Electronic Excited States of Benzene and Ethylene

Abstract

Energies of low-lying excited electronic states of benzene have been calculated using the semiempirical SCF scheme of Pople, Santry, and Segal. Semiquantitatively transferable parameters are chosen by fitting the theory to the ionization potential and the position of the 3B1u state. The principal requisite modification of parameters concerns a reduction in scale of resonance integrals from those suggested by Pople et al.; subsidiary small adjustments are required in diagonal core parameters. The theory predicts several interesting features which are at variance with the usual interpretation of the near-ultraviolet spectrum but which are not in irrevocable conflict with experiment. When these are taken into account, positions of other states, calculated without scaling of atomic-orbital Coulomb repulsion integrals, are in good agreement with experiment. Representative results are also presented for ethylene.