Conjugation and the Intensity of the Infrared Carbonyl Band

Abstract

Data are given for the absolute intensities, in chloroform and carbon tetrachloride solutions and in the vapor state, of the carbonyl infrared absorption bands of a number of compounds in which the carbonyl group is conjugated by varying amounts. The absolute intensity differs in the different solvents and in the vapor, but the increase in intensity of a conjugated carbonyl over that of a simple ketone or aldehyde seems to be independent of the state. For most of these compounds a simple relation is shown to exist between this increase in carbonyl intensity with conjugation and the resonance energy of conjugation. This relation is expected from a consideration of the electronic configurations available to the normal and distorted molecule. That such compounds as the amides do not necessarily fit this trend is attributed to the geometric requirements on the molecule of these electronic configurations.