The role of spin-forbidden processes in N+(3P)+NH3 reactions in the gas phase

The singlet and triplet potential energy surfaces involved in the gas-phase reactions between N+ and ammonia have been explored through the use of high-level ab initio techniques. Our calculations at the CCSD(T)/6-311+G(3df,2p) level of theory provide mechanisms for the formation of both NH3+ and NH2+ products in these reactions. Quite importantly, spin-forbidden processes associated with the cross-over between both potential energy surfaces play an important role in the formation of NH2+. This cross-over is favored by the fact that, although the entrance channel has an overall triplet multiplicity, some of the [H-3, N-2](+) singlet state molecular ions are more stable than the corresponding triplet analogs, because they exhibit N-N bonds significantly stronger. On the other hand, the triplet-singlet transition involves a typical one-center spin-orbit coupling, through the corresponding one-center orbital rotation, which renders this transition particularly effective.