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 T^0.33 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.

Figure 1

Equilibrium constant of the C₃H₃ + O₂ ⇌ C₃H₃O₂ reaction in the 450–368 K temperature range.

Figure 2

Detailed potential energy surface of the C₃H₃ + O₂ system calculated at the CCSD(T,Q,5)/CBS//B3LYP/6-311++G(3df,2p) + ZPEs level of theory (energies are in kcal·mol^–1).

Figure 3

Geometries of some main species optimized at the B3LYP/6-311++G(3df,2p) level (bond lengths are in Å, bond angles are in degrees).

Figure 4

Plots of the predicted individual rate constants of the C₃H₃ + O₂ (RA) reaction forming P₃, P₅–P₉ products in the 300–2000 K range and 7.6 Torr (N₂). The k₁, k₂, and k₃ lines are hidden by the k₆ line.

Figure 5

Plots of the predicted individual rate constants of the C₃H₃ + O₂ (RA) reaction forming the P₃, P₅–P₉ products in the 300–2000 K range and 76 Torr (N₂). The k₁, k₂, and k₃ lines are hidden by the k₆ line.

Figure 6

Plots of the predicted individual rate constants of the C₃H₃ + O₂ (RA) reaction forming the P₃, P₅–P₉ products in the 300–2000 K range and 760 Torr (N₂). The k₂ and k₃ lines are hidden by the k₆ line.

Figure 7

Plots of the predicted individual rate constants of the C₃H₃ + O₂ (RA) reaction forming the P₃, P₅–P₉ products in the 300–2000 K range and 7600 Torr (N₂). The k₃ line is hidden by the k₆ line.

Figure 8

Plots of the predicted individual rate constants of the C₃H₃ + O₂ (RA) reaction forming the P₃, P₅–P₉ products in the 300–2000 K range and 76,000 Torr (N₂).

Figure 9

Branching ratios of C₃H₃ + O₂ (RA) → P₃, P₅–P₉ reactions in the 300–2000 K temperature range and P = 760 Torr (N₂). The k₃ curve is hidden by the k₆ curve.

Figure 10

Calculated rate constants of the C₃H₃ + O₂ reaction at very low pressures (P = 0.7 and 2 Torr) and T = 250–375 K in comparison with the experimental data measured by Slagle and Gutman.

Figure 11

Calculated rate constants of the C₃H₃ + O₂ reaction at very low pressures (P = 1.04–5.56 Torr) and T > 400 K in comparison with the experimental data measured by Slagle and Gutman.

Figure 12

Calculated P-dependent rate constants of the C₃H₃ + O₂ reaction at T = 295 K in comparison with the experimental data measured by Slagle and Gutman and Atkinson and Hudgens.