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

Abstract

Ab initio CCSD(T)/CBS(T,Q,5)//B3LYP/6-311++G(3df,2p) calculations have been conducted to map the C3H3O2 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 + O2 → CH2CO + 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 T0.33 exp[(−2.8 ± 0.03 kcal·mol–1)/(RT)], of the C3H3 + O2 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 + O2 reaction proceeds by simple addition, making an equilibrium of C3H3 + O2 ⇌ C3H3O2. The calculated equilibrium constants, 2.60 × 10–16–8.52 × 10–16 cm3·molecule–1, were found to be in good agreement with the experimental data, being 2.48 × 10–16–8.36 × 10–16 cm3·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.