Reactions of a distonic peroxyl radical anion influenced by SOMO-HOMO conversion: an example of anion-directed channel switching.

In free radicals the singly occupied molecular orbital (SOMO) typically has the highest energy. Recent examples of distonic radical anions were found, however, to disobey the usual orbital configuration, with the singly occupied molecular orbital buried energetically underneath doubly occupied orbitals. This unusual ordering of electrons, which contradicts the aufbau principle, has been characterized as SOMO-HOMO orbital conversion and is expected to perturb radical anion reactivity by branching toward anion-driven over radical-driven processes. Here, we use ion trap mass spectrometry and ab initio calculations to demonstrate that SOMO-HOMO orbital conversion influences the reactivity of a distonic peroxyl radical anion. Experimentally, we generated a distonic radical anion of ß-hydroxy glutaric acid, ˙CH2CH(OH)CH2C(O)O-, and investigated its subsequent reaction with O2 in the gas phase. Theoretical calculations predict that reactions proceed through five isomeric C4H6O5˙- intermediates, two of which exhibit SOMO-HOMO conversion. The detected product ions, corresponding to loss of ˙OH + CO2, ˙OH + HCHO, HO2˙, and HO2˙ + CO2 from the peroxyl radical, can all be reconciled by the proposed reaction mechanism. Finally, we compare the oxygen recombination reaction of the distonic radical ion to the corresponding neutral radical (i.e., ˙CH2CH(OH)CH2C(O)OH). These calculations demonstrate that SOMO-HOMO conversion results in channel switching in the distonic radical anion, suppressing radical-driven mechanisms and promoting pathways that directly involve the anion site.

Decomposition kinetics of perfluorinated sulfonic acids.

Perfluorooctanesulfonic acid (PFOS) is a widespread and persistent pollutant of concern to human health and the environment. Although incineration is often used to treat material contaminated with PFOS and related per- and polyfluoroalkyl substances (PFAS), little is known about the precise chemical mechanism for the thermal decomposition of these substances of concern. Here, we present the first study of the thermal decomposition kinetics of PFOS and related perfluorinated acids, using computational chemistry and reaction rate theory methods. We discover that the preferred channel for PFOS decomposition is via an α-sultone that spontaneously decomposes to form perfluorooctanal and SO2. At 1000 K the halflife for PFOS is predicted to be 0.2 s, decreasing sharply as temperature increases further. These results show that the acid headgroup in PFOS can be efficiently destroyed in incinerators operating at relatively modest temperatures. The new insights provided into the exact decomposition mechanism and kinetics of PFOS will help to improve remediation technologies actively under development.

Gas-Phase Mechanisms of the Reactions of Reduced Organic Nitrogen Compounds with OH Radicals.

Research on the fate of reduced organic nitrogen compounds in the atmosphere has gained momentum since the identification of their crucial role in particle nucleation and the scale up of carbon capture and storage technology which employs amine-based solvents. Reduced organic nitrogen compounds have strikingly different lifetimes against OH radicals, from hours for amines to days for amides to years for isocyanates, highlighting unique functional group reactivity. In this work, we use ab initio methods to investigate the gas-phase mechanisms governing the reactions of amines, amides, isocyanates and carbamates with OH radicals. We determine that N-H abstraction is only a viable mechanistic pathway for amines and we identify a reactive pathway in amides, the formyl C-H abstraction, not currently considered in structure-activity relationship (SAR) models. We then use our acquired mechanistic knowledge and tabulated literature experimental rate coefficients to calculate SAR factors for reduced organic nitrogen compounds. These proposed SAR factors are an improvement over existing SAR models because they predict the experimental rate coefficients of amines, amides, isocyanates, isothiocyanates, carbamates and thiocarbamates with OH radicals within a factor of 2, but more importantly because they are based on a sound fundamental mechanistic understanding of their reactivity.

A Theoretical Study of the Photoisomerization of Glycolaldehyde and Subsequent OH Radical-Initiated Oxidation of 1,2-Ethenediol.

It has recently been discovered that carbonyl compounds can undergo UV-induced isomerization to their enol counterparts under atmospheric conditions. This study investigates the photoisomerization of glycolaldehyde (HOCH2CHO) to 1,2-ethenediol (HOCH═CHOH) and the subsequent (•)OH-initiated oxidation chemistry of the latter using quantum chemical calculations and stochastic master equation simulations. The keto-enol tautomerization of glycolaldehyde to 1,2-ethenediol is associated with a barrier of 66 kcal mol(-1) and involves a double-hydrogen shift mechanism to give the lower-energy Z isomer. This barrier lies below the energy of the UV/vis absorption band of glycolaldehyde and is also considerably below the energy of the products resulting from photolytic decomposition. The subsequent atmospheric oxidation of 1,2-ethenediol by (•)OH is initiated by addition of the radical to the π system to give the (•)CH(OH)CH(OH)2 radical, which is subsequently trapped by O2 to form the peroxyl radical (•)O2CH(OH)CH(OH)2. According to kinetic simulations, collisional deactivation of the latter is negligible and cannot compete with rapid fragmentation reactions, which lead to (i) formation of glyoxal hydrate [CH(OH)2CHO] and HO2(•) through an α-hydroxyl mechanism (96%) and (ii) two molecules of formic acid with release of (•)OH through a ß-hydroxyl pathway (4%). Phenomenological rate coefficients for these two reaction channels were obtained for use in atmospheric chemical modeling. At tropospheric (•)OH concentrations, the lifetime of 1,2-ethenediol toward reaction with (•)OH is calculated to be 68 h.

Unimolecular reaction chemistry of a charge-tagged beta-hydroxyperoxyl radical.

ß-Hydroxyperoxyl radicals are formed during atmospheric oxidation of unsaturated volatile organic compounds such as isoprene. They are intermediates in the combustion of alcohols. In these environments the unimolecular isomerization and decomposition of ß-hydroxyperoxyl radicals may be of importance, either through chemical or thermal activation. We have used ion-trap mass spectrometry to generate the distonic charge-tagged ß-hydroxyalkyl radical anion, ˙CH2C(OH)(CH3)CH2C(O)O(-), and investigated its subsequent reaction with O2 in the gas phase under conditions that are devoid of complicating radical-radical reactions. Quantum chemical calculations and master equation/RRKM theory modeling are used to rationalize the results and discern a reaction mechanism. Reaction is found to proceed via initial hydrogen abstraction from the γ-methylene group and from the ß-hydroxyl group, with both reaction channels eventually forming isobaric product ions due to loss of either ˙OH + HCHO or ˙OH + CO2. Isotope labeling studies confirm that a 1,5-hydrogen shift from the ß-hydroxyl functionality results in a hydroperoxyalkoxyl radical intermediate that can undergo further unimolecular dissociations. Furthermore, this study confirms that the facile decomposition of ß-hydroxyperoxyl radicals can yield ˙OH in the gas phase.

Atmospheric chemistry of enols: a theoretical study of the vinyl alcohol + OH + O(2) reaction mechanism.

Enols are emerging as trace atmospheric components that may play a significant role in the formation of organic acids in the atmosphere. We have investigated the hydroxyl radical ((•)OH) initiated oxidation chemistry of the simplest enol, vinyl alcohol (ethenol, CH2═CHOH), using quantum chemical calculations and energy-grained master equation simulations. A lifetime of around 4 h was determined for vinyl alcohol in the presence of tropospheric levels of (•)OH. The reaction proceeds by (•)OH addition at both the α (66%) and ß (33%) carbons of the π-system, yielding the C-centered radicals (•)CH2CH(OH)2, and HOCH2C(•)HOH, respectively. Subsequent trapping by O2 leads to the respective peroxyl radicals. About 90% of the chemically activated population of the major peroxyl radical adduct (•)O2CH2CH(OH)2 is predicted to undergo fragmentation to produce formic acid and formaldehyde, with regeneration of (•)OH. The minor peroxyl radical HOCH2C(OO(•))HOH is even less stable and undergoes almost exclusive HO2(•) elimination to form glycolaldehyde (HOCH2CHO). Formation of the latter has not been proposed before in the oxidation of vinyl alcohol. A kinetic mechanism for use in atmospheric modeling is provided, featuring phenomenological rate coefficients for formation of the three main product channels ((•)O2CH2CH(OH)2 [8%]; HC(O)OH + HCHO + (•)OH [56%]; HOCH2CHO + HO2(•) [37%]). Our study supports previous findings that vinyl alcohol should be rapidly removed from the atmosphere by reaction with (•)OH and O2 with glycolaldehyde being identified as a previously unconsidered product. Most importantly, it is shown that direct chemically activated reactions can lead to (•)OH and HO2(•) (HOx) recycling.