The reaction of CH3Â +Â O2: experimental determination of the rate coefficients for the product channels at high temperatures

The reaction of methyl radicals (CH3) with molecular oxygen (O2) has been investigated in high-temperature shock tube experiments. The overall rate coefficient, k1 = k1a + k1b, and individual rate coefficients for the two high-temperature product channels, (1a) producing CH3O + O and (1b) producing CH2O + OH, were determined using ultra-lean mixtures of CH3I and O2 in Ar/He. Narrow-linewidth UV laser absorption at 306.7 nm was used to measure OH concentrations, for which the normalized rise time is sensitive to the overall rate coefficient k1 but relatively insensitive to the branching ratio of the individual channels and to secondary reactions. Atomic resonance absorption spectroscopy measurements of O-atoms were used for a direct measurement of channel (1a). Through the combination of measurements using the two different diagnostics, rate coefficient expressions for both channels were determined. Over the temperature range 1590-2430 K, k1a = 6.08 × 107T1.54 exp (−14005/T) cm3 mol−1 s−1 and k1b = 68.6 T2.86 exp (−4916/T) cm3 mol−1 s−1. The overall rate coefficient is in close agreement with a recent ab initio calculation and one other shock tube study, while comparison of k1a and k1b to these and other experimental studies yields mixed results. In contrast to one recent experimental study, reaction (1b) is found to be the dominant channel over the entire experimental temperature range.