A laser flash photolysis study of the reaction methyl + molecular oxygen .fwdarw. methylperoxy (CH3O2) at 298 K

Abstract

The reaction CH3 + O2 k1/ CH3O2 has been studied at room temperature over a wide range of conditions (initial methyl radical concentration (c0), oxygen concentration, and total pressure) by using 193-nm laser flash photolysis of azomethane, coupled with absorption spectroscopy. The rate constant, k1, was measured under conditions of low c0/(O2), where the contribution to the overall methyl radical decay from the reaction CH3 + CH3O2 k3/ 2CH3O was shown to be small (<5%) and allowance was made for this contribution. Fall-off plots based on the Troe factorization method and obtained by using k1 values over the pressure range 31.9 - 490 torr of argon, combined with the low-pressure data of Selzer and Bayes, give limiting low- and high-pressure rate constants of k1 = (4.8 +/- 0.6) x 10 T cmW molecule S s and k1/sup infinity/ = (1.05 +/- 0.12) x 10 S cmT molecule s (95% confidence limits). An analysis of decay data obtained at high c0/(O2) gives k3 = (4.5 +/- 1.1) x 10 cmT molecule s . Dual monitoring of the CH3 decay at 216 nm and the CH3H2 buildup at 254 nm confirms the validity of the analysis, but the CH3O2 decay is toomore » rapid to be ascribed to CH3O2 + CH3O2 and it is suggested that CH3O2 and CH3O react with a rate constant of approx. 1 x 10 cmT molecule s . 35 references, 8 figures, 5 tables.« less