The low-lying singlet-triplet transitions of phenylacetylene and styrene: Analysis of the vibronic structure

The ground state (S-0) and lowest tripler state (T-1) geometries and normal coordinates of phenylacetylene and styrene have been calculated using the MCSCF method with 4-31G basis set. Franck-Condon factors derived from these results reproduce the vibronic structure of S-0--> T-1 excitation spectra previously obtained by electron-energy-loss spectroscopy. The analysis of the spectra shows that the excitation in phenylacetylene takes place within the ring with only minor contributions from the side group. The side group multiple C-C bond is not noticably involved in the S-0--> T-1 excitation. In styrene the excitation is mainly localized in the exocyclic part of the molecule. The spectra are therefore dominated by vibrations involving the stretching motion of the exocyclic C-C bonds.