Separation of 1La Excitation Energies of Conjugated Hydrocarbons into Orbital Energy Difference and Two-Electron Term Contributions

Abstract

Working within the framework of the self-consistent-field approximation, an estimate is given for the value of experimental counterparts of the orbital energy difference and the two-electron term contribution to the 1→−1 excitation energy for eight conjugated hydrocarbons. The two-electron term is almost constant for the benzenoid hydrocarbons examined but markedly different in the case of azulene. The orbital energy difference is responsible for the variation of the excitation energy in the benzenoid series, and, in all cases examined, it is simply related to the topology of the molecule. Semiempirical limited configuration-interaction calculations in the π-electron approximation (Pariser—Parr—Pople) are in qualitative agreement with these findings. The well-known facts that simple Hückel calculations lead to a good correlation with the excitation energies of the benzenoid hydrocarbons if the regression line is not forced to go through the origin and that azulene is far off the regression line are shown to be in accordance with the findings.