Reactions of the HO2 Radical with CH3CHO and CH3C(O)O2 in the Gas Phase

Flash photolysis UV absorption techniques were used to study the HO2 + CH3C(O)O2 reaction. It was found that the reaction HO2 + CH3CHO ⇌ CH3CH(OH)O2 (2, −2) can interfere with the kinetic measurements. Thus, the kinetics and thermochemistry of this reaction were investigated. The UV spectrum of the CH3CH(OH)O2 radical was determined, and the rate constant for the association reaction was measured to be k 2 = 4.4 × 10-14 cm3 molecule-1 s-1 at 298 K, with an uncertainty factor of about 2. The reaction was found to be equilibrated near room temperature, and the equilibrium constant was determined between 298 and 373 K: K 2 = k 2/k - 2 = 1.9 × 10-(27±1) exp[+(6925 ± 840 K)/T] cm3 molecule-1. This corresponds to ΔH o 298 = −64 ± 8 kJ mol-1 and ΔS o 298 = −157 ± 6 J K-1 mol-1 (errors are 2σ), the latter value being in good agreement with the calculated value, using the AM1 semiempirical method. The kinetics of the reactions CH3C(O)O2 + HO2 → CH3C(O)OOH + O2 (1a) and CH3C(O)O2 + HO2 → CH3C(O)OH + O3 (1b) were determined in the temperature range 273−403 K, using low acetaldehyde concentrations, so that reaction 2 could be neglected. The initial radical concentrations were measured carefully and were consistent with the concentrations of the precursors (acetaldehyde and methanol). The rate expression is k 1 = (6.4 ± 2.5) × 10-13 exp[+(925 ± 120 K)/T] cm3 molecule-1 s-1, yielding k 1 = (1.42 ± 0.07) × 10-11 cm3 molecule-1 s-1 at 298 K. The quoted errors are 2σ, and the global uncertainty is estimated at 15%. The branching ratio of channel 1b, β1 = k 1b/k 1, was determined by measuring the residual absorption, from 230 to 280 nm, at sufficiently long time, so that all radicals had recombined. It was verified that the UV spectrum of the residual absorption was the same as that of ozone, within uncertainty. The resulting value β1 = 0.20 ± 0.02 was derived, independent of temperature over the range studied.