Question

What is shielding and deshielding in NMR? Give an example?

Answer 1

Shielding:

• The greater the electron density all around the nucleus, the bigger the opposing magnetic field to the electrons, and hence the greater the shielding.
• Because the proton is subjected to a reduced external magnetic field ${\mathrm{B}}_0$, it requires a lower frequency to establish resonance, causing the chemical shift to shift upfield (lower ppm).

Example of shielding :

• Shielding occurs in nuclear fission when electrons farthest from the center of the atom are drawn away.

Deshielding:

• When the electron density around a nucleus falls, the opposing magnetic field ${\mathrm{B}}_0$ gets smaller, and the nucleus feels more of the external magnetic field.

Example of De - shielding:

• The chemical shift of ${\mathrm{CH}}_4$ protons and ${\mathrm{CH}}_3\mathrm{Cl}$ protons, can be taken into account here. The chlorine atom is an electronegative atom that attracts electron density and causes the hydrogen nucleus to be deshielded. As a result, the movement will be toward greater ppm. Because the hydrogen nucleus in ${\mathrm{CH}}_4$ is protected, the peak appears on the lower ppm side.