Ab initio study on the oxidation of NCN by O (³P): Prediction of the total rate constant and product branching ratios

The reaction of NCN with O is relevant to the formation of prompt NO according to the new mechanism, CH + N₂ → cyclic-C(H)NN - → HNCN → H + NCN. The reaction has been investigated by ab initio molecular orbital and transition state theory calculations. The mechanisms for formation of possible product channels involved in the singlet and triplet potential energy surfaces have been predicted at the highest level of the modified GAUSSIAN-2 (G2M) method, G2M (CC1). The barrierless association/dissociation processes on the singlet surface were also examined with the third-order Rayleigh-Schrödinger perturbation (CASPT3) and the multireference configuration interaction methods including Davidson's correction for higher excitations (MRCI+Q) at the CASPT3(6,6)/6-311+G(3df)//UB3LYP/6-311G(d) and MRCI+Q(6,6)/6-311+G(3df)// UB3LYP/6-311G(d) levels. The rate constants for the low-energy channels producing CO + N₂, CN + NO, and N(₄S) + NCO have been calculated in the temperature range of 200-3000 K. The results show that the formation of CN + NO is dominant and its branching ratio is over 99% in the whole temperature range; no pressure dependence was noted at pressures below 100 atm. The total rate constant can be expressed by: k _t = 4.23 × 10^-11 T^0.15 exp(17/T) cm ³ molecule^-1 s^-1.