Question

What is shielding and deshielding in NMR? Give one Example.

Answer 1

Hint :We know that the basic principle of NMR is to apply an external magnetic field called $B_0$ and measure frequency at which nucleus achieves resonance. Electrons orbiting around the nucleus generate a small magnetic field that opposes the $B_0.$ In this case we say that the electrons are shielding the nucleus from $B_0.$

Complete Step By Step Answer:
It is based on the fact that nuclei of most of the atoms shows spin and all nuclei are electrically charged. NMR-spectroscopy is based on the absorption of electromagnetic radiation in the radio frequency region $3kHz-300GHz$ . Nuclei of atoms have spin and electrical charge, so they generate magnetic field In presence of external magnetic fields.
Shielding: The higher the electron density around the nucleus, the higher the opposing magnetic fields to $B_0.$ from the electrons, the greater the shielding. Because the proton experiences a lower external magnetic field, it needs a lower frequency to achieve resonance, and therefore, the chemical shift shifts up field (lower ppms) .
Deshielding: If the electron density around a nucleus decreases, the opposing magnetic field becomes small and therefore, the nucleus feels more the external magnetic field $B_0.$ , and therefore it is said to be deshielded. Because the proton experiences higher external magnetic field, it needs a higher frequency to achieve resonance, and therefore, the chemical shift shifts downfield (higher ppms) .
Example: Chlorine atom is an electronegative atom that will pull the electron density toward it (electron withdrawing), resulting in a deshielding of the hydrogen nucleus; an edit will fell higher external magnetic field $B_0.$ increasing the resonance frequency and therefore, shifting to higher ppms. Hydrogen nucleus is shielded in the case of $C{H}_4$ and therefore, the peak appears on the lower ppm side.

Note :
Remember that because its nucleus has a very powerful attractive (effective nuclear charge) force directed towards all of its electrons. All of the electron levels are pulled very close to the nucleus, so there is very little shielding between the nucleus and the electrons.