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

The singlet and triplet potential energy surfaces involved in N+ + SH2 reac tions have been explored using high-level ab initio techniques. The geometr ies of the stationary points were optimized at the QCISD/6-311G(dfp) level. The final energies were obtained in CCSD(T)/6-311+G(3df,2p) single-point c alculations. The results obtained show that, although the N+ (D-1) + SH2 en trance channel is higher in energy than the N+(P-3) + SH2 one, most of the [H-2, S, N](+) singlet state cations are lower in energy than the correspon ding triplets, due to their different bonding characteristics. Both singlet and triplet potential energy surfaces are quite close each other, and cros sover between them can occur. The minimum energy crossing points were locat ed by means of CASSCF(6,5) calculations. The spin-orbit couplings show that the transition probability from the triplet to the singlet potential energ y 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 t ypical spin-forbidden processes. Since all the relevant structures along th ese pathways are much lower in energy than the reactants, this mechanism sh ould be accessible even at low impact energies and therefore could be impor tant in processes taking place in interstellar media. (C) 2002 John Wiley & Sons, Inc.