Ultra-short chain fluorocarboxylates exhibit wide ranging reactivity with hydrated electrons.

Recent reports demonstrate that technologies generating hydrated electrons (eaq-; e.g., UV-sulfite) are a promising strategy for destruction of per- and polyfluoroalkyl substances, but fundamental rate constants are lacking. This work examines the kinetics and mechanisms of eaq- reactions with ultra-short chain (C2-C4) fluorocarboxylates using experimental and theoretical approaches. Laser flash photolysis (LFP) was used to measure bimolecular rate constants (k2; M-1 s-1) for eaq- reactions with thirteen per-, and for the first time, polyfluorinated carboxylate structures. The measured k2 values varied widely from 5.26 × 106 to 1.30 × 108 M-1s-1, a large range considering the minor structural changes among the target compounds. Molecular descriptors calculated using density functional theory did not reveal correlation between k2 values and individual descriptors when considering the whole dataset, however, semiquantitative correlation manifests when grouping by similar possible initial reduction event such as electron attachment at the α-carbon versus ß- or γ-carbons along the backbone. From this, it is postulated that fluorocarboxylate reduction by eaq- occurs via divergent mechanisms with the possibility of non-degradative pathways being prominent. These mechanistic insights provide rationale for contradictory trends between LFP-derived k2 values and apparent degradation rates recently reported in UV-sulfite constant irradiation treatment experiments.

Conformational distributions of helical perfluoroalkyl substances and impacts on stability.

Per- and polyfluoroalkyl substances (PFAS) have been widely used the past 70 years in numerous applications due to their chemical and thermal stability. Due to their robust stability, they are environmentally recalcitrant which made them one of the most persistent environmental contaminants. In addition to strong CF bond strength, oleophobicity, hydrophobicity, and high reduction-oxidation (redox) potential of PFAS has led to their inefficient degradation by traditional means. A characteristic of their structure is also their preference to adopt helical conformations along the carbon backbone, contrary to their hydrocarbon analogues. This work investigates the helical nature of perfluoroalkanes through their conformational distributions, especially as a benchmark for determining the impact of polar head groups, heteroatoms, and radical center on helical conformations. Since structure governs reactivity and molecular properties, it is important to assess if minor chemical perturbations in the structure will lead to changes in the conformations. Based on density functional theory calculations and comprehensive conformational distributions, it was concluded that the helicity is a local structural property which changes significantly with the presence of heteroatoms in the perfluoroalkyl chain as well as with the presence of radical centers.

Role of Explicit Hydration in Predicting the Aqueous Standard Reduction Potential of Sulfate Radical Anion by DFT and Insight into the Influence of pH on the Reduction Potential.

Sulfate radical anion (SO4•-) is a potent oxidant capable of destroying recalcitrant environmental contaminants such as perfluoroalkyl carboxylic acids. In addition, it is thought to participate in important atmospheric reactions. Its standard reduction potential (E°) is fundamental to its reactivity. Using theoretical methods to accurately predict the aqueous phase E° requires solvation with explicit water molecules. Herein, using density functional theory, we calculated the aqueous E° of SO4•- and evaluated sensitivity to explicit water count. The E° increased considerably with more waters until ca. 24 were included, after which change in E° was small. When a proton was added to these systems, the E° was similar regardless of the explicit water count and this value was similar to the E° for systems with a large number of explicit waters but no proton. This result agrees with literature evidence that the E° is pH independent. Natural Bond Orbital natural population analysis indicated that in the case of both SO42- and SO4•-, considerable charge was donated from the SO4 center to the explicit solvation shells.

Role of pH in the Transformation of Perfluoroalkyl Carboxylic Acids by Activated Persulfate: Implications from the Determination of Absolute Electron-Transfer Rates and Chemical Computations.

Perfluoroalkyl carboxylic acids (PFCAs) are ubiquitous contaminants known for their bioaccumulation, toxicological harm, and resistance to degradation. Remediating PFCAs in water is an ongoing challenge with existing technologies being insufficient or requiring additional disposal. An emergent approach is using activated persulfate, which degrades PFCAs through sequential scission of CF2 equivalents yielding shorter-chain homologues, CO2 and F-. This transformation is thought to be initiated by single electron transfer (SET) from the PFCA to the activate oxidant, SO4•-. A pronounced pH effect has been observed for thermally activated persulfate PFCA transformation. To evaluate the role of pH during SET, we directly determined absolute rate constants for perfluorobutanoic acid and trifluoroacetic acid oxidation by SO4•- in the pH range of 0.5-4.0 using laser flash photolysis. The average of the rate constants for both substrates across all pH values was 9 ± 2 × 103 M-1 s-1 (±2σ), implying that acid catalysis of thermal persulfate activation may be the primary culprit of the observed pH effect, instead of pH influencing the SET step. In addition, density functional theory was used to investigate if SO4•-protonation might enhance PFCA transformation kinetics. We found that when calculations include explicit water molecules, direct SO4•- protonation does not occur.

1,2-H Atom Rearrangements in Benzyloxyl Radicals.

The rate constants for solvent-assisted 1,2-H atom rearrangements in para-substituted benzyloxyl radicals were studied with density functional theory. The rate of the radical rearrangement, calculated through transition state theory with Eckhart tunneling corrections, was shown to be drastically impacted by the presence of both implicit and explicit solvent molecules, with a quantitative agreement with laser flash photolysis studies for a variety of electron-donating and -withdrawing substituents. The rate of rearrangement was found to be correlated to the distance between the rearranging hydrogen atom and the α-carbon in the transition state, which could be modified through the para substituent and the type of assisting solvent molecule (e.g., water, ethanol, methanol, acetic acid, or a mixture of the latter). Natural bond orbital analysis showed that the rearrangement does not proceed through a hydrogen radical but through a quasi-proton exchange and charge transfer between the benzyl carbon and the adjacent oxygen atom. Energetic and spin population results indicated that electron-withdrawing groups induce faster rearrangement kinetics. Understanding 1,2-H atom shifts in benzyloxyl radicals are essential for tuning the rate of superoxide production in aqueous systems, as the resonance-stabilized carbon radical produced from the rearrangement can bind oxygen and decompose to produce superoxide radical anion, an important reactive intermediate in environmental and biological systems.

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

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.

Impact of Conjugation and Hyperconjugation on the Radical Stability of Allylic and Benzylic Systems: A Theoretical Study.

Resonantly stabilized radicals are some of the most investigated chemical species due to their preferential formation in a wide variety of chemical environments. Density functional theory and post-Hartree-Fock calculations were utilized to elucidate the chemical interactions that contribute to the stability of two ubiquitous, resonantly stabilized radicals, allyl and benzyl radicals. The relative stability of these radical species was quantified through bond dissociation energies and relative rotational energy barriers, with a difference of only 0.1 kcal/mol. To clarify and contextualize the energetic results, natural bond orbitals were used to evaluate the atomic spin density distribution in the given molecules. The benzyl radical was found to be ∼3 kcal/mol less stable than the allyl radical, which was attributed to the inability to efficiently delocalize the spin on a phenyl unit, starkly contrary to general chemistry knowledge. Increasing the degree of π-conjugation and hyperconjugation was shown to benefit allyl radicals to a greater degree than benzyl radicals, again due to more efficient radical delocalization in allyl radicals. This work highlights that more resonance structures do not always lead to a more stabilized radical species, and provides fundamental knowledge about how conjugation and hyperconjugation impact the stabilization of nonbonding electrons in these systems.