Temperature and Pressure-Dependent Rate Constants for the Reaction of the Propargyl Radical with Molecular Oxygen.

Ab initio CCSD(T)/CBS(T,Q,5)//B3LYP/6-311++G(3df,2p) calculations have been conducted to map the CHO potential energy surface. The temperature- and pressure-dependent reaction rate constants have been calculated using the Rice-Ramsperger-Kassel-Marcus Master Equation model. The calculated results indicate that the prevailing reaction channels lead to CHCO + CO and CHCO + HCO products. The branching ratios of CHCO + CO and CHCO + HCO increase both from 18 to 29% with reducing temperatures in the range of 300-2000 K, whereas CCCHO + HO (0-10%) and CHCCO + HO (0-17%) are significant minor products. The desirable products OH and HO have been found for the first time. The individual rate constant of the CH + O → CHCO + HCO channel, 4.8 × 10 exp[(-2.92 kcal·mol)/()], is pressure independent; however, the total rate constant, 2.05 × 10 T exp[(-2.8 ± 0.03 kcal·mol)/)], of the CH + O reaction leading to the bimolecular products strongly depends on pressure. At = 0.7-5.56 Torr, the calculated rate constants of the reaction agree closely with the laboratory values measured by Slagle and Gutman [Symp. (Int.) Combust.1988, 21, 875-883] with the uncertainty being less than 7.8%. At < 500 K, the CH + O reaction proceeds by simple addition, making an equilibrium of CH + O ⇌ CHO. The calculated equilibrium constants, 2.60 × 10-8.52 × 10 cm·molecule, were found to be in good agreement with the experimental data, being 2.48 × 10-8.36 × 10 cm·molecule. The title reaction is concluded to play a substantial role in the oxidation of the five-member radicals and the present results corroborate the assertion that molecular oxygen is an efficient oxidizer of the propargyl radical.