Extensive ab Initio Study of the C₂O₂, C₂S₂, and C₂OS Systems: Stabilities and Singlet−Triplet Energy Gaps

Ab initio molecular orbital theory has been used to study the three lowest open shell states of ethylenedione, C₂O₂, 2-thiooxoethen-1-one, C₂OS, and ethylenedithione, C₂S₂. To treat the singlet and triplet states in an even-handed manner, multiconfigurational self-consistent field theory (MCSCF), which included all the important configurations for a quantitative description of these states, was used as the basis of the investigation. Further correlation effects have been included using a multireference configuration interaction (MRCI) approach. Basis sets of triple-ζ quality with d- and f-polarization functions were employed. Equilibrium geometries were obtained from density functional theory (DFT) calculations using the B3LYP exchange correlation functional and are presented for the ³Σ_g^-, ¹Δ_g states of all three molecules. Harmonic frequencies are also presented for these states and were calculated at the MCSCF and DFT levels. The reported relative energies for the states were obtained from MRCI calculations. As expected from previous work, the ground states are confirmed to be of ³Σ_g^- symmetry for all molecules, but it is only for the C₂S₂ that this state lies below the energy of the dissociation products in their own ground states. For C₂OS, though, this energy gap is small (5.7 kcal/mol). In contrast to a number of other calculations, the ¹Δ_g states for all three molecules are also shown to be minima on the MCSCF potential energy surfaces. The ¹Δ_g states are all close to the ground states (<10 kcal/mol). Consequently, with C₂S₂, this singlet state also lies below the ground state of the dissociated products. The relative energies of the ¹Σ_g⁺ states at the optimized geometries for the ¹Δ_g states have also been determined.