The singlet states of styrene. Theoretical vibrational analysis of the ultraviolet spectrum

Vibrational analyses of the ground and lowest excited singlet states of styrene are performed using an extended PPP-CI model. Franck-Condon factors calculated from the model are used to analyze the intensity distribution of the ultraviolet absorption bands of the jet-cooled molecule from 34 000-46 000 cm^-1. For the weak first absorption system (S₁←S ₀) the small amount of vibrational excitation found experimentally is well described by the theoretical model. The relatively intense second absorption band shows more extended vibrational development. The model predicts that two electronic transitions S₂←S₀ and S ₃←S₀ contribute to the absorption spectrum in this region and that a large number of vibrational excitations involving carbon-carbon stretching motions are active. For the S₁←S ₀ and S₂←S₀ transitions a refinement of the theoretical model is performed to calculate the excited state equilibrium geometries from the measured spectra. Because of the large number of vibrations and the evidence for mixing among them in the excited states (Duschinsky effect), the full multimode treatment of the ground and excited state dynamics is required for an analysis of the spectrum.