PRECISE PPP MOLECULAR-ORBITAL CALCULATIONS OF EXCITATION-ENERGIES OF POLYCYCLIC AROMATIC-HYDROCARBONS .3. EVALUATION OF THE SPECTROCHEMICALSOFTNESS 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 spect rochemical softness parameter k of a novel two-centre electron repulsi on integral, new-gamma, were evaluated based on the model of composite molecules (MCM). The spectroactive portion (SP), namely the part whic h 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 P AHs together with cata-condensed PAHs, The calculated excitation energ ies 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 calc ulations were performed. (C) 1997 Elsevier Science Ltd.