Kinetics of the reactions of CH3 with O(3P) and O2 at 295 K

The reactions of CH3 radicals with O(3P) and O2 have been studied at 295 K in a gas flow reactor sampled by a mass spectrometer. For the reaction between CH3 and O, conditions were such that [O] » [CH3] and the methyl radicals decayed under pseudo‐first‐order conditions giving a rate coefficient of (1.14 ± 0.29) × 10−10 cm3/s. The reaction between CH3 and O2 was studied in separate experiments in which CH3 decayed under pseudo‐first‐order conditions. In this case, the rate coefficient obtained increased with increasing concentration of the helium carrier gas. This was varied over the range of 2.5–25 × 1016 cm−3, resulting in values for the apparent two‐body rate coefficient ranging from 1 × 10−14 to 5.2 × 10−14 cm3/s. No evidence was found for the production of HCHO by a direct two‐body process involving CH3 + O2, and an upper limit of 3 × 10−16 cm3/s was placed on the rate coefficient for this reaction. The experimental results for the apparent two‐body rate coefficient exhibit the curvature one would expect for an association reaction in the fall‐off region. Calculations used to extrapolate these measurements to the low‐pressure limit yield a value for k0 of (3.4 ± 1.1) × 10−31 cm6/s, which is more than a factor of 2 higher than previous estimates.