Ab initio study of the two lowest triplet potential energy surfaces involved in the N(4S) + NO(X2Π) reaction

This work presents ab initio electronic structure calculations of the two possible N(4S) + NO(X2) abstraction reaction channels on the lowest 3A' and 3A' potential energy surfaces (PESs). Complete active space self-consistent-field (CASSCF) calculations, second-order perturbation calculations (CASPT2) and multireference configuration interaction calculations (MR-CI) based on CASSCF wave functions, along with some coupled cluster (CC) calculations were carried out by using the standard correlation-consistent (cc-pVnZ and aug-cc-pVnZ, n=D,T,Q,5) Dunning's basis sets. It was shown that there was no energy barrier along the minimum energy path in the 3A' PES for the N-abstraction reaction channel. However, an energy barrier (6.74 kcal/mol) was located in the 3A' PES. This energy barrier was considerably smaller than the previously reported MR-CCI value (14.4 kcal/mol). It was established that the N and O 2s electron correlation, neglected in previous studies of these authors, was the main source of this energy decrease. As a result, the present ab initio data will produce larger values of the thermal rate constants at high temperatures. High-energy barriers were found for the O-abstraction reaction channel in both PESs (41.13 and 30.77 kcal/mol for 3A' and 3A', respectively), which agree with the accepted idea that this channel will be only important at high collision energies. Nonetheless, current ab initio results show that this channel will be open at not very high collision energies (e.g., over 30 kcal/mol could take place). Experimental studies on the O-abstraction reaction channel are missing and would be useful to confirm its ab initio expected importance