NMR AND ESR

NUCLEAR MAGNETIC RESONANCE(NMR)
Spin magnetic moment of proton is given by
µp= +/- 2.793 µN where µN=eh/4pMp is the nuclear magneton.
Spin magnetic moment of nuetron is given by
µn=-/+1.913µN

At first glance, it seems odd that the neutron, with no net charge, has a spin magnetic moment. But if we assume that the neutrons contain equal amounts of positive and negative charges, a spin magnetic moment could arise even with no net charge.
Let E0 be the energy of the nucleus. When the nucleus is in a constant magnetic field B, magnetic moment of protons interacts with magnetic field B and the energy of interaction is given by µpB. As a result, the energy level E0 splits into two components E0+µpB and E0-µpB.
At this stage, magnetic moment vector M of the nucleus makes precession around the magnetic field direction. This precision is called Larmour precision. Larmour frequency for protons,
nL=ΔE/h=2µpB/h

If an electromagnetic radiation of frequency nL is now incident on the sample, the nuclei will receive the energy to flip their spins and reach the higher state. This phenomenon is called nuclear magnetic resonance (NMR). Radio frequency range electromagnetic radiations are used here.
Experimental method for the detection of NMR

The specimen material is taken in a glass vessel and placed between the pole pieces of the magnet. The magnetic field acting between the pole pieces can be varied by changing the current through the sweep coil. Radio frequency generator supplies Radio frequency electromagnetic radiations to the specimen through Radio frequency coil wound round the specimen. At the same time Radio frequency generator supplies the signal to the external circuit which measures the power absorbed by the specimen. At resonance, the specimen absorbs energy from electromagnetic radiation and hence there occurs a drop in its amplitude. The absorption signal is amplified by the amplifier and finally displayed on a CRO.
Applications of NMR
To study the molecular structure.
In chemical structure analysis of materials.
In the study of chemical reactions.
To study the internal and local electric fields in an alloy.
In the study of diffusion co-efficient.
For measuring nuclear magnetic moment.
For measuring magnetic field.
Electron spin resonance
Spin magnetic moment of electrons is measured in Bohr magneton. Bohr magneton is given by, µB=eh/4pme.
Let E0 be the energy of the electron. When such an electron is placed in a uniform magnetic field of strength B, the energy level E0 splits into two components E0+1/2gµBB and E0-1/2gµBB, where ‘g’ is called Lande’s factor. g=1 for pure orbital motion. g=2 for pure spin motion.
At this stage, magnetic moment vector M of the electron makes precession around the magnetic field direction B. This precision is called Larmour precision. Larmour frequency for protons,
nL=ΔE/h=gµBB/h

If an electromagnetic radiation of frequency nL is now incident on the sample, the electrons will receive the energy to flip their spins and reach the higher state. This phenomenon is called electron spin resonance (ESR). This occurs in elements having unpaired electrons.
ESR exhibited by paramagnetic substances is called electron paramagnetic resonance (EPR). ESR exhibited by ferromagnetic substances is called ferromagnetic resonance (FMR).
Experimental method for the detection of ESR
A strong magnet with pole pieces M1 and M2 provides uniform magnetic field B. The low frequency modulator controls the magnetic field. A very high frequency generator supplies the energy at the time of resonance. Since high frequency oscillations are used, specimen is taken in a waveguide. At resonance, the radio frequency signal is noted and detected in detector. It is suitably amplified in an amplifier and finally recorded in a recorder.

Applications of ESR
In the study of organic, inorganic and biological systems.
To identify paramagnetic reaction intermediate in chemical reactions.
To locate the paramagnetic species in polymers, catalysts etc.
To determine the electronic structure of free radicals.