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ABSTRACT Rate coefficients, k(T), for the gas‐phase Cl atom reaction with hexamethyldisiloxane ((CH3)3SiOSi(CH3)3, L2), k1; octamethyltrisiloxane ([(CH3)3SiO]2Si(CH3)2, L3), k2; decamethyltetrasiloxane ((CH3)3SiO[Si(CH3)2O]2Si(CH3)3, L4, k3; dodecamethylpentasiloxane ((CH3)3SiO[Si(CH3)2O]3Si(CH3)3, L5, k4; hexamethylcyclotrisiloxane ([‐Si(CH3)2O‐]3, D3), k5; octamethylcyclotetrasiloxane ([‐Si(CH3)2O‐]4, D4), k6; decamethylcyclopentasiloxane ([‐Si(CH3)2O‐]5, D5, k7), and dodecamethylcyclohexasiloxane ([‐Si(CH3)2O‐]6, D6, k8) were measured over a range of temperature (273–363 K) using a pulsed laser photolysis (PLP) – resonance fluorescence (RF) technique. The obtained k(296 K) and Arrhenius expressions with 2σ uncertainties including estimated systematic errors are (in units of 10−10 cm3 molecule−1 s−1): L2: k1(296 K) = (1.58 ± 0.07) k1(273–363 K) = (1.36 ± 0.10) exp((43 ± 249)/T) L3: k2(296 K) = (1.93 ± 0.09) k2(273–353 K) = (1.95 ± 0.18) exp((1 ± 632)/T) L4: k3(296 K) = (2.36 ± 0.12) k3(273–353 K) = (2.09 ± 0.16) exp((43± 522)/T) L5: k4(296 K) = (2.84 ± 0.19) k4(296–353 K) = (2.70 ± 0.18) exp((15 ± 490)/T) D3: k5(296 K) = (0.588 ± 0.028) k5(273–353 K) = (0.47 ± 0.03) exp((78 ± 470)/T) D4: k6(296 K) = (1.13 ± 0.06) k6(273–353 K) = (1.14 ± 0.07) exp((4 ± 432)/T) D5: k7(296 K) = (1.72 ± 0.09) k7(273–353 K) = (1.60 ± 0.10) exp((22 ± 432)/T) D6: k8(296 K) = (2.16 ± 0.14) k8(296–353 K) = (1.73 ± 0.14) exp((71 ± 562)/T) The cyclic permethyl siloxanes (cyclic PMS) were found to be less reactive than the analogous linear permethyl siloxane (linear PMS) with an equal number of CH3‐ groups. Both linear and cyclic compounds show a linear relationship between the measured rate coefficient and the number of CH3‐ groups in the molecule. A structure–activity relationship (SAR) is presented that reproduces the experimental data to within ∼10% at all temperatures. For [Cl] ≈ 104 atom cm−3, an approximate free troposphere abundance, the PMS loss due to Cl atom reaction leads to relatively short estimated lifetimes of 7, 6, 5, 4, 20, 10, 7, and 5 days for L2, L3, L4, L5, D3, D4, D5, and D6, respectively. Therefore, the PMSs included in this study are classified as atmospherically very short‐lived substances and Cl atom reaction represents a significant loss process. [ABSTRACT FROM AUTHOR]

Radiative efficiency (RE) is a key climate metric used in the evaluation of a greenhouse gases global warming potential. However, experimental methods are commonly limited to measurements in the spectral region greater than ∼500 cm−1. In this study, density functional theory with the wB97X-D/def2TZVPPD level of theory was employed to calculate the infrared absorption spectra and the contribution of the 0–500 cm−1 spectral region to the total RE for the nearly 800 greenhouse gases, 20 different classes of primarily halogenated compounds, included in the World Meteorological Organization (WMO) Scientific Assessment of Ozone Depletion Annex [1]. In general, the results indicate that the contribution of the low-frequency regime to the total overall RE is not significant, < 2%, with a few exceptions, e.g. several compounds in the hydrocarbon class. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 CF 2 equivalents yielding shorter-chain homologues, CO 2 and F - . This transformation is thought to be initiated by single electron transfer (SET) from the PFCA to the activate oxidant, SO 4 •- . 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 SO 4 •- 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 × 10 3 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 SO 4 •- protonation might enhance PFCA transformation kinetics. We found that when calculations include explicit water molecules, direct SO 4 •- protonation does not occur.