| Abstract: | 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. |
