Nuclear magnetic resonance proton nmr

Nuclear
Magnetic
Resonance
Spectroscopy -
PMR

1

CONTENTS
 Introduction

 Nuclear spin and magnetic movement

 Theory and principle

 Applied field and precession

 Precessional frequency

 Width of absorption line in NMR

 Shielding and Deshielding

Reference standard
January 26, 2013 M.M.C.P. 3

 Chemical shift

 Factor affecting chemical shift

 Interpretation of PMR

 Instrumentation of NMR

 Splitting of the signals

 Spin-spin coupling

 Intensities of Multiplet Peaks

 Spin Decoupling

INTRODUCTION

NMR spectroscopy is different from the interaction of
electromagnetic radiation with matter.

In this spectroscopy the sample is subjected simultaneously with
two magnetic field, One is a stationary and another is varying at
same radio frequency.

The particular combination of these two field energy is absorbed
by sample and signal is obtained when electromagnetic field is
provided to the nucleus of sample. The nucleus start to spin
around the nuclear axies and generate an another magnetic field.
And particular combination of this two field the energy is
absorbed by nucleus this technique is called as a NMR
spectroscopy.

 This transition of nucleus occurs in radio frequency region. The
radio waves are considered for lowest energy and this energy is
just sufficient to affect the nuclear spin of the atom in a molecule.
Hence, this is a most fundamental part of NMR spectroscopy.

 In general, the study of radio frequency radiation by nuclei is
called nuclear magnetic resonance.

 The method of NMR was first developed by E.M. Purcell and
Felix Bloch (1946).


In synthetic organic and organometallic chemistry, solution-state
NMR means a 300-500 MHz NMR spectrometer, high-precision
glass sample tubes, 2 ml of deuterated solvent (typically fully
deuterated chloroform, acetone, benzene, or dichlorobenzene),
several milligrams of pure sample, and a reference substance,
NMR experiments with several hours of spectrometer time and
data interpretation.

The structures of new compounds with molecular weights up to
2000 Da can be determined, especially when analyzed along with
results from NMR databases and mass spectroscopy.


 It is a well known fact that spectra given by all forms of
spectroscopy may be described in terms of the following three
important factors.

1. Frequency of spectral lines or bands.
2. Intensity of spectral lines or bands.
3. Shape of spectral lines or bands.

All above properties depends on the molecular parameters of the
system. In case of the NMR these molecular parameters are found
to be:

1. Shielding constant of nuclei.
2. Coupling constant of nuclei.
3. Lifetime of energy level.


NUCLEAR SPIN AND MAGNETIC MOMENT

Nucleus axis

Nucleus

Magnetic field
Fig: Spinning of Nucleus

All nuclei carry a charge. In some nuclei this charge spins on the
nuclear axis and this circulation of nuclear charge generates a
magnetic dipole along the axies.

The nuclei of atoms are composed of protons and neutrons. Like
electrons, these particle also have the properties to spin on their
own axis and each of them possesses angular momentum1/2(h/2π)

in accordance with the quantum theory. The net resultant of the
angular momentum of all nuclear particles is called nuclear spin.

For a nucleus having a spin quantum number I, these are(2I +1)
spin states.

January 26, 2013 M.M.C.P. 10

Two properties of nuclear particles which are important in
understanding of NMR spectroscopy are:

• The net spin associated with the proton and neutron.
• The distribution of positive charge.

The net spin number or spin quantum number I of a particular
nucleus can be obtained by adding spin numbers of individual
proton and neutron of ½ each, assuming that neutrons cancel only
neutrons and protons cancel only protons, because of pairing or
spinning in opposite directions.

The spin number I have values 0,1/2, 1, 3/2, 5/2 and so forth. If
I=0 that represent no spin.

January 26, 2013 M.M.C.P. 11

PRINCIPLE FOR NUCLEAR SPIN
If the sum of protons and neutrons is even, I is zero or
integral (0,1,2,3 …..)

If the sum of proton and neutrons is odd, I is a half
integral (1/2, 3/2, 5/2….)

If the both protons and neutrons are even numbered, I is
zero.

January 26, 2013 M.M.C.P. 12

35
Cl,
17
16
O
17

January 26, 2013 M.M.C.P. 13

THEORY AND PRINCIPLE
•The NMR is mostly consult with nucleus spin quantum no. (I)= ½ .
The proton having a I = ½ when place in external magnetic field
(Ho) it’s start to spin around the nuclear axis and generate a
another magnetic field.
•According to quantum mechanics there are 2I + 1 so two spin stage
+ ½ and - ½ for the proton.
E
I=-½
E2

I=½

I=+½
E1
Ho
January 26, 2013 Spin state of proton M.M.C.P.
14

• When a charge particle place in magnetic field. It’s start to revel
and therefore it’s pusses angular movement due to generation of
another magnetic field. The charge particle with nucleus spin has
magnitude and direction. Both this property is describe by the
factor called as magnetic movement (µ).

•So, when the proton take place in magnetic field . It has two spin
steps + ½ and - ½ so, there are two energy level for spin steps + ½
& -½.

E1 = + ½ µ Ho ………………….1
E2 = - ½ µ Ho ……………….….2
where, Ho = magnetic field strength.
µ = magnetic movement
ΔE = E1 – E2 …………………....3

January 26, 2013 M.M.C.P. 15

ΔE = µ Ho …………………………4
by Boher’s frequency eq. we can write
ΔE = hv ……………………….5 v= frequency
from the eq. 4 &5
hv = µ Ho ………………………… 6

so, µ Ho ………………………….7
v=
h
This is a basic eq. in NMR spectroscopy.

1.41 Tesla = 60 MHz

2.35 Tesla = 100 MHz

7.05 Tesla = 300 MHz
January 26, 2013 M.M.C.P. 16

APPLIED FIELD AND PRECESSION
 Spinning nuclei-magnetic moments

 Some elements have isotopes with nuclei that behave as though
they were spinning about an axis much like the earth. The
spinning of charge particle generates a magnetic field. As a
consequence, the spinning nuclei behave as though they were tiny
bar magnets having a north and a south pole.

α ᵦ
ᵦ αα ᵦ
α Applied magnetic
field (Ho)
Nuclear magnetic movement with
No magnetic field January 26, 2013 M.M.C.P.
Ho 17

•Since a nucleus or an electron bears a charge, its spin gives rise to a
magnetic field that is analogous to the field produce when an electric
current is passed through a coil of wire. The resulting magnetic
dipole (µ) is oriented along the axis of spin and has a value that is
characteristic for each kind of particle.

Applied
field Ho
E= µᵦHo
No
I= - ½
field

energy 0 ∆E = µᵦHo

I= + ½
E= µᵦHo
18
January 26, 2013 M.M.C.P.

PRECESSION

PRECESSIONAL MOTION
•Because the proton is behaving as a spinning magnet, it can align
itself either with or opposed to an external magnetic field. It can
also
move a characteristic way under the influence of the external
magnet.

•Considering the behavior of a spinning top, the top has a spinning
motion around its axis. It also performs a slower waltz like motion
in which the spinning axis of top moves slowly around the vertical.
Thus is called “precessional motion” and the top is said to be
precessing around the vertical axis of the gravitational force of the
earth.
January 26, 2013 M.M.C.P. 19

•Precession arises due to the interaction of spin and the gravitational
force acting downwards. This is the reason why only a spinning top
will precess; where as a static top will topple over.

•Since the proton is a spinning magnet, it will precess around the axis
of an applied external magnetic field. It will precess in two main
orientations.

•Aligned or parallel with the field-low energy.

•Opposed or anti parallel to the field-high energy.

January 26, 2013 M.M.C.P. 20

PRECESSIONAL FREQUENCY
The precessional frequency of the nucleus is directly proportional
to the strength of external field and also depends on the nature of
the nuclear magnet.

Magnetic nuclei different atoms have different characteristic
precessional frequency.
according to Larmor precessional theory
ω = γH0 ………………1
where, ω= Larmor precessional frequency.
ω = 2 πV ………………2
2 πV = γH0 ………………..3
γ H0
V= 2π ………………….4

V α H0 ..................................5
Intrinsic magnetic dipole momentum
Where, γ = is gyromagnetic ratio = Spin angular momentum
M.M.C.P. 21
January 26, 2013

ENERGY TRANSITIONS

 A proton when kept in an external magnetic field will precess
and can take one of the two orientations with respect to the axis
of
the external field. Either aligned or opposed.

If a proton is precessing in the aligned orientation it can absorbed
energy and pass into the opposed orientation and vice versa by
losing energy.

If we irradiate the precessing nuclei with a beam of radio
frequency, the low energy nuclei may absorb this energy and
move
to a higher energy state.
January 26, 2013 M.M.C.P. 22

The precessing proton will absorb energy from the radio frequency
source, if the precessing frequency same as the frequency of the
radio frequency beam.

When this occurs, the nucleus and radio frequency beam are said to
be resonance, hence the term “ nuclear magnetic resonance”.

January 26, 2013 M.M.C.P. 23

WIDTH OF ABSORPTION LINES IN NMR

The separation of two absorption lines depends on how close
they are to each other and the absorption line width.

The width of the absorption line is affected by a number of
factors, only some of which we can control.

These are the factors:

I.The homogeneous field
II.Relaxation time
III.Magic angle NMR
IV.Other source of line broadening
January 26, 2013 M.M.C.P. 24

1. THE HOMOGENEOUS FIELD
The most important factor controlling the absorption line
width is the applied magnetic field H0.

It is very important that this field be constant over all
parts of the sample, which may be 1-2 inch long. If it is
not, H0 is different parts of the sample and therefore v,
the frequency of the absorbed radiation, will vary in
different parts of sample.

This variation results in a wide absorption line. For
qualitative or quantitative analysis a wide absorption line
is very undesirable, since we may get overlap between
neighbouring peaks.
January 26, 2013 M.M.C.P. 25

2. RELAXATION TIME
The second important feature that influences the absorption line
width is the length of time that an excited nucleus stays in the
exited state.
ΔE Δt = constant
where, ΔE is the uncertain in the value of E
Δt is the length of time a nucleus of time a nucleus spends
in the excited state.

Since ΔE Δt is a constant, when Δt is small, ΔE is large. But we
know that E = hv and that h is constant.

Therefore any variation in E will result in a variation in v. If E is not
an exact number but varies over the range E+ ΔE, then v will not be
exact but vary over the corresponding range v + Δv. Then we have
E + ΔE = h(v + Δv)
January 26, 2013 M.M.C.P. 26

We can summarize this relationship by saying that when Δt is
small, ΔE is large and therefore Δv is large. If Δv is large, then the
frequency range over which absorption takes place is wide and a
wide absorption line results.

There are two principle modes of relaxation,

1. Longitudinal relaxation /spin lattice
2. Transverse relaxation /spin spin

January 26, 2013 M.M.C.P. 27

A. LONGITUDINAL RELAXATION
When the nucleus loses its excitation energy to the surrounding
molecules, the system becomes warm as the energy is changed to
heat.

This process is quite fast when the molecules are able to move
quickly. This is the state of affairs in liquid.

The excitation energy becomes dispersed throughout the whole
system of molecules in which the sample finds it self. No radiation
energy appears, no other nuclei become exited. Instead, as
numerous nuclei lose their energy in this fashion, the temperature
of the sample goes up. This process is called longitudinal
relaxation
T1.
January 26, 2013 M.M.C.P. 28

B. TRANSVERSE RELAXATION

An excited nucleus may transfer its energy to an unexcited nucleus
of a similar molecules that is nearby.

In this process, the nearby unexcited nucleus becomes excited a
the previously excited nucleus become unexcited.

There is no net change in energy of the system, but the length of
the time that one nucleus stays excited is shortened. This process,
which is called transverse relaxation T2.

January 26, 2013 M.M.C.P. 29

3. MAGIC ANGLE NMR
A problem with the examination of solids that the nuclei can be
frozen in space and cannot freely line up in magnetic field.

The NMR signals generated are dependent among other things, on
the orientation of the nuclei. The randomly oriented nuclei
therefore give broad band spectra which are not very useful
analytically.

It can be shown that when one rotates a solid sample such that its
axis of rotation 54.70 to the direction of the applied magnetic field,
the broadening caused by random nuclear orientations tends to be
average out, resulting in narrower spectra.

January 26, 2013 M.M.C.P. 30

 This is more useful analytically because it allow better
resolution and therefor better measurement of chemical shift
and spin-spin splitting. In turn, this is very informative of the
functional group and their positions relative to each other in the
sample molecule.

January 26, 2013 M.M.C.P. 31

4. OTHER SOURCES OF LINE BROADENING

Any process of deactivating, or relaxing, an excited molecule
results in a decrease in a lifetime of the excited state. This is turn
causes line broadening.

Other causes of deactivation include
1. The presence of ions
2. Paramagnetic molecules
3. Nuclei with quadrupole moment

January 26, 2013 M.M.C.P. 32

SHIELDING AND DESHIELDING

1.INDUCED MAGNETIC FIELD :-
In the applied magnetic field, the valence electrons around the
nucleus are cause to circulates and they generates their own
secondary magnetic field is known as induced magnetic field.

2.SHIELDING:-
The circulation of electron around the protons itself generates field
in a such way that , it oppose the applied field.
The field felt by the protons is thus diminished and the proton is
said to be shielded and the absorption said to be upfield.

January 26, 2013 M.M.C.P. 33

DESHIELDING:-
DESHIELDING
If the induced magnetic field reinforced the applied magnetic
field ,then the field felt by the proton is augmented and the proton
is said to be deshielded and the absorption is known as downfield.

January 26, 2013 M.M.C.P. 34

 Compare with naked proton, a shielded proton required
higher applied field strength than the deshielded protons.

Shifts in the position of NMR absorption arising from
shielding and deshielding by electron, due to different chemical
environments around protons are called chemical shift.

Generally chemical shift measured from the signal of
reference standard such as TMS

January 26, 2013 M.M.C.P. 35

The extent of shielding is represented in terms of
shielding parameter α. When absorption occurs, the
field H felt by the proton is represented as,

H = H0 (1 - α )………………. 1
where, H0 = applied field strength.

Greater value of α, greater will be the value of
applied strength which has to be applied to get the
effective field required for absorption and vice
versa. ………………. 2

From 1 and 2 H0

January 26, 2013 M.M.C.P. 36

It is clear that the proton with different electronic
environments or with shielding parameter can be brought
into resonance in two ways
1. The strength of external field is kept steady and the radio
frequency is constantly varied
2. The radio frequency is kept steady and strength of the
applied field is constantly varied.

Clearly at constant radio frequency, shielding shift the
absorption upfield in the molecules where these is a
spherical distribution of electrons around the proton,
It is called positive shielding.

January 26, 2013 M.M.C.P. 37

January 26, 2013 38
M.M.C.P.

REFERENCE STANDARDS

CHARACTERISTICS:-

 Chemical inertness
 Magnetically neutral
 Gives single sharp peak
 Easily recognizable peak
 Miscible with wide range of solvents
 Volatility –to facilitate recovery from valuable samples

January 26, 2013 M.M.C.P. 39

1.TMS(Tetra Methyl Silan):-
 It is generally used as internal standard for measuring the
position of 1H,13C, 29Si in NMR

 TMS at 0.5% concentration is used normally

 TMS has 12 protons which are uniformly shielded because of
highly electro-positive nature of silicon at centre

 Hence this 12 protons gives single sharp peak at oδ which
require maximum magnetic field than protons of the most
organic compounds

 It is chemically inert and miscible with large range of solvents

 Highly volatile and easily removed to get back the sample
January 26, 2013 M.M.C.P. 40

It does not take part in intermolecular association
with the sample

 It’s all protons are magnetically equivalent

TMS can be used as an external reference also.

January 26, 2013 M.M.C.P. 41

2.Sodium salts of 3-(trimethyl silyl)propane
sulphonate:-

 It is a water soluble compound.
 It is used as internal standard for running PMR
spectra of water soluble substances in the duterium
oxide solvent.

January 26, 2013 M.M.C.P. 42

CHEMICAL SHIFT

“Chemical shift is the difference between the absorption position
of the sample proton and the absorption position of reference
standard”
Variations of the positions of NMR absorptions due to the
electronic shielding and deshielding.

January 26, 2013 M.M.C.P. 43

Chemical Shifts….
• Measured in parts per million (ppm).

• It is the ratio of shift downfield from TMS (Hz) to total
spectrometer frequency (MHz).

• The chemical shift is independent of the operating
frequency of the spectrometer.

• Same value for 60, 100, or 300 MHz machine.

• Common scale used is the delta (δ) scale.

January 26, 2013 M.M.C.P. 44

MEASUREMENT OF CHEMICAL SHIFT

Each proton in a molecule has slightly different
chemical environment and consequently has a slightly
different amount of electronic shielding, which results
in a slightly different resonance frequency. These
differences in resonance frequency are very small.

For example, the difference between the resonance
frequencies of the protons in Chloromethane and in
Fluromethane is only 60 MHz when the applied field is
1.41 Tesla.

January 26, 2013 M.M.C.P. 45

Since the radiation used to induce proton spin
transitions at that magnetic field strength is of a frequency near
60 MHz, the difference between Chloromethane and
Fluoromethane represents a change in frequency of only slightly,
not more than one part per million.

It is very difficult to measure the exact resonance frequencies to
that precision. Hence instead of measurement of the exact radio
frequency of any proton, a reference compound is placed in the
solution of the substance to be measured and the resonance
frequency of each proton in the sample is measured relative to the
resonance frequency of the protons of the substance.

January 26, 2013 M.M.C.P. 46

The standard reference substance used universally
is TETRAMETHYLSILANE (TMS), the standard
reference is also known as Internal Standard.

Shift from TMS in Hz
Chemical Shift, ppm δ = X 106
Spectrometer frequency (MHz)

Eg: for CH3Br protons, chemical shift from TMS = 162Hz in a
60 MHz instrument and 270Hz in a 100 MHz instrument.
Calculate δ value.

δ = 162Hz / 60 MHz = 270 / 100 = 2.7ppm

Hence, δ Value remains same irrespective of the spectrometer.

January 26, 2013 M.M.C.P. 47

Chemical shift is measure in three major spectra.
Delta(δ)
Tau scale(τ)
Hertz (Hz)

Up field shielding

Down field shielding

5 4 3 2 1 0 δ scale

5 6 7 8 9 10 Τ scale

1000 800 400 100 HZ

January 26, 2013 M.M.C.P. 48

-5
1 ppm = 60 Hz = 6 ͯ 10MHz

-4
1 ppm = 300 Hz = 3 ͯ 10 MHz

Each δ unit is 1 ppm difference from TMS 60Hz and 300Hz

January 26, 2013 M.M.C.P. 49

CHART FOR DIFFERENT TYPES OF
PROTON CHEMICAL SHIFT VALUES

January 26, 2013 M.M.C.P. 50

BASIC CONCEPTS….
1) The chemical shift or position of line in NMR spectrum gives
information on molecular environment of the nuclei from
which it arises.

2) The chemical shift of nuclei in the different molecules are
similar. If the molecular magnetic environment are similar.

3) The intensity of lines gives directly the relative number of
magnetically active nuclei undergoing the different chemical
shift.

4) The chemical shift is used for the identification of functional
groups and as an aid in determining structural arrangement of
groups.
January 26, 2013 M.M.C.P. 51

5)Greater is the deshielding of proton 
higher will be the value of delta.
Greater is the shielding of proton 
lower will be the value of delta.

6)Electron withdrawing substituents like halogens
which deshielded the protons.
Electron releasing substituents like alkyl groups 
which shielded the protons.

7)The delta unit is independent of shield strength.
Chemical shift position measured in the Hz are field
dependent.

January 26, 2013 M.M.C.P. 52

FACTORS INFLUENCING CHEMICAL SHIFT

• Electronegativity Effects.
• Van der Waal’s Deshielding.
• Hydrogen Bonding.
• Magnetic Anisotropy.
• Concentration, Tempareture and Solvent
Effect.

January 26, 2013 M.M.C.P. 53

ELECTRONEGATIVITY EFFECTS :-
• The chemical shift simply increase as the electronegativity of
the attached element increases.

• Following table illustrates this relationship for several
compounds of the type CH3X.

January 26, 2013 M.M.C.P. 54

• Multiple substituents have a stronger effect than a single
substituent. The influence of the substituent drops off rapidly
with distance, an electronegative element having little effect
on protons that are more than three carbons distant. This effect
is illustrated in the following table.

January 26, 2013 M.M.C.P. 55

Electronegative substituents attached to a carbon atom reduces
that valence electron density around the protons attached to
that carbon due to their electron withdrawing effects.

Electronegative substituents on carbon reduce the diamagnetic
shielding in the neighborhood of the attached protons because
they reduce the electron density around those protons.

The greater the electronegativity of the substituents, the more
deshielding of protons and hence the greater is the Chemical
Shift of those protons.

January 26, 2013 M.M.C.P. 56

January 26, 2013 M.M.C.P. 57

HYDROGEN BONDING :-

Hydrogen atom exhibit property of hydrogen bonding in a
compound which absorbs at a low field in comparison to the one
which does not shows hydrogen bonding.

Hydrogen bonded proton being attached to a highly
electronegative atom will have smaller electron density around
it.  less shielded  resonance will occurs downfield and
downfield shift depends up on the strength of hydrogen bonding.

Intramolecular and Intermolecular hydrogen bonding can be
easily distinguished as the latter does not show any shift in
absorption due to change in concentration.

January 26, 2013 M.M.C.P. 58

 In case of phenols. Absorption occurs between 4-8 δ but if the
concentration is decrease and volume of carbon tetrachloride is
increase then absorption of OH proton occurs upfield,
 Exchangeable Hydrogen: protons that exhibit hydrogen bonding (
eg. Hydroxyl or amino protons ) show resonance over a wide
range. These protons are usually found to attached to a
heteroatom.
 The more hydrogen bonding that takes place, the more deshielded
proton becomes.

January 26, 2013 M.M.C.P. 59

MAGNETIC ANISOTROPY :-

Circulation of electrons, especially the π electrons near by
nuclei generates an induced field which can either oppose or
reinforced the applied field at proton, depending upon location
of proton or space occupied by the protons.

In case of alkynes, shielding occurs but in case of alkenes,
benzene and aldehydes deshielding takes place.

The occurrence of shielding and deshielding can be determined
by the location of proton in the space and so this effect is known
as space effect.

January 26, 2013 M.M.C.P. 60

• There are some types of protons whose chemical shifts are not
easily explained by simple consideration of the electronegativity of
the attached groups.

• For example, when benzene is placed in magnetic field, the π
electrons in the aromatic ring system are induced to circulate
around the ring. This circulation is called as Ring current. The
moving electrons generate a magnetic field which influence the
shielding of the benzene hydrogens.

January 26, 2013 M.M.C.P. 61

Circulating π
electron

Secondary magnetic
field generated by
circulating π electrons
which deshields
aromatic protons

Applied field B0
Diamagnetic anisotropy in Benzene

January 26, 2013 M.M.C.P. 62

The benzene hydrogens are said to be deshielded by the
diamagnetic anisotropy of the ring.

In electromagnetic terminology; an Isotropic field is one
of either uniform density or spherically symmetric
distribution.

Anisotropic field is nonuniform. In case of benzene
labile electrons in the ring interact with the applied field
and thus rendered it anisotropic.

January 26, 2013 M.M.C.P. 63

Thus a proton attached to a benzene ring is influenced by three
magnetic fields:

1)The strong magnetic field applied by the electromagnets of the
NMR spectrophotometer.

2)Weak magnetic field due to shielding by the valence electrons
around the proton.

3)Anisotropy generated by the ring-system π electrons.

So anisotropic effect gives the benzene protons at higher
resonance δ value.

January 26, 2013 M.M.C.P. 64

• All groups in a molecule that have π electrons generate
secondary anisotropic fields.
• In Acetylene the magnetic field generated by induced
circulation of the π electrons has a geometry such that the
acetylenic hydrogens are shielded. Hence acetylenic
hydrogens have resonance at higher field.

Diamagnetic anisotropy
in Acetylene
π

January 26, 2013 M.M.C.P. 65

VAN DER WAAL’S DESHIELDING :-

In the overcrowded molecules. It is possible that some
proton may be occupying stearic hindered position.

Clearly electron cloud of bulky group or hindering
group will tend to repel the electron cloud surrounding
the proton and such proton will shielded and will
resonate at slightly higher value of δ than expected in
the absence of this effect.

January 26, 2013 M.M.C.P. 66

CONCENTRATION, TEMPERATURE AND
SOLVENT EFFECT :-

In ccl4 and cdcl3 chemical shift of proton attached to carbon is
independent of concentration and temperature, while proton of
-OH, –NH2, –SH groups exhibits a substantial conc. and
temperature effects due to the hydrogen bonding

 The intermolecular hydrogen bonding is less affected than
intramolecular bonding by concentration change

Both type of hydrogen bonding affected by the temperature
variation

January 26, 2013 M.M.C.P. 67

INTERPRETATION OF PMR SPECTRA

NMR spectrum of a substance gives very valuable information about
its molecular structure. This information is gathered as follows :

(1)The number signals in PMR spectrum tell us how many kinds
of protons in different chemical environments are present in
structure under examination
(2)The position of signal tell us about the electronic environment
of each kind of proton
(3)The intensities of different signals tell us about the relative
number of protons of different kind
(4)The splitting of signals tell us about environment of the
absorbing protons with respect to the environments of neighboring
protons
January 26, 2013 M.M.C.P. 68

 Triplet = δ-1.7,3H (CH3 )
 Quartet= δ-3.4, 2H (CH2 )

January 26, 2013 M.M.C.P. 69

 Doublet = δ-2.5,3H (CH3 )
 Quartet= δ-5.8,1H (CH )

January 26, 2013 M.M.C.P. 70

 Triplet = δ-1.2,3H (CH3 )
 Quartet= δ-3.6, 2H (CH2 )
 Singlet= δ-4.8, 1H (OH)
January 26, 2013 M.M.C.P. 71

 Quartet= δ-9.8,1H (CHO )
January 26, 2013 M.M.C.P. 72

 Singlet= δ-2.3, 3H (CH3)
 Doublet = δ-8.2,2H (NO2 )
January 26, 2013 M.M.C.P. 73

INSTRUMENTATION OF
NMR

January 26, 2013 M.M.C.P. 74

CLASSIFICATION OF THE NMR
SPECTROPHOTOMETERS

1. Conventional/Continuous NMR spectrophotometer
Minimal type.
Multiple type.
Wide line.
Or
It can also be classified as
a. Single coil spectrophotometer
b. Two coil spectrophotometer

2. Pulsed Fourier transforms NMR spectrophotometer

January 26, 2013 M.M.C.P. 75

COMPONENTS OF THE
SPECTROPHOTOMETER
Basically NMR instrumentation involves the following units.

1.A magnet to separate the nuclear spin energy state.

2. Two RF channels, one for the field/frequency stabilization and one to
supply RF irradiating energy.

3. A sample probe containing coils for coupling the sample with the RF
field;
it consists of Sample holder, RF oscillator, Sweep generator and RF
receiver.

4. A detector to process the NMR signals.

5. A recorder to display the spectrum.

January 26, 2013 M.M.C.P. 76

M.M.C.P. 77
January 26, 2013

77

MAGNETS
•It is used to supply the principal part of the field Ho, which determines the
Larmer frequency of any nucleus.

•The stronger the magnetic field, the better the line separation of chemically
shifted nuclei on the frequency scale.

•The relative populations of the lower energy spin level increases with the
increasing field, leading to a corresponding increase in the sensitivity of the
NMR experiment.
FEATURES:
1. It should give homogeneous magnetic field i.e.; the strength and direction of
the magnetic field should be constant over longer periods.

2. The strength of the field should be very high at least 20,000 gaus.

January 26, 2013 M.M.C.P. 78

TYPES OF MAGNETS:
1. PERMANENT magnets
2. ELECTRO magnets and
3. SUPER CONDUCTING magnets

MAGNETIC COILS
It is not easy or convenient to vary the magnetic field of large stable
magnets, however this problem can be overcome by superimposing a
small variable magnetic field on the main field.

Using a pair of Helmholtz coils on the pole faces of the permanent
magnet does this. These coils induce a magnetic field that can be varied
by varying the current flowing through them.

The small magnetic field is produced in the same direction as the main
field and is added to it. The sample is exposed to both fields, which
appear one field to the nucleus.
January 26, 2013 M.M.C.P. 79

THE PROBE UNIT
It is a sensing element of the spectrophotometer system. It is inserted between the
pole faces of the magnet in X-Y plane of the magnet air gap an adjustable probe
holder.

So the sample in NMR experiment experiences the combined effect of two
magnetic fields ie Ho and RF (EMR).

The usual NMR sample cell is generally made up of the glass, which is strong and
cheap. It consist of a 5 mm outer diameter and 7.5 cm long glass tube containing
0.4 ml of liquid.

The sample tube in NMR is held vertically between the poles faces of the magnet.
The probe contains a sample holder, sweep source and detector coils, with the
reference cell. The detector and receiver coils are orientated at 90 to each other.
The sample probe rotates the sample tube at a 30-40 revolutions on the longitudinal
axis. Each part of the sample tube experiences the same time average the field.

January 26, 2013 M.M.C.P. 80

THE RADIOFREQUENCY GENERATOR
Using an RF oscillator creates the radio frequency radiation, required to
induce transition in the nuclei of the sample from the ground state to
excited states.

The source is highly stable crystal controlled oscillator. It is mounted at
the right angles to the path of the field of wound around the sample tube
perpendicular to the magnetic field to get maximum interaction with the
sample. The oscillator irradiates the sample with RF radiation.

Radio frequencies are generated by the electronic multiplication of natural
frequency of a quartz crystal contained in a thermo stated block.

In order to generate radiofrequency radiation, RFO is used. To achieve
the maximum interaction of the RFradiation with the sample, the coil of
oscillator is wound around the sample container.

January 26, 2013 M.M.C.P. 81

The RFO coil is installed perpendicular (90 ºC) to the applied magnetic field
and transmits radio waves of fixed frequency such as 60,100,200 or 300
MHz to a small coil that energies the sample in the probe.

This is done so that the applied RF field should not change the effective
magnetic field in the process of irradiation.

January 26, 2013 M.M.C.P. 82

SWEEP GENERATOR
 Resonance

 This can achieved by two methods

•Frequency sweep method

If the applied magnetic field is kept constant, the precession frequency is
fixed. In order to bring about resonance, the frequency of the RF field
should be changed so that it is becomes equal to the resonance
frequency.

Thus resonance condition is reached by the holding the applied magnetic
field Ho constant and scanning the Rf transmitter through the
frequencies, until the various nuclei come to resonance in turn as their
precessional frequency matched by the scanning source.

January 26, 2013 M.M.C.P. 83

•Field sweep method

•There is a relationship between the resonance frequency of the nucleus
and the strength of the magnetic field in which the sample is placed.

•If the RF radiation is constant, in order to bring their resonance, the
precession of the nucleus is to be changed by changing the applied
magnetic field.

•Generally the field sweep method is regarded as better because it is easier
to vary the magnetic field than the RF radiation so as to bring about
resonance in nuclei.

January 26, 2013 M.M.C.P. 84

•Practically it is not very easy to vary the magnetic field of a large stable
magnet. This is technical problem is solved by superimposing a small
variable magnetic field on the main field.

•Helmholtz coils

January 26, 2013 M.M.C.P. 85

RADIO FREQUENCY RECEIVER OR
DETECTOR
A few turns of wire is wound around the sample tube lightly. The receiver
coil is perpendicular to both the external magnetic and radiofrequency
transmitter coil.
When RF radiation is passed through the magnetised sample, resonance
occurs which cause the current voltage across the coil to drop.
This electrical signal is small and is usually amplified before recording.
Detection of NMR.
When the radiofrequency radiation is passed through the magnetised
sample two phenomena namely absorption and dispersion may occur.
The absorption of either signal will enable the resonance frequency to be
determined. It is found that the interpretation of absorption spectrum is
easier as compared to the dispersion spectrum.
The detector should be capable of separating absorption signal from
dispersion signals.
January 26, 2013 M.M.C.P. 86

THERE ARE TWO WAYS OF DETECTING THE NMR PHENOMENA
1. Radio frequency bridge (single coil detection)
2. Nuclear detection (crossed coil detection)

SINGLE COIL METHOD
Single coil probe has one coil that not only supplies the RF radiation to the
sample but also serves as part of the detector circuit for the NMR
absorption signal. To detect the resonance absorption and to separate the
NMR signal from the imposed RF field, a RF bridge is used.
At the fixed frequency the current flowing through the coils wrapped
around the pole pieces of the magnet is varied. At the resonance there is
a imbalance generated in this coil by virtue of the developing
magnetization of the sample and this out of balance is detected in RF
circuit.
This technique is widely used in modern NMR spectrophotometer.

January 26, 2013 M.M.C.P. 87

January 26, 2013 M.M.C.P. 88

CROSSED COIL PROBES

Nuclear induction has two coils, one for the irradiating the sample and second
coil mounted orthogonally for the signal detection.

The irradiating coil oriented with its axis perpendicular to the magnetic field (i.e.
along the x-axis). The detector coil is wound around the sample tube with its
axis is the (y-axis) perpendicular to the both Ho (z-axis).

The RF current in the first coil wound around the x-axis excites the nuclei.
The nuclei induction in the second coil wound around the y-axis is
detected. The number of turns in the coil determines the particular
frequency involved.

The RF detector can be tuned to detect either a signal in the absorption
mode or in the dispersion mode. Phase sensitive detector is used which
helping the operator to select the phase of the signal to be detected.

January 26, 2013 M.M.C.P. 89

January 26, 2013 M.M.C.P. 90

THE CONTINUOUS –WAVE (CW)
INSTRUMENT

January 26, 2013 M.M.C.P. 91

WORKING
In the CW spectrometers the spectra can be recorded either with field sweep or
frequency sweep.

Keeping the frequency constant, while the magnetic field is varied, (swept) is
technically easier than holding the magnetic field constant and varying the
frequency.

The sample (0.5 mg) is dissolved in a solvent containing no interfering protons
usually CCl4 or CdCl3 0.5 ml and a small amount of TMS is added to serve as
an internal reference.
The sample cell is a small cylindrical glass tube that is suspended in the gap
between the faces of the pole pieces of the magnet. The sample cell is rotated
around its axis to ensure that all parts of the solution experience a relatively
uniform magnetic field. This increases the resolution of the spectrum.

January 26, 2013 M.M.C.P. 92

Also in the magnetic gap, the radio frequency oscillator coil is installed
perpendicular (90˚) to the applied magnetic field.

This coil supplies the electromagnetic energy used to change the spin
orientations
of the protons.

Detector coil is arranged perpendicular to the RF oscillator coil. As the magnetic

field strength is increased, the precessional frequencies of all the nucleus
increases (a peak or series of peaks)
As the magnetic field strength is increased linearly, a pen travels from left to the
right on a recording chart.

As each chemically distinct type of proton comes into resonance, it is record as a
peak on the chart. The peak δ=0 ppm is due to the internal reference compound
TMS.

January 26, 2013 M.M.C.P. 93

Since highly shielded protons precess more slowly than relatively deshielded
protons. Hence highly shielded protons appear to the right of the chart, and less
shielded or dishelded protons appear to the left.

The region of the chart to the left is sometimes said to be downfield and that to
the right is said to be upfield.
Instruments which vary the magnetic field in a continuos fashion scanning from
the downfield end to upfield end of the spectrum, are called continuous wave
instruments.

Because the chemical shifts of the peaks in this spectrum are calculated from the
frequency differences from the TMS, this type of spectrum is said to be frequency

domain spectrum.

January 26, 2013 M.M.C.P. 94

Peaks generated by a CW instrument have ringing. Ringing occurs because the
excited nuclei do not have time to relax back to their equilibrium state. And pen
of the instrument have advanced to a new position. Ringing is
most noticeable when a peak is a sharp singlet.

January 26, 2013 M.M.C.P. 95

TYPES OF CONTINUOUS –WAVE (CW)
INSTRUMENT
1. Minimal-type NMR spectrometer

 This basic instrument often utilizes a permanent of 14, 21 or 23 K gaus field
strength and RF fields of 60, 90 or 100 MHz respectively.

 Each frequency needed for the selected magnetic nuclei is synthesized from a
suitable harmonic of a 15 MHz crystal oscillator and mixed with the output of
an appropriate low frequency incremental oscillator.

 The minimal type has,

1. Stressed reliability
2. Ease of operation
3. High performance
4. Low cost

January 26, 2013 M.M.C.P. 96

2. Multipurpose NMR spectrometers
These instruments are designed primarily for research, high performance,
expensive and versatility better than minimal type.

The high precision comes through the use of homonuclear and heteronuclear
lock systems and frequency synthesizers.

They are also characterized by high intrinsic sensitivity and the ability to study a
variety of nuclei.

The strength of the magnetic field is quite important since sensitivity, resolution
and the separation of chemically shifted peaks increase as the field strength
increases.

These instruments uses RF field of 220,300 or even 500MHz.

January 26, 2013 M.M.C.P. 97

3. Wild-line CW NMR spectrometer

The wild line NMR spectrometer uses a frequency synthesizer to generate the RF
field and a permanent magnet or a compact lightweight electromagnet.
Slowly varying scan voltages are directly injected in the regulator for the magnet
power supply for the electromagnet. Sample probe temperatures may be varied
over the range 170 to 2000 ºC.
Sample tubes are 15-18mm in outer diameter. The std magnetic field is 9.4 K
gaus for protons and 10 K gaus for F19;the RF field is 40 MHz.
Instruments are also available in which RF applied field is continuously
adjustable over a basic frequency range of 300 Hz to 31MHz usually in steps of
10 Hz.
For signal detection a sweep unit generates audio-modulation voltages
that have selectable frequencies of 20,40,80,200 and 400 MHz.
The output is amplified for simultaneous application to the probe modulation
coils and to the oscilloscope.

January 26, 2013 M.M.C.P. 98

THE PULSED FOURIER TRANSFORM (FT )
INSTRUMENT
•The continuous wave type of NMR spectrometer operates by exciting the nuclei
of the isotope under observation one type at a time.

•In the case of H1 nuclei each distinct type of proton (phenyl, vinyl, methyl and so
on) is excited individually and its resonance peak is observed and recorded,
independently of all the others. As we look at first one type of hydrogen and then
another scanning until all of the types have come into resonance.

•An alternative approach common to modern sophisticated instrument is to use a
powerful but short burst of energy called a pulse that excites all of the magnetic
nuclei in the molecule simultaneously and all the signals are collected at the same
time with a computer.

•In an organic molecule for instance all of the H1 nuclei are induced to undergo
resonance at the same time.

January 26, 2013 M.M.C.P. 99

•The pulse actually contains a range of frequencies centered about the hydrogen in
the molecule at once this signal burst of energy.
•When the pulse is discontinued the excited nuclei begin to lose their excitation
energy and return to the original state or relax. As each excited nucleus relaxes it
emits EMR.

•Since the molecule contains many different nuclei many different frequencies of
EMR are emitted simultaneously. This emission is called a free-induction decay
(FID) signal.

•The intensity of FID decays with the time as all of the frequencies emitted and can
be quite complex. We usually extract individual frequencies due to different
nuclei
by using a computer and a mathematical method called a Fourier-transform
analysis.

•The Fourier transform breaks the FID into its separate since or cosine wave
components. This procedure is too complex to be carried out by eye or by hand; it
requires a computer.
January 26, 2013 M.M.C.P. 100

ADVANTAGES OF FT-NMR

FT-NMR is more sensitive and can measure weaker signals.

The pulsed FT-NMR is much faster (seconds instead of min) as compared to
continuous wave NMR.

FT-NMR can be obtained with less than 0.5 mg of compound. This is important
in the biological chemistry, where only μg quantities of the material may be
available.

The FT method also gives improved spectra for sparingly soluble compounds.

Pulsed FT-NMR is therefore especially suitable for the examination of nuclei
that are magnetic or very dilute samples.

January 26, 2013 M.M.C.P. 101

January 26, 2013 M.M.C.P. 102

COMPONENTS OF FT-NMR
A simplified form of the block diagram showing the instrument components of a
typical Fourier transform NMR spectrometer.

The central component of the instrument is a highly stable magnet in which the
sample is placed.

The sample is surrounded by the transmitter/receiver coil.

A crystal controlled frequency synthesizer having an output frequency of Vc
produces radio-frequency radiation.

This signal passes into a pulse switch and power amplifier, which creates an
intense and reproducible pulse of RF current in the transmitter coil.

Resulting signal is picked up by the same coil which now serves a as
receiver.
January 26, 2013 M.M.C.P. 103

The signal is then amplified and transmitted to a phase sensitive detector .

The detector circuitry produced the difference between the nuclear signals
Vn and the crystal oscillator output Vc which leads to the low frequency
time-domain signal as shown in the fig.

This signal is digitalized and collected in the memory of the computer for
analysis by a Fourier transform program and other data analysis software.

The output from this program is plotted giving a frequency domain
spectrum.

January 26, 2013 M.M.C.P. 104

SAMPLE HANDLING TECHNIQUES IN
NMR SPECTROSCOPY
The sample is placed in the probe, which contains the transmitter and receiver coils
and a spinner to spin the tube about its vertical axis in order to average out field in
homogeneities. In the electromagnet, the tube spins at right angles to the Z axis,
which is horizontal, where as in the superconducting magnet, the tube fits in the
bore.
A routine sample for proton NMR on a scanning
60 MHz instrument consists about 5 – 20mg of the sample in about 0.4ml of the
solvent in a 5mm glass tube.
500MHz instrument consists about less than 1μg of the sample of modest
molecular weight in a microtube.

IDEAL SAMPLE SIZE
For continuous wave spectra – less than 50mg.
For FT spectra 1 – 10mg

January 26, 2013 M.M.C.P. 105

IDEAL SOLVENTS
Inert
Non polar
Low boiling point
Inexpensive
Should contain no protons

COMMONLY USED SOLVENTS
CCl4
CdCl3
DMSO
D2 O
Cd3OD

January 26, 2013 M.M.C.P. 106

SPLITTING OF THE SIGNALS

• Each signal in an NMR spectrum represents
one kind or one set of protons in a molecule.
• It is found that in certain molecules, a single
peak (singlet) is not observed, but instead, a
multiplet (groups of peaks) is observed.

January 26, 2013 M.M.C.P. 107

E.g. A molecule of CH3CH2Br, ethyl bromide.

January 26, 2013 M.M.C.P. 108

SPIN-SPIN COUPLING

• The interaction between two or more protons, most
often through the bonds, results in splitting of the
spectral lines.

• It is related to the number of possible combinations of
the spin orientations of the neighboring protons.

• The magnitude of the spin coupling interaction
between protons in general decreases as the number
of bonds between the coupled nuclei increases.

January 26, 2013 M.M.C.P. 109

 Consider a molecule of ethyl bromide (CH3-CH2-Br).the
spin of two protons (-CH2-) can couple with the
adjacent methyl group (-CH3-) in three different ways
relative to the external field . The three different ways of
alignment are ;

 Thus a triplet of peaks results with the intensity ratio of
1 : 2 : 1 which corresponds to the distribution ratio of
alignment .
January 26, 2013 M.M.C.P. 110

 Similarly the spin of three protons (CH3-) can couple
with the adjacent methylene group (-CH2-) in four
different ways relative to the external field

Thus a quartet of peaks results with an intensity ratio of 1:3:3:1
which corresponds to the distribution ratio of all the alignment.

January 26, 2013 M.M.C.P. 111

• The relative intensities of the individual lines of a
multiplet corresponds to the lines in the binomial
expression .

• If n=1, then (1+x)n = 1 + x.

• If n=2, then (1+ x )2 = 1+2x + x2, thus the lines of
triplet have relative intensities 1: 2 :1.

• If n=3, then ( 1 + x )3 = 1 +3X + 3X + X3, the lines
of quartet have relative intensities 1 : 3: 3 : 1.

January 26, 2013 M.M.C.P. 112

Often a group of hydrogen's will appear as a multiplet
rather than as a single peak.

Multiplets are named as follows:

Singlet Quintet
Doublet Sextet Septet
Triplet Octet
Quartet Nonet

This happens because of interaction with neighboring
hydrogens and is called,
SPIN-SPIN SPLITTING.
M.M.C.P.
January 26, 2013 113

1,1,2-Trichloroethane
The two kinds of hydrogens do not appear as single peaks,
rather there is a “triplet” and a “doublet”.

integral = 2

Cl H
H C C Cl
integral = 1 Cl H

The sub peaks are due to
triplet doublet spin-spin splitting and are
predicted by the n+1 rule.
January 26, 2013 M.M.C.P. 114

n + 1 RULE

January 26, 2013 M.M.C.P. 115

1,1,2-Trichloroethane

integral = 2

Cl H
H C C Cl
integral = 1 Cl H

Where do these multiplets come from ?
….. interaction with neighbors
January 26, 2013 M.M.C.P. 116

this hydrogen’s peak these hydrogens are MULTIPLETS
is split by its two neighbors split by their single
neighbor

singlet
doublet
H H H H
triplet
C C C C quartet
H H quintet
sextet
two neighbors
n+1 = 3 one neighbor septet
triplet n+1 = 2
doublet

January 26, 2013 M.M.C.P. 117

EXCEPTIONS TO THE n+1 RULE
IMPORTANT !

1) Protons that are equivalent by symmetry
usually do not split one another

X CH CH Y X CH2 CH2 Y

no splitting if x=y no splitting if x=y

2) Protons in the same group
usually do not split one another
H H
C H or C
H H 118
January 26, 2013 M.M.C.P.

Con….
3) The n+1 rule applies principally to protons in
aliphatic (saturated) chains or on saturated rings.

CH3
CH2CH2CH2CH2CH3 or H

YES YES

but does not apply (in the simple way shown here)
to protons on double bonds or on benzene rings.

H CH3 CH3

H H
NO NO
January 26, 2013 M.M.C.P. 119

INTENSITIES OF
MULTIPLET PEAKS
PASCAL’S TRIANGLE

January 26, 2013 M.M.C.P. 120

PASCAL’S TRIANGLE
Intensities of

1
multiplet peaks singlet

The interior
entries are 1 1 doublet
the sums of
the two 1 2 1 triplet

1 3 3 1
numbers
immediately
quartet
above.
1 4 6 4 1 quintet

1 5 10 10 5 1 sextet

1 6 15 20 15 6 1 septet

1 7 21 35 35 21 7 1
M.M.C.P.
January 26, 2013 octet
121

 The simple rule to find the multiplicity of the signal from a
group of protons, is to count the number of neighbours (n) &
add 1. That is (n+1) .
No coupled C A singlet
hydrogen C –C – C –H
C J

One coupled A doublet
H
hydrogen
C- C – C –H
C
J
J

H A triplet
Two coupled
hydrogen H - C –C-H
J
C J J
H A quartet
Three coupled
hydrogen H - C – C- H
January 26, 2013 H M.M.C.P. 122

THE ORIGIN OF
SPIN-SPIN SPLITTING

HOW IT HAPPENS ?

January 26, 2013 M.M.C.P. 123

THE CHEMICAL SHIFT OF PROTON HA IS
AFFECTED BY THE SPIN OF ITS NEIGHBOURS
aligned with Bo opposed to Bo
50 % of +1/2 -1/2 50 % of
molecules molecules
H HA H HA
C C C C

Bo
downfield upfield
neighbor aligned neighbor opposed
At any given time about half of the molecules in solution will
have spin +1/2 and the other half will have spin -1/2.
January 26, 2013 M.M.C.P. 124

SPIN ARRANGEMENTS

one neighbor one neighbor
n+1 = 2 n+1 = 2
doublet doublet

H H H H
C C C C

The resonance positions (splitting) of a given hydrogen is
affected by the possible spins of its neighbor.
January 26, 2013 M.M.C.P. 125

SPIN ARRANGEMENTS

two neighbors one neighbor
n+1 = 3 n+1 = 2
triplet doublet

H H H H
C C C C
H H

methine spins
methylene spins

January 26, 2013 M.M.C.P. 126

SPIN ARRANGEMENTS

three neighbors two neighbors
n+1 = 4 n+1 = 3
quartet triplet

H H H H
C C H C C H
H H H H

January 26, 2013
methyl spins M.M.C.P.
methylene spins 127

Advanced Spin-spin Coupling NMR Spectroscopy

NOMENCLATURE

• The spacing between the two adjacent peaks of a multiplet is referred to as
the J or coupling constant

• The value of J for a given coupling is constant, regardless of the field
strength or operating frequency of the instrument

• Coupling between nuclei of the same type
is referred to as homonuclear coupling

• Coupling between dissimilar nuclei is
referred to as heteronuclear coupling

128
• The magnitude of this effect is dependent
on the number of bonds intervening between
two nuclei – in general it is a distance effect, where one-bond couplings
January 26, 2013 be the strongest
would M.M.C.P. 128


Con….
There are many variations of the subscripts and superscripts associated with J
constants

In general, the superscript numeral to the left of J is the number of intervening
bonds through which the coupling is taking place
3
J is a coupling constant operating through three bonds

Subscripts to the right of J can be used to show the type of coupling, such as HH for
homonuclear between protons or HC for heteronuclear between a carbon and
proton

Often, this subscript will be used to define the various J-constants within a
complex multiplet: J1, J2, J3, etc. or JAB, JBC, JAC]

Although J values are referred to as positive numbers, they may in actuality be
positive or negative
January 26, 2013 M.M.C.P. 129


MECHANISM OF COUPLING
• The most coherent theory of how spin information is transferred from one
nucleus to another is the Dirac vector model

• In this model, there is an energetic relationship between the spin of the
electrons and the spin of the nuclei

• An electron near the nucleus has the lowest energy of interaction if its spin is
opposite to that of the nucleus

Nuclear spin electron spin

Energy

Nuclear spin electron spin

January 26, 2013 M.M.C.P. 130

MECHANISM OF COUPLING – ONE BOND COUPLINGS, 1J
• Here, a single bond (two electrons) joins two spin-active nuclei – such as 13C-
1
H

• The bonding electrons will tend to avoid one another, if one is near the 13C
nucleus the other will be near the 1H nucleus

• By the Pauli principle, these electrons must be opposite in spin

• The Dirac model then predicts that the most stable condition between the two
nuclei must be one in which they too are opposite in spin:

C spin
13
H spin
1

electrons opposite in spin

January 26, 2013 M.M.C.P. 131



• These alignments can be used for any heteronuclear pair of spin-active nuclei –
13
P-13C, etc.

• When two nuclei prefer an opposed alignment, as in this example, the J is
positive

• If the two nuclei have parallel spins, the J will be negative (remember spin
information is transferred through the electrons!)

January 26, 2013 M.M.C.P. 132


• The Dirac model predicts the observed spin-spin coupling for the methine 13C-
1
H system

• It is important to note that the electron spins must be opposite
13
C
1
H

13
C
1
H

Excited state is
13
C nuclear resonance of lower energy

13
C
1
H

13
C
1
H
Dirac model Dirac model
favored ground less-favored
state ground state

January 26, 2013 M.M.C.P. 133


• It is these two upper energy states, and the two DEs that generated them that
result in the doublet for an undecoupled methine in a 13C spectrum

13
C
1
H

13
C
1
H

13
C
1
H

13
C
1
H

January 26, 2013 M.M.C.P. 134

MECHANISM OF COUPLING – TWO BOND COUPLINGS, 2J
• As the bond angle H-C-H decreases, the amount of electronic interaction
between the two orbitals increases, the electronic spin correlations also
increase, and J becomes larger. They are sometimes called geminal coupling,
because the two nuclei that interact are attached to the same central
atom(Latin gemini = “twins”)
H H-C-H 109o In general:
2J
H HH = 12-18 Hz
40

JHH
H H-C-H 118o
2J
20
HH = 5 Hz
H

H
H-C-H 120o 90 100 110 120
2J
HH = 0-3 Hz
H

January 26, 2013 M.M.C.P. 135


MECHANISM OF COUPLING – TWO BOND COUPLINGS, 2J

• Variations in J also result from ring size

• As ring size decreases, the C-C-C bond angle decreases,
the resulting H-C-H bond angle increases, – J becomes
smaller
H H H
H H H H
H H
C
H H H

2
JHH (Hz) = 3 5 9 11 13 9 to 15

January 26, 2013 M.M.C.P. 136

MECHANISM OF COUPLING – THREE BOND COUPLINGS, 3J

• These couplings are the one most common to introductory studies in NMR, and are
observed as the coupling through a C-C bond between two C-H bonds - vicinal
coupling.

• Observe the two possible spin intra C-C cations:

-1/2 +1/2

+1/2 +1/2

January 26, 2013 M.M.C.P. 137



Observe that the orbitals must overlap for this communication to take place

The magnitude of the interaction, it can readily be observed, is greatest when the
orbitals are at angles of 0o and 180o to one another:

0o dihedral angle 180o dihedral angle
Maximum overlap
January 26, 2013 M.M.C.P. 138

8. Examples of this effect in operation:

H

HH
H

Jdiaxial = 10-14 Hz
3 3
Jdiequitorial = 4-5 Hz
α = 180ο α = 60ο

H

H

3
Jaxial-eq. = 4-5 Hz
α = 60ο

January 26, 2013 M.M.C.P. 139


MECHANISM OF COUPLING – LONG RANGE COUPLINGS, ≥4J

• the greater the number of intervening bonds the greater the reduction in opportunity
for orbital overlap – long range couplings are uncommon

• In cases where a rigid structural feature preserves these overlaps, however, long
range couplings are observed

January 26, 2013 M.M.C.P. 140


• Examples include the meta- and para- protons to the observed proton on an
aromatic ring and acetylenic systems:

H H
H
H
C C C C
H H H
H
J = 0-1 Jz Hz
5 3
J = 7-10 Hz 4
J = 1-3 Hz 5
J = 0-1 Hz
ortho meta para

January 26, 2013 M.M.C.P. 141


• Rigid aliphatic ring systems exhibit a specialized case of long range coupling – W-
coupling – 4JW

• The more heavily strained the ring system, the less “flexing” can occur, and the
ability to transmit spin information is preserved

H
H H O

H H H

J = 0-1
4 4
J =3 4
J = 7 Hz

January 26, 2013 M.M.C.P. 142

SPIN DECOUPLING
• It is a powerful tool for simplifying a spectrum & is of great value to
organic chemists working with complex molecules. It helps in the
identification of coupled protons in spectra that are too complex for
detailed analysis.

• This technique involves the irradiation of a proton or a group of
equivalent proton with sufficiently intense radio frequency energy to
eliminate completely the observed coupling of the neighboring
protons.

• The simplification of the complex spectrum for easy interpretation is
done by,

1) By using an instrument with a more powerful homogeneous
magnetic field, e.g. a 100 MHz instrument in preference to 60
MHz
instruments.
2) By spin- spin decoupling techniques.
January 26, 2013 M.M.C.P. 143

ISOTOPE EXCHANGE
• Deuterium (2H or D ), the heavy isotope of hydrogen, has been used
extensively in proton NMR spectroscopy for two reasones. First it is
easily introduced into a molecule. Second, the presence of deuterium in a
molecule is not detected in the proton NMR spectrum.
• Deuterium has a much smaller magnetic dipole moment than hydrogen &
therefore, it absorbs at different field strengths. In case of ethylbromide
the deuterium replaces the methyl hydrogens & the following changes
occurs.
2H 3H

Br-CH2-CH3 2H 2H

Br-CH2-CH2D
2H 1H

Br-CH2-CHD2 2H

Br-CH2-CD3
January 26, 2013 M.M.C.P. 144

SHIFT REAGENTS
• Lanthanide series of elements are used as shift reagents. A lanthanide ion
can increase its co-ordination number by interacting with unshared electrons.
As a result the NMR spectrum of the comp. that contains a group
possessing unshared pair of electron undergoes change & large chemical
shift as a difference in peaks is observed.

• All the shift reagents are mild Lewis acids. Shift reagent separates NMR
signals those normally overlap. Thus it gives more simplified spectrum.

• Shift reagent are paramagnetic, so large chemical shift take place.

• Shift reagents is normally used in non polar solvents like CdCl3, CCl4 etc.

• Shift reagents, provide a useful technique for spreading out proton NMR
absorption patterns which normally overlap, without increasing the strength
of the applied magnetic field.

January 26, 2013 M.M.C.P. 145

• In the proton NMR spectrum of n – hexanol, the
high field triplet is distorted which represents the
absorption of a methyl group adjacent to a - CH2 –
group. The low field broad multiplet is due to the
methylene group adjacent to the hydroxyl group.
The proton of the remaining methylene groups are
all burried in the methylene envelope between δ
1.2 & 1.8 .

January 26, 2013 M.M.C.P. 146

• When the same spectrum is recorded after addition of a
soluble europium (III) complex, that is the shift reagent , the
spectrum is spread out over a wider range of frequencies. So
that it is now simplified almost to first order. In the spectrum
OH absorption signal is shifted too far to be.

January 26, 2013 M.M.C.P. 147

COMPARISIONS BETWEEN 13C-NMR & 1H-NMR

13C-NMR 1H-NMR
1. Pulse Fourier Technique is used 1. Continuous wave method is followed.
2. Very fast. 2. Slow process.
3. No peak overlapping observed 3. Peak overlapping observed in case of
in the spectrum. complex samples.
4. Sweep generator & sweep coil 4. Required.
are not required in the NMR
instrument.
5. Chemical shift range is wide 5. δ range is very narrow (δ  0-15).
(δ 0-200).
6. Wide band RF is applied rather 6. Tuned to one frequency.
than tuned to a precise frequency.
7. Work on frequency sweep 7. Works on either field sweep
technique. or frequency sweep techniques.

QUESTIONS :-
 2o marks:-

1. (a) Explain the basic principles involved in NMR spectroscopy.
(b) Write an account of NMR spectra. How its interpretation ? Explain
with examples. (Sep’07)(Apr’08).
1o marks:-

1. Write a note on splitting of signals in NMR spectra. (May’10).
2. Briefly indicate the functions of various units of NMR spectrometer. (Apr’08).
3. Explain shielding & deshielding effect in NMR spectroscopy. (Apr’08).
4. What is chemical shift ? Explain the factors affecting chemical shift. (Apr’08).

January 26, 2013 M.M.C.P. 149

Con….
 5 marks:-

1. Explain chemical shifts in NMR. (‘03)
2. Explain advantages and applications of FT NMR. (‘97)

January 26, 2013 M.M.C.P. 150

REFERENCES :-
1. Sharma YR. Elementary organic spectroscopy principles and
chemical applications. 1st ed. S. Chand and Company ltd; New
Delhi :2008.

2. Chatwal GR, Anand SK. Instrumental methods of chemical
analysis. 1st ed. Himalaya Publishing house; Mumbai: 2004.

3. Jag Mohan. Organic spectroscopy principles and applications. 1 st
ed. Narosa publishing House; New Delhi: 2001.

4. Sharma BK. Instrumental methods of chemical analysis. 24th ed.
Goel Publishing house; Meerut: 2005.

5. S. Ravi Shankar. Text book of pharmaceutical analysis. 3rd ed. Rx
publication; Tirunelveli: 2006.
January 26, 2013 M.M.C.P. 151

6. O.V.K. Reddy. Pharmaceutical analysis. Pulse publication;
Hyderabad.

7. Willey. Handbook of spectroscopy. 2003.

8. Pavia, Lampman, Kriz. Introduction of spectroscopy. 3ed edition.

9. Skoog DA, West DM. principle of instrumental analysis. 2ed
edition.

10.Willard HH, Merritt LL, Dean JA, Settle FA. Instrumental
methods of analysis. Jr CBS publishing and distributors, 7 th
edition.

11. Kasture AV, Mahadik KR, More HN, Wadodkar SG.
Pharmaceutical analysis. Nirali Prakashan. 17th edition 2008.
January 26, 2013 M.M.C.P. 152

12. Silverstein R.M, Webmaster F.A, Spectrometric identification of
organic compounds, 6th edition.148-150.

13. Kemp W. organic spectroscopy. 3rd edition.1996.

14. www.google.co.in

January 26, 2013 M.M.C.P. 153

Any Questions……..?????

154

January 26, 2013 M.M.C.P. 155

Nuclear magnetic resonance proton nmr

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