Parametric method in the theory of vibronic spectra of polyatomic molecules: Absorption and fluorescence spectra and structure of styrene in the excited state

The excited states and absorption and fluorescence spectra of styrene and beta-d(2)-styrene are calculated by the parametric method. It is shown that parametric models of these molecules in the excited states adequately describe their real structure and predict the fine structure of their electronic spectra at the quantitative level, which is sufficient for its detailed interpretation and the refinement of parameters of the molecular model by solving inverse problems. In a second approximation (using only one additional parameter), the method provides a more exact calculation of angular deformations of the molecule upon excitation and, in particular, describes characteristic differences in the intensities of low-frequency spectral components in a series of diphenylpolyene-stilbene-styrene. The number of parameters for molecular fragments is small (2 and 3 in the first and second approximations, respectively), and they are the same for a series of related compounds. Compared to direct ab initio calculations, the parametric method yields substantially better results already in the first approximation, and it is more efficient for modeling molecules in excited states and description of their vibronic spectra. (C) 2000 MAIK "Nauka/Interperiodica".