Ab initio (1)A ' ground potential energy surface and transition state theory kinetics study of the O(D-1)+N2O -> 2NO, N-2+O-2(a (1)Delta(g)) reactions

An ab initio study of the (1)A(') ground potential energy surface (PES) of the O(D-1)+N2O(X (1)Sigma (+)) system has been performed at the CASPT2//CAS SCF (complete active space second-order perturbation theory//complete activ e space self-consistent field) level with Pople basis sets. The two reactio ns leading to 2 NO(X (2)Pi) [reaction (1)] and N-2(X (1)Sigma (+)(g))+O-2(a (1)Delta (g)) [reaction (2)] products have been investigated. In both reac tions a trans-approach of the attacking oxygen to the N2O moiety is found t o be preferred, more markedly in reaction (1). For this reaction also a cis -path is feasible and is possibly connected with the trans-path by a transi tion state placed below reactants. A thorough characterization of the entra nce zone has been performed to allow for subsequent kinetics calculations. Fixed angle and minimum energy paths have been constructed and transition s tate geometries have been refined at the CASPT2 level, thus obtaining appro ximate structures and frequencies for the latter. From these calculations i t can be inferred that both reactions proceed without an energy barrier. Ra te constant calculations in the 100-1000 K temperature range based on CASPT 2 structures and using the transition state theory yield values in good agr eement with experiment for the two reactions, especially when a proper scal ing of the energy barriers is performed. Also, for comparative purposes qua siclassical trajectory calculations were performed on reaction (1) in the s ame temperature range, using a previous pseudotriatomic analytical potentia l energy surface, obtaining good agreement with experiment. (C) 2001 Americ an Institute of Physics.