Calculation of the vibronic fine structure in electronic spectra at higher temperatures. 1. Benzene and pyrazine

A program HOTFCHT for computing the vibronic fine structure of electronic spectra at different temperatures has been developed for a theoretical investigation of the temperature dependence of absorption and fluorescence spectra of organic molecules and a discussion of the temperature dependence of their photophysical properties. The program is based on the time-independent approach using the adiabatic and harmonic approximations. A Taylor series expansion of the electronic transition dipole moment takes into account vibronic coupling similar to a first-order Herzberg-Teller treatment. For the calculation of the Franck-Condon and Herzberg-Teller integrals, the recurrence formulae of Doktorov et al. (J. Mol. Spectrosc. 1977, 64, 302) were used while the derivatives of the electronic transition dipole moment were obtained numerically. As a first application of this program the vibronic fine structure of the So-SI transitions of benzene and pyrazine were calculated at different temperatures. The equilibrium geometries and frequencies determined at the CASSCF level as well as the calculated spectra are in good agreement with experimental data; the main features of the spectra and especially "hot" bands are well-reproduced-and can be assigned to the corresponding vibronic transitions.