The electronic structure of bond-alternating and nonalternant conjugated hydrocarbons: Diphenylpolyenes and azulene

The electronic structures of the first four diphyenylpolyenes (stilbene, diphenylbutadiene, diphenylhexatriene, and diphenyloctatetraene) and of azulene are evaluated using the new "spectroscopic" SCF CNDO/S2 molecular orbital model. In the case of the diphenylpolyenes the higher-energy π and σ ionization potentials and the strong allowed ¹B_u optical transitions (in the vicinity of hv=3-4 eV) are predicted to within about 0.1 eV by this model. In addition, the inclusion in our analysis of σ as well as π electronic orbitale permits the detailed interpretation of the electronic origin of all prominent maxima in their ultraviolet photoemission spectra. In the case of azulene, all major peaks in the photoemission and absorption spectra are predicted to within ΔE ≤ 0.4 eV. The predicted dipole moment is 2.1 D, as compared with a Pariser-Parr-Pople value of 1.9 D. We conclude that the CNDO/S2 model can describe the electronic structure of nonalternant and bond-alternating alternant hydrocarbons, as well as that of the symmetric alternant hydrocarbons considered earlier.