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Nuclear Magnetic
Resonance Spectroscopy-A Review
P.V.R.SATISH KUMAR
ASSISTANT PROFESSOR OF ORGANIC CHEMISTRY
DEPARTMENT OF CHEMISTRY
MRPG COLLEGE, VIZIANAGARAM
M.Sc.(Previous)Chemistry,
Second semester-Organic Chemistry
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PVRSK 2
Introduction NMR is the most powerful tool available for
organic structure determination.
It is used to study a wide variety of nuclei: 1H
13C
15N
19F 31P
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PVRSK 3
Nuclear Spin A nucleus with an odd atomic number or
an odd mass number has a nuclear spin.
The spinning charged nucleus generates
a magnetic field.
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The nuclei of some atoms have a property called SPIN.
NUCLEAR SPINNUCLEAR SPIN
These nuclei behave as if
they were spinning.
This is like the spin property
of an electron, which can have
two spins: +1/2 and -1/2 .
Each spin-active nucleus has a number of spins defined by
its spin quantum number, I.
.. we dont know if they actually do spin!
The spin quantum numbers of some common nuclei follow .
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Element 1H 2H 12C 13C 14N 16O 17O 19F
Nuclear Spin
Quantum No 1/2 1 0 1/2 1 0 5/2 1/2( I )
No. of Spin 2 3 0 2 3 0 6 2
States
Spin Quantum Numbers of Some Common NucleiSpin Quantum Numbers of Some Common Nuclei
Elements with either odd mass or odd atomic number
have the property of nuclear spin.
The number of spin states is 2I + 1,
where I is the spin quantum number.
The most abundant isotopes of C and O do not have spin.
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NUCLEAR SPIN STATES - HYDROGEN NUCLEUS
+ 1/2 - 1/2
The two states
are equivalent
in energy in theabsence of a
magnetic or an
electric field.
+ +
The spin of the positively
charged nucleus generates
a magnetic moment vector, Q.Q
Q
TWO SPIN STATES
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When a charged particle such as a proton spins on its axis, itcreates a magnetic field. Thus, the nucleus can be considered
to be a tiny bar magnet.
Normally, these tiny bar magnets are randomly oriented in
space. However, in the presence of a magnetic field B0, they
are oriented with or against this applied field.
The energy difference between these two states is very small
(
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PVRSK 8
External Magnetic FieldWhen placed in an external field, spinning
protons act like bar magnets.
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THE RESONANCE PHENOMENONTHE RESONANCE PHENOMENON
absorption of energy by the
spinning nucleus
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Nuclear Spin Energy LevelsNuclear Spin Energy Levels
Bo
+1/2
-1/2
In a strongmagnetic field (Bo)
the two spin states
differ in energy.
aligned
unaligned
N
S 10PVRSK
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Absorption of EnergyAbsorption of Energy
Bo
+1/2
-1/2
+1/2
-1/2
(E = hR
(E
quantized
Radiofrequency
Applied
Field
Aligned
Opposed
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Bo
(E
+ 1/2
- 1/2
= kBo = hR
degenerate
at Bo = 0
increasing magnetic field strength
THE ENERGY SEPARATIONTHE ENERGY SEPARATION
DEPENDS ON BDEPENDS ON Boo
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TheThe LarmorLarmor Equation!!!Equation!!!
B0
R!
T
KR !
T
Bo
K is a constant which is different foreach atomic nucleus (H, C, N, etc)
(E = kBo = hR can be transformed into
gyromagnetic
ratio K
strength of the
magnetic field
frequency of
the incoming
radiation that
will cause a
transition
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WHEN A SPIN-ACTIVE HYDROGEN ATOM IS
PLACED IN A STRONG MAGNETIC FIELD
.. IT BEGINS TO PRECESS
A SECOND EFFECT OF A STRONG MAGNETIC FIELDA SECOND EFFECT OF A STRONG MAGNETIC FIELD
OPERATION OF AN NMR SPECTROMETER DEPENDS
ON THIS RESULT
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N
S
[ Nuclei precess at
frequency[ when
placed in a strong
magnetic field.
hR
IfR = [ thenenergy will be
absorbed and
the spin will
invert.
NUCLEAR
MAGNETIC
RESONANCE
NMR
RADIOFREQUENCY
40 - 600 MHz
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1H 99.98% 1.00 42.6 267.53
1.41 60.0
2.35 100.0
7.05 300.0
2H 0.0156% 1.00 6.5 41.1
7.05 45.8
13C 1.108% 1.00 10.7 67.282.35 25.0
7.05 75.0
19F 100.0% 1.00 40.0 251.7
Resonance Frequencies of Selected NucleiResonance Frequencies of Selected Nuclei
Isotope Abundance Bo (Tesla) Frequency(MHz) K(radians/Tesla)
4:1
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The strength of the NMR signal depends on the
Population Difference of the two spin states
resonanceinduced
emission
excess
population
Radiation
induces both
upward and
downward
transitions.
For a net positive signalthere must be an excess
of spins in the lower state.
Saturation = equal populations = no signal
POPULATION AND SIGNAL STRENGTHPOPULATION AND SIGNAL STRENGTH
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PVRSK 18
Two Energy States
The magnetic fields ofthe spinning nucleiwill align eitherwith
the external field, oragainstthe field.
A photon with the rightamount of energy
can be absorbedand cause thespinning proton toflip. =>
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PVRSK 19
(Eand Magnet Strength Energy difference is proportional to the
magnetic field strength.
(E= hR = K h B0
2T
Gyromagnetic ratio, K, is a constant for
each nucleus (26,753 s-1gauss-1 for H).
In a 14,092 gauss field, a 60 MHzphoton is required to flip a proton.
Low energy, radio frequency. =>
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PVRSK 20
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 inducedmagnetic field that opposes the external
magnetic field. =>
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PVRSK 21
Shielded Protons
Magnetic field strength must be increased
for a shielded proton to flip at the same
frequency.
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PVRSK 22
Protons in a MoleculeDepending on their chemical
environment, protons in a molecule are
shielded by different amounts.
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PVRSK 23
NMR Signals The numberof signals shows how many
different kinds of protons are present.
The location of the signals shows howshielded or deshielded the proton is.
The intensityof the signal shows the
number of protons of that type. Signal splittingshows the number of
protons on adjacent atoms. =>
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PVRSK 24
The NMR Spectrometer
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PVRSK 25
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PVRSK 26
The NMR Graph
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PVRSK 27
Internal standards:
Tetramethylsilane(TMS)
TMS is added to the sample.
Since silicon is less electronegative than carbon, TMS protonsare highly shielded. Signal defined as zero.
It gives a single sharp line from twelve magnetically equivalentprotons.
It is chemically inert and miscible with a large range of solvents.
It is highly volatile and hence can be easily be removed if thesample has to be recovered.
It does not involve in intermolecular association with the sample. Organic protons absorb downfield (to the left) of the TMS signal.
=>
Si
CH3
CH3
CH3
H3C
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Internal standards
Sodium salt of 3-(trimethyl silyl)
propane sulphonate.
Used as an internal standard for water
soluble organic compounds in D2O
solvent.
PVRSK 28
O
S
O
-O
Si
sodium 3-(trimethylsilyl) propane sulphonate
Na+
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NMR Solvents
CCl4 - Ideal Solvent (Aprotic And Cheap)
CDCl3 - Useful To Dissolve Varying Polarityi.e., From Non Polar To Considerable Polar
DMSO-d6 Dissolving Relatively InsolubleHeterocyclic Compounds . But Have Disadvantages
(Viscous, Mild Oxidising , Nonvolatile(140c), Affinity
With Water.)
CH3OH-d4 - Polar solvent for high polar compounds& salts .Its disadvantage is susceptibility to exchange
ionizable protons like -OH, -NH2 and CONH2, -CH2-CO- etc.
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NMR Solvents
D2O- high polar solvent limited in practice to salts only.Disadvantage : it exchanges all acidic protons readily.
C6D6- anisotropic specialised solvent .used in routinework.it is often added with CDCl3.
CF3COOH- for amines and heterocyclic compounds.
PVRSK 30
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PVRSK 31
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.
=>
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PVRSK 32
Delta Scale
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PVRSK 33
Location of Signals
More electronegative
atoms deshield more and
give larger shift values.
Effect decreases withdistance.
Additional electronegative
atoms cause increase in
chemical shift.
=>
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PVRSK 34
Typical Values
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PVRSK 35
Aromatic Protons, H7-H8
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PVRSK 36
Vinyl Protons, H5-H6
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PVRSK 37
Acetylenic Protons, H2.5
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PVRSK 38
Aldehyde Proton, H9-H10
=>
Electronegative
oxygen atom
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PVRSK 39
O-H and N-H Signals Chemical shift depends on concentration.
Hydrogen bonding in concentrated
solutions deshield the protons, so signal
is around H3.5 for N-H and H4.5 for O-H.
Proton exchanges between the molecules
broaden the peak.=>
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PVRSK 40
Carboxylic Acid
Proton, H10+
=>
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PVRSK 41
Number of Signals
Equivalent hydrogens have the samechemical shift.
=>
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PVRSK 42
Intensity of Signals
The area under each peak isproportional to the number of protons.
Shown by integral trace.
=>
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PVRSK 43
How Many Hydrogens?
When the molecular formula is known,each integral rise can be assigned to aparticular number of hydrogens.
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PVRSK 44
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. =>
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PVRSK 45
1,1,2-Tribromoethane
Nonequivalent protons on adjacent carbons.
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PVRSK 46
Doublet: 1 Adjacent Proton
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PVRSK 47
Triplet: 2 Adjacent Protons
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PVRSK 48
The N+ 1 Rule
If a signal is split by Nequivalent protons,
it is split into N+ 1 peaks.
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PVRSK 49
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 notequivalent.
Protons on adjacent carbons normally willcouple.
Protons separated by four or more bondswill not couple.
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PVRSK 50
Splitting for Ethyl Groups
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PVRSK 51
Splitting for
Isopropyl Groups
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PVRSK 52
Coupling Constants
When protons couple to each other, they do sowith a certain intensity. This is called the
coupling constant. Coupling constants can
vary from 0 Hz to 16 Hz.
Distance between the peaks of multiplet
Measured in Hz
Not dependent on strength of the external field
Multiplets with the same coupling constants maycome from adjacent groups of protons that split
each other.
=>
V l f
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PVRSK 53
Values for
Coupling Constants
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PVRSK 54
Complex Splitting
Signals may be split by adjacent
protons, different from each other, withdifferent coupling constants.
Example: Ha of styrene which is split by
an adjacent H trans to it (J = 17 Hz) andan adjacent H cis to it (J = 11 Hz).
=>
C C
H
H
Ha
b
c
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PVRSK 55
Splitting TreeC C
H
H
Ha
b
c
=>
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PVRSK 56
Spectrum for Styrene
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PVRSK 57
Stereochemical
Nonequivalence Usually, two protons on the same C are
equivalent and do not split each other.
If the replacement of each of the protons of
a -CH2 group with an imaginary Z gives
stereoisomers, then the protons are non-
equivalent and will split each other.=>
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PVRSK 58
Some Nonequivalent
Protons
C C
H
H
Ha
b
cOH
H
H
H
a
b
c
d
CH3
H Cl
H H
Cl
a b=>
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PVRSK 59
Time Dependence
Molecules are tumbling relative to themagnetic field, so NMR is an averaged
spectrum of all the orientations.
Axial and equatorial protons oncyclohexane 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.
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PVRSK 60
Hydroxyl
Proton Ultrapure samples
of ethanol show
splitting. Ethanol with a small
amount of acidic or
basic impurities will
not show splitting.
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PVRSK 61
N-H Proton
Moderate rate of exchange.
Peak may be broad.
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PVRSK 62
Identifying the O-H
or N-H Peak Chemical shift will depend onconcentration and solvent.
To verify that a particular peak is due toO-H or N-H, shake the sample with D2O
Deuterium will exchange with the O-H
or N-H protons.
On a second NMR spectrum the peak
will be absent, or much less intense.
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PVRSK 63
Fourier Transform NMR
Nuclei in a magnetic field are given aradio-frequency pulse close to their
resonance frequency.
The nuclei absorb energy and precess(spin) like little tops.
A complex signal is produced, then
decays as the nuclei lose energy.
Free induction decay is converted to
spectrum. =>
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PVRSK 64
Hydrogen and Carbon
Chemical Shifts
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PVRSK 65
MRI
Magnetic resonance imaging, noninvasive
Nuclear is omitted because of publics
fear that it would be radioactive. Only protons in one plane can be in
resonance at one time.
Computer puts together slices to get 3D. Tumors readily detected.
=>
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PVRSK 66
THANK YOU
AND PLESE REFER TEXTBOOKS
1)PAVIA ,LAMPMAN &KRIZ2)SILVERSTEIN
3)WILLAM KEMP
4)JAGMOHAN5)P.S.KALSI