Ultraviolet spectra of non-alternant hydrocarbons: The significance of non-neighbour resonance integrals and of configuration-interaction

Abstract

A study has been made of the way in which the calculated positions of excited states of non-alternant hydrocarbons are affected by (a) including all resonance integrals rather than just those for nearest neighbours (b) including configuration-interaction. The VESCF molecular-orbital technique, with two alternative methods for deriving basic integrals, was employed. Predicted positions of excited states are found to be sometimes appreciably altered when all resonance integrals are included, and are somewhat dependent upon the method of evaluation of basic integrals. We conclude that one should be cautious in interpreting U.V. spectra from an isolated molecular-orbital calculation on a non-alternant hydrocarbon. The study of the effect of configuration-interaction for the excited states of fulvene and dimethylenecyclobutene yields results analogous to those found by Koutecky and co-workers for benzene. The predicted relative positions of some excited states are altered when doubly excited configurations are included, but the general level of energies predicted for lower excited states when only singly excited configurations are used is probably close to the result that would be found when up to triply excited configurations are included.