Ab initio study on the oxidation of NCN by OH: Prediction of the individual and total rate constants

The mechanism for the reaction of NCN with OH has been investigated by ab initio molecular orbital and transition-state theory calculations. The potential energy surface (PES) was calculated by the highest level of the modified GAUSSIAN-2 (G2M) method, G2M(CCI). The barrierless association process of OH + NCN → OH⋯NCN (van der Waals, vdw) was also examined at the UCCSD(T)/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) and CASPT2(13,13)/ANO-L//B3LYP/6- 311+G(d,p) levels. The predicted heats of reaction for the production of H + NCNO, HNC + NO, HCN + NO, and N₂ + HOC, 7.8, -53.2, -66.9, and -67.7, respectively, are in excellent agreement with the experimental values, 8.2 ± 1.3, -52.3 ±1.7 (or 55.7 ± 1.7), -66.3 ± 0.7, and -68.1 ± 0.7 kcal/mol. The kinetic results indicate that, in the temperature range of 300-1000 K, the formation of trans, trans-HONCN (LM2) is dominant. Over 1000 K, formation of H + NCNO is dominant, while the formation of HCN + NO becomes competitive. The rate constants for the low-energy channels given in units of cm³ molecule-^' s-¹ can be represented by the following: k₁(LM2) = 1.51 × 10 ¹⁵T^-8.72 exp(-2531/7) at 300-1500 K in 760 Torr N ₂; k₂(H+NCNO) = 5.54 × 10 ^-147- ⁰⁹⁷ exp(-3669/T) and k₃(HCN+NO) = 7.82 × 10 ^-14T^0.44 exp(-2013/T) at 300-2500 K, with the total rate constant of k_t = 3.18 × 10 ²7_-4.63 exp(-740/T), 300-1000 K, and k_t = 2.53 × 10 ^-14T^1-13 exp(-489/T) in the temperature range of 1200-2500 K. These results are recommended for combustion modeling applications.