AB-INITIO STUDY OF THE LOW-LYING ELECTRONIC STATES OF INDENE

An ab initio calculation of the properties of the ground state and som e low-lying electronic states of indene is reported. The results are c ompared with experimental data obtained by infrared and Raman spectros copy, and with a recent fluorescence and resonance enhanced multiphoto n ionization spectra obtained in a supersonic jet. The results show th at the configuration interaction singles method can be used as a good approximation to calculate the molecular frequencies of the electronic ally excited states. It is found that as in other small aromatic molec ules, the frequencies of out-of-plane modes tend to decrease, while th ose of in-plane ones are not changed significantly, except for a Kekul e type vibration, that shows a dramatic increase in the S-1 state. In contrast with the S-1 state, which is found to be bound, the S-2 State is reactive. This is compatible with the diffuse absorption spectrum and the photochemical activity of this state. These results are compar ed with a similar recent study of styrene.