Allowed and Forbidden Transition in UV Spectroscopy

A process which determined the absorption or transmission of light in the ultraviolet region of the electromagnetic spectrum is known as UV (ultraviolet) spectroscopy. In uv spectroscopy light absorbed is used to shift electron from low energy level to high energy level.

ALLOWED TRANSITION

High probability transitions that are seen as powerful absorption bands are considered to be allowed transitions. The transitions that entail the promotion of electrons from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) are those that are permitted in UV spectroscopy. These transitions are also referred to as π → π* and n → π* transitions. The π → π*  transition involves the excitation of electrons from a non-bonding orbital to an anti-bonding orbital causes the π*  transitions. As part of the n → π* transitions, electrons are excited from a lone pair (n) orbital to an anti-bonding π* orbital.

FORBIDDEN TRANSITION

On the other hand

• forbidden transitions have a limited likelihood of happening
• and are seen as weak or nonexistent absorption bands.

In UV spectroscopy, transitions that go against the selection rules are considered forbidden. These transitions, which have a limited probability of happening, entail modifications to the molecule’s spin or parity. They thus tend to be weak or missing in the UV spectrum.