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 CH3CO + CO and CH2CO + HCO products. The branching ratios of CH3CO + CO and CH2CO + HCO increase both from 18 to 29% with reducing temperatures in the range of 300-2000 K, whereas CCCHO + H2O (0-10%) and CHCCO + H2O (0-17%) are significant minor products. The desirable products OH and H2O have been found for the first time. The individual rate constant of the C3H3 + O-2 -> CH2CO + HCO channel, 4.8 x 10(-14) exp[(-2.92 kcalmiddotmol(-1))/(RT)], is pressure independent; however, the total rate constant, 2.05 x 10-14 T-0.33 exp[(-2.8 +/- 0.03 kcalmiddotmol-1)/(RT)], of the C3H3 + 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 C3H3 + O-2 reaction proceeds by simple addition, making an equilibrium of C3H3 + O-2 ? C3H3O2. The calculated equilibrium constants, 2.60 x 10(-16)- 8.52 x 10(-16) cm(3)middotmolecule(-1), were found to be in good agreement with the experimental data, being 2.48 x 10(-16)-8.36 x 10(-16 )cm3middotmolecule(-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.