Ab initio derived analytical fits of the two lowest triplet potential energy surfaces and theoretical rate constants for the N(4S) plus NO(X 2Π) system

This work presents two new analytical fits of the ground potential energy surface (PES) ((3)A(')) and the first excited PES ((3)A(')) involved in the title reaction, considering the N-abstraction (1) and the O-abstraction (2) reaction channels, and the reverse reaction (-1). The PESs are derived from ab initio electronic structure calculations by means of second-order perturbation theory on a complete active-space self-consistent-field wave function (CASPT2 method). Stationary points and extensive grids of ab initio points (about 5600 points for the (3)A(') PES and 4900 points for the (3)A(') PES) were fitted along with some diatomic spectroscopic data to better account for the experimental exoergicity. Thermal rate constants were calculated (200-5000 K) for all mentioned reaction processes by means of the variational transition-state theory with the inclusion of a semiclassical tunneling correction. Excellent agreement with the experimental data was observed for reaction (1) and its reverse, within all the temperature range, substantially improving the results derived from previous analytical PESs. The contribution of the (3)A(') PES to the reaction rate constant (k(1)) was small even at high temperatures (e.g., only 10.8% at 2500 K). Moreover, the main contribution to reaction rate constant (k(2)) was due to the (3)A(') PES, differing from what happens for reaction (1). The O-abstraction reaction channel accounts for a 3.0% of the total reaction (k=k(1)+k(2)) at 5000 K, consistent with the very limited experimental information available. (C) 2003 American Institute of Physics.