Mechanistic study of the reaction of CH2F2 with Cl atoms in the absence and presence of CH4 or C2H6: decomposition of CHF2OH and fate of the CHF2O radical

Abstract

To assess the atmospheric fate of fluorinated compounds, chamber experiments were performed with Fourier transform infrared spectroscopy investigating the products of difluoromethane, CH₂F₂, at 296 ± 2 K. The reactions were initiated by reaction of CH₂F₂ with Cl atoms in the absence and presence of CH₄ or C₂H₆ in air or O₂. No evidence of formation of the fluorinated alcohol, CHF₂OH, from the reactions of the CHF₂O₂ radical with either CH₃O₂ or CH₃CH₂O₂ was observed. However, evidence of an alkoxy radical pathway was observed to form CHF₂OH. The alkoxy radical, CHF₂O, abstracts a hydrogen atom from CH₂F₂ (with reaction mixtures of high initial CH₂F₂ concentrations) to give the alcohol CHF₂OH that in turn decomposes with a rate coefficient of k(CHF₂OH) = (1.68 × 10⁻³ ± 0.19 × 10⁻³) s⁻¹, giving a half-life of the alcohol of (412 ± 48) s. Theoretical calculations indicate that the CHF₂OH decomposition is unlikely to be a unimolecular process, and we instead propose that it is catalyzed by –OH groups present in molecules, or on particles or surfaces. HC(O)F is formed in a yield indistinguishable from 100% from the decomposition of CHF₂OH. The competition between the reaction of CHF₂O radicals with O₂ and with CH₂F₂ was investigated and an experimental rate coefficient ratio of 0.57 ± 0.08 of reaction with O₂ over reaction with CH₂F₂ was determined. Ab initio calculations support a larger reaction barrier for the O₂ reaction by 0.5 kcal mol⁻¹, with transition state theory predicting a rate coefficient ratio of 0.35, in reasonable agreement with experiment. The primary product of the atmospheric degradation of CH₂F₂ is expected to be C(O)F₂ formed by the reaction of CHF₂O with O₂.