Precise PPP molecular orbital calculations of excitation energies of polycyclic aromatic hydrocarbons. Part 6. Spectrochemical atomic softness parameter

Abstract

The excitation energies of the α-, β-, and p-bands of benzo-annelated polyacenes were estimated by the Pariser–Parr–Pople (PPP) molecular orbital calculation method using a novel concept for evaluating the two-centre electron repulsion integral, new-γ^A, in which the spectrochemical atomic softness (SCAS) parameter k_r^A was taken into account. The value of k_r^A was determined using the concept of the spectroactive portion (SP) and allotted to each carbon atom. The calculated results using new-γ^A were excellent for the simultaneous calculations of the p-, α- and β-bands of benzo-annelated polyacenes and were better than those obtained from the conventional N·M-γ or the usual new-γ. The C–C bond lengths calculated using new-γ^A reproduced quantitatively the observed values.