Nuclear Magnetic Resonance in Organic Chemistry [Autosaved].ppt
1. Special Topics In Organic Chemistry (2 Credit Hours)
Course Code: PPGQ01
Instructor: Prof. Haroon Ur Rashid, PhD
Center for Chemical, Pharmaceutical and Food Sciences, Federal University of
Pelotas (UFPel), Pelotas Rio Grande do Sul, Brazil
Email: haroongold@gmail.com
Course Contents
Chapter 1: Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry
Chapter 2: Stereochemistry & Stereoisomerism
Chapter 3: Photochemistry
Chapter 4: Pericyclic Reactions
2. Chapter 1: Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry
Course Contents:
Introduction
Nuclear Spins
Relaxation Process
Chemical Shift
Spin Spin Splitting
The N + 1 Rule
Coupling Constant
13C-NMR
Magnetic Resonance Imagine (MRI)
1H-1H COSY (Correlation Spectroscopy)
HETCOR Spectrum or (1H- 13C COSY)
DEPT 13C NMR SPECTRA
Other Types Of 1H-13C COSY
References
3. Introduction
Nuclear magnetic resonance (NMR) is a physical phenomenon
based upon the quantum mechanical magnetic properties of an
atom's nucleus.
NMR also commonly refers to a family of scientific methods that
exploit nuclear magnetic resonance to study molecules ("NMR
spectroscopy").
The method of NMR was first developed by E.M. Purcell and
Felix bloch(1946)
Major application of NMR spectroscopy lies in the area of
synthetic organic chemistry, inorganic chemistry, bio-organic
chemistry, bio-inorganic chemistry, 3
4. Introduction
Nuclear magnetic Resonance (NMR): Magnetic Properties of certain atomic
nuclei.
All atomic nuclei: Positively charged, some of them also spin about their axes,
similar to the spinning of electrons
Spin quantum number = 0, do not spin
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
=>
4
5. Nuclear spin is characterized by a quantum number I, which may be
integral, half-integral or 0.
Only nuclei with spin quantum number I 0 can absorb/emit
electromagnetic radiation. The magnetic quantum number mI has
values of –I, -I+1, …..+I .
( e.g. for I=3/2, mI=-3/2, -1/2, 1/2, 3/2 )
– 1. A nucleus with an even mass A and even charge Z nuclear
spin I is zero
– Example: 12C, 16O, 32S No NMR signal
– 2. A nucleus with an even mass A and odd charge Z integer
value I
– Example: 2H, 10B, 14N NMR detectable
5
6. – 3. A nucleus with odd mass A I=n/2, where n is an odd integer
– Example: 1H, 13C, 15N, 31P NMR detectable
• Nuclear magnetic moments
• Magnetic moment is another important parameter for a nuclei
• = I (h/2)
• I: spin quantum number; h: Plank constant;
• : gyromagnetic ratio (property of a nuclei, the ratio of its magnetic
moment to its angular momentum)
• 1H: I=1/2 , = 267.512 *106 rad⋅s−1⋅T−1
• 13C: I=1/2 , = 67.264*106
• 15N: I=1/2 , = 27.107*106
6
7. Nuclear Zeeman effect
• Zeeman effect: when an atom is placed in an external
magnetic field, the energy levels of the atom are split into
several states.
• The energy of a give spin sate (Ei) is directly proportional
to the value of mI (component of the magnetic quantum
number) and the magnetic field strength B0
Spin State Energy EI=- . B0 = mIB0 r(h/2p)
• Notice that, the difference in energy will always be an
integer multiple of B0r(h/2p). For a nucleus with I = 1/2, the
energy difference between two states is
ΔE = E-1/2-E+1/2 = B0 r(h/2p)
7
8. 8
The Zeeman splitting is proportional to the strength of the magnetic
field
m=–1/2
m=+1/2
9. 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.
9
=>
12. In a magnetic field, there are now two energy states for a proton: a
lower energy state with the nucleus aligned in the same direction as
B0, and a higher energy state in which the nucleus aligned against
B0.
When an external energy source (h) that matches the energy
difference (E) between these two states is applied, energy is
absorbed, causing the nucleus to “spin flip” from one orientation to
another.
The energy difference between these two nuclear spin states
corresponds to the low frequency RF region of the electromagnetic
spectrum.
12
13. Two Energy States
The magnetic fields of the
spinning nuclei will align either
with the external field, or
against the field.
A photon with the right amount
of energy can be absorbed and
cause the spinning proton to flip.
13
15. E and Magnet Strength
• Energy difference is proportional to the magnetic field strength.
• E = h = h B0
2
• Gyromagnetic ratio, , is a constant for each nucleus (26,753 s-
1gauss-1 for H).
• The gyromagnetic ratio (also sometimes known as the
magnetogyric ratio in other disciplines) of a particle or system is
the ratio of its magnetic moment to its angular momentum, and
it is often denoted by the symbol γ, gamma
• In a 14,092 gauss field, a 60 MHz photon is required to flip a
proton.
• Low energy, radio frequency. =>
15
16. 16
NMR Spectroscopy
Where is it?
1nm 10 102 103 104 105 106 107
(the wave) X-ray UV/VIS Infrared Microwave Radio
Frequency
(the transition) electronic Vibration Rotation Nuclear
(spectrometer) X-ray UV/VIS Infrared/Raman NMR
Fluorescence
17. 17
Properties of the Nucleus
Nuclear spin
Nuclear magnetic moments
The Nucleus in a Magnetic Field
Precession and the Larmor frequency
Nuclear Zeeman effect & Boltzmann distribution
When the Nucleus Meet the right Magnet and radio wave
Nuclear Magnetic Resonance
Before using NMR
What are N, M, and R ?
18. 18
Nuclear magnetic moments
Magnetic moment is another important
parameter for a nuclei
= I (h/2)
I: spin number; h: Plank constant;
: gyromagnetic ratio (property of a nuclei)
1H: I=1/2 , = 267.512 *106 rad T-1S-1
13C: I=1/2 , = 67.264*106
15N: I=1/2 , = 27.107*106
19. Principle of NMR
Subatomic particles (electrons, protons and neutrons) can be
imagined as spinning on their axes.
In many atoms (such as 12C) these spins are paired against each
other, such that the nucleus of the atom has no overall spin.
However, in some atoms (such as 1H and 13C) the nucleus does
possess an overall spin. The rules for determining the net spin of
a nucleus are as follows;
19
20. 1. If the number of neutrons and the number of protons are both
even, then the nucleus has NO spin.
2 . If the number of neutrons plus the number of protons is odd,
then the nucleus has a half-integer spin (i.e. 1/2, 3/2, 5/2)
3. If the number of neutrons and the number of protons are both
odd, then the nucleus has an integer spin (i.e. 1, 2, 3)
20
Isotope
Natural %
Abundanc
e
Spin
(I)
Magnetic
Moment
(μ)*
Magnetogyri
c
Ratio (γ)†
1H 99.9844 1/2 2.7927 26.753
2H 0.0156 1 0.8574 4,107
11B 81.17 3/2 2.6880 --
13C 1.108 1/2 0.7022 6,728
17O 0.037 5/2 -1.8930 -3,628
19F 100.0 1/2 2.6273 25,179
29Si 4.700 1/2 -0.5555 -5,319
31P 100.0 1/2 1.1305 10,840
21. 21
Nuclear Spin and Magnetism
• A nucleus with an even mass A and even charge Z
nuclear spin I is zero
• Example: 12C, 16O, 32S No NMR signal
• A nucleus with an even mass A and odd charge Z
integer value I
• Example: 2H, 10B, 14N NMR detectable
• A nucleus with odd mass A I=n/2, where n is an
odd integer
• Example: 1H, 13C, 15N, 31P NMR detectable
22. 22
1. A spinning charge generates a magnetic field. The resulting spin-
magnet has a magnetic moment (μ) proportional to the spin.
2. In the presence of an external magnetic field (B0), two spin
states exist, +1/2 and -1/2.
3.The magnetic moment of the lower energy +1/2 state is aligned
with the external field, but that of the higher energy -1/2 spin state
is opposed to the external field. Note that the arrow representing
the external field points North.
24. 24
In an Applied Magnetic Field
•Nuclei with 2 allowed spin states
can align either with or against the
field, with slight excess of nuclei
aligned with the field
•The nuclei precess about an axis
parallel to the applied magnetic field,
with a frequency called the Larmor
Frequency (w)
25. Larmor Frequency is Proportional to the Applied
Magnetic Field
Slow precession in small
magnetic field
Faster precession in larger
magnetic field
26. 26
Zeeman effect: when an atom is placed in an external magnetic
field, the energy levels of the atom are split into several states.
• The energy of a give spin sate (Ei) is directly proportional to the
value of mI and the magnetic field strength B0
Spin State Energy EI=- . B0 =-mIB0 r(h/2)
•For a nucleus with I=1/2, the energy difference between two states
is
ΔE=E-1/2-E+1/2 = B0 r(h/2)
The Zeeman splitting is proportional to the strength of the
magnetic field
m=1/2
m=-1/2
27. 27
Boltzmann distribution
Quantum mechanics tells us that, for net absorption of radiation to occur,
there must be more particles in the lower-energy state than in the higher
one.
If no net absorption is possible, a condition called saturation.
When it’s saturated, Boltzmann distribution comes to rescue:
Pm=-1/2 / Pm=+1/2 = e -DE/kT
where P is the fraction of the particle population in each state,
T is the absolute temperature,
k is Boltzmann constant 1.381*10-28 JK-1
Anything that increases the population difference will give rise to a more
intense NMR signal.
28. Magnetization
Magnetization or magnetic polarization is
the vector magnetic field, expresses the
density of permanent or induced magnetic
dipole moments in a magnetic material.
The nuclei create a bulk magnetization
along the z-axis
28
29. Relaxation Process
Relaxation refers to the phenomenon of nuclei returning to their
thermodynamically stable states after being excited to higher
energy levels. The energy absorbed when a transition from a lower
energy level to a high energy level occurs is released when the
opposite happens.
How NMR Signals are generated
Absence of applied field, two spin states equally distributed, (50 :
50).
Relative number of nuclei in two spin states is called Boltzmann
equilibrium.
After application of magnetic field, excess of nuclei builds up in
lower energy α spin state (aligned with the field and more stable).
Excess of nuclei in lower energy sate will generate NMR signals.
29
30. 30
Free Induction decay (FID) is the observable NMR signal generated by non-
equilibrium nuclear spin magnetization precessing about the magnetic
field (conventionally along z).
When all of the excess nuclei absorb energy, saturation is achieved, a condition
in which the populations of both spin states are once again equal. Population of
the upper spin state can not be increased further.
31. Supply of radio waves having frequency equal to the processional frequency of
nuclei, disturb Boltzmann equilibrium.
Excess nuclei are excited to upper spin state and when they return to the lower
energy spin state (relax) to reestablish Boltzmann equilibrium, FID (Free
Induction Decay) Signal which is then processed to give NMR signal.
31
32. Spin-Lattice/ Longitudnal Relaxation
A process in which a nucleus in higher energy spin states transfers its
energy to the Lattice (framework of the molecule) as kinetic energy. The
nucleus is transitioned from higher energy spin state to lower energy spin
state causing an excess of nuclei in the lower energy spin state (Boltzmann
equilibrium is achieved) a necessary condition for the phenomenon of
NMR.
Occurs in the direction of the field
The system as whole becomes warm.
Molecules of sample and solvent undergo rotational, vibrational and
translational motions.
32
33. Magnetically active nuclei of these molecules precess, producing
small magnetic field in the lattice.
A small magnetic field properly oriented and precessing with a
comparable frequency induces transition of a nearby nucleus from
higher energy spin state to lower energy spin state.
Energy from this transition is transferred to the components of
lattice as translational, vibrational and rotational energy.
Spin-Lattice/Longitudinal Relaxation is denoted by T1.
1/T1 denotes Spin-Lattice/Longitudinal Relaxation rate.
33
34. Spin-Spin/Transverse Relaxation
Occurs when a nucleus in higher energy spin state transfers it
energy to another nucleus in lower energy spin state.
One nucleus is transitioned from higher energy state to lower
energy state while another is transitioned from lower to higher
energy spin state.
34
35. There will be no net change in the populations of nuclei in the two
spin states.
The two nuclei exchanging the energy must be precessing with the
same frequency.
It occurs in a plane perpendicular to direction of the field (xy-
Plane).
Denoted by T2.
Spin-Spin or Transverse relaxation rate 1/T2
35
36. 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.
36
37. Shielded Protons
Magnetic field strength must be increased for a shielded proton
to flip at the same frequency.
37
=>
38. Protons in a Molecule
Depending on their chemical environment, protons in a molecule
are shielded by different amounts.
38
=>
39. 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. =>
39
40. INSTRUMENTATION
1. MAGNET (Permanent magnets, Conventional electromagnets and
Super conducting magnets)
2. SAMPLE PROBE
3. FIELD SWEEP GENARETOR
4. THE RADIO FREQUENCY SOURCE
5. THE SIGNAL DETECTOR &
6. RECORDER SYSTEM
40
46. Tetramethylsilane
TMS is added to the sample, as a standard
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.
Tetramethyl silane (TMS) is used as reference because it is soluble
in most organic solvents, inert, volatile, and has 12 equivalent 1Hs
and 4 equivalent 13Cs:
Other references can be used, such as the residual solvent peak,
dioxane for 13C 46
Si
CH3
CH3
CH3
H3C
47. Chemical Shift
The chemical shift of a nucleus is the difference between the
resonance frequency of the nucleus and a standard, relative to the
standard. This quantity is reported in ppm and given the symbol
delta,
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.
47
49. 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.
=>
49
51. Regions of the 1H NMR Spectrum
51
Qualitatively, this is a chart you might want to know.
52. Integration
52
• The integration quantifies the relative number of
protons giving rise to that signal a under a peak
• A computer will calculate the area of each peak
• The curve height represents the integration
53. 53
• The computer operator sets one of the peaks to a whole
number to let it represent a number of protons
• The computer uses the integration ratios to set the values
for the other peaks
1.00 1.05
1.48 1.56
54. 54
• Integrations represent numbers of protons, so you
must adjust the values to whole numbers
• If the integration of the first peak is doubled, the
computer will adjust the others according to the
ratio
2.00 2.10
3.12
55. 55
• he integrations are relative quantities rather than
an absolute count of the number of protons
• Predict the 1H shifts and integrations for tert-butyl
methyl ether
• Symmetry can also affect integrations
• Predict the 1H shifts and integrations for 3-
pentanone
61. 61
0
TMS
ppm
2
10 7 5
15
Aliphatic
Alcohols, protons a
to ketones
Olefins
Aromatics
Amides
Acids
Aldehydes
Deshielded
(low field)
Shielded
(up field)
HO-CH2-CH3
wo
low
field
high
field
Notice that the intensity
of peak is proportional to
the number of H
64. Intensity of Signals
• The area under each peak is proportional to the number of protons.
• Shown by integral trace.
64
=>
65. How Many Hydrogens?
When the molecular formula is known, each integral rise can
be assigned to a particular number of hydrogens.
65
=>
66. Spin-Spin Splitting (Multiplicity)
• 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. =>
66
67. • When a signal is observed in the 1H NMR, often it is
split into multiple peaks
• Multiplicity or a splitting patterns results
67
68. 68
Multiplicity results from magnetic effects that protons have on each
other
Consider protons Ha and Hb
We already saw that protons align with or against the external
magnetic field
Hb will be aligned with the magnetic field in some molecules. Other
molecules in the sample will have Hb aligned against the magnetic
field
Some Hb atoms have a slight shielding affect on Ha and others have
a slight deshielding affect
69. 69
The resulting multiplicity or splitting pattern for Ha is a doublet
A doublet generally results when a proton is split by only one other
proton on an adjacent carbon
70. 70
Consider an example where there are two protons on the adjacent
carbon
There are three possible effects the Hb protons have on Ha
71. 71
Ha appears as a triplet
The three peaks in the triplet have an integration ratio of
1:2:1
WHY?
72. 72
Consider a scenario where Ha has three
equivalent Hb atoms splitting it
Explain how the magnetic fields cause
shielding or deshielding.
73. • By analyzing the splitting pattern of a signal in the
1H NMR, you can determine the number of
equivalent protons on adjacent carbons
73
74. • The trend in table 16.3 also allows us to predict
splitting patterns
• Explain how the n+1 rule is used
74
75. Multiplicity
75
• Remember three key rules
1. Equivalent protons can not split one another
– Predict the splitting patterns observed for
1,2-dichloroethane
2. To split each other, protons must be within a 2 or 3 bond
distance
76. 76
Remember three key rules
3. The n+1 rule only applies to protons that are all equivalent
The splitting pattern observed for the proton shown below will be
more complex than a simple triplet
Complex splitting will be discussed later in this section
77. • Predict splitting patterns for all of the protons in the
molecule below.
77
–Ha is a doublet
–Hb is a septet
–Hc is a doublet
–Hd is a triplet
–He is a doublet
–Hf is a doublet
78. 78
•The degree to which a neighboring proton will shield or deshield its
neighbor is called a coupling constant
•The coupling constant or J value is the distance between peaks of a
splitting pattern measured in units of Hz
• When protons split each other, their coupling constants will be equal
• Jab = Jba
79. 79
because the coupling
constant is a smaller
percentage of the
overall Hz available
• The coupling constant will be constant even if an
NMR instrument with a stronger or weaker
magnetic field is used
• Higher field strength instruments will give better
resolution between peaks,
83. The N + 1 Rule
83
If a signal is split by N equivalent protons, it is split into N + 1 peaks.
=>
84. 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.
84
87. Coupling Constants
• Distance between the peaks of multiplet
• Measured in Hz
• Not dependent on strength of the external field
• Multiplets with the same coupling constants may come
from adjacent groups of protons that split each other.
=>
87
89. Complex Splitting
• Signals may be split by adjacent protons, different from
each other, with different coupling constants.
• Example: Ha of styrene which is split by an adjacent H
trans to it (J = 17 Hz) and an adjacent H cis to it (J = 11
Hz).
=>
89
C C
H
H
H
a
b
c
92. 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.
=>
92
94. 94
Complex Splitting: Doublet of Doublets, (dd)
Doublet of doublets: In NMR spectroscopy, a signal that is split into a doublet, and
each line of this doublet split again into a doublet. Occurs when coupling constants
are unequal. 1H-NMR spectrum of methyl acrylate
95. 95
Consider the Hc signal, which is centered at 6.21 ppm
Hc signal is actually composed of four sub-peaks.
Hc is coupled to both Ha and Hb , but with two different coupling constants.
Ha is trans to Hc across the double bond, and splits the Hc signal into a doublet
with a coupling constant of Jac = 17.4 Hz.
In addition, each of these Hc doublet sub-peaks is split again by Hb (geminal
coupling) into two more doublets, each with a much smaller coupling constant of
Jbc = 1.5 Hz.
96. 96
The signal for Ha at 5.95 ppm is also a doublet of doublets, with
coupling constants 3Jac= 17.4 Hz and 3Jab = 10.5 Hz.
97. 97
The signal for Hb at 5.64 ppm is split into a doublet by Ha, a cis coupling with
3Jab = 10.4 Hz.
Each of the resulting sub-peaks is split again by Hc, with the same geminal
coupling constant 2Jbc = 1.5 Hz that we saw previously when we looked at the Hc
signal.
The overall result is again a doublet of doublets, this time with the two `sub-
doublets` spaced slightly closer due to the smaller coupling constant for the cis
interaction
98. 98
When a proton is coupled to two different neighbouring proton sets with identical or very
close coupling constants, the splitting pattern that emerges often appears to follow the simple
`n + 1 rule` of non-complex splitting.
1,1,3-trichloropropane, we would expect the signal for Hb to be split into a triplet by Ha, and
again into doublets by Hc, resulting in a 'triplet of doublets'.
Ha and Hc are not equivalent (their chemical shifts are different), but it turns out that 3Jab is
very close to 3Jbc. If we perform a splitting diagram analysis for Hb, we see that, due to the
overlap of sub-peaks, the signal appears to be a quartet, and for all intents and purposes
follows the n + 1 rule.
99. 99
The Hc peak in the spectrum of 2-pentanone appears as a
sextet, split by the five combined Hb and Hd protons.
Technically, this 'sextet' could be considered to be a 'triplet of
quartets' with overlapping sub-peaks.
100. Aromatic Protons
100
In many cases, it is difficult to fully analyze a complex splitting pattern. In the
spectrum of toluene, for example, if we consider only 3-bond coupling we would
expect the signal for Hb to be a doublet, Hd a triplet, and Hc a triplet.
all three aromatic proton groups have very similar chemical shifts and their signals
overlap substantially, making such detailed analysis difficult. In this case, we
would refer to the aromatic part of the spectrum as a multiplet.
101. Molecular tumbling
• Molecules are tumbling relative to the magnetic field, so NMR is
an averaged spectrum of all the orientations.
• Tumbling is rotation, vibration and translation of the molecule in
which the hydrogen proton resides.
• Axial and equatorial protons on cyclohexane interconvert so
rapidly that they give a single signal.
.
101
102. O-H and N-H Signals
• Chemical shift depends on concentration.
• Hydrogen bonding in concentrated solutions deshield the
protons, so signal is around 3.5 for N-H and 4.5 for O-H.
• Proton exchanges between the molecules broaden the peak.
102
103. Hydroxyl
Proton
• Proton transfers for OH and NH
may occur so quickly that the
proton is not split by adjacent
protons in the molecule
• Ultrapure samples of ethanol
show splitting.
• Ethanol with a small amount of
acidic or basic impurities will not
show splitting.
103
=>
105. Identifying the O-H or N-H Peak
• To verify that a particular peak is due to O-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.
=>
105
106. Carbon-13
12C has no magnetic spin.
13C has a magnetic spin, but is only 1% of the carbon in a sample.
The gyromagnetic ratio of 13C is one-fourth of that of 1H.
Hundreds of spectra are taken, averaged.
13C nucleus is about 400 times less sensitive than H nucleus to the
NMR phenomena
Due to the low abundance, we do not usually see 13C-13C
coupling
Chemical shift range is normally 0 to 220 ppm.
Chemical shifts are also measured with respect to
tetramethylsilane, (CH3)4Si (i.e. TMS). 106
107. Similar factors affect the chemical shifts in 13C as seen for H-
NMR
Long relaxation times (excited state to ground state) mean no
integrations
"Normal" 13C spectra are "broadband, proton decoupled" so the
peaks show as single lines
Number of peaks indicates the number for different types of C.
107
108. Differences in 13C Technique
Resonance frequency is ~ one-fourth, 15.1 MHz instead of 60 MHz.
Peak areas are not proportional to number of carbons.
May require several hours, compared to 15–30 minutes for 1H NMR.
The nuclear dipole is weaker, the difference in energy between alpha and
beta states is one-quarter that of proton NMR, and the Boltzmann
population difference is correspondingly less.
13C-NMR spectra take longer to acquire than H-NMR, though they tend
to look simpler.
Accidental overlap of peaks is much less common than for H-NMR
which makes it easier to determine how many types of C are present.
108
109. Fourier Transform NMR
• Nuclei in a magnetic field are given a radio-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.
109
112. Spin-Spin Splitting
It is unlikely that a 13C would be adjacent to
another 13C, so splitting by carbon is negligible.
The resonances due to 13C nuclei are split by
neighbouring H atoms. These splittings would
complicate the appearance of the spectra
making them harder to interpret.
112
113. 113
Therefore, in a "normal" 13C spectra, these couplings
are "removed" by applying a continuous second radio
frequency signal of a broad frequency range that
excites all the H nuclei and cancels out the coupling
patterns due to the interaction of the H with the 13C.
This means that each C is seen as a single line. Of
course information is being lost by doing this, such as
how many H are attached to each C.
Proton Spin Decoupling
114. • In off-resonance decoupling the one bond C-H
couplings are retained.
• So the signal for a particular C is given by the
number of attached H in accord with n+1 rule.
• For example, a -CH3 shows as a quartet and a -
CH2- as a triplet.
114
Off-Resonance Decoupling
116. 116
when a proton peak is irradiated at its resonance frequency, there
is an enhancement in the peak intensity of a proton that is close
in space to that irradiated proton is known as the Nuclear
Overhauser effect (NOE effect).
Nuclear Overhauser Effect
117. 117
Irradiation of the 5-methyl group resulted in the enhancement of both the H4 and
H6 peaks, whereas irradiation of the 3-methyl group enhanced only the H4 peak.
3-methyl proton is close in space with the H4 proton. Thus, when the 3-methyl
proton is irradiated by applying its resonance frequency, the intensity of the H4-
peak increases as shown below figure.
118. Interpreting 13C NMR
• The number of different signals indicates the number of
different kinds of carbon.
• The location (chemical shift) indicates the type of functional
group.
• The peak area indicates the numbers of carbons (if integrated).
• The splitting pattern of off-resonance decoupled spectrum
indicates the number of protons attached to the carbon. =>
118
119. A table of typical chemical shifts in C-13 NMR spectra
119
carbon environment
chemical shift
(ppm)
C=O (in ketones) 205 - 220
C=O (in aldehydes) 190 - 200
C=O (in acids and esters) 170 - 185
C in aromatic rings 125 - 150
C=C (in alkenes) 115 - 140
RCH2OH 50 - 65
RCH2Cl 40 - 45
RCH2NH2 37 - 45
R3CH 25 - 35
CH3CO- 20 - 30
R2CH2 16 - 25
RCH3 10 - 15
The most significant factors
affecting the chemical shifts are:
Electronegativity of the groups
attached to the C
Hybridisation of C
121. 121
The carbon in the CH3 group
is attached to 3 hydrogens
and a carbon.
The carbon in the CH2 group
is attached to 2 hydrogens, a
carbon and an oxygen.
But which is which?
122. Magnetic Resonance Imaging (MRI)
• Detection of NMR signals emitted by protons of water and fat
molecules.
• Noninvasive imaging modality, 3D images, safer for patients,
avoids the use of X-rays and other ionizing radiations,
• Malignant tumors, cerebral abnormalities, multiple sclerosis,
infarcted artery and lesions.
• Drawbacks of MRI (a) weak signal intensity and (b) longer time to
acquire a scan.
• Overcome: High relaxivity contrast agents are used, .
•
122
123. 1H-1H COSY (Correlation Spectroscopy)
• 2-D NMR spectra have two frequency axes and one intensity axis. The
most common 2-D spectra involve 1H-1H shift correlation; they identify
protons that are coupled (i.e., that split each other’s signal). This is called
1H-1H shift-correlated spectroscopy, which is known by the acronym
COSY.
1H-1H correlation spectra
2-D dimensional plot with 1H spectrum along each axis and on the
diagonal
Protons coupling to one another produce off diagonal correlations
This allows assignment of proton groups that are connected in the
molecule
Shows connectivity in the compound 123
126. 1H-1H COSY Spectrum of Ethyl Vinyl Ether
126
Fig1. It looks like a mountain range viewed from the air because intensity is the
third axis.
These “mountain-like” spectra (known as stack plots) are not the spectra actually
used to identify a compound.
Instead, the compound is identified using a contour plot Fig:2, where each
mountain in Fig:1 is represented by a large dot (as if its top had been cut off).
The two mountains shown in Fig:1 correspond to the dots labelled B and C in
Fig: 2
Fig:2, the usual one-dimensional 1HNMR spectrum is plotted on both the x- and y-
axes.
To analyse the spectrum, a diagonal line is drawn through the dots that bisect the
spectrum.
127. 1H-1H COSY Spectrum of Ethyl Vinyl Ether
The dots that are not on the diagonal (A, B, C) are called cross peaks.
Cross peaks indicate pairs of protons that are coupled.
For example, if we start at the cross peak labelled A and draw a
straight line parallel to the y-axis back to the diagonal, we hit the
dot on the diagonal at ~ 1.1 ppm produced by the Ha protons
If we next go back to A and draw a straight line parallel to the x-axis
back to the diagonal, we hit the dot on the diagonal at ~ 3.8 ppm
produced by the Hb protons. This means that the Ha and Hb protons
are coupled.
127
128. 1H-1H COSY Spectrum of Ethyl Vinyl Ether
If we then go to the cross peak labelled B and draw two
perpendicular line back to the diagonal, we see that the Hc and
He protons are coupled; the cross peak labelled C shows that
the Hd and He protons are coupled.
Notice that we used only cross peaks below the diagonal; the
cross peaks above the diagonal give the same information.
Notice also that there is no cross peak due to the coupling of Hc
and Hd, consistent with the absence of coupling for two
protons bonded to an sp2 carbon.
128
129. 129
1H−1H COSY NMR spectrum of L-alanine in D2O at concentration of ∼20 mg/mL
134. HETCOR Spectrum or (1H- 13C COSY)
2-D NMR spectra that show 13C-1H shift correlation are called HETCOR from heteronuclear
correlation) spectra. HETCOR spectra indicate coupling between protons and the carbon
to which they are attached.
Example: 2-methyl-3-pentanone
The 13C NMR spectrum is shown on the x-axis and the 1H NMR spectrum is shown on the y-
axis. The cross peaks in a HETCOR spectrum identify which hydrogens are attached to
which carbons.
For example, cross peak A indicates that the hydrogens that shows a signal at ~ 0.9 ppm in the
1H NMR are bonded to the carbon that shows a signal at ~ 6 ppm in the 13CNMR spectrum.
Cross peak B shows that the hydrogens that show a signal at ~ 1 ppm are bonded to the carbon
that shows a signal at ~ 19 ppm
Cross peak C shows that the hydrogens that show a signal at ~ 2.5 ppm are bonded to the
carbon that shows a signal at ~ 34 ppm
134
137. DEPT 13C NMR SPECTRA
Distortionless Enhancement of Polarization Transfer
(DEPT)
13C DEPT spectra enable different carbon (CH3, CH2, CH, and
quaternary)
Types to be identified
DEPT 45: -CH, - CH2 and –CH3 all positive
DEPT 90: only CH peaks visible?
DEPT 135: -CH2 peaks negative
-CH and CH3 peaks positive
PENDANT: -CH2 and quaternary peaks negative
-CH3 and CH peaks positive
137
139. DEPT 13C NMR SPECTRA
DEPT: stands for distortionless enhancement by polarization transfer.
This technique to distinguish among CH3, CH2, and CH group
It is now much more widely used than proton coupling to determine the
number of hydrogens attached to a carbon.
DEPT 13C spectrum does not show a signal for a carbon that is not
attached to a hydrogen.
For example: 13C NMR spectrum of 2-butanone shows 4 signals because
it has 4 nonequivalent carbons, whereas the DEPT 13C NMR of 2-
butanone shows only three signals because the carbonyl carbon is not
bonded to a hydrogen, so it will not produce a signal.
Normal 13C NMR gives 4 signals
4 3 2 1 DEPT 13C NMR gives 3 signal
CH3-CH2-CO-CH3
139
140. DEPT 13C NMR Spectra of Ipsenol
In CDCl3 at 75.6 MHz:
Subspeectrum A, CH up.
Subspectrum B, CH3 and CH up, CH2 down. The conventional 13C
NMR spectrum is at the bottom.
140
HO
1
2
3
4
5
6
7
8
3X CH, 2X CH3
4X CH2
3X CH-
142. Other Types Of 1H-13C COSY
1. HMBC (Heteronuclear Multiple Bond Correlation): 1H-13C
several bond correlation
2. HSQC (Heteronuclear Single Quantum Coherence): 1H-13C
carbon and protons direct correlation
3. HMQC (Heteronuclear multiple quantum coherence): correlation
between protons and other nuclei such as 15N or 31P
142
143. Limitations of NMR spectroscopy
Its lack of sensitivity. fairly large numbers are required. minimum
sample size is about 0.1 ml having minimum concentrations of about
on 1%
Limited number of nuclei which may be usefully studied with this
technique.
In most of the cases, the technique is limited to liquid samples or to a
liquid capable of dissolving in a suitable solvents or of melting at a
temperature below 260 oc
In some compounds two different types of hydrogen atoms resonance
at similar resonance frequencies. This results in an overlap of spectra.
Hence the interpretation of spectra becomes difficult.
143
144. APPLICATIONS
• 1. Determination of optical purity
• 2. Study of molecular interactions
• 3. Quantative analysis: assay components, surfactant chain length
Determination, hydrogen analysis, iodine value, moisture analysis
• 4. Elemental analysis
• 5. Multicomponent mixture analysis
• 6. Magnetic Resonance Imaging
• 7. NMR has also been used in various special fields that includes
industrial quality control, biology, engineering and medicine
• 8. Structure elucidation 144
145. • Other applications
Molecular conformation in solution
Quantitative analysis of mixtures containing known
compounds.
Determining the content and purity of a sample.
Through space connectivity (over Hauser effect).
Chemical dynamics (Line shapes, relaxation phenomena)
Solid State NMR is widely popular for the
characterization of polymers, rubbers, ceramics, and
molecular sieves. 145
146. References:
D.L. Pavia, G.M. Lampman, G.S. Kriz, J.R. Vyvyan, Nuclear Magnetic
Resonance Spectroscopy, Advanced NMR Techniques, in: Introduction to
Spectroscopy (4th Ed.), Cengage Learning, USA, 2009,
146
