Nuclear magnetic resonance

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 µ_pB. As a result, the energy level E0 splits into two components E₀+µ_pB and E₀-µ_pB.

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,

n_L=ΔE/h=2µ_pB/h

If an electromagnetic radiation of frequency n_L 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

1. To study the molecular structure.
2. In chemical structure analysis of materials.
3. In the study of chemical reactions.
4. To study the internal and local electric fields in an alloy.
5. In the study of diffusion co-efficient.
6. For measuring nuclear magnetic moment.
7. For measuring magnetic field.