The intensity of carbonyl bands in the infrared spectra of cyclic anhydrides of dicarboxylic acids

Abstract

It is shown that for cyclic anhydrides of aromatic dicarboxylic acids the ratio between the intensities of the low-frequency and the high-frequency carbonyl stretching bands is considerably smaller than for aliphatic anhydrides with anhydride rings of the same size. This observation can be related to the ease with which the π-cloud of the unsaturated portion of the molecule can be delocalised in such a way that it can enter conjugation with both carbonyl groups in the symmetric streching mode.