Decomposition of Pyruvic Acid on the Ground-State Potential Energy Surface

A potential energy surface is reported for isomerization and decomposition of gas-phase pyruvic acid (CH₃C­(O)­C­(O)­OH) in its ground electronic state. Consistent with previous works, the lowest energy pathway for pyruvic acid decomposition is identified as decarboxylation to produce hydroxymethylcarbene (CH₃COH), with overall barrier of 43 kcal mol^–1. This study discovers that pyruvic acid can also isomerize to the α-lactone form with a barrier of only 36 kcal mol^–1, from which CO elimination can occur at 49 kcal mol^–1 above pyruvic acid. An additional novel channel is identified for the tautomerisation of pyruvic acid to the enol form, via a double H-shift mechanism. The barrier for this process is 51 kcal mol^–1, which is around 20 kcal mol^–1 lower than the barrier for conventional keto–enol tautomerization via a 1,3-H shift transition state. Rate coefficients are calculated for pyruvic acid decomposition through RRKM theory/master equation simulations at 800–2000 K and 1 atm, showing good agreement with the available experimental data. The dissociation of vibrationally excited pyruvic acid produced through photoexcitation and subsequent internal conversion to the ground state is also modeled under tropospheric conditions and is seen to produce appreciable quantities of CO (∼1–4%) in addition to CH₃COH via the dominant CO₂ loss channel

Atmospheric Chemistry of 2‑Aminoethanol (MEA): Reaction of the NH₂^•CHCH₂OH Radical with O₂

The alkanolamine 2-aminoethanol (NH₂CH₂CH₂OH), otherwise known as monoethanolamine (MEA), is a widely used solvent for carbon capture, yet relatively little is known about its atmospheric chemistry. The hydroxyl radical initiated oxidation of MEA is thought to predominantly form the α-aminoalkyl radical NH₂^•CHCH₂OH, which will subsequently react with O₂ in the atmosphere to produce a peroxyl radical. We have investigated the reaction of O₂ with the NH₂^•CHCH₂OH radical using quantum chemical calculations and master equation kinetic modeling. This reaction is found to proceed predominantly via a chemically activated mechanism under tropospheric conditions to directly produce the imine 2-iminoethanol (NHCHCH₂OH) + HO₂^•, with lesser amounts of the collisionally deactivated peroxyl radical NH₂CH­(O₂^•)­CH₂OH. By largely bypassing a peroxyl radical intermediate, this process avoids ozone-promoting conversion of NO to NO₂ and makes the oxidation of MEA to 2-iminoethanol HO_<i>x</i>-neutral overall. The imine product of MEA oxidation is proposed as an important intermediate in the formation of aerosols via uptake to water droplets and subsequent hydrolysis to ammonia and glycolaldehyde

Formation of Nitrosamines and Alkyldiazohydroxides in the Gas Phase: The CH₃NH + NO Reaction Revisited

Aminyl free radicals of the form RN^•H are formed in the photochemical oxidation of primary amines, and their reaction with ^•NO is an important tropospheric sink. Reaction of the parent methylamidogen radical (CH₃N^•H) with ^•NO in the gas phase has been studied using quantum chemical techniques and RRKM theory/master equation based kinetic modeling. Calculations with the G3X-K composite theoretical method indicate that reaction proceeds via exothermic formation of a primary nitrosamine intermediate, CH₃NHNO, which can isomerize to an alkyldiazohydroxide, CH₃NNOH, and further eliminate water to form diazomethane, CH₂NN. Master equation simulations conducted at tropospheric conditions identify that the collisionally stabilized CH₃NHNO and CH₃NNOH isomers are the major reaction products, with smaller yields of CH₂NN + H₂O. A previously proposed mechanism in which the primary nitrosamine is destroyed via isomerization to CH₂NHNOH, followed by reaction with O₂ to produce CH₂NH + HO₂^• + ^•NO, is disproved. In the atmosphere, CH₂NN may be formed with sufficient vibrational energy to directly dissociate to singlet methylene (¹CH₂) and N₂, whereas under combustion conditions this is expected to be the dominant pathway. This study suggests that stabilized primary nitrosamines can indeed form in the photochemical oxidation of amines, along with alkyldiazohydroxides and diazoalkanes. Both classes of compound are potent alkylating agents that may need to be considered in future atmospheric studies

Mystery of 1‑Vinylpropargyl Formation from Acetylene Addition to the Propargyl Radical: An Open-and-Shut Case

The addition of acetylene (C₂H₂) to the propargyl radical (C₃H₃) initiates a cascade of molecular weight growth reactions that result in the production of polycyclic aromatic hydrocarbons (PAHs) in flames. Although it is well-established that the first reaction step produces the cyclic C₅H₅ radical cyclopentadienyl (<i>c</i>-C₅H₅), recent studies have also detected significant quantities of the open-chain form, 1-vinylpropargyl (<i>l</i>-C₅H₅). This work presents a mechanism for the C₃H₃ + C₂H₂ reaction from <i>ab initio</i> calculations, which includes pathways for the formation of both the open and shut isomers as well as for their interconversion. Formation of both isomers proceeds from the initial HCCCH₂CHCH^• reaction adduct with similar barriers, both well below the entrance channel energy. Subsequent isomerization of <i>l</i>-C₅H₅ with <i>c</i>-C₅H₅ also transpires at below the energy of the reactants, although this process connects two deep wells (being resonance stabilized radicals), and must compete with collisional energy transfer. An RRKM theory/master equation model is developed for the reported C₅H₅ reaction mechanism. Master equation simulations suggest that both cyclic and open-chain isomers are expected to form from the C₃H₃ + C₂H₂ reaction across a range of temperatures, although the lifetime of <i>l</i>-C₅H₅ is relatively short for rearrangement to <i>c</i>-C₅H₅

Reaction of Methacrolein with the Hydroxyl Radical in Air: Incorporation of Secondary O₂ Addition into the MACR + OH Master Equation

Methacrolein (MACR) plays an important role in atmospheric chemistry within the planetary boundary layer, as it is one of the major oxidation products of isoprene and has a short lifetime toward the hydroxyl radical (OH). In this study, quantum chemical techniques and statistical reaction rate theory have been used to simulate the addition of OH to MACR at conditions representative of the troposphere. In this chemically activated reaction, the time scales for product formation versus collisional deactivation of the vibrationally excited adduct are explicitly considered. Furthermore, the subsequent addition of O₂ is also incorporated within a single master equation, so as to investigate doubly activated peroxyl radical formation. The major reaction product of OH addition to MACR is the HOCH₂C^•(CH₃)­CHO radical formed via addition to the outer (β) carbon. This radical is predominantly in the <i>Z</i> isomer although around a third of the population is quenched as the higher-energy <i>E</i> isomer. Calculated rate constants agree well with experiment when using M06–2X/aug-cc-pVTZ barrier heights, but are somewhat overpredicted using G3SX energies. The overall rate constant is controlled by competition between dissociation of the MACR···OH van der Waals complex back to reactants and isomerization on to MACR–OH adducts, which takes place on a time scale of several nanoseconds, but collisional deactivation of the MACR–OH adducts occurs on a time scale that is around an order of magnitude longer. When O₂ addition is included in the master equation, we observe that the MACR–OH adducts are removed by reaction with O₂ on a similar time scale to collisional deactivation. Around 50% of the subsequent peroxyl radical population is formed with some identifiable excess vibrational energy above singly activated [MACR–OH–O₂]*, with around 20% provided with an additional 20 kcal mol^–1 (>40 kcal mol^–1 relative to quenched MACR–OH–O₂) that can go into further unimolecular reaction. This double activation process is expected to lead to some prompt unimolecular decomposition of excited [MACR–OH–O₂]** peroxyl radicals to yield products including hydroxyacetone and methylglyoxal, regenerating the initiating OH radical in the process

Reaction of Benzene with Atomic Carbon: Pathways to Fulvenallene and the Fulvenallenyl Radical in Extraterrestrial Atmospheres and the Interstellar Medium

The reaction of benzene with ground-state atomic carbon, C­(³P), has been investigated using the G3X-K composite quantum chemical method. A suite of novel energetically favorable pathways that lead to previously unconsidered products are identified. Reaction is initiated by barrierless C atom cycloaddition to benzene on the triplet surface, producing a vibrationally excited [C₇H₆]* adduct that can dissociate to the cycloheptatrienyl radical (+ H) via a relatively loose transition state 4.4 kcal mol^–1 below the reactant energies. This study also identifies that this reaction adduct can isomerize to generate five-membered ring intermediates that can further dissociate to the global C₇H₅ minima, the fulvenallenyl radical (+ H), or to <i>c</i>-C₅H₄ and acetylene, with limiting barriers around 20 and 10 kcal mol^–1 below the reactants, respectively. If intersystem crossing to the singlet surface occurs, isomerization pathways that are lower-yet in energy are available leading to the C₇H₆ minima fulvenallene, with all barriers over 40 kcal mol^–1 below the reactants. From here further barrierless fragmentation to fulvenallenyl + H can proceed at <i>ca</i>. 25 kcal mol^–1 below the reactants. In the reducing atmospheres of planets like Jupiter and satellites like Titan, where benzene and C­(³P) are both expected, it is proposed that fulvenallene and the fulvenallenyl radical would be the dominant products of the C₆H₆ + C­(³P) reaction. Fulvenallenyl may also be a significant reaction product under collision-free conditions representative of the interstellar medium, although further work is required here to confirm the identity of the C₇H₅ radical product

Photoisomerization of Methyl Vinyl Ketone and Methacrolein in the Troposphere: A Theoretical Investigation of Ground-State Reaction Pathways

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

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 <i>Z</i> 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

Photoisomerization of Methyl Vinyl Ketone and Methacrolein in the Troposphere: A Theoretical Investigation of Ground-State Reaction Pathways

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

Atmospheric Chemistry of Enols: A Theoretical Study of the Vinyl Alcohol + OH + O₂ 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 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^• (HO_<i>x</i>) recycling