Question

What is shielding and deshielding in NMR? Can you give me an example?

Answer 1

See explanation.

Explanation:

The basic principle of NMR is to apply an external magnetic field called `B_0` and measure the frequency at which the nucleus achieves resonance.

Electrons orbiting around the nucleus generate a small magnetic field that opposes `B_0`. In this case we say that electrons are shielding the nucleus from `B_0`.

Shielding:
The higher the electron density around the nucleus, the higher the opposing magnetic field to `B_0` from the electrons, the greater the shielding. Because the proton experiences lower external magnetic field, it needs a lower frequency to achieve resonance, and therefore, the chemical shift shifts upfield (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) .

How would this affect the H NMR spectrum?

Let us compare the chemical shift of `CH_4` protons and `CH_3Cl` protons.

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 `CH_4` and therefore, the peak appears on the lower ppm side.

Images source: Organic Chemistry-Janice Gorzynski Smith 3rd Ed