Mechanistic study of the reaction of CH 2 F 2 with Cl atoms in the absence and presence of CH 4 or C 2 H 6: Decomposition of CHF 2 OH and fate of the CHF 2 O radical
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To assess the atmospheric fate of fluorinated compounds, chamber experiments were performed with Fourier transform infrared spectroscopy investigating the products of difluoromethane, CH 2 F 2 , at 296 ± 2 K. The reactions were initiated by reaction of CH 2 F 2 with Cl atoms in the absence and presence of CH 4 or C 2 H 6 in air or O 2 . No evidence of formation of the fluorinated alcohol, CHF 2 OH, from the reactions of the CHF 2 O 2 radical with either CH 3 O 2 or CH 3 CH 2 O 2 was observed. However, evidence of an alkoxy radical pathway was observed to form CHF 2 OH. The alkoxy radical, CHF 2 O, abstracts a hydrogen atom from CH 2 F 2 (with reaction mixtures of high initial CH 2 F 2 concentrations) to give the alcohol CHF 2 OH that in turn decomposes with a rate coefficient of k(CHF 2 OH) = (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 CHF 2 OH 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 CHF 2 OH. The competition between the reaction of CHF 2 O radicals with O 2 and with CH 2 F 2 was investigated and an experimental rate coefficient ratio of 0.57 ± 0.08 of reaction with O 2 over reaction with CH 2 F 2 was determined. Ab initio calculations support a larger reaction barrier for the O 2 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 CH 2 F 2 is expected to be C(O)F 2 formed by the reaction of CHF 2 O with O 2 .
|Enheter & grupper|
Ämnesklassifikation (UKÄ) – OBLIGATORISK
|Tidskrift||Physical Chemistry Chemical Physics|
|Status||Published - 2019|
|Peer review utförd||Ja|