Precise PPP molecular orbital calculations of the excitation energies of polycyclic aromatic hydrocarbons .2. Evaluation of the spectrochemical softness parameter based on the spectroactive partial structure of a molecule

For precise Pariser-Parr-Pople molecular orbital (PPP MO) calculations, the values of the spectrochemical softness parameter (k) of a new two centre electron repulsion integral (new-gamma) were evaluated based on an appropriate partial structure of cata-condensed polycyclic aromatic hydrocarbons (PAHs). The spectroactive aromatic sextet resonance system [ASRS] was defined as a spectroactive partial structure of a molecule. The calculated excitation energies of the p-band of the cata-condensed PAHs accurately reproduced the observed values. In particular, the calculated energies of branched PAHs were improved compared with those obtained using our previous method. Copyright (C) 1996 Elsevier Science Ltd