NUCLEAR MAGNETIC RESONANCE (NMR) SPECTROSCOPY

Introduction to NMR
Spectroscopy
• The study of absorption of
radiofrequency radiation by nuclei in a
magnetic field is called Nuclear
Magnetic Resonance.

• When a charged particle such as a proton spins on its axis,
it creates a magnetic field.
• Thus, the nucleus can be considered to be a tiny bar
magnet.
• Normally, these tiny bar magnets are randomly oriented in
space. However, in the presence of a external magnetic
field, they can absorb electromagnetic radiation in radio-
frequency region.
• They are oriented with or against this applied field.
• More nuclei are oriented with the applied field.

• The angular momentum associated with the
spin quantum number, I (in unit of h/2п)
• The I for isotopes would be an integral values
or a half-integral values.
• I value equal to zero indicates no spin.
• Spin Number of isotopes determine by these
rules:
1. Nuclei with even number of proton and
neutron=zero spin
2. Nuclei with odd number of proton and
neutron=integral spin
3. Nuclei with odd mass number= half-integral
spin

Spin quantum number for various
nuclei
Number of
protons
Number of
Neutrons
Spin
Quantum
Number (I)
Examples
Even Even 0 12C, 16O, 32S
Odd Even 1/2 1H, 19F, 31P
" " 3/2
11B,35Cl, 79Br,
127I
Even Odd 1/2 13C
" " 5/2 17O

PRINCIPLE
• Keeping the magnetic field constant;
• Varying the radiation frequency
through the substance.
• Observing the frequency at which
radiation is absorbed is the principle
way of NMR Spectroscopy.

PRINCIPLE
• Keeping the radiation frequency constant;
• Varying the magnetic field through the
substance.
• At some value of the field strength where
the energy required to flip the proton
matches the radiation energy,
• Absorption occurs and signal is observed.
• Due to convenience it is the practical way
of NMR Spectroscopy.

Downfield direction
The NMR spectrum

INSTRUMENTATIO
N
• There are two general types of NMR
instrument; continuous wave and
Fourier transform.
• Early experiments were conducted with
continuous wave (C.W.) instruments,
and in 1970 the first Fourier transform
(F.T.) instruments became available.
• This type now dominates the market.

Magnet
Normally superconducting. Some
electromagnets and permanent magnets
(EM-360, EM-390) still around.

RADIOFREQUENCY
OSCILLATOR
• The rf field is provided by a transmitter coil
whose magnetic vector component moves in a
plane perpendicular to the direction of HO the rf
field induces nuclear transition when its
frequency equal to angular precissional velocity

RADIO FREQUENCY RECEIVER
(DETECTOR)
• The flipping of nuclei as a result of
irradiation induces a voltage in receiving
coil

RECORDER
The voltage from the receiving coil is
amplified and observed in a recorder. The
peaks of an NMR spectrum are result of
plotting intensity of absorption vs frequency
of strength (field strength).

ADVANTAGES OF FTNMR
• Rapid functioning with repetition of every 2
secs.
• FTNMR can easily take:
 The spectra of 16 samples at very low conc.
 NMR studies on nuclei with very low natural
abundance (13C).
 NMR studies on nuclei with low abundance
and small magnetic moments (13C,15N).

NMR SPECTRUM
• All protons absorbed at the same
effective field strengths but they
absorb at different applied field
strengths.
• The applied field strength is
measured and the graph is plotted
between the applied field strength
and the absorption.
• This plot is called NMR spectrum.

THE NUMBER OF THE
SIGNALS
• The number of signals depends on the
number of equivalent protons
(hydrogens)
• In a molecule, protons in the same
magnetic environment absorb at the
same applied magnetic field strength
• For example:
 CH₃-CH₂-Cl CH₃-CH₂-CH₂-Cl
2 NMR signals 3 NMR signals
Ethyl Chloride n-Propyl chloride

THE POSITION OF THE
SIGNALS
• The position of a signal in the spectrum helps
to reveal
• What "type" of proton(s) gives rise to the
signal. The
• position of a signal – its chemical shift – is
measured in ppm (parts per million) relative
to the proton signal
• Equivalent protons have the same chemical
shift.
• Also, protons in similar environments, but in
different molecules, will absorb at about the
same place in the spectrum.

• The reference point from which chemical shifts are
measured is, for practical reason, not the signal from the
naked proton but the signal from the actual compound,
usually Tetra Methyl Saline (TMS) is used because;
 TMS is chemically inert,
 Has low boiling point,
 Easily removed from a recoverable sample of valuable
organic compound,
 Soluble in most organic solvents,
 Can be added to the sample solution as an internal
standard,
 TMS is not soluble in H20 or D2O, for solution in these
solvent the sodium salt of 3propane sulfonic acid is used.

• The most commonly used scale is the δ
(delta) scale on which the TMS signal is
taken as 0.0ppm.
Small δ value = Small down field shift
Large δ value = Large down field shift
• There is another scale known as τ (tau) on
which the TMS is taken as 10.0ppm.
• The two scales are related by the expression
τ = 10-δ

INTENSITIES OF THE SIGNALS
(PEAK AREA AND PROTON
COUNTING):Consider NMR spectra of toluene and p-xylene
(p-xylene)
Each compound possess two type of proton:
(1)Methyl (2)aromatic protons
These protons shows two signal in NMR spectra nearly δ 2.3 and δ 7.2
values
Intensities of methyl proton and aromatic proton signal in NMR spectra
on comparison based on the areas under the peak show that they have
peak in 3:5 of toluene while 6:4 (3:2) of p-xylene
Area under NMR signal are measured by electronic
integrator and are usually given on the spectrum chart in the form of
stipped curve.

THE SPLITTINGS OF THE
SIGNALS• (SPIN-SPIN COUPLING)
• Splitting is a phenomenon which result by the interaction of
proton(H)with the adjacent proton in a compound or
molecule.
E.g.: 1,1,2tri bromomethane
Br-CH-CH2-Br
• When secondary proton feel magnetic field by the spin of
neighbouring tertiary proton inc if the tertiary proton is
aligned with the applied field r dec if tertiary proton against
the field
• For half the molecule absorption by a secondary proton shift
downfield and other half of the molecule the absorption shift
up field the signal is split into two peaks a doublet with equal
peak intensities
• Similarly absorption of tertiary proton is affected by the spin
of the neighbouring secondary proton…

Sample Handling
• The sample which are use in NMR are must be
clear liquid or non-viscous
• Material which are use they should be near to
liquid
• The solvent which are use they must be not
contain any of that molecule which may
contain proton(H) because in process this
proton give there peak which cause error so
avoid the use of such molecule like alcohol,
water so we prefers the solvent like
deuterated chloroform(CDCL3), deuterated
benzene(C6D6), D2O, D6-DMSO (Dimethyl
sulphoxide)in order to diminish the error.

APPLICATIONS OF
NMR• The most important application of proton
NMR is identification, structural elucidation
of organic, metal-organic and biochemical
molecules.
• It is use for the identification of compounds.
• It is useful in quantitative analysis of
absorbing species.
• The number of nuclei in the spectrum α the
peak area.
• NMR is useful in determination of functional
groups such as Aldehydes, Ketones,
carboxylic acids, alcohols and phenols.

Elemental Analysis
• Elemental analysis is done for the
elements like carbon, hydrogen,
nitrogen.
• Sometimes it is done for the analysis
of elements like sulpher and oxygen.
• NMR spectroscopy can be employed
to determine the total concentration
of given kind of magnetic nucleus in a
sample.

Carbon13 NMR
There are two basic advantages of
C13 NMR:
• Provide information about the
backbone of molecule (proton NMR
gives periphery information.)
• The scale limit in C13 NMR is about
200ppm (proton NMR limits about
10ppm – 15ppm)

NUCLEAR MAGNETIC RESONANCE (NMR) SPECTROSCOPY

NUCLEAR MAGNETIC RESONANCE (NMR) SPECTROSCOPY

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NUCLEAR MAGNETIC RESONANCE (NMR) SPECTROSCOPY