Question

Explain how the polarity of the solvent can affect a π to π* transition. Why are orbitals affected differently?

Answer 1

Atomic orbitals combines to form molecular orbitals which may be bonding or antibonding and during bond formation the type of interaction among the molecular orbital leads to formation of bonding or antibonding bonds, such as σ, σ*, π, π* etc. Besides a nonbonding orbitals do not participate in bond formation in anyway. The energy content of these bonding, antibonding and nonbonding orbitlas are different from each other, which makes them different from each other. The energy required for the transition follows the order (decreasing order)

σ - σ* > n- σ* > π- π* > n- π*

Impact of Solvent polarity

     Non polar compounds shows no changes in absorption maximum(λ_max) with change in polarity of the solvents ( same in alcohol(polar) and hexane (non polar)). However in case of polar compounds (basically carbonyls) shows appreciable shifting of bands with polarity of solvents.

π -π^*: In π -π^* transition, the absorption band moves to longer wavelength with increasing the solvent polarity. This is due to the fact that the excited state in such type of transition is more polar and hence receives more stabilization from a polar solvent thus shifting the absorption wabelength towards red end i.e- bathochromically shifted.