FLASH-PHOTOLYSIS STUDY OF THE CH3O2 + CH3O2 REACTION - RATE CONSTANTS AND BRANCHING RATIOS FROM 248 TO 573-K

The reactions 2CH3O2 → 2CH3O + O2 (1a), 2CH3O2 → CH3OH + HCHO + O2 (1b), and 2CH3O2 → CH3OOCH3 + O2 (1c) have been studied at temperatures between 248 and 573 K. At temperatures above 373 K, the resulting decay traces were distorted away from pure second order at short wavelengths (around 210 nm), owing to the presence of the hydroperoxy radicals formed via the nonterminating pathway (1a) and the subsequent rapid step CH3O + O2 → HCHO + HO2 (2). This distortion enabled the nonterminating/terminating branching ratio, β, to be determined. Combining the present results with previously published work on the branching ratios gave 1n β = 3.80 - 1470/T. Thus, although reaction 1 acts as a termination reaction under atmospheric conditions, it largely serves to convert CH3O2 into HO2 under combustion conditions. The temperature dependence of β enabled the real rate constant for the reaction, k1, to be obtained over the entire experimental temperature range, giving k1 = 1.3 × 1013 exp(365/T) cm3 molecule-1 s-1, with σ2A/cm6 molecule-2 s-2 = 2.00 × 10-28, σ2E/R/K2 = 1712, and σ2AE/R/cm3 molecule-1 s-1 = -5.61 × 10-13. Absolute uncertainties, including contributions from both the experimental measurements and the dependence of k1 on various analysis parameters, are estimated to be 22%, independent of temperature. No dependence of either the branching ratio or k1 on the total pressure was found. The mechanism of the title reaction is discussed and the present results are compared with existing studies of alkylperoxy self-reactions. The implications for combustion and atmospheric modeling are also discussed. © 1990 American Chemical Society.