A DICD (dispersion-induced circular dichroism) and normal absorption study of the n → π* carbonyl transition for the isoelectronic substituent series -CH₃, -NH₂, -OH

An experimental DICD (dispersion-induced circular dichroism) and parallel normal absorption study of the lowest n → π* transition of the carbonyl chromophore in simple carbonyls of the form R₁R₂CO for the isoelectronic substituent series R_i = -CH₃, -OH and -NH₂ is presented. The results indicate that, contrary to conventional expectations, the energetic position of the transition is progressively shifted to lower energy for the above substituent ordering. The weakness of the absorption bands in acetic acid and urea provides a rationale for why these bands have not been previously reported. The results suggest that the relative shift is ascribable to resonance (through space) coupling between the substituent and the carbonyl π-system.