Practice question

Electronic transitions require distinct excitation energies determined by orbital energy gaps. σ–σ* involves strongly bonded electrons with very large separation, absorbing in vacuum UV below 150 nm. n–σ* is intermediate, involving lone pair to antibonding sigma orbitals around 150-200 nm. π–π* conjugated systems and n–π* have smallest gaps, appearing in near UV 200-300 nm. Therefore decreasing energy follows σ–σ* highest, then n–σ*, then π–π*, with n–π* lowest. This energetic hierarchy explains why biomolecular spectra show peptide backbone in far UV and aromatic, nucleic acid absorptions in near UV.