MULTICONFIGURATIONAL PERTURBATION-THEORY (CASPT2) APPLIED TO THE STUDY OF THE LOW-LYING SINGLET AND TRIPLET EXCITED-STATES OF CYCLOPROPENE

The electronic spectrum of cyclopropene has been studied using multico nfigurational second-order perturbation theory (CASPT2) with extended ANO-type basis sets. The calculation comprises two valence states and the 3s, 3p, 3d members of the Rydberg series converging to the pi and sigma ionization limits. A total of twenty singlet and twenty triplet excited states have been analyzed. The results confirm the valence nat ure of the lowest energy singlet-singlet band and yield a conclusive a ssignment: the first dipole-allowed transition in cyclcopropene is due to absorption to a (sigma --> eta) state. The (eta --> eta*) (V) sta te is interleaved among a number of Rydberg states in the most intense band of the system. The remaining spectral bands are due to Rydberg t ransitions of higher energy. The two lowest singlet-triplet transition s involve the same valence states. The results are in agreement with a vailable experimental data and provide a number of new assignments of the experimental spectra.