Ab initio study of core excited cyclopropane

Ab initio configuration interaction calculations have been carried out in order to assign the bands observed in the carbon K-shell spectrum of gaseous cyclopropane, measured using the inner-shell electron energy loss spectroscopy (ISEELS) method. Simple Franck-Condon calculations, based on the linear coupling approximation, were performed in order to reproduce the observed vibrational structure. The good agreement between the theoretical and the measured spectra allows to assign precisely most of the peaks. The spectrum is dominated by an intense band, corresponding to the 1s→σ∗(CC)(1a2′) transition, with large vibrational excitation. This is explained by the trimethylenic equilibrium geometry of this core state, very different from the D3h ground state structure. On the other hand, the Rydberg core-excited states and the core ion are only slightly distorted. Finally, the calculated structural and energetic parameters of the NC2H6 valence states match well those of the corresponding core states of C3H6, illustrating the validity of the equivalent core approximation.