number(Charge P)

OddEvenEvenOdd

Spin QuantumNumber (I)

12 32 52 hellip12 32 52 hellip

01 2 3

FORMS OF ENERGY

Mechanical Energy

- Mass number of oxygen = 16 - Atomic number of oxygen = 8

- Number of neutrons = 16 ndash 8 = 8- Both even numbers so I = 0 and no spin

- Oxygen does not possess a magnetic moment

- Nuclei with I = 0 do not absorb RF radiation and do not give an NMR signal

12C 16O 32S 28Si

FORMS OF ENERGY

Mechanical Energy

- The actual spin number such as 32 52 2 or 3 aredetermined experimentally

- The two most important nuclei in organic chemistry and biochemistry are 13C and 1H (I = 12 for both)

- Elements in the first six rows of the periodic table have at least one stable isotope with a nonzero I

(exceptions Ar Tc Ce Pm Bi Po)

FORMS OF ENERGYMagnetic Energy

- Nuclear energy attributed to the electrical charge of the nucleus

- Electrical charge in motion sets up a magnetic field

- Magnetogyric (gyromagnetic ratio) = γ = microI

- micro = nuclear magnetic moment

- The value of γ is different for each type of nucleus

- NMR is based on the response of magnetically active nuclei to an external applied magnetic field

QUANTIZATION OF NUCLEI

- Given a strong and uniform external magnetic field Bo

- Nucleus lines up in a definite direction relative to the direction of the external field

- Each relative direction is associated with an energy level

- Only certain well-defined energy levels are permitted(energy levels are quantized)

- The number of magnetic quantum states = 2I + 1(number of orientations)

QUANTIZATION OF NUCLEI

Consider 1H

I = 12

- Number of orientations = 2(12) + 1 = 2

- Implies only two energy levels are permitted

- Magnetic quantum number (m) = 12 and -12

Zeeman Splitting- The splitting of the energy levels in a magnetic field

QUANTIZATION OF NUCLEI

- A nucleus with I = 12 can exist in one of two discrete energy levels in an external magnetic field (Bo)

- Magnetic moment of the lower energy level (m = 12) is aligned with the magnetic field

- Magnetic moment of the higher energy level (m = -12) is aligned against the magnetic field

- The two energy levels are separated by ΔE (ΔE is proportional to Bo)

- The lower energy level is magnetically more favored

QUANTIZATION OF NUCLEI

Precession

- Spinning of nucleus causing axis of rotation to move in a circular path about the external magnetic field

- Direction of precession is either with or against the applied magnetic field

- The absorption of radiation causes a transition between these two energy states

- Radiation frequency equals ΔE for transition to occur

QUANTIZATION OF NUCLEI

- ΔE also depends on the magnetic moment micro of the nucleus

The absorption frequency that can result in a transition of ΔE

equation)(LarmorγBωor2π

Bγν o

o

- ω = frequency in radianssecond

- Frequency of RF radiation is directly proportional to the external magnetic field

QUANTIZATION OF NUCLEI

- Energy of precessing nucleus (E) = microBocosθ

- θ = angle between axis of rotation and Bo

- θ changes when a nucleus absorbs energy in the form of RF radiation

Larmor Frequency- The frequency of radiation that can be absorbed by a spinning

charged nucleus in a magnetic field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- Absorption involves flipping of the magnetic moment

- Magnetic moment aligned with the field becomes aligned against the field

- Absorption of RF radiation occurs when the rate of precession becomes equal to the frequency of RF radiation applied

- Nucleus is therefore in an excited state when it becomes aligned against the field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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PROPERTIES OF NUCLEI

- It is assumed that nuclei rotate about an axis

- Spinning of nuclei of atoms in a molecule in a magnetic field absorbs RF radiation

- The direction of the spinning axis changes

- Different atoms in a molecule have unique absorption frequencies (or resonances) if the nuclei possess magnetic moment

PROPERTIES OF NUCLEI

- Nuclei have a nuclear spin represented by I

- I is referred to as the spin quantum number

- Nuclei are also charged so produce magnetic moment along the axis of rotation upon spinning

- Nuclei must have a nonzero spin quantum number and a magnetic dipole moment to produce an NMR signal

FORMS OF ENERGY

Two forms of energy are displayed by nuclei

Mechanical Energy

and

Magnetic Energy

FORMS OF ENERGY

Mechanical Energy

- Nuclear energy due to mass and spin of the nuclei

Mechanical energy of the hydrogen atom

1II2π

hmomentumangularSpin

I = spin quantum number

I = 12 for the proton (1H)

FORMS OF ENERGY

Mechanical Energy

Rules for Predicting I

Mass Number(P+N)

OddOddEvenEven

Atomic Number(Charge P)

OddEvenEvenOdd

Spin QuantumNumber (I)

12 32 52 hellip12 32 52 hellip

01 2 3

FORMS OF ENERGY

Mechanical Energy

- Mass number of oxygen = 16 - Atomic number of oxygen = 8

- Number of neutrons = 16 ndash 8 = 8- Both even numbers so I = 0 and no spin

- Oxygen does not possess a magnetic moment

- Nuclei with I = 0 do not absorb RF radiation and do not give an NMR signal

12C 16O 32S 28Si

FORMS OF ENERGY

Mechanical Energy

- The actual spin number such as 32 52 2 or 3 aredetermined experimentally

- The two most important nuclei in organic chemistry and biochemistry are 13C and 1H (I = 12 for both)

- Elements in the first six rows of the periodic table have at least one stable isotope with a nonzero I

(exceptions Ar Tc Ce Pm Bi Po)

FORMS OF ENERGYMagnetic Energy

- Nuclear energy attributed to the electrical charge of the nucleus

- Electrical charge in motion sets up a magnetic field

- Magnetogyric (gyromagnetic ratio) = γ = microI

- micro = nuclear magnetic moment

- The value of γ is different for each type of nucleus

- NMR is based on the response of magnetically active nuclei to an external applied magnetic field

QUANTIZATION OF NUCLEI

- Given a strong and uniform external magnetic field Bo

- Nucleus lines up in a definite direction relative to the direction of the external field

- Each relative direction is associated with an energy level

- Only certain well-defined energy levels are permitted(energy levels are quantized)

- The number of magnetic quantum states = 2I + 1(number of orientations)

QUANTIZATION OF NUCLEI

Consider 1H

I = 12

- Number of orientations = 2(12) + 1 = 2

- Implies only two energy levels are permitted

- Magnetic quantum number (m) = 12 and -12

Zeeman Splitting- The splitting of the energy levels in a magnetic field

QUANTIZATION OF NUCLEI

- A nucleus with I = 12 can exist in one of two discrete energy levels in an external magnetic field (Bo)

- Magnetic moment of the lower energy level (m = 12) is aligned with the magnetic field

- Magnetic moment of the higher energy level (m = -12) is aligned against the magnetic field

- The two energy levels are separated by ΔE (ΔE is proportional to Bo)

- The lower energy level is magnetically more favored

QUANTIZATION OF NUCLEI

Precession

- Spinning of nucleus causing axis of rotation to move in a circular path about the external magnetic field

- Direction of precession is either with or against the applied magnetic field

- The absorption of radiation causes a transition between these two energy states

- Radiation frequency equals ΔE for transition to occur

QUANTIZATION OF NUCLEI

- ΔE also depends on the magnetic moment micro of the nucleus

The absorption frequency that can result in a transition of ΔE

equation)(LarmorγBωor2π

Bγν o

o

- ω = frequency in radianssecond

- Frequency of RF radiation is directly proportional to the external magnetic field

QUANTIZATION OF NUCLEI

- Energy of precessing nucleus (E) = microBocosθ

- θ = angle between axis of rotation and Bo

- θ changes when a nucleus absorbs energy in the form of RF radiation

Larmor Frequency- The frequency of radiation that can be absorbed by a spinning

charged nucleus in a magnetic field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- Absorption involves flipping of the magnetic moment

- Magnetic moment aligned with the field becomes aligned against the field

- Absorption of RF radiation occurs when the rate of precession becomes equal to the frequency of RF radiation applied

- Nucleus is therefore in an excited state when it becomes aligned against the field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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PROPERTIES OF NUCLEI

- Nuclei have a nuclear spin represented by I

- I is referred to as the spin quantum number

- Nuclei are also charged so produce magnetic moment along the axis of rotation upon spinning

- Nuclei must have a nonzero spin quantum number and a magnetic dipole moment to produce an NMR signal

FORMS OF ENERGY

Two forms of energy are displayed by nuclei

Mechanical Energy

and

Magnetic Energy

FORMS OF ENERGY

Mechanical Energy

- Nuclear energy due to mass and spin of the nuclei

Mechanical energy of the hydrogen atom

1II2π

hmomentumangularSpin

I = spin quantum number

I = 12 for the proton (1H)

FORMS OF ENERGY

Mechanical Energy

Rules for Predicting I

Mass Number(P+N)

OddOddEvenEven

Atomic Number(Charge P)

OddEvenEvenOdd

Spin QuantumNumber (I)

12 32 52 hellip12 32 52 hellip

01 2 3

FORMS OF ENERGY

Mechanical Energy

- Mass number of oxygen = 16 - Atomic number of oxygen = 8

- Number of neutrons = 16 ndash 8 = 8- Both even numbers so I = 0 and no spin

- Oxygen does not possess a magnetic moment

- Nuclei with I = 0 do not absorb RF radiation and do not give an NMR signal

12C 16O 32S 28Si

FORMS OF ENERGY

Mechanical Energy

- The actual spin number such as 32 52 2 or 3 aredetermined experimentally

- The two most important nuclei in organic chemistry and biochemistry are 13C and 1H (I = 12 for both)

- Elements in the first six rows of the periodic table have at least one stable isotope with a nonzero I

(exceptions Ar Tc Ce Pm Bi Po)

FORMS OF ENERGYMagnetic Energy

- Nuclear energy attributed to the electrical charge of the nucleus

- Electrical charge in motion sets up a magnetic field

- Magnetogyric (gyromagnetic ratio) = γ = microI

- micro = nuclear magnetic moment

- The value of γ is different for each type of nucleus

- NMR is based on the response of magnetically active nuclei to an external applied magnetic field

QUANTIZATION OF NUCLEI

- Given a strong and uniform external magnetic field Bo

- Nucleus lines up in a definite direction relative to the direction of the external field

- Each relative direction is associated with an energy level

- Only certain well-defined energy levels are permitted(energy levels are quantized)

- The number of magnetic quantum states = 2I + 1(number of orientations)

QUANTIZATION OF NUCLEI

Consider 1H

I = 12

- Number of orientations = 2(12) + 1 = 2

- Implies only two energy levels are permitted

- Magnetic quantum number (m) = 12 and -12

Zeeman Splitting- The splitting of the energy levels in a magnetic field

QUANTIZATION OF NUCLEI

- A nucleus with I = 12 can exist in one of two discrete energy levels in an external magnetic field (Bo)

- Magnetic moment of the lower energy level (m = 12) is aligned with the magnetic field

- Magnetic moment of the higher energy level (m = -12) is aligned against the magnetic field

- The two energy levels are separated by ΔE (ΔE is proportional to Bo)

- The lower energy level is magnetically more favored

QUANTIZATION OF NUCLEI

Precession

- Spinning of nucleus causing axis of rotation to move in a circular path about the external magnetic field

- Direction of precession is either with or against the applied magnetic field

- The absorption of radiation causes a transition between these two energy states

- Radiation frequency equals ΔE for transition to occur

QUANTIZATION OF NUCLEI

- ΔE also depends on the magnetic moment micro of the nucleus

The absorption frequency that can result in a transition of ΔE

equation)(LarmorγBωor2π

Bγν o

o

- ω = frequency in radianssecond

- Frequency of RF radiation is directly proportional to the external magnetic field

QUANTIZATION OF NUCLEI

- Energy of precessing nucleus (E) = microBocosθ

- θ = angle between axis of rotation and Bo

- θ changes when a nucleus absorbs energy in the form of RF radiation

Larmor Frequency- The frequency of radiation that can be absorbed by a spinning

charged nucleus in a magnetic field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- Absorption involves flipping of the magnetic moment

- Magnetic moment aligned with the field becomes aligned against the field

- Absorption of RF radiation occurs when the rate of precession becomes equal to the frequency of RF radiation applied

- Nucleus is therefore in an excited state when it becomes aligned against the field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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FORMS OF ENERGY

Two forms of energy are displayed by nuclei

Mechanical Energy

and

Magnetic Energy

FORMS OF ENERGY

Mechanical Energy

- Nuclear energy due to mass and spin of the nuclei

Mechanical energy of the hydrogen atom

1II2π

hmomentumangularSpin

I = spin quantum number

I = 12 for the proton (1H)

FORMS OF ENERGY

Mechanical Energy

Rules for Predicting I

Mass Number(P+N)

OddOddEvenEven

Atomic Number(Charge P)

OddEvenEvenOdd

Spin QuantumNumber (I)

12 32 52 hellip12 32 52 hellip

01 2 3

FORMS OF ENERGY

Mechanical Energy

- Mass number of oxygen = 16 - Atomic number of oxygen = 8

- Number of neutrons = 16 ndash 8 = 8- Both even numbers so I = 0 and no spin

- Oxygen does not possess a magnetic moment

- Nuclei with I = 0 do not absorb RF radiation and do not give an NMR signal

12C 16O 32S 28Si

FORMS OF ENERGY

Mechanical Energy

- The actual spin number such as 32 52 2 or 3 aredetermined experimentally

- The two most important nuclei in organic chemistry and biochemistry are 13C and 1H (I = 12 for both)

- Elements in the first six rows of the periodic table have at least one stable isotope with a nonzero I

(exceptions Ar Tc Ce Pm Bi Po)

FORMS OF ENERGYMagnetic Energy

- Nuclear energy attributed to the electrical charge of the nucleus

- Electrical charge in motion sets up a magnetic field

- Magnetogyric (gyromagnetic ratio) = γ = microI

- micro = nuclear magnetic moment

- The value of γ is different for each type of nucleus

- NMR is based on the response of magnetically active nuclei to an external applied magnetic field

QUANTIZATION OF NUCLEI

- Given a strong and uniform external magnetic field Bo

- Nucleus lines up in a definite direction relative to the direction of the external field

- Each relative direction is associated with an energy level

- Only certain well-defined energy levels are permitted(energy levels are quantized)

- The number of magnetic quantum states = 2I + 1(number of orientations)

QUANTIZATION OF NUCLEI

Consider 1H

I = 12

- Number of orientations = 2(12) + 1 = 2

- Implies only two energy levels are permitted

- Magnetic quantum number (m) = 12 and -12

Zeeman Splitting- The splitting of the energy levels in a magnetic field

QUANTIZATION OF NUCLEI

- A nucleus with I = 12 can exist in one of two discrete energy levels in an external magnetic field (Bo)

- Magnetic moment of the lower energy level (m = 12) is aligned with the magnetic field

- Magnetic moment of the higher energy level (m = -12) is aligned against the magnetic field

- The two energy levels are separated by ΔE (ΔE is proportional to Bo)

- The lower energy level is magnetically more favored

QUANTIZATION OF NUCLEI

Precession

- Spinning of nucleus causing axis of rotation to move in a circular path about the external magnetic field

- Direction of precession is either with or against the applied magnetic field

- The absorption of radiation causes a transition between these two energy states

- Radiation frequency equals ΔE for transition to occur

QUANTIZATION OF NUCLEI

- ΔE also depends on the magnetic moment micro of the nucleus

The absorption frequency that can result in a transition of ΔE

equation)(LarmorγBωor2π

Bγν o

o

- ω = frequency in radianssecond

- Frequency of RF radiation is directly proportional to the external magnetic field

QUANTIZATION OF NUCLEI

- Energy of precessing nucleus (E) = microBocosθ

- θ = angle between axis of rotation and Bo

- θ changes when a nucleus absorbs energy in the form of RF radiation

Larmor Frequency- The frequency of radiation that can be absorbed by a spinning

charged nucleus in a magnetic field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- Absorption involves flipping of the magnetic moment

- Magnetic moment aligned with the field becomes aligned against the field

- Absorption of RF radiation occurs when the rate of precession becomes equal to the frequency of RF radiation applied

- Nucleus is therefore in an excited state when it becomes aligned against the field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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FORMS OF ENERGY

Mechanical Energy

- Nuclear energy due to mass and spin of the nuclei

Mechanical energy of the hydrogen atom

1II2π

hmomentumangularSpin

I = spin quantum number

I = 12 for the proton (1H)

FORMS OF ENERGY

Mechanical Energy

Rules for Predicting I

Mass Number(P+N)

OddOddEvenEven

Atomic Number(Charge P)

OddEvenEvenOdd

Spin QuantumNumber (I)

12 32 52 hellip12 32 52 hellip

01 2 3

FORMS OF ENERGY

Mechanical Energy

- Mass number of oxygen = 16 - Atomic number of oxygen = 8

- Number of neutrons = 16 ndash 8 = 8- Both even numbers so I = 0 and no spin

- Oxygen does not possess a magnetic moment

- Nuclei with I = 0 do not absorb RF radiation and do not give an NMR signal

12C 16O 32S 28Si

FORMS OF ENERGY

Mechanical Energy

- The actual spin number such as 32 52 2 or 3 aredetermined experimentally

- The two most important nuclei in organic chemistry and biochemistry are 13C and 1H (I = 12 for both)

- Elements in the first six rows of the periodic table have at least one stable isotope with a nonzero I

(exceptions Ar Tc Ce Pm Bi Po)

FORMS OF ENERGYMagnetic Energy

- Nuclear energy attributed to the electrical charge of the nucleus

- Electrical charge in motion sets up a magnetic field

- Magnetogyric (gyromagnetic ratio) = γ = microI

- micro = nuclear magnetic moment

- The value of γ is different for each type of nucleus

- NMR is based on the response of magnetically active nuclei to an external applied magnetic field

QUANTIZATION OF NUCLEI

- Given a strong and uniform external magnetic field Bo

- Nucleus lines up in a definite direction relative to the direction of the external field

- Each relative direction is associated with an energy level

- Only certain well-defined energy levels are permitted(energy levels are quantized)

- The number of magnetic quantum states = 2I + 1(number of orientations)

QUANTIZATION OF NUCLEI

Consider 1H

I = 12

- Number of orientations = 2(12) + 1 = 2

- Implies only two energy levels are permitted

- Magnetic quantum number (m) = 12 and -12

Zeeman Splitting- The splitting of the energy levels in a magnetic field

QUANTIZATION OF NUCLEI

- A nucleus with I = 12 can exist in one of two discrete energy levels in an external magnetic field (Bo)

- Magnetic moment of the lower energy level (m = 12) is aligned with the magnetic field

- Magnetic moment of the higher energy level (m = -12) is aligned against the magnetic field

- The two energy levels are separated by ΔE (ΔE is proportional to Bo)

- The lower energy level is magnetically more favored

QUANTIZATION OF NUCLEI

Precession

- Spinning of nucleus causing axis of rotation to move in a circular path about the external magnetic field

- Direction of precession is either with or against the applied magnetic field

- The absorption of radiation causes a transition between these two energy states

- Radiation frequency equals ΔE for transition to occur

QUANTIZATION OF NUCLEI

- ΔE also depends on the magnetic moment micro of the nucleus

The absorption frequency that can result in a transition of ΔE

equation)(LarmorγBωor2π

Bγν o

o

- ω = frequency in radianssecond

- Frequency of RF radiation is directly proportional to the external magnetic field

QUANTIZATION OF NUCLEI

- Energy of precessing nucleus (E) = microBocosθ

- θ = angle between axis of rotation and Bo

- θ changes when a nucleus absorbs energy in the form of RF radiation

Larmor Frequency- The frequency of radiation that can be absorbed by a spinning

charged nucleus in a magnetic field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- Absorption involves flipping of the magnetic moment

- Magnetic moment aligned with the field becomes aligned against the field

- Absorption of RF radiation occurs when the rate of precession becomes equal to the frequency of RF radiation applied

- Nucleus is therefore in an excited state when it becomes aligned against the field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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FORMS OF ENERGY

Mechanical Energy

Rules for Predicting I

Mass Number(P+N)

OddOddEvenEven

Atomic Number(Charge P)

OddEvenEvenOdd

Spin QuantumNumber (I)

12 32 52 hellip12 32 52 hellip

01 2 3

FORMS OF ENERGY

Mechanical Energy

- Mass number of oxygen = 16 - Atomic number of oxygen = 8

- Number of neutrons = 16 ndash 8 = 8- Both even numbers so I = 0 and no spin

- Oxygen does not possess a magnetic moment

- Nuclei with I = 0 do not absorb RF radiation and do not give an NMR signal

12C 16O 32S 28Si

FORMS OF ENERGY

Mechanical Energy

- The actual spin number such as 32 52 2 or 3 aredetermined experimentally

- The two most important nuclei in organic chemistry and biochemistry are 13C and 1H (I = 12 for both)

- Elements in the first six rows of the periodic table have at least one stable isotope with a nonzero I

(exceptions Ar Tc Ce Pm Bi Po)

FORMS OF ENERGYMagnetic Energy

- Nuclear energy attributed to the electrical charge of the nucleus

- Electrical charge in motion sets up a magnetic field

- Magnetogyric (gyromagnetic ratio) = γ = microI

- micro = nuclear magnetic moment

- The value of γ is different for each type of nucleus

- NMR is based on the response of magnetically active nuclei to an external applied magnetic field

QUANTIZATION OF NUCLEI

- Given a strong and uniform external magnetic field Bo

- Nucleus lines up in a definite direction relative to the direction of the external field

- Each relative direction is associated with an energy level

- Only certain well-defined energy levels are permitted(energy levels are quantized)

- The number of magnetic quantum states = 2I + 1(number of orientations)

QUANTIZATION OF NUCLEI

Consider 1H

I = 12

- Number of orientations = 2(12) + 1 = 2

- Implies only two energy levels are permitted

- Magnetic quantum number (m) = 12 and -12

Zeeman Splitting- The splitting of the energy levels in a magnetic field

QUANTIZATION OF NUCLEI

- A nucleus with I = 12 can exist in one of two discrete energy levels in an external magnetic field (Bo)

- Magnetic moment of the lower energy level (m = 12) is aligned with the magnetic field

- Magnetic moment of the higher energy level (m = -12) is aligned against the magnetic field

- The two energy levels are separated by ΔE (ΔE is proportional to Bo)

- The lower energy level is magnetically more favored

QUANTIZATION OF NUCLEI

Precession

- Spinning of nucleus causing axis of rotation to move in a circular path about the external magnetic field

- Direction of precession is either with or against the applied magnetic field

- The absorption of radiation causes a transition between these two energy states

- Radiation frequency equals ΔE for transition to occur

QUANTIZATION OF NUCLEI

- ΔE also depends on the magnetic moment micro of the nucleus

The absorption frequency that can result in a transition of ΔE

equation)(LarmorγBωor2π

Bγν o

o

- ω = frequency in radianssecond

- Frequency of RF radiation is directly proportional to the external magnetic field

QUANTIZATION OF NUCLEI

- Energy of precessing nucleus (E) = microBocosθ

- θ = angle between axis of rotation and Bo

- θ changes when a nucleus absorbs energy in the form of RF radiation

Larmor Frequency- The frequency of radiation that can be absorbed by a spinning

charged nucleus in a magnetic field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- Absorption involves flipping of the magnetic moment

- Magnetic moment aligned with the field becomes aligned against the field

- Absorption of RF radiation occurs when the rate of precession becomes equal to the frequency of RF radiation applied

- Nucleus is therefore in an excited state when it becomes aligned against the field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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FORMS OF ENERGY

Mechanical Energy

- Mass number of oxygen = 16 - Atomic number of oxygen = 8

- Number of neutrons = 16 ndash 8 = 8- Both even numbers so I = 0 and no spin

- Oxygen does not possess a magnetic moment

- Nuclei with I = 0 do not absorb RF radiation and do not give an NMR signal

12C 16O 32S 28Si

FORMS OF ENERGY

Mechanical Energy

- The actual spin number such as 32 52 2 or 3 aredetermined experimentally

- The two most important nuclei in organic chemistry and biochemistry are 13C and 1H (I = 12 for both)

- Elements in the first six rows of the periodic table have at least one stable isotope with a nonzero I

(exceptions Ar Tc Ce Pm Bi Po)

FORMS OF ENERGYMagnetic Energy

- Nuclear energy attributed to the electrical charge of the nucleus

- Electrical charge in motion sets up a magnetic field

- Magnetogyric (gyromagnetic ratio) = γ = microI

- micro = nuclear magnetic moment

- The value of γ is different for each type of nucleus

- NMR is based on the response of magnetically active nuclei to an external applied magnetic field

QUANTIZATION OF NUCLEI

- Given a strong and uniform external magnetic field Bo

- Nucleus lines up in a definite direction relative to the direction of the external field

- Each relative direction is associated with an energy level

- Only certain well-defined energy levels are permitted(energy levels are quantized)

- The number of magnetic quantum states = 2I + 1(number of orientations)

QUANTIZATION OF NUCLEI

Consider 1H

I = 12

- Number of orientations = 2(12) + 1 = 2

- Implies only two energy levels are permitted

- Magnetic quantum number (m) = 12 and -12

Zeeman Splitting- The splitting of the energy levels in a magnetic field

QUANTIZATION OF NUCLEI

- A nucleus with I = 12 can exist in one of two discrete energy levels in an external magnetic field (Bo)

- Magnetic moment of the lower energy level (m = 12) is aligned with the magnetic field

- Magnetic moment of the higher energy level (m = -12) is aligned against the magnetic field

- The two energy levels are separated by ΔE (ΔE is proportional to Bo)

- The lower energy level is magnetically more favored

QUANTIZATION OF NUCLEI

Precession

- Spinning of nucleus causing axis of rotation to move in a circular path about the external magnetic field

- Direction of precession is either with or against the applied magnetic field

- The absorption of radiation causes a transition between these two energy states

- Radiation frequency equals ΔE for transition to occur

QUANTIZATION OF NUCLEI

- ΔE also depends on the magnetic moment micro of the nucleus

The absorption frequency that can result in a transition of ΔE

equation)(LarmorγBωor2π

Bγν o

o

- ω = frequency in radianssecond

- Frequency of RF radiation is directly proportional to the external magnetic field

QUANTIZATION OF NUCLEI

- Energy of precessing nucleus (E) = microBocosθ

- θ = angle between axis of rotation and Bo

- θ changes when a nucleus absorbs energy in the form of RF radiation

Larmor Frequency- The frequency of radiation that can be absorbed by a spinning

charged nucleus in a magnetic field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- Absorption involves flipping of the magnetic moment

- Magnetic moment aligned with the field becomes aligned against the field

- Absorption of RF radiation occurs when the rate of precession becomes equal to the frequency of RF radiation applied

- Nucleus is therefore in an excited state when it becomes aligned against the field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

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FORMS OF ENERGY

Mechanical Energy

- The actual spin number such as 32 52 2 or 3 aredetermined experimentally

- The two most important nuclei in organic chemistry and biochemistry are 13C and 1H (I = 12 for both)

- Elements in the first six rows of the periodic table have at least one stable isotope with a nonzero I

(exceptions Ar Tc Ce Pm Bi Po)

FORMS OF ENERGYMagnetic Energy

- Nuclear energy attributed to the electrical charge of the nucleus

- Electrical charge in motion sets up a magnetic field

- Magnetogyric (gyromagnetic ratio) = γ = microI

- micro = nuclear magnetic moment

- The value of γ is different for each type of nucleus

- NMR is based on the response of magnetically active nuclei to an external applied magnetic field

QUANTIZATION OF NUCLEI

- Given a strong and uniform external magnetic field Bo

- Nucleus lines up in a definite direction relative to the direction of the external field

- Each relative direction is associated with an energy level

- Only certain well-defined energy levels are permitted(energy levels are quantized)

- The number of magnetic quantum states = 2I + 1(number of orientations)

QUANTIZATION OF NUCLEI

Consider 1H

I = 12

- Number of orientations = 2(12) + 1 = 2

- Implies only two energy levels are permitted

- Magnetic quantum number (m) = 12 and -12

Zeeman Splitting- The splitting of the energy levels in a magnetic field

QUANTIZATION OF NUCLEI

- A nucleus with I = 12 can exist in one of two discrete energy levels in an external magnetic field (Bo)

- Magnetic moment of the lower energy level (m = 12) is aligned with the magnetic field

- Magnetic moment of the higher energy level (m = -12) is aligned against the magnetic field

- The two energy levels are separated by ΔE (ΔE is proportional to Bo)

- The lower energy level is magnetically more favored

QUANTIZATION OF NUCLEI

Precession

- Spinning of nucleus causing axis of rotation to move in a circular path about the external magnetic field

- Direction of precession is either with or against the applied magnetic field

- The absorption of radiation causes a transition between these two energy states

- Radiation frequency equals ΔE for transition to occur

QUANTIZATION OF NUCLEI

- ΔE also depends on the magnetic moment micro of the nucleus

The absorption frequency that can result in a transition of ΔE

equation)(LarmorγBωor2π

Bγν o

o

- ω = frequency in radianssecond

- Frequency of RF radiation is directly proportional to the external magnetic field

QUANTIZATION OF NUCLEI

- Energy of precessing nucleus (E) = microBocosθ

- θ = angle between axis of rotation and Bo

- θ changes when a nucleus absorbs energy in the form of RF radiation

Larmor Frequency- The frequency of radiation that can be absorbed by a spinning

charged nucleus in a magnetic field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- Absorption involves flipping of the magnetic moment

- Magnetic moment aligned with the field becomes aligned against the field

- Absorption of RF radiation occurs when the rate of precession becomes equal to the frequency of RF radiation applied

- Nucleus is therefore in an excited state when it becomes aligned against the field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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FORMS OF ENERGYMagnetic Energy

- Nuclear energy attributed to the electrical charge of the nucleus

- Electrical charge in motion sets up a magnetic field

- Magnetogyric (gyromagnetic ratio) = γ = microI

- micro = nuclear magnetic moment

- The value of γ is different for each type of nucleus

- NMR is based on the response of magnetically active nuclei to an external applied magnetic field

QUANTIZATION OF NUCLEI

- Given a strong and uniform external magnetic field Bo

- Nucleus lines up in a definite direction relative to the direction of the external field

- Each relative direction is associated with an energy level

- Only certain well-defined energy levels are permitted(energy levels are quantized)

- The number of magnetic quantum states = 2I + 1(number of orientations)

QUANTIZATION OF NUCLEI

Consider 1H

I = 12

- Number of orientations = 2(12) + 1 = 2

- Implies only two energy levels are permitted

- Magnetic quantum number (m) = 12 and -12

Zeeman Splitting- The splitting of the energy levels in a magnetic field

QUANTIZATION OF NUCLEI

- A nucleus with I = 12 can exist in one of two discrete energy levels in an external magnetic field (Bo)

- Magnetic moment of the lower energy level (m = 12) is aligned with the magnetic field

- Magnetic moment of the higher energy level (m = -12) is aligned against the magnetic field

- The two energy levels are separated by ΔE (ΔE is proportional to Bo)

- The lower energy level is magnetically more favored

QUANTIZATION OF NUCLEI

Precession

- Spinning of nucleus causing axis of rotation to move in a circular path about the external magnetic field

- Direction of precession is either with or against the applied magnetic field

- The absorption of radiation causes a transition between these two energy states

- Radiation frequency equals ΔE for transition to occur

QUANTIZATION OF NUCLEI

- ΔE also depends on the magnetic moment micro of the nucleus

The absorption frequency that can result in a transition of ΔE

equation)(LarmorγBωor2π

Bγν o

o

- ω = frequency in radianssecond

- Frequency of RF radiation is directly proportional to the external magnetic field

QUANTIZATION OF NUCLEI

- Energy of precessing nucleus (E) = microBocosθ

- θ = angle between axis of rotation and Bo

- θ changes when a nucleus absorbs energy in the form of RF radiation

Larmor Frequency- The frequency of radiation that can be absorbed by a spinning

charged nucleus in a magnetic field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- Absorption involves flipping of the magnetic moment

- Magnetic moment aligned with the field becomes aligned against the field

- Absorption of RF radiation occurs when the rate of precession becomes equal to the frequency of RF radiation applied

- Nucleus is therefore in an excited state when it becomes aligned against the field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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QUANTIZATION OF NUCLEI

- Given a strong and uniform external magnetic field Bo

- Nucleus lines up in a definite direction relative to the direction of the external field

- Each relative direction is associated with an energy level

- Only certain well-defined energy levels are permitted(energy levels are quantized)

- The number of magnetic quantum states = 2I + 1(number of orientations)

QUANTIZATION OF NUCLEI

Consider 1H

I = 12

- Number of orientations = 2(12) + 1 = 2

- Implies only two energy levels are permitted

- Magnetic quantum number (m) = 12 and -12

Zeeman Splitting- The splitting of the energy levels in a magnetic field

QUANTIZATION OF NUCLEI

- A nucleus with I = 12 can exist in one of two discrete energy levels in an external magnetic field (Bo)

- Magnetic moment of the lower energy level (m = 12) is aligned with the magnetic field

- Magnetic moment of the higher energy level (m = -12) is aligned against the magnetic field

- The two energy levels are separated by ΔE (ΔE is proportional to Bo)

- The lower energy level is magnetically more favored

QUANTIZATION OF NUCLEI

Precession

- Spinning of nucleus causing axis of rotation to move in a circular path about the external magnetic field

- Direction of precession is either with or against the applied magnetic field

- The absorption of radiation causes a transition between these two energy states

- Radiation frequency equals ΔE for transition to occur

QUANTIZATION OF NUCLEI

- ΔE also depends on the magnetic moment micro of the nucleus

The absorption frequency that can result in a transition of ΔE

equation)(LarmorγBωor2π

Bγν o

o

- ω = frequency in radianssecond

- Frequency of RF radiation is directly proportional to the external magnetic field

QUANTIZATION OF NUCLEI

- Energy of precessing nucleus (E) = microBocosθ

- θ = angle between axis of rotation and Bo

- θ changes when a nucleus absorbs energy in the form of RF radiation

Larmor Frequency- The frequency of radiation that can be absorbed by a spinning

charged nucleus in a magnetic field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- Absorption involves flipping of the magnetic moment

- Magnetic moment aligned with the field becomes aligned against the field

- Absorption of RF radiation occurs when the rate of precession becomes equal to the frequency of RF radiation applied

- Nucleus is therefore in an excited state when it becomes aligned against the field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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QUANTIZATION OF NUCLEI

Consider 1H

I = 12

- Number of orientations = 2(12) + 1 = 2

- Implies only two energy levels are permitted

- Magnetic quantum number (m) = 12 and -12

Zeeman Splitting- The splitting of the energy levels in a magnetic field

QUANTIZATION OF NUCLEI

- A nucleus with I = 12 can exist in one of two discrete energy levels in an external magnetic field (Bo)

- Magnetic moment of the lower energy level (m = 12) is aligned with the magnetic field

- Magnetic moment of the higher energy level (m = -12) is aligned against the magnetic field

- The two energy levels are separated by ΔE (ΔE is proportional to Bo)

- The lower energy level is magnetically more favored

QUANTIZATION OF NUCLEI

Precession

- Spinning of nucleus causing axis of rotation to move in a circular path about the external magnetic field

- Direction of precession is either with or against the applied magnetic field

- The absorption of radiation causes a transition between these two energy states

- Radiation frequency equals ΔE for transition to occur

QUANTIZATION OF NUCLEI

- ΔE also depends on the magnetic moment micro of the nucleus

The absorption frequency that can result in a transition of ΔE

equation)(LarmorγBωor2π

Bγν o

o

- ω = frequency in radianssecond

- Frequency of RF radiation is directly proportional to the external magnetic field

QUANTIZATION OF NUCLEI

- Energy of precessing nucleus (E) = microBocosθ

- θ = angle between axis of rotation and Bo

- θ changes when a nucleus absorbs energy in the form of RF radiation

Larmor Frequency- The frequency of radiation that can be absorbed by a spinning

charged nucleus in a magnetic field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- Absorption involves flipping of the magnetic moment

- Magnetic moment aligned with the field becomes aligned against the field

- Absorption of RF radiation occurs when the rate of precession becomes equal to the frequency of RF radiation applied

- Nucleus is therefore in an excited state when it becomes aligned against the field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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QUANTIZATION OF NUCLEI

- A nucleus with I = 12 can exist in one of two discrete energy levels in an external magnetic field (Bo)

- Magnetic moment of the lower energy level (m = 12) is aligned with the magnetic field

- Magnetic moment of the higher energy level (m = -12) is aligned against the magnetic field

- The two energy levels are separated by ΔE (ΔE is proportional to Bo)

- The lower energy level is magnetically more favored

QUANTIZATION OF NUCLEI

Precession

- Spinning of nucleus causing axis of rotation to move in a circular path about the external magnetic field

- Direction of precession is either with or against the applied magnetic field

- The absorption of radiation causes a transition between these two energy states

- Radiation frequency equals ΔE for transition to occur

QUANTIZATION OF NUCLEI

- ΔE also depends on the magnetic moment micro of the nucleus

The absorption frequency that can result in a transition of ΔE

equation)(LarmorγBωor2π

Bγν o

o

- ω = frequency in radianssecond

- Frequency of RF radiation is directly proportional to the external magnetic field

QUANTIZATION OF NUCLEI

- Energy of precessing nucleus (E) = microBocosθ

- θ = angle between axis of rotation and Bo

- θ changes when a nucleus absorbs energy in the form of RF radiation

Larmor Frequency- The frequency of radiation that can be absorbed by a spinning

charged nucleus in a magnetic field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- Absorption involves flipping of the magnetic moment

- Magnetic moment aligned with the field becomes aligned against the field

- Absorption of RF radiation occurs when the rate of precession becomes equal to the frequency of RF radiation applied

- Nucleus is therefore in an excited state when it becomes aligned against the field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

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QUANTIZATION OF NUCLEI

Precession

- Spinning of nucleus causing axis of rotation to move in a circular path about the external magnetic field

- Direction of precession is either with or against the applied magnetic field

- The absorption of radiation causes a transition between these two energy states

- Radiation frequency equals ΔE for transition to occur

QUANTIZATION OF NUCLEI

- ΔE also depends on the magnetic moment micro of the nucleus

The absorption frequency that can result in a transition of ΔE

equation)(LarmorγBωor2π

Bγν o

o

- ω = frequency in radianssecond

- Frequency of RF radiation is directly proportional to the external magnetic field

QUANTIZATION OF NUCLEI

- Energy of precessing nucleus (E) = microBocosθ

- θ = angle between axis of rotation and Bo

- θ changes when a nucleus absorbs energy in the form of RF radiation

Larmor Frequency- The frequency of radiation that can be absorbed by a spinning

charged nucleus in a magnetic field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- Absorption involves flipping of the magnetic moment

- Magnetic moment aligned with the field becomes aligned against the field

- Absorption of RF radiation occurs when the rate of precession becomes equal to the frequency of RF radiation applied

- Nucleus is therefore in an excited state when it becomes aligned against the field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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QUANTIZATION OF NUCLEI

- ΔE also depends on the magnetic moment micro of the nucleus

The absorption frequency that can result in a transition of ΔE

equation)(LarmorγBωor2π

Bγν o

o

- ω = frequency in radianssecond

- Frequency of RF radiation is directly proportional to the external magnetic field

QUANTIZATION OF NUCLEI

- Energy of precessing nucleus (E) = microBocosθ

- θ = angle between axis of rotation and Bo

- θ changes when a nucleus absorbs energy in the form of RF radiation

Larmor Frequency- The frequency of radiation that can be absorbed by a spinning

charged nucleus in a magnetic field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- Absorption involves flipping of the magnetic moment

- Magnetic moment aligned with the field becomes aligned against the field

- Absorption of RF radiation occurs when the rate of precession becomes equal to the frequency of RF radiation applied

- Nucleus is therefore in an excited state when it becomes aligned against the field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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QUANTIZATION OF NUCLEI

- Energy of precessing nucleus (E) = microBocosθ

- θ = angle between axis of rotation and Bo

- θ changes when a nucleus absorbs energy in the form of RF radiation

Larmor Frequency- The frequency of radiation that can be absorbed by a spinning

charged nucleus in a magnetic field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- Absorption involves flipping of the magnetic moment

- Magnetic moment aligned with the field becomes aligned against the field

- Absorption of RF radiation occurs when the rate of precession becomes equal to the frequency of RF radiation applied

- Nucleus is therefore in an excited state when it becomes aligned against the field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- Absorption involves flipping of the magnetic moment

- Magnetic moment aligned with the field becomes aligned against the field

- Absorption of RF radiation occurs when the rate of precession becomes equal to the frequency of RF radiation applied

- Nucleus is therefore in an excited state when it becomes aligned against the field

QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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QUANTIZATION OF NUCLEI

Consider a Proton (1H)

- The nucleus losses the energy absorbed and returns to the unexcited state

- The nucleus reabsorbs and losses energy alternatively

- The nucleus becomes excited and unexcited alternatively and is said to be in a state of resonance

QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

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QUANTIZATION OF NUCLEI

Units of Magnetic Field Strength

- Tesla (T) or gauss (G)

- 1 T = 104 G

- The RF radiation absorbed by a proton = 60 MHz if Bo = 141 T

- 60 MHz 13C NMR will use Bo of 56 T

- Proton is assumed if the nucleus is not specified

THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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THE BOLTZMANN DISTRIBUTION

- Defines the ratio of excited nuclei to unexcited nuclei

kT2πγhB-ΔEkT

o

oee

N

N

- N = number of excited nuclei- No = number of unexcited nuclei

- The Boltzmann ratio in NMR is typically close to 100(low sensitivity technique)

- Signal is seen only when there is an excess of molecules in the ground state

THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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THE BOLTZMANN DISTRIBUTION

- The excess unexcited state nuclei is called the Boltzmann excess

- Boltzmann excess is maximum in the absence of radiation

- The sample is said to be saturated if all excess nuclei is excitedand absorption approaches zero

(occurs when RF radiation intensity is too high)

- NMR signal intensity increases with increasing field strength

ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

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ABSORPTION LINES

Factors Affecting Absorption Linewidth- Homogeneous field

- Relaxation time

- Magic angle spinning (MAS)

Resolution of absorption lines depends on - closeness of the absorption lines to each other

- absorption linewidth

ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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ABSORPTION LINES

Homogeneous Field

- Bo must be constant throughout sample

- Bo must be stable over required data collection time

- This produces constant frequency in all parts of sample

- Results in narrow absorption lines

ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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ABSORPTION LINES

Relaxation Time

- The length of time that an excited nucleus stays in the excited state

- Large ΔE results in large frequency changes resulting in broad absorption lines

Relaxation- The process of an excited state nucleus losing its energy and

returning to the ground state

Two modes of relaxation longitudinal (T1) and transverse (T2)

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- An excited state nucleus losses energy and drops to the low spin state

- The energy lost is absorbed by the lattice in the form of increased vibrational and rotational motion

- A very small increase in sample temperature results in T1

- Occurs in most liquid samples

- T1 is large for crystalline solids and viscous liquids

ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

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ABSORPTION LINES

Longitudinal Relaxation (T1 Spin-lattice)

- T1 depends on the magnetogyric ratio (γ) and the lattice mobility

Lattice- Consists of both absorbing and nonabsorbing nuclei

ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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ABSORPTION LINES

Transverse Relaxation (T2 Spin-spin)

- An excited nucleus losses energy to a nearby unexcited nucleus

- The excited nucleus goes to the ground state and the unexcited nucleus goes to the excited state

- There is no net change in energy of the system

- Average excited state lifetime decreases and broad linewidth results

- T2 is very short for solids

ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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ABSORPTION LINES

Magic Angle Spinning (MAS)

- Absorption linewidths are generally broad in solids

- Nuclei in solids cannot freely line up in Bo (have fixed orientation)

- NMR signals depend on the orientation of nuclei to Bo

- Solid samples spin at 5476o (the magic angle) to Bo which resultsin narrow line spectra (the usual angle is 90o for liquids)

- Very high frequencies are used for spinning (5-15 kHz)

ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

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ABSORPTION LINES

Other Factors

- The presence of ions

- The presence of paramagnetic molecules (O2)

- Nuclei with quadrupole moments (I gt 12) (14N)

FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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FTNMR

- Employs pulse wave instead of continuous wave

- Short and strong RF pulses are applied to sample (few seconds between pulses)

- An RF pulse through a coil of wire around the sample is used to generate a second magnetic field B1

- B1 is at right angles (90o) to Bo (gives highest signal)

- Current is induced in the wire when the pulse is discontinued

FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

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FTNMR

- The induced current is the NMR signal

- The signal undergoes free induction decay (FID)

- The FID signal is processed using FT to produce the spectrum

Advantages- All resonances are excited simultaneously

- Improved signal-to-noise ratio

- Improved sensitivity

CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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CHEMICAL SHIFT

- The variation in absorption frequency

- Nuclei in different chemical environments within a molecule absorb at slightly different frequencies

- Absorption frequency depends on the chemical structure of the molecule

Chemical Shift Anisotropy- The phenomenon in solids of nuclei having different chemical

shifts as a result of orientation in space

CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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CHEMICAL SHIFT

- Electrons of the nuclei rotate and generate a small magnetic field σBo

- σ = screening constant (diamagnetic shielding constant)

- σBo opposes Bo hence nuclei are shielded slightly from Bo by the orbiting electrons

- The effective magnetic field exposed to the nucleusBeff = Bo ndash σBo

- The change in absorption frequency due to shielding is the chemical shift

CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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CHEMICAL SHIFT

- Chemical shifts are measured relative to a standard nucleus

- Tetramethylsilane [TMS Si(CH3)4] is the most common

ppm10xν

νν)(shiftChemical 6

NMR

Rs δ

νs = resonant frequency of a given nucleusνR = resonant frequency of the reference nucleus

νNMR = the spectrometer frequency

- Chemical shift is expressed as ppm

CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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CHEMICAL SHIFT

- The x-axis of NMR spectra has units of chemical shift

- TMS signal is a single peak located at 00 ppm

- Magnetic field increases from left to right along the x-axis

- Shielding increases from left to right

- A nucleus to the right is more shielded than to the left

- δ increases from right to left

- δ is constant for constant Bo

CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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CHEMICAL SHIFT

- Deshielded means higher δ and shielded means lower δ

- Chemically identical nuclei are shifted from chemically different nuclei

- δ is used for characterizing unknown compounds or compounds whose empirical formulas are known but structures are unknown

- δ of unknowns are compared to those of known compounds under the same conditions

SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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SPIN-SPIN COUPLING

- Each peak in a spectrum under high resolution is seen to compose of several peaks due to spin-spin coupling (spin-spin splitting)

- Protons on a given carbon are chemically equivalent to each other

- Protons on different carbons may be chemically different

Consider butanal (CH3CH2CH2CHO)- There are 4 chemically different proton groups

- Low resolution spectrum would show 4 proton peaks- Spin-spin splitting also results

- Protons on a given carbon split the peak of adjacent protons

MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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MULTIPLICITY

- The number of peaks that results due to splitting of a given peak

- Multiplicity = 2nI + 1- n = number of equivalent protons on the adjacent carbon atoms

- I = 12 for proton implies multiplicity = n + 1

For two adjacent groups causing splitting- Multiplicity = (2nI + 1)(2nacuteI + 1)

- Protons on the same carbon cannot split themselves (no coupling)

SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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SPIN-SPIN COUPLING CONSTANT (J)

- The magnitude of separation between split peaks

- Is a measure of the magnetic interaction between the nuclei

- Usually expressed in Hz

- The magnitude of J remains the same if Bo changes

- J decreases as protons move further apart

- J provides structural information about a compound

For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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For the 13C spectra

I = 12

- C-C spin-spin coupling is not usually observed in C NMR spectra

- J values range between 20 and 200 Hz

- Spin-spin relaxation between 13C and 1H is possible

- Peak area gives relative numbers of each type of nucleus(shown as step-like line on spectrum)

SPIN-SPIN COUPLING CONSTANT (J)

COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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COMPONENTS OF FTNMR

- Magnet- RF generator

- Sample chamber (probe)- Pulse generator

- RF receiver- Computer

- Signal processing electronics

- Older NMR instruments were CWNMR (continuous wave)- Modern NMR instruments are FTNMR

SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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SAMPLE HOLDER

- Tube-shaped so is called the sample tube

- Transparent to RF radiation

- Chemically inert

- Commonly made of glass or pyrex with a plastic cap

- About 6-7 inches long and 18 inches diameter

- Flow cells are used for hyphenated techniques(HPLC-NMR)

SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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SAMPLE PROBE

- Is the sample chamber and the heart of the NMR system

- Holds the sample fixed in the magnetic field

- Spins the sample holder with the aid of air turbines

- Contains the coils for transmission and detection of NMR signals

- The RF transmitting and receiving coil surrounds the sample holder

- A single coil is used and is centered in the magnet

SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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SAMPLE PROBE

- Have variable temperature control

- The coil is tuned to the precession frequency of the nucleus

- A strong RF pulse is transmitted to the sample for excitation

- The same coil detects the signal by picking up the FID signal from the relaxing nuclei when the pulse is stopped

- Separate fixed frequency probes are used for each nucleus for high sensitivity (1H 13C 19F 29Si etc)

- Variable frequency probes have low sensitivity

MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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MAGNET

- Must produce a homogeneous magnetic field throughout sample

- Must be strong and stable

- Commercial magnets range from 14 T to 164 T (may be more)

- Modern instruments use superconducting solenoid magnets(coil wire is made of NbSn or NbTi)

- Superconducting SHIM COILS are wound around the main coil to improve homogeneity

MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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MAGNET

- The superconducting coils are then submerged in liquid helium

- The liquid helium reservoir is encased in a liquid nitrogen reservoir

Shimming- Adjusting shim coils for field homogeneity

- Compensates for sample composition volume and temperature

- Done automatically in modern instruments

BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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BORE

- Contains the air conduits for spinning the sample holder

- Bore size is usually between 5-10 cm in diameter

- Larger diameters are used for solids

- Very large bore size is used for human body (MRI systems)

- Field homogeneity is better in narrow bore magnets

- The magnet quenches when the superconducting coils warm up so must be kept cold

RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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RF GENERATION AND DETECTION

- RF crystal oscillator is used for generation of RF radiation

- Output is amplified and filtered to produce monochromatic RF radiation

- Pulsed radiation (~ 10 micros duration) falls on the sample

- Rectangular pulse of about 500 MHz frequency may be used

- Pulse provides a range of frequencies that is able to simultaneously excite all nuclei whose resonances fall within the range

SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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SIGNAL INTEGRATOR AND COMPUTER

- Signal integrator measures the peak area

- Area is usually superimposed on the spectrum displayed on the computer

- Much of the instrumentation is under computer control(shimming autosamplers pulse sequences spinning etc)

- A single NMR data file can be processed in less than one second

Hand-held NMR instruments are currently available

SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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SAMPLE PREPARATION

- Neat nonviscous samples are run as is

- About 05 mL of the liquid sample is placed in the sample tube

- Suitable solvents may be used to mix samples

Suitable Solvents- Chemically inert toward sample and sample holder

- Very simple or no NMR absorption spectrum- Easily recovered for nondestructive analysis

- Contain no protons for proton NMR(CCl4 CS2 D2O CDCl3)

SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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SAMPLE PREPARATION

- Analyte concentration is generally ~ 2-10

- Sample are purged to remove O2 and filtered to remove Fe(O2 and Fe are paramagnetic so cause line broadening)

- Suitable soluble solid sample size is 2-3 mg in 05 mL solvent

- Solid samples are solvated to give narrow bandwidth(soaked in solvent to swell and become jelly-like)

- Solid samples are run in instruments equipped with MAS

- NMR is not used for gas samples (sample is dissolved in liquid)

CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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CHEMICAL EXCHANGELabile proton

- Proton that exchanges between substances in a solution

Consider CH3OH and H2O- H in the OH group exchanges with H in H2O

- Spin-spin splitting due to labile proton is not observed if exchange rate is greater than change in resonance frequency

- Compounds with NH and OH protons have spectrum dependent on temperature concentration and solvent polarity

- Chemical exchange also includes bond-formation and cleavage

DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

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DOUBLE RESONANCE

- Employs two different RF sources and a variety of pulse sequences

- Used to simplify NMR spectra

- Used to sort out complex splitting patterns (complex multiplets)

2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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2D NMR

- Information is obtained in a second frequency dimension

- Easier to interpret

- More structural information is provided

- Employs selective manipulation of specific nuclear spins followed by interaction between nuclear spins

2D experiments consist of- Pulse followed by evolution period t1

- A second pulse followed by acquisition period t2

2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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2D NMR

COSY- Correlated spectroscopy

- Homonuclear 2D experiment- Plot of δ vs δ identifies spin-coupled resonances

HETCOR- Heteronuclear chemical shift correlated experiment

- Heteronuclear 2D experiment- Usually connects 1H resonances with 13C resonances

- Plots 13C chemical shift vs 1H chemical shift

2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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2D NMR

INADEQUATE- Incredible natural abundance double quantum transfer

- Multipulse- Allows observation of natural abundance 13C-13C coupling

APT- Attached proton test

- Multipulse- Used to distinguish between even and odd numbers of protons

coupled to 13C through one bond- Even numbers of bound protons give positive peaks- Odd numbers of bound protons give negative peaks

2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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2D NMR

DEPT- Distortionless enhancement by polarization transfer

- Multipulse- Only 13C nuclei with the same number of bound protons have

enhanced resonances

NOESY- Nuclear overhauser effect spectroscopy

- Identifies dipolar coupled nuclei within certain distances- Identifies connectivities through cross correlation

(3D 4D 5D NMR are also available)

HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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HYPHENATED NMR TECHNIQUES

HPLC-NMR

- HPLC separates the components of a mixture

- NMR takes the spectrum and identifies the structure of each component

- Uses a flow cell but not a static cell

- Used for analysis of metabolites in body fluids

HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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HYPHENATED NMR TECHNIQUES

HPLC-NMR-MS

- For pharmaceutical research

- Samples are separated in the HPLC column

- NMR determines the structural information of components

- MS determines the molecular weight and additional structural information

- MS is a destructive technique so placed after the NMR

MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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MAGNETIC RESONANCE IMAGING (MRI)

- For imaging of the human body

- A highly uniform magnetic field is used (field strength 08-3 T)

- A linear magnetic field gradient is superimposed

- Bore of the magnet is large enough to accommodate human body

- Physical position of nuclei is located in three dimensions

- Various body tissue components are revealed

MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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MAGNETIC RESONANCE IMAGING (MRI)

- High resolution 3D measurements reveal abnormalies such as fractures and cancerous growth

- Noninvasive and nondestructive

- No side effects

- Better than X-ray which can cause body damage

- Used for studying cancer epilepsy heart problems stroke etc

APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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APPLICATIONS OF NMRQualitative

- For the determination of the molecular structure of compounds

- Positions and shape of peaks are used to study reaction kinetics

- For studying the lifetimes of reactants intermediates and products

- Relative proton counts are obtained from NMR spectra but not the actual numbers

- Actual numbers can be determined if the empirical formula (from elemental analysis) and molar mass

(from mass spectrometry) are known

APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

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APPLICATIONS OF NMRQuantitative

- Determination of percent purity of chemicals

- Quality control of raw materials and finished products

- Determination of the amount of fat in milk and F in toothpaste

- Analysis of polymers and ceramics (amorphous vs crystalline)

- 31P NMR for analysis of phosphorus-based insecticides

- Determination of the octane number in gasoline

LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

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LIMITATIONS OF NMR

- Limited to the measurement of nuclei with magnetic moments

- May be less sensitive to other spectroscopic methods

- Ions do not respond to NMR but contribute to line broadening

THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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THE PROTON SPECTRA

- Used to identify functional groups

- Absorbance of TMS is set to 00 ppm

- Multiplicity = n + 1 or (n + 1)(nacute + 1)

NMR absorption spectra are characterized by- Chemical shift of peaks

- Spin-spin splitting of peaks

THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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THE PROTON SPECTRA

The spectrum provides the following information

- Number of different types of protons in a molecule(number of proton resonances)

- The type of proton identified by the chemical shift

- The number of adjacent equivalent protons (from multiplicity)

- Relative number of each type of proton (from area)

- Peaks of adjacent protons lsquoleanrsquo towards each other(unsymmetrical multiplets)

THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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THE PROTON SPECTRA

Aliphatic Alkanes- Methyl group (CH3) absorbs at ~ 088 ppm

- Methylene group (CH2) absorbs at ~ 126 ppm

Alkyle Halides- Halides substituted on an alkane

- The halides deshield the protons attached to the same C atom- Peaks are shifted to higher resonance frequencies

- F shows the largest deshielding effect and I the least- I = 12 for F nucleus

- F couples but the other halogens do not (H-F amp F-F coupling)

THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

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THE PROTON SPECTRA

Alkenes- Have two characteristic types of protons

Vinyl Proton- Proton attached to the double bond

- Chemical shift of vinyl protons at ~ 45-6 ppm- Lack of free rotation so spin-spin coupling is complicated

Allylic Proton- Proton on C atom adjacent to the double bond

- Slightly deshielded by the double bond- Chemical shift ~ 12-25 ppm

THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

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THE PROTON SPECTRA

Alkynes- The hydrogen atom at the end of the triple bond of a terminal

alkyne is known as the acetylenic hydrogen

- Absorbs at ~ 18 ppm

- Protons on C next to the triple bond are similarly affected as those in alkenes

- Chemical shifts are similar to those of the allylic protons in alkenes

THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

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THE PROTON SPECTRA

Aromatic Compounds

Ring Protons- Protons on the aromatic ring

- Highly deshielded by π electrons- Absorb peaks between 65 and 80 ppm

- Few other protons may absorb in this region

Benzylic Protons- Protons on the C adjacent to the aromatic ring

- Deshielded by the ring but to a lesser extent than the ring protons- Absorb peaks between 22 and 28 ppm

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Alcohols- Contain OH group attached to a C atom

- δ of the OH proton depends on temperature solvent concentration- Covers 05-50 ppm range for aliphatic alcohols

- Aromatic ring deshields the OH protons- Protons on the C to which the OH is attached absorb ~ 31-38 ppm

- No splitting occurs between OH proton and other protons on the same C (rapid exchange)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

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THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Aldehydes- Have terminal CHO group

- Aldehydic proton occurs between 90 and 100 ppm- Protons on the adjacent C to the C=O group are slightly deshielded

(occur between 20-25 ppm)

Ketones- Contains a nonterminal C=O group

- Protons on adjacent C to the C=O group are similar to the aldehyde(between 20 and 25)

THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

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THE PROTON SPECTRA

Oxygen Containing Organic Compounds

Carboxylic Acids- Contain the COOH functional group

- Acidic COOH proton absorbs between 10 and 13 ppm

- Protons on adjacent C atom absorb between 21 and 25 ppm

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

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THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

- Have complicated NMR spectra- δ depends on temperature solvent and concentration

14N nucleus has I = 1- Proton on N should split into 2(1)+1 = 3 peaks

- Proton on C adjacent to N should split into 3 peaks- Fast exchange range so splitting is not usually seen- NH groups are broad due to quadrupole moments

- Peaks are sharp singlets or weak broad signals

THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

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THE PROTON SPECTRA

Nitrogen Containing Organic Compounds

Amines RNH2

- N-H peak occurs between 05 and 40 ppm- Proton on adjacent C to N is shifted between 23 and 3 ppm

Amides RCONH2

- N-H peak occurs between 5 and 9 ppm- Proton on adjacent C to N is shifted between 2 and 25 ppm

THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

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THE 13C SPECTRA

- C is the central element in organic chemistry

- 12C has zero NMR signal so 13C is very essential

- 13C has 11 abundance so has very weak signal

- 13C chemical shift is up to 200 ppm (vs 10 ppm for 1H)

- Overlaps are minimized due to wide range

- FTNMR makes 13C readily determined

- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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- Unique peaks are seen for chemically different C atoms

- Absence of spin-spin splitting of adjacent C atoms

- Techniques are available to decouple 13C and 1H coupling

- Singlets are therefore seen for each different chemically different C atom

THE 13C SPECTRA

Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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Heteronuclear Decoupling

- Coupling between 13C and 1H gives complex spectra

- Multiplets overlap

- Broad band decoupling eliminates this problem

- Decoupling is by irradiation with a wide RF frequency range and chemically different C atoms give singlets

THE 13C SPECTRA

Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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Nuclear Overhauser Effect (NOE)

- The increase in peak areas due to elimination of peak splitting by broad band decoupling

- NOE can double 13C peak intensity

- Hence peak intensity may not be related to number of C atoms

- NOE must be eliminated when quantitative analysis is required

THE 13C SPECTRA

Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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Solid Samples

- Line broadening is observed due to the different orientations

- Broadening is eliminated by MAS (5-15 kHz at 5475o)

- Line broadening also occurs due to interaction between 13C and 1H

- Requires longer time due to long spin-lattice relaxation

- FTNMR systems use high-power dipolar decoupling MAS and cross-polarization to produce narrow line spectra

THE 13C SPECTRA

- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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- Chemical shift of alkane C atoms between 0 and 75 ppm

- Aromatic and alkene between 100 and 160 ppm

- COOH carbon between 170 and 180 ppm

- Aldehydic carbon between 190 and 210 ppm

THE 13C SPECTRA

Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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Monosubstituted Benzene Rings

- Ortho C atoms are equivalent

- Meta C atoms are equivalent

- Substituted C and para C are each unique

- Should show C peaks in the aromatic region

THE 13C SPECTRA

• INSTRUMENTAL ANALYSIS CHEM 4811
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