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

Abstract

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.

Sequential C–F bond functionalizations of trifluoroacetamides and acetates via spin-center shifts

“…S4). 1,2-F radical rearrangement has been posited during the isomerization of perfluorinated radicals (69). However, in our case, density functional theory (DFT) calculations revealed that such a 1,2-F migration of Int-2 requires a high activation free energy (DG ‡ = 23.9 kcal/mol) and is endergonic by 12.1 kcal/ mol (fig.…”

Mediator and Additive Free Trifluoromethyl‐Fluorination of Terminal Alkenes by Persistent Perfluoroalkyl Radical

“…The mechanism of this unprecedent intermolecular fluorine atom transfer resulted from the C(sp 3 )–F bond cleavage is not yet fully understood, and it will be a subject of our further study. We cannot completely exclude possible formation of HF traces under the reaction conditions, which can potentially assist the fluorine transfer step . However, the observation of only compound C as a product of the fluorine elimination from PPFR is a clear evidence that fluorine anion is unlikely to be involved.…”

Conformational distributions of helical perfluoroalkyl substances and impacts on stability

“… 7,15 Therefore, we calculated the PES scans of C4 to C6 RPFAs to evaluate their helical conformations and compare them against their PFA analogues. Probing helical conformations of RPFAs is more complex than PFAs, since RPFAs may undergo 1,2‐F atom shifts, causing the formation of more isomers of RPFAs 26 . For example, C4 RPFAs can have the chemical structures and , which are referred as α‐C4RPFA and β‐C4RPFA (Figure S1, inset).…”

“…Probing helical conformations of RPFAs is more complex than PFAs, since RPFAs may undergo 1,2-F atom shifts, causing the formation of more isomers of RPFAs. 26 The most notable result of the PES scan of α-C4RPFA…”