12.6 : 紫外可见光谱分子电子跃迁

Upon absorption of radiation, electrons in a molecule move from the highest occupied molecular orbital, HOMO, to the lowest unoccupied molecular orbital, LUMO, with higher potential energy.

In most molecules, the energy difference between the electronic levels of the HOMO and the LUMO varies from 125–650 kJ/mol.

Usually, the sigma orbitals are the lowest energy occupied molecular orbitals, followed by the pi and n or nonbonding orbitals. The unoccupied or antibonding orbitals contain the highest energy.

In alkanes, only sigma-to-sigma antibonding electronic transition is observed, while saturated molecules like alcohols, amines, and ethers show n-to-sigma antibonding transitions.

Alkenes and alkynes show pi to antibonding pi transitions, whereas carbonyl compounds show n to antibonding pi transitions.

As per the selection rule, transitions like the n to antibonding pi transition, which involves a change in an electron's spin quantum number, are forbidden.