The results of ab initio G2(MP2), QCISD(T)/6-311+G(3df) and full valence active space CASSCF(16,12)/6-311+G(3df) calculations of the stationary points on the lowest singlet and triplet potential energy surfaces of carbon dioxide and their intersystem crossing show that spin-forbidden unimolecular decomposition of CO2 can proceed by two different mechanisms. The non-collinear channel goes through a C2v-symmetric minimum energy crossing point MSX1 in the vicinity of the bent local minimum 4 in the triplet state. Once on the triplet surface, the molecule has to overcome the barrier of 131.3 kcal/mol at transition state TS3 before yielding the products, Full-size image (<1 K). The collinear channel directly leads from CO21 to the products via linear MSX2. The barrier at MSX2, estimated as ∼135 kcal/mol, is higher than that for the non-collinear channel, but the probability of intersystem crossing for the collinear mechanism is expected to be higher than for the non-collinear channel, since the spin–orbit coupling value for MSX2 (Full-size image (<1 K)) is much higher than that for MSX1 (Full-size image (<1 K)). The two mechanisms of unimolecular decomposition of CO2 are expected to compete with each other and exhibit different mode-specific dynamics. Spin-forbidden fragmentation of CO2 is compared with the fragmentation of the isoelectronic N2O molecule. The mechanisms for the reverse Full-size image (<1 K) and Full-size image (<1 K) reactions are also discussed.
