AB-INITIO VARIATIONAL TRANSITION-STATE THEORY CALCULATIONS FOR THE H+NH2[--]H-2+NH HYDROGEN ABSTRACTION REACTION ON THE TRIPLET POTENTIAL-ENERGY SURFACE

We report high-level ab initio calculations that characterize the triplet potential energy surface for the H + NH2 <----> H-2 + NH direct hydrogen abstraction reaction. A minimum energy reaction pathway has been computed at the full-valence complete active space self-consistent field (CASSCF) level using a correlation-consistent polarized valence double zeta basis set. Energies along this reaction path have been further refined by second-order multireference configuration interaction calculations based on the full-valence CASSCF reference wave function and using a correlation-consistent polarized valence triple zeta basis set. The temperature dependence of the reaction rates for the forward and reverse reactions are determined by performing canonical variational transition state theory calculations using the ab initio potential energy surface information as input. These rate calculations incorporate tunneling in the zero-curvature approximation through the ground state vibrationally adiabatic potential energy curve and incorporate anharmonic contributions to the vibrational partition functions through an independent normal mode approximation. The general agreement between the calculated and measured high-temperature rates for both the forward and reverse reaction indicate this reaction proceeds as a direct abstraction under these conditions and not via the spin-forbidden addition/elimination route.