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 β (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.

No abstract

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.

The ground-state rearrangement and decomposition of methyl vinyl ketone (MVK) and methacrolein (MACR) has been investigated using quantum chemical calculations and RRKM theory/master equation simulations. MVK and MACR absorb actinic radiation at around 380–280 nm, and we have identified a number of isomerization pathways with barriers that are accessible from the longer wavelength end of this range (visible/near-UV). Assuming that radiationless transitions dominate, master equation simulations of the reactions on the vibrationally excited ground-state potential-energy surface predict that isomerization to 2-hydroxybutadiene and 1-hydroxymethylallene from MVK, and isomerization to dimethylketene from MACR, are the major tropospheric reaction channels. Despite these processes having low quantum yields, they are prevalent because of the coincidence of high absorption cross sections with significant solar photon fluxes at around 320–330 nm, where photodissociation does not occur. This work suggests that photoisomerization may be an important process in the photolysis of these compounds in the troposphere, particularly for MVK, which, in comparison with MACR, has both a shorter lifetime with respect to photolysis and a longer lifetime with respect to reaction with the •OH radical.

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 β-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.

β-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 β-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 β-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 β-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 β-hydroxyl functionality results in a hydroperoxyalkoxyl radical intermediate that can undergo further unimolecular dissociations. Furthermore, this study confirms that the facile decomposition of β-hydroxyperoxyl radicals can yield ˙OH in the gas phase.

We report a novel mechanism for the interconversion of 2-hydroxypropanal with its more-stable ketone isomer hydroxyacetone. Reaction proceeds via concerted transfer of two H atoms, requires a barrier of only 40 kcal mol 21 , bypasses the enediol intermediate, and is general for a-hydroxy carbonyls. A similar isomerization mechanism is shown to persist for b, g, and d-hydroxy carbonyls; these compounds are skeletal forms of the monosaccharides and this work, therefore, discloses the gas-phase mechanism for aldose-ketose isomerization. As an example, the isomerization of glyceraldehyde to dihydroxyacetone is shown to proceed via this mechanism with a barrier of 31 kcal mol 21 . Rate coefficients and thermochemical properties are reported for the isomerization of 2-hydroxypropanal and hydroxyacetone for use in detailed kinetic models. Additionally, RRKM theory k(E) values for this reaction suggest that it may transpire in the troposphere following solar excitation. K E Y W O R D S computational chemistry, gas-phase reactions, pyrolysis, reaction mechanisms 1 | I N T R O D U C T I O N Hydroxycarbonyls are common oxygenated intermediates formed in the photochemical oxidation of volatile organic compounds and in the thermal processing of lignocellulosic material. Hydroxyacetone (HOCH 2 C(O)CH 3 ), the simplest a-hydroxy ketone, is a widely-detected trace gas in the troposphere, [1] and its photochemical oxidation mechanisms have been studied extensively. [2-5] 2-Hydroxypropanal (CH 3 CHOH-CHO), an isomer of hydroxyacetone, is an intermediate in the pyrolysis and combustion chemistry of glyceral. [6] More broadly, the open-chain forms of the monosaccharides are hydroxycarbonyls, with an aldehyde (aldose) or ketone (ketose) moiety and a hydroxyl group at each remaining carbon. Hydroyxacetone and 2-hydroxypropanal thus serve as model compounds for the a-hydroxy carbonyl functionality in these simple sugars.Recently, we reported on the isomerization mechanism of glycolaldehyde (HOCH 2 CHO), [7] the archetypal a-hydroxy carbonyl and an important tropospheric compound in its own right. [8] This work revealed that glycolaldehyde could undergo keto-enol tautomerization via intramolecular transfer of two H atoms, mediated by the hydroxyl group. This process is reminiscent of a bimolecular mechanism previously shown to catalyze keto-enol tautomerizations. [9] In the course of our investigation into glycolaldehyde it became apparent that this compound could undergo degenerate isomerization reactions in which the unique O atoms (i.e., the alcohol and aldehyde functional groups) were effectively scrambled. The present work extends this analysis to hydroxycarbonyl compounds in which the alcohol and aldehyde/ketone groups are not interchangeable, starting with the hydroxyacetone/2-hydroxypropanal pairing, through the application of ab initio calculations and reaction rate theory modelling. This study expands our understanding of rearrangement reactions available to hydroxycarbonyl compounds in thermal environments and in the troposph...

Deprotonation of a remote site in a peroxyl radical energetically buries the singly occupied molecular orbital, suppressing radical-driven oxidation and promoting reactions involving the anion site.