Question

What is anisotropy in NMR?

Answer 1

Hint: NMR is a type of spectroscopy, in which the nucleus of the protons of the compounded are studied and the anisotropy is a property of the compound which tells the shielding and deshielding effects in NMR.

Complete answer:
NMR whose full form is nuclear magnetic resonance, is a type of spectroscopy in which we study the interaction of the magnetic oscillating field of the electromagnetic radiation and the magnetic energy of the hydrogen nucleus when the sample is placed in the external magnetic field.
The anisotropy is a property of the compound which tells the shielding and deshielding effects in NMR. This anisotropy can be best explained in the benzene ring, as we know that the pi-electrons in the benzene ring are delocalized due to which the resonance is possible. So when the benzene is placed in an external magnetic field, then there is a movement of pi-electrons around the benzene ring. This movement of electrons will oppose the applied magnetic field but this is only with respect to the interior of the ring. But with respect to the exterior of the ring, the movement of the electrons reinforces the external magnetic field.
So, the protons of the aromatic compounds become deshielded with respect to the conventional protons and the absorption occurs downfield. Therefore, this is known as an anisotropic effect.

Note:
The property of anisotropy in NMR (nuclear magnetic resonance) is not possible for the aliphatic compounds, only for the aromatic protons are studied. The anisotropy affects the chemical shift of the compound.