Can the Low-Lying Electronic States of Benzenoid Hydrocarbons Be Described by the Semiempirical Valence Bond Approach?

The low-lying excited states of benzenoid hydrocarbons with up to 22 π-electrons have been obtained from exact semiempirical valence bond (VB) calculations. The effective valence bond (EVB) model, which implements nonadjacent and cyclic six-body interactions on the basis of the classical valence bond (CVB) model, is shown to give satisfactory predictions of the low-lying excited spectra of small benzenoids with no more than 14 π-electrons. For larger benzenoid systems, the CVB model can account well for their lowest excited spectra, being in good agreement with the available experimental data and semiempirical PPP molecular orbital results. In addition, a good quadratic relationship between the energy difference between the ground and first excited states and the number of Kekulé valence structures is revealed among the isomeric benzenoid systems.