Structure and bonding in proteins: Part 1. The electron organisation in the amide unit

A consistent set of numerical information on the electron organisation in the amide unit has been obtained for a number of small amides, using the readily available Gaussian-70 computer package and a modest amount of computer time. There is good agreement with experimental facts concerning bond lengths, bond angles, rotation barriers and cis/trans energy differences. The results have been analysed in terms of simple molecular orbital theory and other simple valence ideas to make the results intelligible in a physical sense, so that they may be used in larger molecules. The rotation barrier about the CN bond seems to be a simple π-electron conjugation effect, whose magnitude is influenced in the main by the internal geometry of the CNO fragment rather than by substituents.