Ab initio study of the low-lying electronic states of indene

An ab initio calculation of the properties of the ground state and some low-lying electronic states of indene is reported. The results are compared with experimental data obtained by infrared and Raman spectroscopy, and with a recent fluorescence and resonance enhanced multiphoton ionization spectra obtained in a supersonic jet. The results show that the configuration interaction singles method can be used as a good approximation to calculate the molecular frequencies of the electronically excited states. It is found that as in other small aromatic molecules, the frequencies of put-of-plane modes tend to decrease, while those of in-plane ones are not changed significantly, except for a Kekulé type vibration, that shows a dramatic increase in the S₁ state. In contrast with the S₁ state, which is found to be bound, the 82 state is reactive. This is compatible with the diffuse absorption spectrum and the photochemical activity of this state. These results are compared with a similar recent study of styrene.