Introduction To Proton NMR and Interpretation

Presented by
Aamir Malik Khalid Lateef
M.PHARM FIRST YEAR(Sem. II)
Department of Pharmaceutical Chemistry

Contents
 Introduction
 1H NMR (PROTON NMR)
 Principle
 Instrumentation
 Relaxation process
 Chemical shift
 Splitting of signals
 NMR spectra and interpretaion
 References
2

Introduction to NMR
 It is the study of absorption of radiofrequency radiation by nuclei in a
magnetic field is called Nuclear Magnetic Resonance.
 Nuclear magnetic resonance spectroscopy is basically another form
of absorption spectrometry. It involve change of the spin state of a
nucleus, when the nucleus absorb electromagnetic radiation in a
strong magnetic field.
 The source of energy in NMR is radio waves which have long
wavelengths, and thus low energy and frequency.
3

Proton NMR
 It is a technique which is based on the absorption of electromagnetic
radiation in the radio frequency region 4 to 900 MHz by nuclei of the
atoms.
 It is used to study a wide variety of nuclei: 1H ,15N, 19F, 13C, 31P.
 The most common form of NMR is based on the hydrogen-1 (1H), nucleus
or proton.
 Shows how many kinds of nonequivalent hydrogen’s are in a compound.
 Equivalent H’s have the same signal while nonequivalent are “different”
and as such may cause additional splitting (diastereotopic effect).
4

Principle
 Protons in different environments absorb at slightly different frequencies, so they
are distinguishable by NMR.
 The frequency at which a particular proton absorbs is determined by its electronic
environment.
 The size of the magnetic field generated by the electrons around a proton
determines where it absorbs.
 Modern NMR spectrometers use a constant magnetic field strength B0, and then a
narrow range of frequencies is applied to achieve the resonance of all protons.
 Only nuclei that contain odd mass numbers (such as 1H, 13C, 19F and 31P) or odd
atomic numbers (such as 2H and 14N) give rise to NMR signals.
5

-spin states-spin states
absorb
Release ∆E
Signals detected by NMR
6

Instrumentation
 A magnet to separate the nuclear spin energy state.
 Two RF channels, one for the field/frequency stabilization and one
to supply RF irradiating energy.
 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.
 A detector, to process the NMR signals.
 A recorder, to display the spectrum.
Basically NMR instrumentation involves the following
units.
7

Schematic diagram of NMR spectrophotometer
8

Relaxation Process
Two kinds of relaxation processes are:
 Spin-spin relaxation: This takes place by transferring energy to neighboring
nucleus. A nucleus in the upper energy state can transfer its energy to a
neighboring nucleus by mutual exchange of spin.
 Spin- Lattice Relaxation: It involves the transfer of energy from the nucleus
in its higher energy state to the molecular lattice, the energy is transferred to
the components of lattice as the additional translational, vibrational &
rotational energy, and this process keeps the excess of nuclei in the lower
energy state which is necessary for NMR Phenomenon.
9

Chemical Shift
 “Chemical shift is the difference between the absorption
position of the sample proton and the absorption position of
reference standard.”
 The position in NMR spectrum where signal occurs is called
chemical shift.
 It indicates how far the signal is from the TMS reference peak.
Chemical Shift, ppm (d )= Shift from TMS in Hz X 106
Spectrometer frequency(Mhz) 10

Splitting of 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.
 E.g. A molecule of CH3CH2Br, ethyl bromide.
11

Pascal’s Triangle
n(no of neighboring H) Name Signal Row
0 Singlet 1
1 Doublet 1 1
2 Triplet 1 2 1
3 Quartet 1 3 3 1
4 Quintet 1 4 6 4 1
5 Sextet 1 5 10 10 5 1
6 Septet 1 6 15 20 15 6 1
7 Octet 1 7 21 35 35 21 7 1
12

1H NMR Spectra
 1H NMR spectra consist of signals originated from hydrogens or
protons present in molecule.
 Only those hydrogens will give signals which are having
different environment.
 Hydrogens having same chemical environment will only appear
as a single signal.
13

Steps in 1H spectra interpretation
1.Number of signals Indicate how many different kinds of
protons present.
2.Position of signals Indicate something about (chemical shift),
magnetic (electronic) environment of
protons.
3.Relative intensity of signals Proportional to number of protons present
4.Splitting of signals (spin spin coupling) Indicate the number of near by nuclei
usually protons.
14

Chemical Shift table for different groups
15

Example of NMR spectra
 CH3CH2Br, ethyl bromide.
Triplet = δ -1.7,3H (CH 3 )
Quartet= δ -3.4, 2H (CH 2 )
17

a b c
 CH3COCH2CH3 , 2-Butanone
c
ab
(a)Singlet at 1.7
(b)Quartet at 2.5
(c)Triplet at 1.1
18

 Toluene,
a
b
(a)Singlet at 2.3
(b)Multiplet at 7.2
19

a b a
 Octane ,CH3(CH2)6CH3
(a)Triplet at 0.88
(b)Singlet at 1.26
a
b
20

CH3
 Alkene, CH3CH2CH2C=CH2
a b d
c
e
(a)Triplet at 0.9
(b) Sixtet at 1.5
(c)Singlet at 1.8
(d)Triplet at 2
(e)Singlet at 4.6
21

 Ester, CH3C-O-CH2-CH
O CH3
CH3
22

 Ethanol , CH3CH2OH
a cb
(a)Triplet at 1.3
(b)Singlet at 2.5
(c)Quartet at 3.8
23

 Propanoic acid, CH3CH2COOH
a b c
(a)Triplet at 1.2
(b)Quartet at 2.3
(c)Singlet at 11.2
a
b
c
24

References
1. Doglas A Skoog, F. James Holler,Timothy A. Nieman, Principles of
Instrumental Analysis - 5th edition, Eastern press, Bangalore, 1998,pp
no.551.
2. Donald L.Pavia, Gary M.Lampman and George S.Kriz, “Introduction to
Spectroscopy”, 3rd edition, Thomsan Learning 60 Albert street, Asia
complex, Singapore.pp.102-111 and 139-150
3. Brian S. Furnis, Antony J.Hannaford, Peter W.G. Smith, Austin R.
Tatchell; Vogel’s textbook of practical organic chemistry, 5th
edition , PEARSON publications.
4. https://en.m.wikipedia.org/wiki/Proton_nuclear_magnetic_resonance
25

Introduction To Proton NMR and Interpretation

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