Nuclear Magnetic Resonance Spectrometry
Nuclear magnetic dipole
Spinning charge in proton generates magnetic
Proton precessing in a magnetic field Ho
Nuclear magnetic dipole axis of nuclear energy
Precessional orbit rotation precession
Low energy spin state (-
component of Nuclear
magnetic field H1 Spin
High energy spin state
Oscillator generates rotating component of Precession -Energy Relationship
magnetic field H1
H1 (Magnetic component of radio frequency from oscillator coil): oscillator frequency
H1 can be resolved into 2 components rotating in opposite directions.
(1) Rotating in the same direction in the precessional orbit of the molecular
(2) Rotating in the opposite direction as the precessional orbit of the nuclear
magnetic dipole ; disregard
Magnetic Properties of Nuclei
Nuclei of certain atoms posses a mechanical spin or angular momentum. The total
angular momentum depends on the nuclear spin or spin number (spin quantum number) I.
The numerical value of the spin number ( I ) is related to the mass number and the
atomic number. Each proton and neutron has its own spin and I is a result of these spins.
Mass Number Atomic Number Spin Number
Odd Even or odd 1/2, 3/2, 5/2,----
Even Even 0
Even Odd 1, 2, 3, ---
The magnetic nucleus may assume any one of ( 2 I + 1) orientations with respect to the
directions of the applied magnetic field.
Therefore, a proton (1/2) will be able to assume only one of two possible orientations that
correspond to energy levels of + or - H in an applied magnetic field, where H is the
strength of the external magnetic field.
If proper v is introduced, the Wo will be resonance with the properly applied radio
frequency (Hi) and the proton will absorb the applied frequency and will be raised to the
high spin (energy) state.
Even though the external magnetic field strength (Ho) applied to the molecule is the
same, the actual magnetic field strength exerted to the protons of the molecule are
different if the protons are in the different electronic chemical environment.
Fundamental NMR Equation of Radio Frequency and Magnetic Field Strengh
The energy difference between the two states is
: (Magnetogyric Ratio) : Constant and a fundamental nuclear constant.
V : Electromagnetic frequency in radio frequency
Ho : An external magnetic field
Wo = Ho
Ho = 2V
Therefore Wo = 2V
= 2 / hI
= Magnetic Moment (Magnetic Dipole Moment)
h = Planck's Constant
I = Spin Number
Relationship between Radio Frequency and Magnetic Field Strength for Proton
Radio Frequency (Mega Hertz) Magnetic Field (Gauss)
E = hv
1.4 T 2.35 T 4.7 T 7.0 T
60 MHz 100 MHz 200 MHz 300 MHz
Schematic Diagram of an NMR Spectrometer
Transmitter coil Receiver coil Recorder
The difference in the absorption frequency of a particular proton of the sample
from the absorption frequency (position) of a reference proton.
The protons at the electron rich environments (strong electronegative
molecules such as oxygen and halogens) will feel less external magnetic
field strength because the magnetic field strength generated by electrons
surrounding the proton will counteract the applied magnetic field strength
(Ho), which can be said deshielded proton.
Therefor, the Wo of the protons in the electron rich chemical
environments will be less and require less radio frequency to be resonance
with the applied radio frequency compared to the protons in the electron
poor chemical environments.
ppm = (reference frequency - sample frequency) 106
Operating instrument frequency
Vi : Resonance Frequency of Proton i
Ho: Applied Magnetic Field
: Shielding Constant
The Reference Compounds : TetraMethylSilane (TMS)
H C H
H C Si C H
H C H
General Regions of Chemical Shifts
10 9 8 7 6 5 4 3 2 1 0
R CH C H C H2 C H CH C H2 C O C H3
5.3 2.7 2.0 3.6
Rest of the protons on CH3 and CH2 absorb at 0.8 - 2.0 very crowded area,
usually see a broad, big peak
Spin-Spin Coupling (Spin-Spin Splitting)
Spin-Spin Coupling is the indirect coupling of proton spins through the intervening
It occurs because there is some tendency for a bonding electron to pair its spins with the
spin of the nearest protons. The spin of a bonding electron having been thus influenced.
Coupling is ordinarily not important beyond 3 bonds unless there is ring strains as in
small rings or bridged systems, or bond delocalization as in aromatic or unsaturated
Signal a is split into a doublet by coupling with one proton; signal b is split into a triplet
by two protons. Spacing in both sets is same (Jab).
C H2Br C HBr2 a
NMR of Fatty Acid Methyl - Ester
Information from NMR Spectrum
The Number of signals
The Position of signals
The Intensity of signals
The Splitting of signals
C 19H32O 2 Methly linolenate
a e e c e e b O
CH3 CH2 CH CH (CH2 CH CH)2 CH2 (CH2)5 CH2 C
Chemical shift (ppm)
a 0.97 e ca.5.38