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 C 3 H 3 O 2 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 CH 3 CO + CO and CH 2 CO + HCO products. The branching ratios of CH 3 CO + CO and CH 2 CO + HCO increase both from 18 to 29% with reducing temperatures in the range of 300-2000 K, whereas CCCHO + H 2 O (0-10%) and CHCCO + H 2 O (0-17%) are significant minor products. The desirable products OH and H 2 O have been found for the first time. The individual rate constant of the C 3 H 3 + O 2 → CH 2 CO + HCO channel, 4.8 × 10 -14 exp[(-2.92 kcal·mol -1 )/( RT )], is pressure independent; however, the total rate constant, 2.05 × 10 -14 T 0.33 exp[(-2.8 ± 0.03 kcal·mol -1 )/ (RT )], of the C 3 H 3 + O 2 reaction leading to the bimolecular products strongly depends on pressure. At P = 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 T < 500 K, the C 3 H 3 + O 2 reaction proceeds by simple addition, making an equilibrium of C 3 H 3 + O 2 ⇌ C 3 H 3 O 2 . The calculated equilibrium constants, 2.60 × 10 -16 -8.52 × 10 -16 cm 3 ·molecule -1 , were found to be in good agreement with the experimental data, being 2.48 × 10 -16 -8.36 × 10 -16 cm 3 ·molecule -1 . 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.