ESR study of the 1,2-migration of F atoms in polyfluorinated radicals

Kovtonyuk, V. N.; Kobrina, L. S.; Volodin, Alexander M.

doi:10.1007/bf00696965

Cited by 5 publications

(5 citation statements)

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“…Reports of fluorine atom shifts in carbon radical systems are rare, but such rearrangements are apparently feasible. − In the accompanying paper, UB3LYP/6-311+G(2df,2p)//UB3LYP/6-31G(d) calculations provided an estimated barrier ( E ⧧ ) of 29.2 kcal/mol for this rearrangement . On the basis of these calculations, we believe that the unimolecular 1,2-fluorine atom shift of CHF 2 CF 2 · should be competitive with its second, bimolecular H atom abstraction.…”

Section: Resultsmentioning

confidence: 88%

Pyrolyses of Chlorodifluoromethane and Trifluoromethane in the Presence of Hydrogen. Mechanism and Optimization of Reaction Conditions

et al. 2001

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When CHClF 2 and CHF 3 are subjected to high-temperature, gas-phase flow pyrolysis in the presence of H 2 , they are converted, via a free radical chain mechanism, to CH 2 F 2 , CHF 2 CHF 2 , and CF 3 CH 2 F in good yield. Optimal conditions for pyrolysis of CHClF 2 involve a high conversion (92%) at 650 °C with an observed yield of products ) 18, 17, and 28%, respectively, whereas optimal conditions for CHF 3 involve a low conversion (24%) at 775 °C, but a higher yield of products (26, 6, and 39%, respectively).

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Section: Resultsmentioning

confidence: 88%

Pyrolyses of Chlorodifluoromethane and Trifluoromethane in the Presence of Hydrogen. Mechanism and Optimization of Reaction Conditions

et al. 2001

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“…60 Furthermore, F atom rearrangements in polyfluorinated cyclohexadienyl radicals generated by photochemical decomposition or heating of perfluoro-p-xylene and pentafluorobenzoyl peroxide was observed via electron paramagnetic resonance (EPR). 55,57,61 An early theoretical study by Fossey and Nedelec found the activation barriers for 1,2-F and 1,2-H shifts to be 107 and 88 kcal/mol respectively, indicating that these processes would not readily occur. 62 Surprisingly, further studies on the 1,2migration of hydrogen and fluorine in • CF 2 −CHFT and • CF 2 −CFHT radicals, where T is a tritium atom, found that 1,2-F atom rearrangements had lower barriers than 1,2-H atom shifts.…”

Section: Introductionmentioning

confidence: 99%

“…Conversely, in the early 1970s, 1,2-migrations of a fluorine atom were thought to be kinetically impossible, although rearrangements of a fluorine atom in carbenes and radical ions has been well documented . The least investigated of the rearrangements, 1,2-fluorine atom shifts in radicals, have since been hypothesized or directly measured in a diverse consortium of chemical systems, both in the gaseous and the condensed phases. − For example, 1,2-fluorine shifts were observed in the condensed phase via NMR for the tertiary fluorine adjacent to a tertiary perfluorocarbanion . Furthermore, F atom rearrangements in polyfluorinated cyclohexadienyl radicals generated by photochemical decomposition or heating of perfluoro- p -xylene and pentafluorobenzoyl peroxide was observed via electron paramagnetic resonance (EPR). ,, An early theoretical study by Fossey and Nedelec found the activation barriers for 1,2-F and 1,2-H shifts to be 107 and 88 kcal/mol respectively, indicating that these processes would not readily occur .…”

Section: Introductionmentioning

confidence: 99%

“…The least investigated of the rearrangements, 1,2-fluorine atom shifts in radicals, have since been hypothesized or directly measured in a diverse consortium of chemical systems, both in the gaseous and the condensed phases. − For example, 1,2-fluorine shifts were observed in the condensed phase via NMR for the tertiary fluorine adjacent to a tertiary perfluorocarbanion . Furthermore, F atom rearrangements in polyfluorinated cyclohexadienyl radicals generated by photochemical decomposition or heating of perfluoro- p -xylene and pentafluorobenzoyl peroxide was observed via electron paramagnetic resonance (EPR). ,, An early theoretical study by Fossey and Nedelec found the activation barriers for 1,2-F and 1,2-H shifts to be 107 and 88 kcal/mol respectively, indicating that these processes would not readily occur . Surprisingly, further studies on the 1,2-migration of hydrogen and fluorine in • CF 2 –CHFT and • CF 2 –CFHT radicals, where T is a tritium atom, found that 1,2-F atom rearrangements had lower barriers than 1,2-H atom shifts .…”

Section: Introductionmentioning

confidence: 99%

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1,2-Fluorine Radical Rearrangements: Isomerization Events in Perfluorinated Radicals

Hoomissen

Vyas

2017

J. Phys. Chem. A

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Devising effective degradation technologies for perfluoroalkyl substances (PFASs) is an active area of research, where the molecular mechanisms involving both oxidative and reductive pathways are still elusive. One commonly neglected pathway in PFAS degradation is fluorine atom migration in perfluoroalkyl radicals, which was largely assumed to be implausible because of the high C-F bond strength. Using density functional theory calculations, it was demonstrated that 1,2-F atom migrations are thermodynamically favored when the fluorine atom migrated from a less branched carbon center to a more branched carbon center. Activation barriers for these rearrangements were within 19-29 kcal/mol, which are possible to easily overcome at elevated temperatures or in photochemically activated species in the gas or aqueous phase. It was also found that the activation barriers for the 1,2-F atom migration are lowered as much as by 10 kcal/mol when common oxidative degradation products such as HF assisted the rearrangements or if the resulting radical center was stabilized by vicinal π-bonds. Natural bond orbital analyses showed that fluorine moves as a radical in a noncharge-separated state. These findings add an important reaction to the existing knowledge of mechanisms for PFAS degradation and highlights the fact that 1,2-F atom shifts may be a small channel for isomerization of these compounds, but upon availability of mineralization products, this isomerization process could become more prominent.

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“…Perhaps the most interesting if not controversial step in the proposed mechanism is the fluorine radical shift in step 7. Reports of fluorine atom shifts in carbon radical systems are rare, but such rearrangements nevertheless appear to be relatively facile, much more so than the analogous H atom shifts. − In 1988, Kotaka examined the specific F-shift of eq 7 by semiempirical calculations, with his results being given in Table . Our own DFT calculations, also given in Table , were consistent with Kotaka's INDO calculations, and they indicated an E ⧧ of only 29.2 kcal/mol for this unimolecular 1,2-F atom shift.…”

Section: Resultsmentioning

confidence: 99%

Thermal Isomerization of 1,1,2,2-Tetrafluoroethane (FC-134) to 1,1,1,2-Tetrafluoroethane (FC-134a) in the Presence of Hydrogen

Romelaer

Baker

2001

J. Am. Chem. Soc.

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The mechanism of the high-temperature, gas-phase isomerization of 1,1,2,2-tetrafluoroethane (FC-134) to 1,1,1,2-tetrafluoroethane (FC-134a) in the presence of H 2 has been explored both experimentally and computationally. Studies of the impact of temperature, H 2 /FC-134 ratio, and contact time on conversion and yield, as well as a study of deuterium incorporation when D 2 was used in place of H 2 , led to the conclusion that a free radical chain mechanism involving rearrangement of CHF 2 CF 2 ‚ to CF 3 CHF ‚ is involved.

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ESR study of the 1,2-migration of F atoms in polyfluorinated radicals

Cited by 5 publications

References 5 publications

Pyrolyses of Chlorodifluoromethane and Trifluoromethane in the Presence of Hydrogen. Mechanism and Optimization of Reaction Conditions

Pyrolyses of Chlorodifluoromethane and Trifluoromethane in the Presence of Hydrogen. Mechanism and Optimization of Reaction Conditions

1,2-Fluorine Radical Rearrangements: Isomerization Events in Perfluorinated Radicals

Thermal Isomerization of 1,1,2,2-Tetrafluoroethane (FC-134) to 1,1,1,2-Tetrafluoroethane (FC-134a) in the Presence of Hydrogen

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