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NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY
Dr.P.L.Prasunamba
Retired Professor in Chemistry
Osmania University
Hyderabad
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NMR Spectroscopy
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STRUCTURE ELUCIDATION OF Organic compounds
• Chemical degradation
• Spectral methods
uv Spectroscopy
IR Spectroscopy
NMR Spectroscopy
Mass Spectrometry
• Synthesis
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Electromagnetic Spectrum
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Radiofrequency waves
wavelength (λ ) 1012 Ao / 108 µ / 104 cm / 100 m
Wave-number (ν = 1/λ) 10-4 cm-1
Frequency (c/ λ) 3 x 106 Hz = 3 MHz
Energy (E = hv) 10-8 eV / 10-4 cal
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NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY
1. This technique is based on Magnetic properties of Nuclei,
hence the name Nuclear Magnetic Resonance Spectroscopy.
2. Different energy levels for absorption of radiant energy and
excitation are not existing in the molecule. They are to be
generated.
3. The two different energy states involved are
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Nuclear Magnetic Resonance
Definition:
Nuclei with I>0 under the influence of magnetic field when exposed to Radiofrequency waves resonance occurs
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Which Nuclei
• Nuclei = Protons + Neutrons
• Nuclear spin quantum number/ Angular momentum of nuclei = I
• Spin quantum number of H = +1/2 & -1/2
• Are all nuclei magnetic ?
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Magnetic & Non-magnetic Nuclei
Nuclei
Odd mass number1H,13C,17O,19F
I=1/2,3/2,5/2
MAGNETIC
Even mass numberOdd(Z)+Odd(n)
2D,10B,14NI=1,2,3
MAGNETIC
Even mass numberEven(Z)+ Even(n)
12C, 18OI=0
NON - MAGNETIC
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1H-NMR / PMR spectroscopy
1H-NMR spectrum of a molecule gives all the
information about the hydrogens present in it.
number of signals
position of the signals
intensities of the signals
splitting of a signal
8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0
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Applications of PMR
• Structural elucidation
• Reaction mechanisms
• Electronegativity information
• Aromaticity
• Sterochemistry
• Confirmational analysis
• Optical purity
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Explaining NMR
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The NMR Spectrometer
A Modern NMR Instrument
Radio Wave
Transceiver
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Principles of NMR
• Measures nuclear magnetism or changes in nuclear magnetism in a molecule
• NMR spectroscopy measures the absorption of light (radio waves) due to changes in nuclear spin orientation
• NMR only occurs when a sample is in a strong magnetic field
• Different nuclei absorb at different energies (frequencies)
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Protons (and other nucleons) Have Spin
Spin up Spin down
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Each Spinning Proton is Like a “Mini-Magnet”
Spin up Spin down
N
S
N
S
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magnetic moment
μ
A Spinning Gyroscope
in a Gravity Field
A Spinning Charge
in a Magnetic Field
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How these two orientations of protons of different
energy generated?
This has become possible due to magnetic properties of nucleus.
Nuclei are spinning around their own axis just like the earth. The
nuclei are positively charged. The circulating charge generates
magnetic field, hence nucleus is equivalent to a tiny magnet
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Bigger Magnets are Better
low frequency high frequency
Increasing magnetic field strength
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Different Isotopes Absorb at Different Frequencies
low frequency high frequency
2H 15N 13C 19F 1H
30 MHz 50 MHz 125 MHz 480 MHz 500 MHz
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Because of the behavior of nucleus as a tiny bar magnet, it will be oriented
in two different ways.
One of this is aligned with the magnetic field (parallel) and the other
aligned against the applied magnetic field (anti parallel).
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In the absence of applied magnetic field the nuclei are oriented
randomly
When we apply an external magnetic
field the magnets alter direction.
Nuclear magnet
External magnet
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Resonance
• The nuclei is precessing in the magnetic field.
• To allow the nuclei to flip from E1 to E2 one must irradiate at the precessional frequency,
ω
• ω is a function of H0
H0
H0
14,000 25,000
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Resonance
• Irradiate with radio frequency to equalize the population then allow to relax back to normal distribution.
r.frelax
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Two Energy States
The magnetic fields of the spinning nuclei will align either withthe external field, or against the field.
A photon with the right amount of energy can be absorbed and cause the spinning proton to flip.
NMR RequiReMeNt (cONtd…)
θ
ΔE
E1 -µHO
E2 µHO
HO
Δ E = E2 – E1
= μH0 – ( - μH0 )
= 2 μH0
E = hn , hn = 2 μH0
n= 2 μ /h. H0
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NMR equation (contd..)
• At Resonance n= 2 μ /h. H0
• For Hydrogen μ = 1.41x 10-23 erg/gauss
h = 6.624 x 10-27
• If H0 = 14,000 gauss
• n = 2 x 1.41 x 10-23 x 14000/6.624 x 10-27
= 60 x 106 Hz or 60 MHz
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Magnetic Shielding
• If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained.
• But protons are surrounded by electrons that shield them from the external field.
• Circulating electrons create an induced magnetic field that opposes the external magnetic field.
Shielded Protons
Magnetic field strength must be increased for a shielded proton to flip at the same frequency.
Spin Relaxation
• There are two primary causes of spin relaxation:
• Spin – lattice relaxation, T1, longitudinal relaxation
lattice
Spin- spin relaxation, T2, transverse relaxation.
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Chemical Shift
• Measured in parts per million.
• Ratio of shift downfield from TMS (Hz) to total spectrometer frequency (Hz).
• Same value for 60, 100, or 300 MHz machine.
• Called the delta scale.
Tetramethylsilane
• We need a reference peak to measure from.
• TMS is added to the sample.
• Since silicon is less electronegative than carbon, TMS protons are highly shielded. Signal defined as zero.
• Organic protons absorb downfield (to the left) of the TMS signal.
Si
CH3
CH3
CH3
H3C
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Delta Scale
=>
NMR Units of Measurement
• Parts per million (d) 0 - 12 ppm (for 1H)
nobs - nref
ppm is proportional to frequency
nref
x 106ppm =
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1H NMR Spectra Exhibit...
• Chemical Shifts (peaks at different frequencies or ppm values)
• Splitting Patterns (from spin coupling)
• Different Peak Intensities (# 1H)
8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 41
The NMR Graph
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=>
1H-NMR spectrum of methyl acetate
Downfield: the shift of an NMR signal to the left on the
chart paper
Upfield: the shift of an NMR signal to the right on the
chart paper
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Typical Values
=>
Factors affecting chemical shift
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Location of Signals
• More electronegative atoms deshield more and give larger shift values.
• Effect decreases with distance.
• Additional electronegative atoms cause increase in chemical shift.
=>
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Vinyl Protons, d5-d6
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=>
Aromatic Protons, d7-d8
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=>
Acetylenic Protons, d2.5
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=>
Aldehyde Proton, d9-d10
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=>
Electronegative
oxygen atom
O-H and N-H Signals
• Chemical shift depends on concentration.
• Hydrogen bonding in concentrated solutions deshield the protons, so signal is around d3.5 for N-H and d4.5 for O-H.
• Proton exchanges between the molecules broaden the peak.
Carboxylic Acid Proton, d10+
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NMR Signals
• The number of signals shows how many different kinds of protons are present.
• The location of the signals shows how shielded or deshielded the proton is.
• The intensity of the signal shows the number of protons of that type.
• Signal splitting shows the number of protons on adjacent atoms.
Equivalent hydrogens: have the same chemical environment
– a molecule with 1 set of equivalent hydrogens gives 1 NMR signal
H3 C
C C
CH3
H3 C CH3
CH3 CCH3 ClCH2 CH2 Cl
Propanone(Acetone)
1,2-Dichloro-ethane
Cyclopentane 2,3-Dimethyl-2-butene
O
a molecule with 2 or more sets of equivalent hydrogens gives
a different NMR signal for each set
CH3CHCl
Cl Cl
C C
CH3
H H
O
Cyclopent-anone
(2 s ignals)
1,1-Dichloro-ethane
(2 signals )
(Z)-1-Chloro-propene
(3 signals)
Cyclohexene (3 signals)
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Number of SignalsEquivalent hydrogens have the same chemical
shift.
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Intensity of Signals• The area under each peak is proportional to
the number of protons.
• Shown by integral trace.
How Many Hydrogens?
When the molecular formula is known, each integral rise can be assigned to a particular number of hydrogens.
NMR Peak Intensities
C - CH C - CH2 C - CH3
Y|
Y|
Y|
X Z X Z X Z
AUC = 1 AUC = 2 AUC = 360
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Spin-Spin Coupling
• Many 1H NMR spectra exhibit peak splitting (doublets, triplets, quartets)
• This splitting arises from adjacent hydrogens (protons) which cause the absorption frequencies of the observed 1H to jump to different levels
• These energy jumps are quantized and the number of levels or splittings = n + 1 where “n” is the number of nearby 1H’s
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Spin-Spin Splitting
• Nonequivalent protons on adjacent carbons have magnetic fields that may align with or oppose the external field.
• This magnetic coupling causes the proton to absorb slightly downfield when the external field is reinforced and slightly upfield when the external field is opposed.
• All possibilities exist, so signal is split.
Spin-Spin Coupling
C - Y C - CH C - CH2 C - CH3
H|
H|
H|
H|
singlet doublet triplet quartet
X ZX Z X Z X Z
J
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1,1,2-Tribromoethane
Nonequivalent protons on adjacent carbons.
Doublet: 1 Adjacent Proton
Triplet: 2 Adjacent Protons
1H-NMR spectrum of 1,1-dichloroethane
CH3 -CH-Cl
Cl
For these hydrogens, n = 1;their signal is split into(1 + 1) = 2 peaks; a doublet
For this hydrogen, n = 3;its s ignal is split into(3 + 1) = 4 peaks; a quartet
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The N + 1 Rule
If a signal is split by N equivalent protons,
it is split into N + 1 peaks.
Spin Coupling Intensities
1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1
1 1 1 12
Pascal’s Triangle
1 13 3
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Range of Magnetic Coupling
• Equivalent protons do not split each other.
• Protons bonded to the same carbon will split each other only if they are not equivalent.
• Protons on adjacent carbons normally will couple.
• Protons separated by four or more bonds will not couple.
Splitting for Ethyl Groups
Splitting for Isopropyl Groups
Time Dependence
• Molecules are tumbling relative to the magnetic field, so NMR is an averaged spectrum of all the orientations.
• Axial and equatorial protons on cyclohexane interconvert so rapidly that they give a single signal.
• Proton transfers for OH and NH may occur so quickly that the proton is not split by adjacent protons in the molecule.
Hydroxyl Proton
• Ultrapure samples of ethanol show splitting.
• Ethanol with a small amount of acidic or basic impurities will not show splitting.
N-H Proton
• Moderate rate of exchange.
• Peak may be broad.
Understanding the Protons Song
We can derive the following information:
• The number of different types of protons (the number of signals)
• The environment of the protons (the chemical shift)
• The number of protons on the neighbouring carbons (the signal multiplicity)
• The number of protons responsible for each signal (the integral)
We shall use Paracetamol as an example
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Paracetamol from an NMR Experiment
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