Computational study of the HCCO + NO reaction: Ab initio MO/vRRKM calculations of the total rate constant and product branching ratios

Accurate thermochemistry for major product channels of the HCCO + NO (R1) reaction was established at the CCSD(T)/6-311 +G(3df,2p) and QCISD(T)/6-311 +G(3df,2p) levels of theory based on the potential energy surface (PES) calculated by density functional B3LYP/6-311G(d,p) method. Two barrierless entrance pathways forming trans- and cis-nitrosoketene (1 and 2) have been mapped out following the minimum energy paths. Using isodesmic reaction analysis, the standard enthalpy of formation for 1 is found to be 17.8 ± 1.7 kcal/ mol. The recently reported PES of the nitrosoketene decomposition (Vereecken, L.; Sumathy, R.; Carl, S. A.; Peeters, J. Chem. Phys. Lett. 2001, 344, 400) has been appended with two additional pathways to form HCNO + CO (R1a) and HCN + CO₂ (R1b) products. A lower-energetic transition state for the decomposition of 2 to HCNO + CO has been found, and a new stepwise pathway for the decomposition of 1,2-oxazet-4-one (3) to HCN + CO₂ has been considered in competition with a more favorable concerted pathway. The effective total rate constant and products distribution have been calculated in the framework of multichannel and multiquantum well kinetic model, and the effects of P and T have been evaluated by weak collision master equation/RRKM approach with a variational treatment of the entrance channels. Our predicted total rate constant, k_R1(T) = 1.37 × 10¹⁶ T^-0.98 exp(-190/T) cm³ mol^-1 s^-1, exhibits a negative temperature dependence in very good agreement with experiment. The α(R1b) branching fraction can be given by 0.5 exp(-T/67.1) + 0.3 exp(-T/2592) in the 250-2500 K temperature range, which is also well-supported by available experimental data.