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The mechanisms and kinetics of the reaction of a thiocyanato radical (NCS) with NO were investigated by a high-level ab initio molecular orbital method in conjunction with variational RRKM calculations. The species involved were optimized at the B3LYP/6-311++G(3df,2p) level, and their single-point energies were refined by the CCSD(T)/aug-cc-PVQZ//B3LYP/6-311+G(3df,2p) method. Our calculated results indicate favorable pathways for the formation of several isomers of an NCSNO complex. Formation of OCS + N 2 also is possible, although this pathway involves a substantial energy barrier. The predicted total rate constants, k total, at a 2 torr He pressure can be represented by the following equations: k total = 9.74 x 10 (26) T (-13.88) exp(-6.53 (kcal mol (-1))/ RT) at T = 298-950 K and 1.17 x 10 (-22) T (2.52) exp(-6.86 (kcal mol (-1))/ RT) at T = 960-3000 K, in units of cm (3) molecule (-1) s (-1), and the predicted values are in good agreement with the experimental results in the temperature range of 298-468 K. The calculated results clearly indicate that the branching ratio for R M1 in the temperature range of 298-950 K has the largest value ( R M1 accounts for 0.53-0.39). However, in the higher temperature range (960-3000 K), the formation of OCS + N 2 ( P5) with branching ratio R P5 (0.40-0.79) becomes dominant. The rate constants for key individual product channels are provided for different temperature and pressure conditions. |
