The electronic structure of polyenes and unsaturated carbonyl compounds

PE spectra of polyene aldehydes are reported and discussed on the basis of a semiempirical LCFO-MO treatment within the π electron approximation. The necessary parameter values including the CC bond alternation were obtained by fitting PPP orbital and transition energies to the PE and UV data of diphenyl polyenes and polyene aldehydes. Thus, for planar systems the assignment of a PE band near 13.5 eV to the π orbital of the CO group could be given, which could be confirmed by the LCFO-MO calculations and by results from ab initio calculations, and which forms the basis of a discussion and comparison of π orbital interactions in planar unsaturated carbonyl compounds and hydrocarbons.