Reaction of CH3 radicals with OH at room temperature and pressure

Abstract

The kinetics of the reaction CH₃+ OH(+M)→ CH₃OH(+M) have been studied by pulse radiolysis combined with transient ultraviolet absorption spectrophotometry. The radical source reactions (2) F + CH₄→ HF + CH₃ and (3) F + H₂O → HF + OH were initiated by pulse radiolysis of Ar/SF₆/CH₄/H₂O mixtures at a total pressure of 1 atm and room temperature. The kinetics of methyl radicals were studied by monitoring the transient absorption signals at 216.4 nm. In the absence of water vapour the observed decay was simple second order in accordance with the self-reaction (4) CH₃+ CH₃(+M)→ C₂H₆(+M). The initial yield of F-atoms was derived from the observed maximum absorbance of CH₃ produced via reaction (2) using a consensus value of σ(CH₃). The relative yields of CH₃ and OH were controlled by varying the [H₂O]/[CH₄] concentration ratio and the ratio of the rate constants k₂/k₃= 3.2 ± 0.2 was derived from the observed variation in the yield of methyl radicals as a function of the [H₂O]/[CH₄] concentration ratio. The decay kinetics of CH₃ were studied as a function of the relative radical yields, G(OH)/G(CH₃). The kinetic features were analysed by computer modelling of reactions (2)–(4) combined with the reaction (1) CH₃+ OH(+M)→ CH₃OH(+M), (5) OH + OH(+M)→ H₂O₂(+M) and (6) OH + OH → O + H₂O. A representative set of experimental decay curves could be fitted within the signal-to-noise ratio after adjustment of the cross-combination rate constant to a value of k₁=(9.4 ± 1.3)× 10^–11 cm³ molecule^–1 s^–1.