NMR spectroscopy.ppt instrumentation, principle

1. Nuclear Magnetic Resonance
Spectroscopy
Pune District Education
Association’s Shankarrao Ursal
College of Pharmaceutical
Sciences & Research Centre.
Presented by : Dr. Vijaya U. Barge
(Vice Principal & Professor )

2. Learning Objectives :
 Nuclear magnetic resonance (NMR) spectroscopy is a very important spectrosc
opio method that provides crucial information relating to chemical structure of
molecules.
 This chapter deals with proton NMR, a technique giving information about the
type and number of magnetically distinct protons in a molecule.
 All aspects of this highly versatile technique have been covered includinginstru
mentation details.Sample handling, solvent selection and use of internal standa
rds have beendiscussed in detail.
 Chemical environment of the protons determines their chemical shifts. Various
aspects affecting the chemical shifts of protons have been highlighted The conc
ept of multiplicity of the peaks has been extensively discussed. It is an importa
nt facet of proton NMR, and its interpretation provides extremely valuable stru
ctural information.The origin and concept of spin-spin coupling has been given
in the chapter and interpretation has been given taking relevant examples of pr

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Contents
• Principle
• Chemical shift
• Reference standard
• Factors affecting chemical shift
• Spin-spin coupling
• Coupling constant
• Chemical and magnetic equivalence
• Solvents used
• Instrumentation
• Applications

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Principle
1. The spinning nucleus – Nucleus of the hydrogen atom behaves as a spinning bar
magnet, because it possesses both electric charge and magnetic spin . Spinning
charged body will generate a magnetic field .
2 . The effect of an external magnetic field – Proton will respond to influence of an
external magnetic field and will tend to align itself with that field . Proton can adopt two
orientations – aligned (parallel) with the field or opposed(antiparallel) with the field

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Principle
3 . Precessional motion – Consider the behavior of a spinning top as
describing its spinning motion , in which the spinning axis of the top moves
slowly around the vertical. This precessional motion, and the top is said to be
precessing around the vertical axis . The precession arises from the
interaction of spin gyroscopic motion . As the proton is a spinning magnet , it
will , precess around the axis of an applied external magnetic field in aligned
with the field or opposed to the field.

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Principle
4 .Precessional frequency – The precessional frequency of the nucleus is directly
proportional to the strength of an external field and depends on the nature of
the nuclear magnet . Magnetic nuclei of different atoms have different
characteristic precessional frequency .
According to Larmor precession theory,
ω = gB0
Where w = Larmor precession frequency
But w = 2pv
2pv = gB0
v a B0

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Principle
5 . Energy transition – If a proton is precessing in the aligned orientation , it can
absorb energy and pass into the opposed orientation , subsequently it can lose
this extra energy and relax back into the aligned position . The transition from
one energy state to the other is called flipping of the proton . The transition of
two energy states can be brought by the radiofrequency wave region .

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When proton is placed in a magnetic field , its electrons are caused to circulate ,
so they produce induced magnetic field.
If induced field opposes(diamagnetic)the applied field , then proton is said to
be shielded . This shifts the absorption up field.
If induced field parallel(paramagnetic) to applied field , then proton feels
higher field strength and such proton is said to be deshielded . This shifts the
absorption down field.
Frequency of sample – Frequency of reference
d = ------------------------------------- x 10 6 ppm
Frequency of reference
t = 10 - d
Chemical shift

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Factors affecting chemical shift
1. Electronegativity – As the electronegativity of the functional group is
increased , the protons come to resonate at a higher delta values .
2. Vander Waal’s deshielding – In a rigid molecule , a proton to occupy a
sterically hindered position . Electron cloud of a hindering group will tend to
repel the electron cloud surrounding the proton by electrostatic repulsion .
The proton will be deshielded and appear at higher delta values that would
be predicted in the absence of the effect .
3. Anisotropic effect – In this case pi electrons circulate under the influence of
the applied field and can lead shifts to higher frequency (down field shift or
paramagnetic shift) or to lower frequency ( up field shift or diamagnetic shift)
. In addition , the effects are paramagnetic in certain direction around the pi
clouds , and diamagnetic in others , so that these effects are described as
anisotropic.
4. Hydrogen bonding – A hydrogen atom exhibiting property of hydrogen
bonding in a compound absorbs at a lower field . The hydrogen bonded proton
being attached to a highly electronegative atom will have smaller electron
density around it . Deshielded in nature , the field felt by such a proton will
be more and hence resonance downfield . The downfield shift depends upon
the strength of hydrogen bonding.

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Reference standard
In TMS , silicon pushes electrons into the methyl groups of tetra methyl silane by
a positive inductive effect and this powerful shielding effect means TMS protons
come to resonance at low frequency . It is referred due to following reasons –
i.TMS contains 12 p all are magnetically equivalent .
ii.It is chemically inert .
iii.It is miscible with a large range of solvents .
iv.It is highly volatile .
v.It can be easily removed to get back the sample .
vi.It does not take part in intermolecular association with the sample .
vii.Its resonance position is far away from absorption due to protons in most organ
ic molecules .

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Spin – spin coupling
 Singlet – The signal for OH proton appears as a singlet because of
exchangeable protons . Exchange of OH protons among alcohol molecules is
catalyzed by acid , base and other impurities commonly found in alcohols .
The frequency of exchange is high and hence such protons are unable to
couple with protons on neighboring atoms . Hence OH protons give singlet .
Pascal’s triangle

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Spin – spin coupling
 Doublet – In dibromoethane , signal of the protons in CH2 group splits as a dou
blet due to the presence of one proton on neighboring carbon atom . When pl
aced in an applied electric field the adjacent proton can be aligned with or ag
ainst B0 . Since the absorbing protons feel two different magnetic fields , they
absorb two different frequencies in the NMR spectrum , thus splitting absorpti
on signal into a doublet .

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Spin – spin coupling
 Triplet – In ethanol , CH2 has 4 possible spin states . In CH 3 both the spins are
aligned against the external magnetic field . The signal will shift up field . But
OH protons has both the spins aligned with external magnetic field . Thus sign
al will shift down field . The effect of methylene protons spin state will be tw
ice that of methyl protons and hydroxyl protons . Thus the area under the pea
k of triplet will be twice that of other two .

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Coupling constant
The distance between two adjacent peaks is constant and is called coupling
constant . It is denoted by letter J . It is measured in Hz or in cps . The value of
coupling constant is independent of the external field .
J = Distance between lines (ppm) x Instrument frequency (Hz)

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Chemical and magnetic equivalence
Chemical equivalence - All the protons found in chemically identical
environments in a molecule have same chemical shift value . These are
chemically equivalent protons . If two or more nuclei are equivalent by symmetry
they are said to be chemical equivalents .
Magnetic equivalence – When chemically equivalent protons do not split each
other they are said to be magnetically equivalent . For protons to be
magnetically equivalent they have to satisfy two conditions –
1.They should have same chemical shift .
2.They must have equal coupling constant .

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Solvents used
 A substance free from protons should be used as a solvent . It should not give
absorption of its own in the spectrum . The solvent must be capable of dissolv
ing at least 10% of the substance under investigation . Following solvents are
commonly used –
1. Carbon tetrachloride
2. Carbon disulphide
3. Deuterochloride
4. Hexachloroacetone

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Instrumentation
 Magnet – It is used to supply principle part of field . The magnet in most
commercial NMR spectrometers may be either electromagnet or permanent
magnet . The magnet used must have a very high degree of field homogeneity
between the pole pieces if high resolution work is needed .
 Magnetic field sweep – An alteration over a small range in the applied field
may be made by making use of a pair of coils located parallel to the magnet .
These coils super impose on the main field of the magnet the additional field
required to bring the total resonance condition . By varying a direct current
through these coils the effective field can be changed by a few hundred
milligauss without any loss in field homogeneity .
 Radio frequency source –The signal from a radio frequency oscillator is fed
into a pair of coils mounted at right angles to the path of the field . A plane
polarized beam of radiation has been obtained .
 Sample holder – A glass tube is used of diameter 5 mm . Its capacity is 0.4 ml
. The sample must be in the liquid state for high resolution spectra . It is
contained in a tube which is spun by means of air turbine .

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Instrumentation

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Instrumentation
 Sample probe –It is a device that holds the sample tube in a fixed position in
the field . It is provided with a air driven turbine for rotating the sample alon
g its longitudinal axis at several hundred RPM . This rotation minimizes the eff
ects of inhomogeneity in the field . It helps in obtaining sharper lines and bett
er resolution . Probe may be either a single coil or a turn coil system.
 The receiver –The resonance signal is detected by one of the two methods . I
n single coil instrument a Wheatstone bridge is employed . The applied signal
is balanced against the received signal . The absorption signal is recorded as a
n out of balance emf which may be amplified and recorded . In double coil tra
nsmitter and receiver coils are set at right angles to each other about the sam
ple .
 Signal detector and recording system - The receiver coil directs the radio fr
equency signal produced by the resonating nuclei . The electrical signal gener
ated into the coil must be amplified before it can be recorded . Resonance lin
e intensity is proportional to the number of nuclei responsible for the signal .
The area under the signal is a direct measure of the intensity . It is determine
d in a cumulative manner by an electronic integrator .

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Applications
1. Structure elucidation – NMR spectroscopy gives information about
types of protons , environment of protons , number of protons .
2. Assay of components – Single component or multicomponent without
separation of compounds can be quantitatively estimated . Specific
peak for each component is identified . The peak area given by
integral value is found by using standard and sample .
3. Detection of aromaticity – Protons attached to heterocyclic
compounds whose pi electrons follow huckels rule . Protons are
deshielded due to circulating ring current of pi electrons . As a result
of this the signals for the aromatic protons appear at a very low field
. From this , the aromatic character of the compound under
investigation can be predicted .
4. Distinction between cis – trans isomerism – The concerned protons
have different values of chemical shift and coupling constant .

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5. Hydrogen bonding – Proton signal is shifted towards low field in case
of hydrogen bonding . This reveals that hydrogen bond formation results
in decrease in the electron shielding of the proton . So it can be used to
study hydrogen bonding in organic compounds .
6. Materials science – It can be used to investigate new materials of
great technological importance such as glasses , ceramics , polymers ,
semiconductors . It can be used to investigate reaction taking place in
catalytic surfaces .
7. Food chemistry – It can be used in verification of wine aging . It is
also used in identification of oil fatty constituents .
8. Clinical applications – It is used in the localization and
characterization of metabolites in biological fluids . So it can be utilized
in the diagnosis of many diseases .
Applications

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Reference
Dr. Shashikant D. Bharhate , Dr.MD. Rageeb MD. Usman , Poonam A. Salunke ,
Shital S. Patil , Mordern Pharmaceutical Analytical Techniques , Edition 2019 ,
S.Vikas and Company Medical Publishers
Dr. Mrinalini C. Damle , Dr.Vandana T. Gawande , Pharmaceutical Analysis – IV ,
First Edition , Nirali Prakshan

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