Singlet and Triplet Electronic States Involved in the Reactions CO₂ + O → CO₃ → CO + O₂

The singlet and triplet electronic states of the CO₃ complex are studied with the best available variational quantum chemical methods, CASSCF and MRCI, using a flexible basis set without imposing any symmetry restriction. A large domain of the potential energy surfaces correlated with the lowest four reactant states of CO₂ + O is explored. Several pathways leading to two different types of isomers (star-shaped and chain-shaped) are found for both singlet and triplet spin states, and their spectroscopic properties are characterized. Among these pathways, three reaction pathways on high-lying singlet potential energy surfaces leading to the CO + O₂ dissociation channel are reported for the first time, as well as four intersystem crossing points between the triplet and singlet surfaces. Change in electronic properties along different reaction paths is analyzed in detail.