Theoretical Analysis of the Electronic Spectra of Benzaldehyde

Abstract

The electronic spectrum of benzaldehyde has been studied by using multiconfigurational second-order perturbation theory through the multistate extension (MS-CASPT2). The nπ* 11A‘ ‘ state, placed vertically at 3.71 eV, is assigned to the lowest band. The 11A‘ → 21A‘ and 11A‘ → 31A‘ transitions, of ππ* nature, located at 4.33 and 4.89 eV, are responsible for the second- and third-energy bands, respectively. The most intense feature involves the 41A‘ and 51A‘ ππ* excited states, calculated to be 5.98 and 6.23 eV above the ground state. In addition, excited states corresponding to the low-lying Rydberg series are related to the available experimental data. Geometry optimizations for the ground state and low-lying excited states of both nπ* and ππ* characters have been carried out at the CASSCF level. The relative ordering of the lowest nπ* and ππ* triplet states varies depending on the geometry employed. Furthermore, the S1(nπ*) and T2(ππ*) potential hypersurfaces are found to intersect upon relaxation in S1, providing a possible explanation for the efficient singlet−triplet intersystem crossing occurring in benzaldehyde.