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Chapter
1
Nuclear Magnetic Resonance (NMR)
Spectroscopy
1.1INTRODUCTION
Nuclear magnetic resonance spectroscopy is a technique which gives us information about the number and
types of atoms of a particular element in the molecule, for example, about the number and types of :
(a)hydrogen atoms using 1H-NMR spectroscopy: also called proton magnetic resonance (PMR)
spectroscopy.
(b) carbon atoms using 13C-NMR spectroscopy.
The technique was developed by Edward M. Purcell and Felix Bloch during late 1940's for which they
shared the 1952 Nobel Prize in physics. Their discovery paved the way for development of NMR spectroscopy
as a useful tool for structure elucidation of organic compounds. Since then this branch of knowledge has
developed by leaps and bounds as evident by another Nobel Prize in Chemistry awarded to Professor Richard
R. Ernest in 1991 for his pioneer work on applications of this technique.
We have learnt the application of UV and IR spectroscopic techniques for elucidation of structures of
organic compounds. But they have limited use, e.g, UV helps us in identifying chromophore and conjugation
and IR helps us in identifying the functional group(s). Contrary to these techniques, NMR gives us a deeper
insight into the structure giving very useful information about the carbon-hydrogen framework of organic
molecules. In a number of cases this technique can be used to establish the complete structure of an unknown
compound.
1.2 THE PRINCIPLE
Like electrons, the nuclei (positively charged particles) of certain atoms or their isotopes possess intrinsic
property of spinning along their axis. These spinning nuclei, being charged, generate a magnetic moment
along the axis of spin. Thus nuclei behave as tiny bar magnets. Here it may be worthwhile to mention that the
nuclei of all atoms do not possess such magnetic property. Only those nuclei which have either (a) odd atomic
numbers and odd mass numbers or (b) odd atomic numbers and even mass numbers or (c) even atomic
numbers and odd mass numbers, possess a quantized spin angular momentum, i.e., a magnetic moment.
Further, all these nuclei possess either half integral (n/2) or full integral (n) spin quantum numbers. It
means that atomic nuclei having I = n or n/2 will have magnetic properties. In contrast, the nuclei of atoms
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ORGANIC CHEMISTRY [VOL-III]
possessing even mass number and even atomic number do not possess a nuclear spin and hence do not have
magnetic properties.
For a nucleus having a spin quantum number I, there are 2I + 1 allowed spin states. The atomic numbers,
mass numbers, spin quantum numbers and number of allowed spin states of a few typical nuclei are listed in
table 1.1.
Table 1.1 Mass numbers, atomic numbers, spin quantum numbers and allowed spin states of some
typical nuclei
11
5B
In the absence of applied field all the allowed spin states of a nucleus are randomly oriented, degenerate
and are also equally populated. (Fig. 1.1(a))
Higher
energy state
E
n
e
r
g
y
No magnetic field
(all spin states are
degenerate)
Lower
energy state
–1/2 (aligned
against the
applied field)
+1/2 (aligned
with the
applied field)
H0
(Applied
magnetic
field)
In presence of applied magnetic field
(two split spin states)
No applied magnetic field
(degenerated spin states)
(b)
(a)
–1/2 (aligned against
the applied field)
E
n
e
r
g
y
14100
gauss
0.0239 J mol
23500
gauss
70460
gauss
–1
0.120 J mol
–1
+1/2 (aligned with
the applied field)
Strength of applied magnetic field (gauss)
(c)
Fig. 1.1 Energy levels of protons in the absence and presence of applied magnetic field
However, when such nuclei are placed under the influence of strong external magnetic field of strength
H0, their different spin states no more remain random and degenerate. The interaction between nuclear spins
and the applied field is quantized. For example, in the case of hydrogen nucleus (the proton having two spin
1
 1
 1
1
states + and −  the magnetic moments will either be aligned with  +  or aligned against  −  the
 2

 2
2
2
applied field, i.e., two energy states will be generated. In the lower energy state the nuclear magnetic moment
 1
will be aligned with the applied field  +  and in the higher energy state it will be aligned against the
 2
1

applied field  −  (Fig. 1.1(b)).
 2
Chapter 1
3
NUCLEAR MAGNETIC RESONANCE (NMR) SPECTROSCOPY
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ORGANIC CHEMISTRY [VOL-III]
The difference in energy between the two energy states is very small being 0.0239 J mole –1 when the
strength of applied magnetic field is 14100 gauss (or 1.41 Tesla). Because of such a small difference , the two
energy states are nearly equally populated. However, calculations have shown that there is always slight
excess of nuclei in the lower energy state (0.001%; Boltzmann distribution). It is this excess which is
responsible for absorption of energy in the radio frequency (rf) region of the spectrum. It may further be
noted that the difference in energy of two states increases with the increase in the strength of applied magnetic
field (Fig. 1.1 (c)).
The frequency of absorption (n) is given by the expression —
ν=
µH0
MHz
hI
where, m = magnetic moment; H0 = strength of the applied field; h = Planck’s constant; I = spin quantum
number.
1
; h = 6.624×10–27; m = 1.41×10–23
2
1.41 × 10 −23 × 14100
∴ν =
MHz
1
× 6.624 × 10 −27
2
Now for a proton H0 = 14100 gauss; I = ;
= 60 MHz
So for we have seen that the effect of applied magnetic field is to segregate the protons into two energy
states—aligned with (lower energy) and aligned against (higher energy). A second effect of the applied
magnetic field on the nucleus is that it begins to ‘Wobble’ or ‘precess’ and traces a conical surface in the same
manner as a spinning top or a gyroscope traces under the influence of earth’s gravitational field (Fig. 1.2).
Precessional
orbit
Axis of rotation
Spinning nucleus
(a)
H0
Direction of
magnetic field
(b )
Fig. 1.2 (a) Spinning top precessing in the earth’s gravitational field; (b) Precession of a spinning nucleus
under the influence of the applied magnetic field.
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1.3The Process of Absorption: Resonance
The precession of the nucleus, which is a charged particle, generates an oscillating electric field of the same
frequency (w) as the frequency of precessing nucleus (w). Now when such a nucleus is irradiated with
electromagnetic radiation also of the same frequency (w), the two frequencies couple and the nucleus attains
1
1
the state of resonance and absorbs energy. The nuclear spin is flipped from spin state + to –� (Fig 1.3).
2
2
The instrument detects and records this as a signal to yield a spectrum called nuclear magnetic resonance
spectrum.
1.4The Chemical Shift : Shielding and Deshielding
So far our discussion on NMR spectroscopy had been based on the presumption that this technique is
related to the study of ‘free’ protons. If this were the case, then any one combination of applied field and
electromagnetic radiation which produces a signal for one particular proton would also produce the same
signal for all the protons in the molecule. In other words, the spectrum would consist of one single signal
which would be of no use in structural elucidation.
However, in a molecule, the protons are not at all ‘free’. They are rather surrounded by electron cloud
which starts circulating under the influence of applied magnetic field. This circulation of electrons, which is
called diamagnetic current, induces a local magnetic field (also called counterfield) which opposes the
applied field near the nucleus (Fig. 1.4). Consequently, the field strength actually felt by the nucleus (H eff) is
always less than the strength of applied field (H0). Such a nucleus (Heff < H0) is said to be shielded. As this
shielding is due to diamagnetic current, it is also called diamagnetic shielding.
Fig. 1.3 Resonance process
Chapter 1
NUCLEAR MAGNETIC RESONANCE (NMR) SPECTROSCOPY
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ORGANIC CHEMISTRY [VOL-III]
Induced magnetic field
opposes the applied field
near the nucleus
Nucleus
Circulating electron produces
diamagnetic current
H0
Induced magnetic field
Applied field
Fig. 1.4 Diamagnetic shielding of nucleus
For example, we know that under the influence of applied magnetic field of 14100 gauss (or 1.41 Tesla)
an isolated proton resonates at 60 MHz. But when we examine the 1H-NMR spectrum of phenylacetone we
find that it exhibits three types of signals as given below:
59.999995 MHz
60 MHz
59.999700 MHz
CH2
CO
CH3
59.99982 MHz
It is quite clear from the above example that each type of proton is shielded as compared to an isolated
proton. Therefore, to cause absorption of radiowaves of 60 MHz, by each type of proton the strength of the
applied magnetic field should be increased (or decreased) to a different extent. Obviously, the degree of
shielding or dishielding of a given proton attached to a carbon atom depends upon the inductive effect of the
other atoms or groups attached to this carbon. The electron - releasing groups (+ I effect) will increase the
electron density around the proton under investigation and would cause its shielding. On the contrary, electron
- withdrawing groups (– I effect) will decrease the electron density around the proton under examination
and would cause its deshielding. In the former case the strength of the applied magnetic field should be
increased while in the latter case it should be decreased.
As different types of protons absorb differently, there is a need to interpret these results by comparing
their absorption with a standard proton. This difference between absorption position of a particular
proton and that of a reference (or standard) proton is called chemical shift of that proton.
1.5 Selection of a Reference Proton
It has been observed that the difference in resonance frequencies of different types of protons due to shielding
and deshielding is generally very small. For instance, in the case of CH3F and CH3Cl, this difference is just
72 Hz under the influence of applied magnetic field of 14,000 gauss which is 1.2 ppm of the irradiating
frequency (60 MHz) and as such it is extremely difficult to measure such a small difference accurately.
To overcome this difficulty, a suitable reference compound is chosen and the resonance frequency of the
given proton is measured relative to a resonance frequency of proton(s) of this standard compound. The most
commonly used reference compound is tetramethylsilane(CH3)4Si (abbreviated as TMS).
CH3
H 3C
Si
CH 3
CH3
TMS
The choice of TMS as a reference compound is associated with the following advantages:
(i) It is relatively inert as it has no polar group.
(ii) It is a highly volatile liquid (b.p. 299.5K) and , therefore, can be easily removed after use thereby
making the recovery of the sample (in pure condition) easy.
(iii) It is miscible with most of the organic solvents and does not form any type of complexes with
them.
(iv) It has only one type of protons, so it exhibits one sharp single peak (a singlet).
(v) Due to, the presence of silicon atom, which is more electropositive than carbon, the chemical
environment of all the protons of TMS is quite different than that of most of the protons. Hence
the signal due to protons of TMS appears outside the normal range of protons of majority of
organic compounds. In fact the protons of TMS are highly shielded so chances of overlapping of
the signal due to these protons with that of other protons are very rare.
1.6Units and scale of chemical shift
The chemical shift can be measured in Hz which is directly proportional to the strength of applied magnetic
field. Since a number of instruments operating at different field strengths (60 MHz, 100 MHz, 200 MHz,
300 MHz, etc.) are available, the chemical shift, measured in Hz, will be different with different instruments
which would create problem in interpreting the results. To overcome this problem a field independent scale,
was developed. The unit used for this is d (delta) which is the ratio of observed chemical shift (in Hz) to the
radio frequency used (in MHz). Since the resulting number is too small, it is multiplied by 106 so that we
obtain a value which can be conveniently handled. Thus
δ=
Observed chemicalshift (Hz) from TMS
× 106
Spectrometer frequency (MHz)
For example, for an instrument operating at 60 MHz the resonance frequency of a particular proton is
120 Hz and for the same proton it is 200 Hz for an instrument operating at 100 MHz. Though the frequencies
in Hz are different, their d value is the same (i.e. 2.0) as shown below.
Chapter 1
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NUCLEAR MAGNETIC RESONANCE (NMR) SPECTROSCOPY
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ORGANIC CHEMISTRY [VOL-III]
First instrument
δ=
120 Hz × 106
= 2.0
60 MHz
Second instrument
δ=
200 Hz × 106
= 2.0
100 MHz
The d value assigned to TMS is 0(zero) and the chemical shifts of majority of protons fall between d 0
to d 10 on a ten point scale. A small d value indicates upfield shift (higher field strength) and the proton under
examination is said to be shielded. On the contrary a large d value indicates downfield shift (lower field
strength) and the proton is said to be deshielded. Here it may be noted that another scale called t (tau) scale
has been developed and on this scale the chemical shift of TMS is assigned the value t as 10.
Thus both d and t values are related to each other as follows:
d = 10–t or t = 10 – d
The relationship between the two scales, position of TMS signal and the correlation of shielding and
deshielding effects with magnetic field effect are shown in Fig. 1.5.
Fig. 1.5 Scales of NMR spectra
In the above figure, a shift of NMR signal to the left on the chart paper is called downfield shift and is
attributed to deshielding. A shift to the right on the chart paper is called upfield shift and is attributed to
shielding.
1.7 SOLVENTS USED IN PMR SPECTROSCOPY
Any solvent which does not contain protons can be used in PMR spectroscopy. The most widely used solvent
is deuterated chloroform (CDCl3). Other solvents which are commonly used in PMR spectroscopy are: CCl4;
deuterated acetone, i.e., (CD3)2CO (acetone-d6); deuterated dimethyl sulphoxide, i.e., (CD3)2SO (DMSO-d6)
and deuterated benzene, i.e., C6D6 (benzene-d6), etc.
Trifluoroacetic acid (F3CCOOH) is also used sometimes even though it contains a proton. The reason
being that this acidic proton absorbs at a very low field (11.2d) and hence does not interfere with the
absorptions of most of the compounds which usually occurs in the range 0–10d.
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NUCLEAR MAGNETIC RESONANCE (NMR) SPECTROSCOPY
Chapter 1
1.8NMR Spectrometer
Generally the following two types of spectrometers are used:
(i) Continuous wave (CW) NMR spectrometer
(ii) Fourier transform (FT) NMR spectrometer
(i)Continuous Wave NMR Spectrometer
In this type of instrument frequency of the radiation is kept constant, while the magnetic field strength is
continuously varied. The essential components of a CW NMR spectrometer (Fig. 1.6) are as follows:
(a) A powerful magnet
(b) A radiofrequency generator
(c) A radiofrequency detector
(d) A radiofrequency recorder
(e) A probe unit
(a) Magnet: It is large, strong magnet and is homogeneous over the area of the sample. The strength
of the magnetic field can be varied by fixing electromagnetic coils on the poles of the main
magnet.
(b) Radiofrequency generator: It generates the electromagnetic waves in the radiofrequency region
and transmits these waves to the sample through a coil fixed in the probe.
(c) Radiofrequency detector: It detects the absorption of radiofrequency when a particular proton
attains the state of resonance through a coil which is also fixed in the probe.
(d) Radiofrequency recorder: The radiofrequency detector transmits the signal received directly to
the oscilloscope which records the spectrum (on a graph paper) as a plot of resonance signal v/s
strength of the applied field.
(e) Probe: It is the sensing device which is inserted between poles of the magnet. It actually houses the
sample tube, (a thin walled glass tube about 15-20 cm long and 5 mm diameter), radiofrequency
generator coil and radiofrequency detector coil.
NMR spectrum
Radiofrequency
generator
Detector and
amplifier
Reco
rder
Probe
S
Variable
magnetic
field
Sample tube
Fig 1.6 Schematic diagram of a typical NMR spectrometer
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ORGANIC CHEMISTRY [VOL-III]
Working: The sample is dissolved in a suitable solvent which is usually NMR inactive and devoid of
protons, eg., CCl4, CDCl3 (chloroform-d1) DMSO-d6 (Dimethyl sulphoxide - d6) CD3OD (Methanol-d4) etc.
A small amount of TMS is added to this solution which is now transferred to the sample tube. The tube is then
capped, inserted in the probe, and spun to ensure that all parts of the solution experience uniform magnetic
field. The coil coming from the radiofrequency generator transmits the electromagnetic energy for flipping to
take place. The other coil connected to radiofrequency detector picks up a signal the moment a proton attains
the state of resonance. This signal is transmitted to the recorder which generates the NMR spectrum. As a
typical example, the NMR spectrum of phenyl acetone is shown in Fig. 1.7.
Fig. 1.7 1HNMR spectrum of phenylacetone
(ii)Fourier Transform NMR (FT NMR) Spectrometer
Nowadays more advanced instruments known as FT NMR spectrometers are available. In these instruments,
the magnetic field is kept constant whereas, the radiofrequency is varied. The greatest advantage of FT NMR
over CW NMR is that the former records the spectrum within 5 seconds whereas the latter takes 2-5 minutes
to scan it.
1.9 Factors Affecting Chemical Shifts
The chemical shifts of different types of protons are effected by local environments. Sometimes these effects
are so strong that the values of chemical shifts can be used as diagnostic tools for characterisation of different
types of protons thereby playing a major role in elucidation of the structure of unknown.
The values of the chemical shifts of different types of protons, listed in table 1.2, are illustrative.
Table 1.2 Chemical shifts of different protons
Proton
d
Proton
CH3—R
0.9
—C—CH2—OR
3.4
CH3—C=C
1.6
—C—CH2—I
3.2
CH3—COOR
2.0
—C—CH2—Br
3.5
CH3—COR
2.1
—C—CH2—Cl
3.6
CH3—S—
2.1
—C—CH2—OH
3.6
d
Contd....