High‐level ab initio study of the N+(3P)+SH2 reactions in the gas phase: Role of spin‐forbidden pathways

The singlet and triplet potential energy surfaces involved in N⁺+SH₂ reactions have been explored using high‐level ab initio techniques. The geometries of the stationary points were optimized at the QCISD/6‐311G(df,p) level. The final energies were obtained in CCSD(T)/6‐311+G(3df,2p) single‐point calculations. The results obtained show that, although the N⁺(¹D)+SH₂ entrance channel is higher in energy than the N⁺(³P)+SH₂ one, most of the H₂, S, N]⁺ singlet state cations are lower in energy than the corresponding triplets, due to their different bonding characteristics. Both singlet and triplet potential energy surfaces are quite close each other, and crossover between them can occur. The minimum energy crossing points were located by means of CASSCF(6,5) calculations. The spin‐orbit couplings show that the transition probability from the triplet to the singlet potential energy surface is significantly large. One of the most important consequences is that some of the products of the reaction, such as SH⁺, can be formed in typical spin‐forbidden processes. Since all the relevant structures along these pathways are much lower in energy than the reactants, this mechanism should be accessible even at low impact energies and therefore could be important in processes taking place in interstellar media. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001