Conformational behaviour of organic carbonyl compounds. Part 1. A molecular orbital approach to the study of internal rotation in conjugated aldehydes and ketones

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Abstract
Semi-empirical and ab initio MO methods are employed in a study of internal rotation of conjugated aldehydes and ketones. In general CNDO/2 does not provide a correct pattern for the energy as a function of the angle of internal rotation. The results from the semi-empirical PClLO method are more consistent with experiment. The calculated energy barriers are noticeably lower than the experimental ones, but roughly proportional. The energy barriers calculated by the ab initio MO approach with the minimal STO-3G basis set behave correctly as regards the relative stability of planar and perpendicular conformations, but quantitatively are not comparable with experimental values. The π-bond order for the C–C bond joining the carbonyl group to the unsaturated system for the planar conformation shows a linear correlation with the experimental free energy of activation for a large number of molecules and this is true of the values obtained both by CNDO/2 and by ab initio STO-3G.

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