Precise PPP molecular orbital calculations of excitation energies of polycyclic aromatic hydrocarbons .3. Evaluation of the spectrochemical softness based on the model of composite molecules

For Pariser-Parr-Pople molecular orbital (PPP MO) calculations of the p-band of polycyclic aromatic hydrocarbons (PAHs), the values of spectrochemical softness parameter k of a novel two-centre electron repulsion integral, new-gamma, were evaluated based on the model of composite molecules (MCM). The spectroactive portion (SP), namely the part which contributes mainly to the p-band, of a molecular framework was used as an index for the evaluation of the parameter k for peri-condensed PAHs together with cata-condensed PAHs, The calculated excitation energies of the p-band of PAHs using the new-gamma included k values based on the SP reproduced accurately the observed ones. The precise MO calculations were performed. (C) 1997 Elsevier Science Ltd.