The Long Wave-Length Spectra of Aldehydes and Ketones Part I. Saturated Aldehydes and Ketones

Abstract

Intensity calculations are used to help in deciding which of two theoretically predicted transitions should be identified with the weak longest wave-length absorption characteristic of the unconjugated ––C = O group. In terms of the localized LCAO MO and the AO approximations used, one of these transitions is forbidden while the calculated intensity for the other, allowed, transition is much too large to be compatible with the low intensities observed for the absorption. The observed absorption should, therefore, be ascribed to the forbidden transition. The Q and S integrals needed for the calculations, involving AO's of the C and O atoms, are tabulated. The intensity for the allowed transition, when expressed using semilocalized LCAO MO's depends somewhat on the nature of the atoms attached to the C=O group. The absorption regions appearing at shorter wave-lengths in ketones and aldehydes are discussed. It is concluded that the λ1900 region in ketones is characteristic of the C=O group and probably is due either to a Rydberg transition or most likely to the allowed transition for which intensity calculations have been made. If the latter is true the upper level of the λ1900 absorption could be largely responsible for the perturbations which cause the appearance of the long wave-length forbidden transition. It is shown that the long wave-length absorptions from the ––C = S group can be explained in the same manner as those for the ––C=O group.