Theoretical study of the excitation spectra of five-membered ring compounds: Cyclopentadiene, furan, and pyrrole

Multireference perturbation theory with complete active space self-consistent field (CASSCF) reference functions was applied to the study of the valence and Rydberg excited states in the range of 5-8 eV of five-membered ring compounds, cyclopentadiene, furan, and pyrrole. The spectra of these molecules have been studied extensively for many years but characterization is far from complete. The present approach can describe all kinds of excited states with the same accuracy. The calculated transition energies are in good agreement with corresponding experimental data. We were able to predict the valence and Rydberg excited states with an accuracy of 0.27 eV or better except for the B₂⁺ of pyrrole. The valence excited states of five-membered ring compounds were interpreted in terms of the covalent minus states and ionic plus states of the alternate symmetry. The unobserved 1A]₁→A₁^- transition with very weak intensity, which is hidden under the strong 1A₁→B₂⁺ transition, is also discussed. Overall, the present theory supports the assignments recently made by Serrano-Andres et al. based on the CASSCF plus second-order perturbation results. However, there remain some discrepancies in the assignment of the spectrum of pyrrole.