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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

Østerstrøm, Freja F.; Andersen, Christina LU and Da Silva, Gabriel (2019) In Physical Chemistry Chemical Physics 21(18). p.9376-9383
Abstract

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 ... (More)

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 .

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organization
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type
Contribution to journal
publication status
published
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in
Physical Chemistry Chemical Physics
volume
21
issue
18
pages
8 pages
publisher
Royal Society of Chemistry
external identifiers
  • scopus:85065566136
ISSN
1463-9076
DOI
10.1039/c8cp06425c
language
English
LU publication?
yes
id
a37e44a3-358e-45eb-a2b5-1d2ccbf43ba3
date added to LUP
2019-06-04 11:26:23
date last changed
2019-06-19 04:14:43
@article{a37e44a3-358e-45eb-a2b5-1d2ccbf43ba3,
  abstract     = {<p>                             To assess the atmospheric fate of fluorinated compounds, chamber experiments were performed with Fourier transform infrared spectroscopy investigating the products of difluoromethane, CH                             <sub>2</sub>                             F                             <sub>2</sub>                             , at 296 ± 2 K. The reactions were initiated by reaction of CH                             <sub>2</sub>                             F                             <sub>2</sub>                              with Cl atoms in the absence and presence of CH                             <sub>4</sub>                              or C                             <sub>2</sub>                             H                             <sub>6</sub>                              in air or O                             <sub>2</sub>                             . No evidence of formation of the fluorinated alcohol, CHF                             <sub>2</sub>                             OH, from the reactions of the CHF                             <sub>2</sub>                             O                             <sub>2</sub>                              radical with either CH                             <sub>3</sub>                             O                             <sub>2</sub>                              or CH                             <sub>3</sub>                             CH                             <sub>2</sub>                             O                             <sub>2</sub>                              was observed. However, evidence of an alkoxy radical pathway was observed to form CHF                             <sub>2</sub>                             OH. The alkoxy radical, CHF                             <sub>2</sub>                             O, abstracts a hydrogen atom from CH                             <sub>2</sub>                             F                             <sub>2</sub>                              (with reaction mixtures of high initial CH                             <sub>2</sub>                             F                             <sub>2</sub>                              concentrations) to give the alcohol CHF                             <sub>2</sub>                             OH that in turn decomposes with a rate coefficient of k(CHF                             <sub>2</sub>                             OH) = (1.68 × 10                             <sup>-3</sup>                              ± 0.19 × 10                             <sup>-3</sup>                             ) s                             <sup>-1</sup>                             , giving a half-life of the alcohol of (412 ± 48) s. Theoretical calculations indicate that the CHF                             <sub>2</sub>                             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                             <sub>2</sub>                             OH. The competition between the reaction of CHF                             <sub>2</sub>                             O radicals with O                             <sub>2</sub>                              and with CH                             <sub>2</sub>                             F                             <sub>2</sub>                              was investigated and an experimental rate coefficient ratio of 0.57 ± 0.08 of reaction with O                             <sub>2</sub>                              over reaction with CH                             <sub>2</sub>                             F                             <sub>2</sub>                              was determined. Ab initio calculations support a larger reaction barrier for the O                             <sub>2</sub>                              reaction by 0.5 kcal mol                             <sup>-1</sup>                             , 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                             <sub>2</sub>                             F                             <sub>2</sub>                              is expected to be C(O)F                             <sub>2</sub>                              formed by the reaction of CHF                             <sub>2</sub>                             O with O                             <sub>2</sub>                             .                         </p>},
  author       = {Østerstrøm, Freja F. and Andersen, Christina and Da Silva, Gabriel},
  issn         = {1463-9076},
  language     = {eng},
  number       = {18},
  pages        = {9376--9383},
  publisher    = {Royal Society of Chemistry},
  series       = {Physical Chemistry Chemical Physics},
  title        = {Mechanistic study of the reaction of CH                         <sub>2</sub>                         F                         <sub>2</sub>                          with Cl atoms in the absence and presence of CH                         <sub>4</sub>                          or C                         <sub>2</sub>                         H                         <sub>6</sub> : Decomposition of CHF                         <sub>2</sub>                         OH and fate of the CHF                         <sub>2</sub>                         O radical},
  url          = {http://dx.doi.org/10.1039/c8cp06425c},
  volume       = {21},
  year         = {2019},
}