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

The excitation energies of the alpha-, beta-, and p-bands of benzo-annelate d polyacenes were estimated by the Pariser-Parr-Pople (PPP) molecular orbit al calculation method using a novel concept for evaluating the two-centre e lectron repulsion integral, new-gamma (A), in which the spectrochemical ato mic 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-gamma (A ) were excellent for the simultaneous calculations of the p-, alpha- and be ta -bands of benzo-annelated polyacenes and were better than those obtained from the conventional N.M-gamma or the usual new-gamma. The C-C bond lengt hs calculated using new-gamma (A) reproduced quantitatively the observed va lues.