Question

Why does a change in polarizability in a molecule allow a vibrational mode to be Raman active? Why does a change in dipole moment allow a vibrational mode to be IR active? Why do they differ?

Answer 1

A molecule gets distorted that is it gets polarized in an electric field. A Raman shift which is Raman transition from one state to another, can be activated optically only in the presence of non-zero polarizability, since, Stokes and Anti-Stokes lines only arise if the polarizability changes during the vibration.

A dipole moment is defined as a vector from the sum of dipole vectors in a molecule.

When a molecule interact with the light source of the instrument, where the light source of the instrument is an oscillating electromagnetic field. In order for the molecule to interact with light and be detected by the IR, it must create its own oscillating electromagnetic field. A dipole moment can oscillate by changing in either its direction or its magnitude or both, because it’s a vector quantity.

Since a dipole moment is the product of as the change in between charges and the change of distance between the atoms. The distance between atoms is changing when a molecule vibrates, the molecule has a changing dipole moment. This change in magnitude creates an oscillating electromagnetic field, and hence the molecule can be detected by the IR.