CALCULATION OF ELECTRONIC SPECTRA OF PORPHIN AND ITS DERIVATIVES BY A MODIFIED INDO/S METHOD

Quantum-chemical calculations of excited electronic states of porphin (H2P) and Mg-porphin (MgP) have been carried out in the framework of the INDO/S method with varying off-diagonal matrix elements of the one-electron Hamiltonian H-mu nu. and electron-electron interaction integrals gamma(mu nu) as functions of the internuclear distance. It has been found that a simultaneous increase in the pi-type overlap factor for H-mu nu. and decrease in the gamma(mu nu) integrals, as compared with gamma(NM)(mu nu). calculated by the Nishimoto-Mataga formula, make it possible to reproduce the positions of the Q-and B-transitions in the experimental absorption spectrum with a precision of similar to 300 cm(-1). In this case, the N-x-transition intensity of the H2P molecule is halved, which means that only two B-transitions should be related to the Soret band as in the four-orbital model. Using the proposed parameterization (INDO/Sm), electronic spectra have been calculated for a number of porphyrin compounds including chlorine, bacteriochlorin, tetrabenzoporphin, and tetraazaporphin. The results obtained agree with the experimental data to an accuracy of 300-700 cm(-1) whereas the accuracy of the standard INDO/S calculation is no better than 3000 cm(-1).