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 b eta-d(2)-styrene are calculated by the parametric method. It is shown that parametric models of these molecules in the excited states adequately descr ibe their real structure and predict the fine structure of their electronic spectra at the quantitative level, which is sufficient for its detailed in terpretation and the refinement of parameters of the molecular model by sol ving inverse problems. In a second approximation (using only one additional parameter), the method provides a more exact calculation of angular deform ations of the molecule upon excitation and, in particular, describes charac teristic differences in the intensities of low-frequency spectral component s in a series of diphenylpolyene-stilbene-styrene. The number of parameters for molecular fragments is small (2 and 3 in the first and second approxim ations, respectively), and they are the same for a series of related compou nds. Compared to direct ab initio calculations, the parametric method yield s substantially better results already in the first approximation, and it i s more efficient for modeling molecules in excited states and description o f their vibronic spectra. (C) 2000 MAIK "Nauka/Interperiodica".