The infrared spectra of amides. Part 1. The stretching vibrations of primary carboxamides

Abstract

The infrared spectra of more than 150 primarycarboxamides in the NH stretching and the carbonyl region have been measured. Comparison is made with Bellamys analysis of the stretching modes of amines. The offset to higher frequencies of amides compared with amines is attributed to the widening of the bond angle. The symmetric and antisymmetric stretching of the NH2 bonds is analysed via a rearrangement of the Linnett equations. It is shown thereby that changes of bond strength vary the symmetric and antisymmetric frequencies approximately proportionately whilst deviations from that relationship are due either to changes of the bond angle, steric hindrance or to hydrogen bonding. The spectral changes of m- and p-substituted amides are correlated with Hammett sigma functions, both in the carbonyl and the NH2 stretching region. Intensities and the influence of steric effects are also discussed.