New data presented regarding the influence of different types of substituents in the benzene ring of aromatic carbonyl compounds (including benzils) on the > C=O frequency have made it possible to revise and rationalise older explanations.

Based on uniform considerations of polarisation, rational explanation can be provided for the effects of ethylene and phenyl conjugation, hydrogen bond formation and complex formation. Abnormal effects are observed in special cases where other important factors are simultaneously involved as for example in N-oxides and flavonoids of various types; but they are explicable on a rational basis. Further, the behaviour of acetylene and nitrile compounds throws insight on the nature of some of these factors.