Theoretical studies on the electronic spectrum of selenium dioxide. Comparison with ozone and sulfur dioxide

MRCI vertical excitations energies and oscillator strengths of SeO2 were calculated for singlet and triplet valence states up to about 8 eV, and for low-lying 5s-Rydberg states starting at 8.5 eV. CCSD(T) and DFT (B3PW91) geometry optimizations were performed for 20 excited states. There is good agreement with the experimental geometry of the ground state and of 11B2. The calculated vibrational frequencies for the ground state are close to the observed frequencies. Relaxed DFT potential curves are given for the lowest singlet states in C2v symmetry, and for the lowest 1A″ state, which bridges the conical intersection between 11A2 and 11B1. Results are reported for adiabatic electron affinity, ionization potential, and dissociation energy. The SeOO structure lies about 3.4 eV above the OSeO ground state. Comparison is made with vertical excitation energies of first singlet and triplet states of O3 and SO2.