The reaction of fluorine atoms with methanol: yield of CH3O/CH2OH and rate constant of the reactions CH3O + CH3O and CH3O + HO2

Abstract

Xenondifluoride, XeF₂, has been photolysed in the presence of methanol, CH₃OH. Two reaction pathways are possible: F + CH₃OH → CH₂OH + HF and F + CH₃OH → CH₃O + HF. Both products, CH₂OH and CH₃O, will be converted to HO₂ in the presence of O₂. The rate constants for the reaction of both radicals with O₂ differ by more than 3 orders of magnitude, which allows an unequivocal distinction between the two reactions when measuring HO₂ concentrations in the presence of different O₂ concentrations. The following yields have then been determined from time-resolved HO₂ profiles: ϕ_CH2OH = (0.497 ± 0.013) and ϕ_CH3O = (0.503 ± 0.013). Experiments under low O₂ concentrations lead to reaction mixtures containing nearly equal amounts of HO₂ (converted from the first reaction) and CH₃O (from the second reaction). The subsequent HO₂ decays are very sensitive to the rate constants of the reaction between these two radicals and the following rate constants have been obtained: k(CH₃O + CH₃O) = (7.0 ± 1.4) × 10⁻¹¹ cm³ s⁻¹ and k(CH₃O + HO₂) = (1.1 ± 0.2) × 10⁻¹⁰ cm³ s⁻¹. The latter reaction has also been theoretically investigated on the CCSD(T)//M06-2X/aug-cc-pVTZ level of theory and CH₃OH + O₂ have been identified as the main products. Using μVTST, a virtually pressure independent rate constant of k(CH₃O + HO₂) = 4.7 × 10⁻¹¹ cm³ s⁻¹ has been obtained, in good agreement with the experiment.