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(CC1). 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 N(2) + 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 cm(3) molecule(-1) s(-1) can be represented by the following: k(1)(LM2) = 1.51 x 10 (15)T(-8.72) exp(-2531/T) at 300-1500 K in 760 Torr N(2); k(2)(H+NCNO) = 5.54 x 10 (-14)T(-0.97) exp(-3669/T) and k(3)(HCN+NO) = 7.82 x 10 (-14)T(0.44) exp(-2013/T) at 300-2500 K, with the total rate constant of k(t) = 3.18 x 10 (2)T(-4.63) exp(-740/T), 300-1000 K, and k(t) = 2.53 x 10 (-14)T(1.13) exp(-489/T) in the temperature range of 1200-2500 K. These results are recommended for combustion modeling applications.