Nuclear Magnetic Nuclear Magnetic Resonance Spectroscopy Resonance Spectroscopy

(NMR)(NMR)

Dr AKM Shafiqul IslamDr AKM Shafiqul Islam

School of Bioprocess School of Bioprocess EngineeringEngineering

X-ray

UV/Visible

Infrared

Microwave

Radio Frequency

Bond-breaking

Electronic

Vibrational

Rotational

Nuclear andElectronic Spin

REGION ENERGY TRANSITIONS

Types of Energy Transitions in Each Region Types of Energy Transitions in Each Region of the Electromagnetic Spectrumof the Electromagnetic Spectrum

(NMR)

What is NMR Spectroscopy?What is NMR Spectroscopy?

• Nuclear Magnetic ResonanceNuclear Magnetic Resonance

• Radio Frequency Absorption Spectra of atomic Radio Frequency Absorption Spectra of atomic nuclei in substances subjected to magnetic fields.nuclei in substances subjected to magnetic fields.

• Spectral Dispersion is Sensitive to the chemical Spectral Dispersion is Sensitive to the chemical environment via “coupling” to the electrons environment via “coupling” to the electrons surrounding the nuclei.surrounding the nuclei.

• Interactions can be interpreted in terms of Interactions can be interpreted in terms of structure, bonding, reactivity structure, bonding, reactivity

Do all nuclei possess spin? No.

Only those with an odd atomic number or odd atomic mass:

13C 1H 14N 15N 31P

12C 16O 32S 6 8 16

NMR active

NMR inactive

6 1 7 7 15

Two common types of NMR spectroscopy Two common types of NMR spectroscopy used are;used are;

1.1. 11H NMR (proton NMR) is used to determine H NMR (proton NMR) is used to determine the number of hydrogen atom in the the number of hydrogen atom in the moleculemolecule

2.2. 1313C NMR (carbon NMR) used to determine C NMR (carbon NMR) used to determine the number of carbon atom in the molecule. the number of carbon atom in the molecule.

• The source of energy in NMR is radio waves. The source of energy in NMR is radio waves. Radiation in the radiofrequency region of Radiation in the radiofrequency region of the electromagnetic spectrum are very high the electromagnetic spectrum are very high wavelength, so it correspond to low wavelength, so it correspond to low frequency and energy.frequency and energy.

• When the low energy radio waves interact When the low energy radio waves interact with the molecule, they change the nuclear with the molecule, they change the nuclear spin of some elements, including spin of some elements, including 11H and H and 1313C.C.

The Use of NMR SpectroscopyThe Use of NMR Spectroscopy

• Used to determine relative location of atoms Used to determine relative location of atoms within a moleculewithin a molecule

• Most helpful spectroscopic technique in organic Most helpful spectroscopic technique in organic chemistrychemistry

• Maps carbon-hydrogen framework of moleculesMaps carbon-hydrogen framework of molecules

• Depends on very strong magnetic fieldsDepends on very strong magnetic fields

1H NMR Tells Us…1H NMR Tells Us…

• The number of different absorptions implies how many The number of different absorptions implies how many different types of hydrogens are present.different types of hydrogens are present.

• The amount of shielding (chemical shift) is determined The amount of shielding (chemical shift) is determined by each hydrogen's environment, and so we get by each hydrogen's environment, and so we get information about the local electronic surroundings for information about the local electronic surroundings for each hydrogen.each hydrogen.

• The intensities of the signals tell us the number of The intensities of the signals tell us the number of identical hydrogens.identical hydrogens.

• The splittings of each signal tells us about the other The splittings of each signal tells us about the other groups proximate to the hydrogens in question. groups proximate to the hydrogens in question.

Magnetic Properties of Nuclei – Magnetic Properties of Nuclei – Nuclear SpinNuclear Spin

• 1313C and C and 11H nuclei spin on their axes.H nuclei spin on their axes.

• Magnetic fields are produced by Magnetic fields are produced by electric currents.electric currents.

• The circulation of electrical charge The circulation of electrical charge generates a magnetic moment along generates a magnetic moment along the axis of spin, so that these nuclei the axis of spin, so that these nuclei act like tiny magnets, having a small act like tiny magnets, having a small magnetic field.magnetic field.

• The The 1313C and C and 11H nuclei both possess spin. H nuclei both possess spin.

• The nucleus can be irradiated with radio The nucleus can be irradiated with radio waves to cause a spin transition. The waves to cause a spin transition. The frequency of the this transition depends on frequency of the this transition depends on the electronic environment of the nucleus. the electronic environment of the nucleus.

• By measuring the frequency, we can learn By measuring the frequency, we can learn about the way in which the nucleus is about the way in which the nucleus is bonded to other atoms in the molecule.bonded to other atoms in the molecule.

Nuclear Spin (cont.)Nuclear Spin (cont.)

In the earth’s very In the earth’s very weak magnetic field weak magnetic field (0.000057 T), the (0.000057 T), the nuclear magnetic nuclear magnetic moments of these moments of these magnetic nuclei are magnetic nuclei are randomly oriented.randomly oriented.

Influence of Magnetic Influence of Magnetic FieldField

No magnetic field with magnetic field

B0

• When an external energy source (When an external energy source (hh) that ) that match the energy difference (match the energy difference (E) between E) between these two states is applied, energy is these two states is applied, energy is absorbed, causing the nucleus to “spin absorbed, causing the nucleus to “spin flip” from one orientation to another.flip” from one orientation to another.

• The energy difference between these two The energy difference between these two nucleus spin states corresponds to the nucleus spin states corresponds to the low-frequency radiation in the RF region of low-frequency radiation in the RF region of the electromagnetic spectra.the electromagnetic spectra.

400 MHz NMR 400 MHz NMR SpectrometerSpectrometer

400 MHz Avance System

Unixcomputer

electroniccontrols

super-conducting

magnet

400 MHz 400 MHz SuperconductiSuperconducti

ng Magnetng Magnet

• magnetic field strength • 9.4 Tesla (94,000 gauss)

400 MHz is the frequency usedfor proton detection in this field

NMR sample tube and holderdescend into center of magnet

Keep metal (ferromagnetic)objects, pacemakers, and

credit cards several feet away!

NMR Sample NMR Sample PositionPosition

(prior to release into (prior to release into probe)probe)

Liquid Helium-269°C (4.2 K)

Liquid Nitrogen-196°C (77.4 K)

NMR samplepositioned

at top of probe

Superconducting magnetsrequire continuous cooling.

5 mm NMRsample tube

RF energy

upper levelof NMR solution

11H NMR spectrumH NMR spectrum

• An NMR Spectrum plot the intensity of a peak An NMR Spectrum plot the intensity of a peak against its chemical shift measured in ppm. against its chemical shift measured in ppm.

• Increasing chemical shift is plotted from right Increasing chemical shift is plotted from right to left.to left.

• The term upfield and downfield describe the The term upfield and downfield describe the relative location of signals. Upfield means to relative location of signals. Upfield means to right. The (CHright. The (CH33))33C- peak is upfield from the C- peak is upfield from the CHCH33O- peak.O- peak.

1. Chemical shift - each nonequivalent hydrogen gives a unique signal along the x-axis.

2. Spin-spin coupling - neighboring NMR active nuclei split each others signal.

3. Integration - peak areas are proportional to # of equivalent nuclei giving a signal.

1H-NMR Data: 3 components

Interpretation of NMR Spectra

Two signals split intomultiple peaks havinga ratio of areas of 2:3.

“quartet” “triplet”

Interpretation of Interpretation of 11H-NMR DataH-NMR Data

• The NMR absorption are measured The NMR absorption are measured tetramethylsilane (TMS). TMS is a volatile tetramethylsilane (TMS). TMS is a volatile and inert compound that give a single peak and inert compound that give a single peak upfield from the typical NMR absorptionupfield from the typical NMR absorption

10 9 8 7 6 5 4 3 2 1 0

TMS reference signaltetramethylsilaneSi(CH3)4 = 0.00

increasing deshielding

increasing shielding

• What makes NMR spectroscopy so useful is the fact What makes NMR spectroscopy so useful is the fact that the same type of magnetic nuclei (that the same type of magnetic nuclei (11H or H or 1313C) in C) in different electronic environments absorb different different electronic environments absorb different amounts of rf energies and have different chemical amounts of rf energies and have different chemical shifts, giving several different signals.shifts, giving several different signals.

• The number of signals indicates the different kinds The number of signals indicates the different kinds (environments) of (environments) of 11H or H or 1313C nuclei.C nuclei.

• The position of the signals (chemical shifts) indicate The position of the signals (chemical shifts) indicate what kind of what kind of 11H or H or 1313C nuclei they are (alkane, vinylic, C nuclei they are (alkane, vinylic, aromatic, etc.).aromatic, etc.).

NMR SPECTROSCOPYNMR SPECTROSCOPY

• An NMR spectrum maps the carbon-hydrogen An NMR spectrum maps the carbon-hydrogen framework of an organic molecule, since each framework of an organic molecule, since each distinct distinct 11H or H or 1313C nucleus is represented by a C nucleus is represented by a distinct NMR signal in the spectrum.distinct NMR signal in the spectrum.

• Some important terms to understand in NMR Some important terms to understand in NMR spectroscopy are:spectroscopy are:a. Equivalent hydrogensa. Equivalent hydrogensb. Line of Integrationb. Line of Integrationc. Chemical Shiftc. Chemical Shiftd. Shieldingd. Shieldinge. Spin-spin splittinge. Spin-spin splitting

1. How many absorptions are there? 1. How many absorptions are there? (indicates number of equivalent kinds of Hs.)(indicates number of equivalent kinds of Hs.)

2. What are their intensities? 2. What are their intensities? (indicates the (indicates the number of Hs of each type.)number of Hs of each type.)

3. What are their chemical shifts? 3. What are their chemical shifts? (indicates (indicates the environment of each type of H.)the environment of each type of H.)

4. What is the splitting pattern: 4. What is the splitting pattern: (indicates (indicates neighboring hydrogens.)neighboring hydrogens.)

Interpretation of NMR Interpretation of NMR SpectraSpectra