For UG/PG students of All Engineering (B Tech/B E) branches, Chemistry, Food Technology, Biochemistry, Biotechnology. The video lecture link of the presentation is https://www.youtube.com/watch?v=bFPhvnW8T18&t=99s

1. Nuclear Magnetic
Resonance (NMR)
Spectroscopy
[Basic Concepts]
Prof. Harish Chopra,
Department of Chemistry,
SLIET, Longowal (Pb) INDIA

2. Introduction
Nuclear magnetic resonance (NMR) spectroscopy involves transition
of a nucleus from one spin state to another with the resultant
absorption of electromagnetic radiation by spin active nuclei
(having spin quantum number > 0) under the influence of
magnetic field.
When the frequency of the rotating magnetic field and the Larmor
frequency (frequency of precessing nucleus) become equal, these are
said to be in resonance leading to the absorption or emission of energy
by the nucleus. The spectrum drawn between peak intensities vs.
frequency of absorption (expressed as 𝛿) is called as NMR
spectrum and the methodology is called as NMR spectroscopy.
❏ NMR is non-destructive technique.
❏ Finest technique for determining the structure of organic
compounds.
❏ Larger amounts of sample are needed for NMR than mass
spectroscopy.
2

3. Theory
3
The nuclei of many elemental isotopes have a characteristic spin
(I). Some nuclei have integral spins (E.g., I = 1, 2, 3....), some have
fractional spins (E.g., I = 1/2, 3/2, 5/2 ....), and a others have no spin,
I = 0 (E.g., 12C, 16O, 32S,....). Only those nuclei exhibit nuclear
magnetic resonance which has the spin quantum number (I) >0.
The proton nucleus has I=1/2 (spin active) and hence shows nuclear
magnetic resonance whereas 12C and 16O have I=0 (spin inactive), so
does not show nuclear magnetic resonance. Isotopes of particular
interest and use to organic chemists are 1H, 13C, 19F and 31P, all of
which have I=1/2. These isotopes have been studied extensively.
The number of spin states for a given nucleus is given by
(2I+1), where I is spin quantum number of the nucleus.

4. Theory
4
A spinning active nucleus such as proton behaves as if it were
spinning about an axis. This spinning charged particle generates a
magnetic field around it and therefore behaves as spinning nuclear
magnets. The resulting spin-magnet has a magnetic moment (m)
proportional to the spin. Such a motion is called as precessional
motion and proton is said to be precessing around the vertical axis of
the earth’s gravitational field. In the presence of an external magnetic
field (B0), two spin states exist, +1/2 and -1/2. 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

5. Theory
5
The difference in energy between the two spin states is dependent
on the external magnetic field strength, and is always very small.
The two spin states have the same energy when the
external magnetic field is zero, but diverge as the external
magnetic field increases. At a magnetic field strength equal to
Bx a formula for the energy difference is given in the figure (for
proton I = 1/2 and 𝜇 is the magnetic moment of the nucleus in the
field).

6. NMR Spectrometer
6
The commonly used NMR spectrometers are
based on two techniques; (i) Continuous
wave (CW) method (ii) Fourier transform
(FT) method. The actual procedure for taking
the spectrum varies, but the simplest is
referred to as the continuous wave (CW)
method. A typical CW-spectrometer
consists of a magnet, radio frequency source,
a detector and an amplifier. A solution of the
sample in a uniform 5 mm glass tube is placed
between the poles of a powerful magnet, and is
spun to average any magnetic field variations.
Radio frequency radiation of appropriate
energy is broadcast from a radio frequency
source into the sample from an antenna coil.
A receiver coil surrounds the sample tube, and
emission of absorbed radio frequency energy
is monitored by electronic devices (detector,
amplifier and recorder) and a computer.

7. NMR Spectrum
7
An NMR spectrum is obtained by changing or sweeping the magnetic
field over a small range of frequency while observing the radio
frequency signal from the sample. An equally effective technique is
to vary the frequency of the radio frequency radiation while holding
the external field constant. NMR spectrum is plot of intensity of
NMR signal vs magnetic field (frequency) with reference to TMS.
.
Intensity
Frequency

8. Shielding/Deshielding
8
Since electrons are charged particles, they move in response to the
external magnetic field (Bo) so as to generate a secondary magnetic field
(induced magnetic field) that opposes the much stronger applied field.
Therefore, the magnetic field felt by proton (nucleus) is diminished
and the proton is said to be shielded and the process is called as
shielding of proton (nucleus). So, Bo must be increased to compensate
for the induced shielding field. But if the induced magnetic field
reinforces the applied external magnetic field, the proton (nucleus)
experiences a higher magnetic field than the applied external
magnetic field and such proton is said to be deshielded and the process
is called as de-shielding of proton (nucleus).

9. Chemical Shift
9
The location of an NMR signal in a spectrum for an organic
compound relative to a reference signal (whose value is taken as zero)
from a standard compound added to the sample is called as chemical
shift of that compound. Such shifts (compared with standard
reference) in the positions of NMR signals which arise due to
shielding or deshielding of protons by electrons is called as chemical
shift. Chemical Shift is having units of parts-per-million (ppm), and
designated by the symbol δ. The value of chemical shift of an organic
compound with respect to TMS can be defined as:
.
The most of the organic compounds show chemical shift values
between 0 and 10. In the 𝛕-scale, the signal for TMS is taken as 10
ppm and the two scales are related to each other as follows:

10. NMR Spectrum
10
Tetramethylsilane, (CH3)4Si, [TMS], is commonly used as the reference
compound of choice for 1H and 13C NMR because of low electronegativity of
silicon which produces more shielding of equivalent protons in TMS as
compared to most of the organic compounds. This high shielding of protons
in TMS shifts the signal up field and taken as zero in NMR spectrum. TMS is
the most convenient reference because it is chemically unreactive, highly
volatile (low boiling) hence, can be easily removed from the sample after
the measurement and it is miscible with almost all organic compounds.
Further, it gives a single sharp NMR signal that does not interfere with the
resonances normally observed for other organic compounds.
.The NMR spectrum is plotted
with magnetic field strength
increasing to the right, thus,
the signal for TMS appearing
at the extreme right (δ=0) of
the spectrum.
.

11. NMR Spectrum - Solvents
11
In order to take the NMR spectra of a solid, it is usually necessary
to dissolve it in a suitable solvent. Early studies used carbon
tetrachloride for this purpose, since it has no hydrogen that could
introduce an interfering signal. Unfortunately, CCl4 is a poor solvent
for many polar organic compounds and is also toxic. Deuterium
labeled compounds are now widely used as NMR solvents. The
Common examples are:
❏ Deuterium oxide (D2O),
❏ Chloroform-d (CDCl3),
❏ Benzene-d6 (C6D6),
❏ Acetone-d6 (CD3COCD3)
❏ DMSO-d6 (CD3SOCD3)
Since the deuterium isotope of hydrogen has a different magnetic
moment and spin, it is invisible in a spectrometer tuned to protons.
.

12. Factors Affecting
Chemical Shift
12
Inductive Effect
VanderWaal’s Deshielding
Anisotropic Effect
Hydrogen Bonding

13. Inductive Effect
[Electronegativity]
13
If the electron density about a proton nucleus is relatively high, the induced
magnetic field due to electron motions will be stronger leading to larger
shielding effect and in such cases therefore a higher external field (Bo) will be
needed for the radio frequency energy to excite the nuclear spin. Since silicon is
less electronegative than carbon, the electron density about the methyl
hydrogens in tetramethylsilane is expected to be greater than the electron
density about the methyl hydrogens in neopentane (2,2-dimethylpropane), and
the characteristic resonance signal from the silane derivative does indeed lie at
a higher magnetic field. Such nuclei are said to be shielded. Elements that are
more electronegative than carbon should exert an opposite effect (reduce the
electron density); and methyl groups bonded to such elements exhibit lower field
signals (they are deshielded). The de-shielding effect of electron withdrawing
groups is roughly proportional to their electronegativity.
.
Additive Deshielding
effect of EWG groups

14. van der Waal’s Deshielding
14
In molecules having bigger substituents showing steric
hindrance, the electron cloud of the bulky group will tend to
repel the electron cloud surrounding the proton. Therefore,
such a proton shall be de-shielded and will resonate at
higher value of chemical shift (δ) than expected in the
absence of this effect which is called as van der-Waal’ de-
shielding.
.

15. Anisotropic Effect
15
The interaction of the 𝜋 electrons with the applied magnetic field is complex
and leads to upfield chemical shifts (lower frequency, diamagnetic shifts) or
to downfield chemical shifts (higher frequency, paramagnetic shifts). As
these effects are paramagnetic in certain directions around the 𝜋
electron cloud and diamagnetic in other directions hence, they are called
as anisotropic effects. The shielding and deshielding regions of the
carbonyl group are similar to alkenes. Two cone shaped regions, centred
on oxygen atom lie parallel to the axis of carbon-oxygen double bond (C=O).
The protons within these cones experiences deshielding and hence aldehydic
protons appear at higher δ values. The protons held below or above these
cones will experience shielding and hence appear at lower δ values.
.

16. Anisotropic Effect
16
In case of alkenes, the orientation of alkene with respect to applied magnetic
field determines the extent of anisotropic effects. If the alkene group is so
oriented that direction of the applied field is at 90° to the plane of the double
bond, the circulation of electrons shall induce a secondary magnetic field which
will be paramagnetic (deshielding) in the region of alkene protons and
diamagnetic (shielding) around the carbon atom. The region where the
direction of induced secondary magnetic field is parallel to the applied
magnetic field (B0), the net magnetic field felt by the protons shall be higher
than the applied field (B0). The protons in these regions, therefore, give an NMR
signal at higher values than expected. In the regions above or below the plane
of the double bond will experience a shielding effect as in these areas induced
secondary magnetic field opposes the applied magnetic field (B0). The protons
in these regions therefore give an NMR signal at lower δ values than expected..
.
So, double bond in alkenes can be divided
into two regions,; deshielding occurring in
cone shaped regions and protons
appearing at higher δ values and shielding
occurring outside these cones and protons
appearing at lower δ values

17. Anisotropic Effect
17
The 𝜋 -electrons cloud of benzene ring above and below the plane of the
ring circulates in reaction to the external field so as to generate an opposing
field at the centre of the ring and a supporting field at the edge of the ring. This
kind of spatial variation is called anisotropy.
Regions in which the induced field supports or adds to the external field are said
to be deshielded, because a slightly weaker external field will bring about
resonance for nuclei in such areas. However, regions in which the induced field
opposes the external field are termed shielded because an increase in the applied
field is needed for resonance.
.
Shielded regions are
designated by a plus
sign (+), and
deshielded regions by
a negative sign (-).

18. Anisotropic Effect
18
The alkyne protons appear at lower 𝛿 values as compared to alkenes or
carbonyl compounds is due to the fact that circulation of 𝜋 electrons
around the triple bond takes place in such a manner that protons
experiences a diamagnetic effect (shielding) which explains the
anisotropy about the triple bond nicely accounts for the relatively high
field chemical shift of ethynyl hydrogens and these appear at lower 𝛿
values in NMR spectrum.
.

19. Hydrogen Bonding
19
Hydrogen bonding shifts the resonance signal of a proton to
lower field (higher frequency). The chemical shift of the hydroxyl
hydrogen of an alcohol varies with concentration. Very dilute
solutions of 2-methyl-2-propanol, (CH3)3COH, in carbon tetrachloride
solution display a hydroxyl resonance signal having a relatively high-
field chemical shift (< 1.0 𝛿). In concentrated solution this signal shifts
to a lower field, usually near 2.5 𝛿 because in concentrated solution
hydrogen bonding plays a major role. Like alcohols, the more acidic
hydroxyl group of phenol generates a lower-field resonance signal,
which shows similar concentration dependence.

20. Hydrogen Bonding
20
The hydroxyl proton of carboxylic acids displays
a resonance signal significantly down-field (10.0 to
13.0 δ and is often broader than other signals) of
other functional groups because of their favoured
hydrogen bonded dimeric association.
Intramolecular hydrogen bonds, especially those defining a six-
membered ring, generally exhibit a very low-field proton resonance. The case of
4-hydroxypent-3-ene-2-one (the enol tautomer of 2,4-pentanedione) is an
example of the sensitivity of the NMR experiment to dynamic change. In the
NMR spectrum of the pure liquid, sharp signals from both the keto and enol
tautomers are seen, their mole ratio being 4:21. The chemical shift of the
hydrogen-bonded hydroxyl proton is δ 14.5, exceptionally downfield.

21. Spin-Spin Interactions
[Spin-Spin Splitting or Coupling]
21
The NMR spectrum, theoretically gives number of peaks equal to the number
of equivalent protons in the molecule and one type of equivalent protons
should give one peak (signal). However, in practical the NMR spectrum is
more complicated than we might have expected. E.g., In 1,1,2-
trichloroethane (Cl2CH-CH2Cl), which has two types of equivalent
protons and NMR spectrum should give two signals for these set of
protons, However, in actual the NMR gives two signals but one signal is
consisting of a set of three peaks whereas the other signal of set of two
peaks. This is a common feature in the spectra of compounds having
different sets of hydrogen atoms bonded to adjacent carbon atoms. So, the
signals expected from the set of equivalent protons do not appear as a
single peak but split up into a group of peaks. This phenomenon is called as
splitting of NMR signal.

22. Spin-Spin Interactions
[Spin-Spin Splitting or Coupling]
22
The splitting of spectral lines takes place because of a coupling
interaction between neighbouring protons having different magnetic
environments. The spin states of the neighbouring protons [parallel (↑)
or anti-parallel (↓¯)] modify the actual magnetic field experienced by the
particular proton. Hence, the different magnetic field experienced by the
absorbing proton depending on its spin orientation causes the split of
the spectral line into a group of peaks. This phenomenon is called as
spin-spin splitting or spin-spin coupling. The number of peaks
formed after splitting of a spectral line depends upon the number of
neighbouring protons. E.g., In 1,1,2-trichloroethane (Cl2CH-CH2Cl), the
signal from two equivalent protons of -CH2Cl group is split into two
peaks (doublet) under the influence of one non-equivalent
neighbouring proton of Cl2CH- group. Similarly, the signal from one
equivalent proton of Cl2CH- group is split into three peaks (triplet)
under the influence of two equivalent neighbouring protons of -CH2Cl
group.

23. Spin-Spin Interactions
[Spin-Spin Splitting or Coupling]
23
The most commonly found splitting patterns have been given
names, such as doublet (two equal intensity signals), triplet (three
signals with an intensity ratio of 1:2:1), quartet (a set of four signals
with intensities of 1:3:3:1) and quintet (a set of five signals with
intensities of 1:4:6:4:1). The line separation during splitting of
spectral lines is always constant within a given multiplet, and is
called as coupling constant (J). The magnitude of J, usually given
in units of Hertz (Hz), is independent of the applied magnetic field
and the operating frequency of the NMR spectrometer.
.

24. Spin-Spin Interactions
[Spin-Spin Splitting or Coupling]
24
Protons having the same chemical shift
(called isochronous) do not exhibit spin-
spin splitting or the spin-spin splitting
is shown only by non-equivalent
neighbouring protons (protons with
different chemical shifts),
Ethane (CH3CH3) or 1,2-
dichloroethane (ClCH2CH2Cl); no
splitting takes place as the protons
present on both the carbons are
equivalent protons
Protons separated by more than two
atoms (non-adjacent) do not undergo
spin-spin splitting.
[(CH3)2C(Br)CH2Br] do not show
splitting of proton signal of methyl
groups by - CH2Br group or vice-
versa as they are not adjacent
protons
2-methylpropene do not show any
splitting whereas 2-halopropenes
exhibit splitting as the two vicinal
protons present in the former are
equivalent protons whereas non-
equivalent in the latter
Equivalent protons present on the
same carbon generally do not split
each other but splitting may take
place if these are non-equivalent..
General Rules

25. Spin-Spin Interactions
[Spin-Spin Splitting or Coupling]
25
General Rules
The magnitude of the spin-spin splitting depends on many factors and is
given by the coupling constant J (units of Hz). J is the same for both
partners in a spin-splitting interaction and is independent of the external
magnetic field strength
The splitting pattern of a given proton (or set of equivalent protons) can be
predicted by the n+1 rule, where n is the number of neighboring spin-
coupled protons with the same (or very similar) coupling constants. Thus, the
number of peaks (N) which a proton will give on splitting is one more than
the vicinal protons (n). E.g., if there are two neighbouring spin-coupled
protons, the observed signal is a triplet (2+1=3); if there are three spin-
coupled neighbors the signal is a quartet (3+1=4); if there are four spin-
coupled neighbours the signal is a quintet (4+1=5).

26. Spin-Spin Interactions
[Spin-Spin Splitting or Coupling]
26
General Rules
The intensity of the peaks is related to the possibility of various spin-
spin combinations for each orientation. The intensity for doublet,
triplet, quartet and quintet is 1:1, 1:2:1, 1:3:3:1 and 1:4:6:4:1

27. Coupling Constant
27
The distance between the two centres of peaks in a given multiplet is
known as coupling constant (J). The coupling constant signifies the
magnitude of splitting of NMR spectral lines and its value is
independent of the external magnetic field applied. Its value however
depends upon the structural relation between the coupled protons and is
expressed in cycles per seconds or Hertz.

28. 28
References
The some contents are taken from:
Chemistry For
Engineers
By
Harish Chopra
Anupama Parmar
[In addition, Internet sources have also been used]

29. Thank You !
29