Question

Why is sigma to pie star and pi to sigma star not possible in visible transitions?

Answer 1

Answer :

The σ to σ* transition requires an absorption of a photon with a wavelength which does not fall in the UV-vis range (see table 2 below). Thus, only π to π* and n to π* transitions occur in the UV-vis region are observed.

Explanation:

Ultraviolet and visible radiation interacts with matter which causes electronic transitions (promotion of electrons from the ground state to a high energy state). The ultraviolet region falls in the range between 190-380 nm, the visible region fall between 380-750 nm.

The following electronic transitions are possible:

π- π* (pi to pi star transition)

n - π* (n to pi star transition)

σ - σ * (sigma to sigma star transition)

n - σ * (n to sigma star transition)