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

Threlfall, T. (2022) The infrared spectra of amides. Part 1. The stretching vibrations of primary carboxamides. Vibrational Spectroscopy, 121, [103386]. (doi:10.1016/j.vibspec.2022.103386).

Record type: Article

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.

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Accepted/In Press date: 20 May 2022

e-pub ahead of print date: 26 May 2022

Published date: 8 June 2022

Additional Information: Funding Information: It is a great pleasure to record the contribution made to this work by numerous colleagues, particularly Dr Bryan Slater and Mrs Marion Dowsett who made many of the measurements and of Mr Peter Isherwood and Mr John Mills, whose help with the grating conversion was invaluable. My grateful thanks to Prof. Philip Threlfall-Holmes of Leeds University for help with the diagrams.

Keywords: Bellamy plots, Hammett. constants, Infrared spectra, Linnett equations, Primary amides