Question

What is shielding effect in NMR?

Answer 1

Since the proton has a high density of electrons it won't experince much external magnet field

Explanation:

So the name is actually just what it is. So when we think of shielding effect in NMR we think of those atoms that are not surrounded to electronegative atoms (Nitrogen, oxygen, sulfide etc.).

When there are a high density of electrons around the nucleus, the higher the opposing magnetic field to `B_0` from the electrons, the greater the shielding. This is because the proton will experience lower external magnetic field `B` and therefore the chemical shift will be to the right (low ppm).

It's the opposite when deshielding. Here the proton has a low density of electrons around its nucleus, therefore the proton will experience higher external magnetic field.