Conformational distributions of helical perfluoroalkyl substances and impacts on stability

Mifkovic, Maleigh; Hoomissen, Daniel J. Van; Vyas, Shubham

doi:10.1002/jcc.26967

Cited by 11 publications

(6 citation statements)

References 23 publications

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“…To determine the effect of not searching for the global minimum, 20 PFASs were used to compare the calculated C–F BDEs from the lowest energy conformers versus those obtained from helical conformers (Scheme b). The helical conformation was chosen because the preferred state of a long-chain PFAS is where the C–F bonds are arranged in a helix . The helicity arises from a 17–20° twist in the C–C–C–C dihedral angle of the carbon backbone from the all-trans 180° observed in hydrocarbons (Figure a).…”

Section: Resultsmentioning

confidence: 99%

“…The helical conformation was chosen because the preferred state of a long-chain PFAS is where the C−F bonds are arranged in a helix. 55 The helicity arises from a 17− 20°twist in the C−C−C−C dihedral angle of the carbon backbone from the all-trans 180°observed in hydrocarbons (Figure 3a). The corresponding radical product geometry was obtained by deleting the relevant fluorine atom from the PFAS and then reoptimizing at the M06-2X/6-31+G(d) level of theory.…”

Section: ■ Computational Detailsmentioning

confidence: 99%

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High-Level Quantum Chemical Prediction of C–F Bond Dissociation Energies of Perfluoroalkyl Substances

Lorpaiboon,

2023

J. Phys. Chem. A

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In this study, 550 C−F bond dissociation energies (BDEs) of a variety of per-and polyfluoroalkyl substances (PFASs) obtained from high-level DLPNO-CCSD(T)/CBS calculations were used to assess the accuracy of contemporary density functional theory (DFT) and semiempirical methods. DLPNO-CCSD(T)/CBS gas phase C−F BDEs fall between 404.9−550.7 kJ mol −1 and M06-2X and ωB97M-V in conjunction with the aug-cc-pVTZ basis set predicted BDEs closest to the benchmark level with a mean absolute deviation (MAD) of 7.3 and 8.3 kJ mol −1 , respectively. It was observed that DFT prediction errors increase with the degree of fluorination and system size. As such, previous model chemistry recommendations based on smaller nonfluorinated systems may not be carried over to modeling the energetics of PFASs and related systems.

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Section: Resultsmentioning

confidence: 99%

Section: ■ Computational Detailsmentioning

confidence: 99%

High-Level Quantum Chemical Prediction of C–F Bond Dissociation Energies of Perfluoroalkyl Substances

Lorpaiboon,

2023

J. Phys. Chem. A

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“…The longer PFCAs adopt a helicity to the carbon backbone, which is increasingly pronounced as the chain length increases and more pronounced in the neutral than in the ions. The effects of helicity in PFCA have been explored elsewhere . Similar helicity in n -perfluoroalkanes has been attributed variously either to electrostatic interactions or to hyperconjugation. , The orientation of the carboxylic head group varies relative to the carbon backbone.…”

Section: Resultsmentioning

confidence: 99%

“…The effects of helicity in PFCA have been explored elsewhere. 36 Similar helicity in n-perfluoroalkanes has been attributed variously either to electrostatic interactions or to hyperconjugation. 37,38 The orientation of the carboxylic head group varies relative to the carbon backbone.…”

Section: ■ Introductionmentioning

confidence: 98%

Elementary Reactions Leading to Perfluoroalkyl Substance Degradation in an Ar⁺/e^– Plasma

et al. 2022

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The reactivities of three perfluoroalkyl carboxylic acids (PFCAs) (perfluoropropanoic acid (C 2 F 5 COOH, PFPA), perfluorobutanoic acid (C 3 F 7 COOH, PFBA), and perfluorooctanoic acid (C 7 F 15 COOH, PFOA)) in a thermal, weakly ionized, argon/electron plasma were investigated from 300 to 600 K using a Langmuir probe−flowing afterglow apparatus. The results are supported by density functional theory calculations of the energetics of PFCA, C n F 2n+1 COOH, from n = 1 to 7. PFPA and PFBA attach electrons at a substantial fraction of the calculated capture rate; PFOA likely attaches electrons with similarly high efficiency, but the low vapor pressure of PFOA resulted in only qualitative results. All three compounds attach electrons dissociatively via HF elimination. The "acidity channel" (i.e., formation of H + C n F 2n+1 COO − ), calculated to be slightly endothermic, is never observed even at higher temperatures where this channel would be energetically allowed. Attachment to perfluorooctanesulfonic acid (n-C 8 F 17 SO 3 H) is rapid, yielding the conjugate base n-C 8 F 17 SO 3 − as proton transfer to the electron is exothermic. At temperatures near 450 K (PFOA), 550 K (PFBA), or 600 K (PFPA), the parent neutrals thermally decompose as evidenced by abrupt changes in rate constants and branching ratios. PFPA and PFBA react with Ar + close to the calculated capture rate, and PFOA likely does as well. The reaction mechanism starts via charge transfer, which can then lead to a range of product ions. Reactions with Ar + yield fluorocarbon radicals, clarifying and supporting a previously proposed mechanism of PFCA degradation in an argon plasma.

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“…Unlike PFPrA, PFCAs with a chain length greater than C4 can exhibit a stable helical conformation, characterized by a torsional twist in the central F−C−C−F or C−C−C−C dihedral angle. 66 To minimize the complexities of performing conformational sampling, only the straight-chain conformers were considered in this study; however, the impact of conformational isomerism on the calculated properties reported here is currently underway and will be reported in a future study. Another chain length effect is the "1,2-F atom rearrangement" that could occur in perfluoroalkyl radicals resulting from cleavage of the carboxyl group.…”

Section: Mechanistic Insights For Pfasmentioning

confidence: 99%

Degradation and Defluorination of Per- and Polyfluoroalkyl Substances by Direct Photolysis at 222 nm

et al. 2023

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The susceptibility of 19 representative per- and polyfluoroalkyl substances (PFAS) to direct photolysis and defluorination under far-UVC 222 nm irradiation was investigated. Enhanced photolysis occurred for perfluorocarboxylic acids (PFCAs), fluorotelomer unsaturated carboxylic acids (FTUCAs), and GenX, compared to that at conventional 254 nm irradiation on a similar fluence basis, while other PFAS showed minimal decay. For degradable PFAS, up to 81% of parent compound decay (photolysis rate constant (k 222 nm) = 8.19–34.76 L·Einstein–1; quantum yield (Φ222 nm) = 0.031–0.158) and up to 31% of defluorination were achieved within 4 h, and the major transformation products were shorter-chain PFCAs. Solution pH, dissolved oxygen, carbonate, phosphate, chloride, and humic acids had mild impacts, while nitrate significantly affected PFAS photolysis/defluorination at 222 nm. Decarboxylation is a crucial step of photolytic decay. The slower degradation of short-chain PFCAs than long-chain ones is related to molar absorptivity and may also be influenced by chain-length dependent structural factors, such as differences in pK a, conformation, and perfluoroalkyl radical stability. Meanwhile, theoretical calculations indicated that the widely proposed HF elimination from the alcohol intermediate (C n F2n+1OH) of PFCA is an unlikely degradation pathway due to high activation barriers. These new findings are useful for further development of far-UVC technology for PFAS in water treatment.

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Conformational distributions of helical perfluoroalkyl substances and impacts on stability

Cited by 11 publications

References 23 publications

High-Level Quantum Chemical Prediction of C–F Bond Dissociation Energies of Perfluoroalkyl Substances

High-Level Quantum Chemical Prediction of C–F Bond Dissociation Energies of Perfluoroalkyl Substances

Elementary Reactions Leading to Perfluoroalkyl Substance Degradation in an Ar⁺/e^– Plasma

Degradation and Defluorination of Per- and Polyfluoroalkyl Substances by Direct Photolysis at 222 nm

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Conformational distributions of helical perfluoroalkyl substances and impacts on stability

Cited by 11 publications

References 23 publications

High-Level Quantum Chemical Prediction of C–F Bond Dissociation Energies of Perfluoroalkyl Substances

High-Level Quantum Chemical Prediction of C–F Bond Dissociation Energies of Perfluoroalkyl Substances

Elementary Reactions Leading to Perfluoroalkyl Substance Degradation in an Ar+/e– Plasma

Degradation and Defluorination of Per- and Polyfluoroalkyl Substances by Direct Photolysis at 222 nm

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Elementary Reactions Leading to Perfluoroalkyl Substance Degradation in an Ar⁺/e^– Plasma