Publications

Using chemical kinetic modeling and statistical analysis, we investigate the possibility of correlating key chemical "markers"-typically small molecules-formed during very lean (φ ∼0.001) oxidation experiments with near-stoichiometric (φ ∼1) fuel ignition properties. One goal of this work is to evaluate the feasibility of designing a fuel-screening platform, based on small laboratory reactors that operate at low temperatures and use minimal fuel volume. Buras et al. [Combust. Flame 2020, 216, 472-484] have shown that convolutional neural net (CNN) fitting can be used to correlate first-stage ignition delay times (IDTs) with OH/HO2measurements during very lean oxidation in low-T flow reactors with better than factor-of-2 accuracy. In this work, we test the limits of applying this correlation-based approach to predict the low-temperature heat release (LTHR) and total IDT, including the sensitivity of total IDT to the equivalence ratio, φ. We demonstrate that first-stage IDT can be reliably correlated with very lean oxidation measurements using compressed sensing (CS), which is simpler to implement than CNN fitting. LTHR can also be predicted via CS analysis, although the correlation quality is somewhat lower than for first-stage IDT. In contrast, the accuracy of total IDT prediction at φ = 1 is significantly lower (within a factor of 4 or worse). These results can be rationalized by the fact that the first-stage IDT and LTHR are primarily determined by low-temperature chemistry, whereas total IDT depends on low-, intermediate-, and high-temperature chemistry. Oxidation reactions are most important at low temperatures, and therefore, measurements of universal molecular markers of oxidation do not capture the full chemical complexity required to accurately predict the total IDT even at a single equivalence ratio. As a result, we find that φ-sensitivity of ignition delay cannot be predicted at all using solely correlation with lean low-T chemical speciation measurements.

The reactivity of carbonyl oxides has previously been shown to exhibit strong conformer and substituent dependencies. Through a combination of synchrotron multiplexed photoionization mass spectrometry experiments (298 K, 4 Torr) and high-level theory (CCSD(T)-F12/cc-pVTZ-F12//B2PLYP-D3/cc-pVTZ with an added CCSDT(Q) correction), we explore the conformer dependence of the reaction of acetaldehyde oxide (CH₃CHOO) with dimethyl amine (DMA). The experimental data supports the theoretically predicted 1,2-insertion mechanism and the formation of an amine-functionalized hydroperoxide reaction product. Tunable-VUV photoionization probing of anti- or anti- + syn-CH₃CHOO reveals a strong conformer dependence of the title reaction. Here, the rate coefficient of DMA with anti-CH₃CHOO is predicted to exceed that for the reaction with syn-CH₃CHOO by a factor of ~34,000, which is attributed to submerged barrier (syn) vs. barrierless (anti) mechanisms for energetically downhill reactions.

Rapid molecular-weight growth of hydrocarbons occurs in flames, in industrial synthesis, and potentially in cold astrochemical environments. A variety of high- and low-temperature chemical mechanisms have been proposed and confirmed, but more facile pathways may be needed to explain observations. We provide laboratory confirmation in a controlled pyrolysis environment of a recently proposed mechanism, radical–radical chain reactions of resonance-stabilized species. The recombination reaction of phenyl (c-C6H5) and benzyl (c-C6H5CH2) radicals produces both diphenylmethane and diphenylmethyl radicals, the concentration of the latter increasing with rising temperature. A second phenyl addition to the product radical forms both triphenylmethane and triphenylmethyl radicals, confirming the propagation of radical–radical chain reactions under the experimental conditions of high temperature (1100–1600 K) and low pressure (ca. 3 kPa). Similar chain reactions may contribute to particle growth in flames, the interstellar medium, and industrial reactors.

Ongoing progress in synthetic biology, metabolic engineering, and catalysis continues to produce a diverse array of advanced biofuels with complex molecular structure and functional groups. In order to integrate biofuels into existing combustion systems, and to optimize the design of next-generation combustion systems, understanding connections between molecular structure and ignition at low-temperature conditions (< 1000 K) remains a priority that is addressed in part using chemical kinetics modeling. The development of predictive models relies on detailed information, derived from experimental and theoretical studies, on molecular structure and chemical reactivity, both of which influence the balance of chain reactions that occur during combustion – propagation, termination, and branching. In broad context, three main categories of reactions affect ignition behavior: (i) initiation reactions that generate a distribution of organic radicals, R˙; (ii) competing unimolecular decomposition of R˙ and bimolecular reaction of R˙ with O2; (iii) decomposition mechanisms of peroxy radical adducts (ROO˙), including isomerization via ROO˙ ⇌ Q˙OOH. All three categories are influenced by functional groups in different ways, which causes a shift in the balance of chain reactions that unfold over complex temperature- and pressure-dependent mechanisms. The objective of the present review is three-fold: (1) to provide a historical account of research on low-temperature oxidation of biofuels, including initiation reactions, peroxy radical reactions, Q˙OOH-mediated reaction mechanisms, and chain-branching chemistry; (2) to summarize the influence of functional groups on chemical kinetics relevant to chain-branching reactions, which are responsible for the accelerated production of radicals that leads to ignition; (3) to identify areas of research that are needed – experimentally and computationally – to address fundamental questions that remain. Results from experimental, quantum chemical, and chemical kinetics modeling studies are reviewed for several classes of biofuels – alcohols, esters, ketones, acyclic ethers and cyclic ethers – and are compared against analogous results in alkane oxidation. The review is organized into separate sections for each biofuel class, which include studies on thermochemistry and bond dissociation energies, rate coefficients for initiation reactions via H-abstraction and related branching fractions, reaction mechanisms and product formation from reactive intermediates, ignition delay times, and chemical kinetics modeling. Each section is then summarized in order to identify areas for which additional functional group-specific work is required. The review concludes with an outline for research directions for improving the fundamental understanding of biofuel ignition chemistry and related chemical kinetics modeling.

Trifluoroacetic acid (TFA), a highly soluble and stable organic acid, is photochemically produced by certain anthropogenically emitted halocarbons such as HFC-134a and HFO-1234yf. Both these halocarbons are used as refrigerants in the automobile industry, and the high global warming potential of HFC-134a has promoted regulation of its use. Industries are transitioning to the use of HFO-1234yf as a more environmentally friendly alternative. We investigated the environmental effects of this change and found a 33-fold increase in the global burden of TFA from an annual value of 65 tonnes formed from the 2015 emissions of HFC-134a to a value of 2220 tonnes formed from an equivalent emission of HFO-1234yf. The percentage increase in surface TFA concentrations resulting from the switch from HFC-134a to HFO-1234yf remains substantial with an increase of up to 250-fold across Europe. The increase in emissions greater than the current emission scenario of HFO-1234yf is likely to result in significant TFA burden as the atmosphere is not able to disperse and deposit relevant oxidation products. The Criegee intermediate initiated loss process of TFA reduces the surface level atmospheric lifetime of TFA by up to 5 days (from 7 days to 2 days) in tropical forested regions.

Oxiranes are a class of cyclic ethers formed in abundance during low-temperature combustion of hydrocarbons and biofuels, either via chain-propagating steps that occur from unimolecular decomposition of β-hydroperoxyalkyl radicals (β-̇QOOH) or from reactions of HOȮ with alkenes. Ethyloxirane is one of four alkyl-substituted cyclic ether isomers produced as an intermediate from n-butane oxidation. While rate coefficients for β-̇QOOH → ethyloxirane + ȮH are reported extensively, subsequent reaction mechanisms of the cyclic ether are not. As a result, chemical kinetics mechanisms commonly adopt simplified chemistry to describe ethyloxirane consumption by convoluting several elementary reactions into a single step, which may introduce mechanism truncation error—uncertainty derived from missing or incomplete chemistry. The present work provides fundamental insight on reaction mechanisms of ethyloxirane in support of ongoing efforts to minimize mechanism truncation error. Reaction mechanisms are inferred from the detection of products during chlorine atom-initiated oxidation experiments using multiplexed photoionization mass spectrometry conducted at 10 Torr and temperatures of 650 K and 800 K. To complement the experiments, calculations of stationary point energies were conducted using the ccCA-PS3 composite method on ̇R + O2 potential energy surfaces for the four ethyloxiranyl radical isomers, which produced barrier heights for 24 reaction pathways. In addition to products from ̇QOOH → cyclic ether + ȮH and ̇R + O2 → conjugate alkene + HOȮ, both of which were significant pathways and are prototypical to alkane oxidation, other species were identified from ring-opening of both ethyloxiranyl and ̇QOOH radicals. The latter occurs when the unpaired electron is localized on the ether group, causing the initial ̇QOOH structure to ring-open and form a resonance-stabilized ketohydroperoxide-type radical. The present work provides the first analysis of ethyloxirane oxidation chemistry, which reveals that consumption pathways are complex and may require an expansion of submechanisms to increase the fidelity of chemical kinetics mechanisms.

The oxidation of neo-pentane was studied by combining experiments, theoretical calculations, and mechanistic developments to elucidate the impact of the 3rd O2 addition reaction network on ignition delay time predictions. The experiments were based on photoionization mass spectrometry in jet-stirred and time-resolved flow reactors allowing for sensitive detection of the keto-hydroperoxide (KHP) and keto-dihydroperoxide (KDHP) intermediates. With neo-pentane exhibiting a unique symmetric molecular structure, which consequently results only in single KHP and KDHP isomers, theoretical calculations of ionization and fragment appearance energies and of absolute photoionization cross sections enabled the unambiguous identification and quantification of the KHP intermediate. Its temperature and time-resolved profiles together with calculated and experimentally observed KHP-to-KDHP signal ratios were compared to simulation results based on a newly developed mechanism that describes the 3rd O2 addition reaction network. A satisfactory agreement was observed between the experimental data points and the simulation results, adding confidence to the model’s overall performance.

Isoprene is the most abundant non-methane hydrocarbon emitted into the Earth's atmosphere. Ozonolysis is an important atmospheric sink for isoprene, which generates reactive carbonyl oxide species (R1R2CO+O-) known as Criegee intermediates. This study focuses on characterizing the catalyzed isomerization and adduct formation pathways for the reaction between formic acid and methyl vinyl ketone oxide (MVK-oxide), a four-carbon unsaturated Criegee intermediate generated from isoprene ozonolysis. syn-MVK-oxide undergoes intramolecular 1,4 H-atom transfer to form a substituted vinyl hydroperoxide intermediate, 2-hydroperoxybuta-1,3-diene (HPBD), which subsequently decomposes to hydroxyl and vinoxylic radical products. Here, we report direct observation of HPBD generated by formic acid catalyzed isomerization of MVK-oxide under thermal conditions (298 K, 10 torr) using multiplexed photoionization mass spectrometry. The acid catalyzed isomerization of MVK-oxide proceeds by a double hydrogen-bonded interaction followed by a concerted H-atom transfer via submerged barriers to produce HPBD and regenerate formic acid. The analogous isomerization pathway catalyzed with deuterated formic acid (D2-formic acid) enables migration of a D atom to yield partially deuterated HPBD (DPBD), which is identified by its distinct mass (m/z 87) and photoionization threshold. In addition, bimolecular reaction of MVK-oxide with D2-formic acid forms a functionalized hydroperoxide adduct, which is the dominant product channel, and is compared to a previous bimolecular reaction study with normal formic acid. Complementary high-level theoretical calculations are performed to further investigate the reaction pathways and kinetics.

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In the context of tropospheric chemistry, Criegee intermediates denote carbonyl oxides with biradical/zwitterionic character (R1R2COO) that form during the ozonolysis of alkenes. First discovered almost 70 years ago, stabilised versions of Criegee intermediates formed via collisional removal of excess energy have interesting kinetic and mechanistic properties. The direct production and detection of these intermediates were not reported in the literature until 2008. However, recent advances in their generation through the ultraviolet irradiation of the corresponding diiodoalkanes in excess O2 and detection by various spectroscopic techniques (photoionisation, ultraviolet, infrared, microwave and mass spectrometry) have shown that these species can react rapidly with closed-shell molecules, in many cases at or exceeding the classical gas-kinetic limit, via multiple reaction pathways. These reactions can be complex, and laboratory measurements of products and the temperature and pressure dependence of the reaction kinetics have also revealed unusual behaviour. The potential role of these intermediates in atmospheric chemistry is significant, altering models of the oxidising capacity of the Earth's atmosphere and the rate of generation of secondary organic aerosol.

Ozonolysis of isoprene, one of the most abundant volatile organic compounds emitted into the Earth's atmosphere, generates two four-carbon unsaturated Criegee intermediates, methyl vinyl ketone oxide (MVK-oxide) and methacrolein oxide (MACR-oxide). The extended conjugation between the vinyl substituent and carbonyl oxide groups of these Criegee intermediates facilitates rapid electrocyclic ring closures that form five-membered cyclic peroxides, known as dioxoles. This study reports the first experimental evidence of this novel decay pathway, which is predicted to be the dominant atmospheric sink for specific conformational forms of MVK-oxide (anti) and MACR-oxide (syn) with the vinyl substituent adjacent to the terminal O atom. The resulting dioxoles are predicted to undergo rapid unimolecular decay to oxygenated hydrocarbon radical products, including acetyl, vinoxy, formyl, and 2-methylvinoxy radicals. In the presence of O2, these radicals rapidly react to form peroxy radicals (ROO), which quickly decay via carbon-centered radical intermediates (QOOH) to stable carbonyl products that were identified in this work. The carbonyl products were detected under thermal conditions (298 K, 10 Torr He) using multiplexed photoionization mass spectrometry (MPIMS). The main products (and associated relative abundances) originating from unimolecular decay of anti-MVK-oxide and subsequent reaction with O2 are formaldehyde (88 ± 5%), ketene (9 ± 1%), and glyoxal (3 ± 1%). Those identified from the unimolecular decay of syn-MACR-oxide and subsequent reaction with O2 are acetaldehyde (37 ± 7%), vinyl alcohol (9 ± 1%), methylketene (2 ± 1%), and acrolein (52 ± 5%). In addition to the stable carbonyl products, the secondary peroxy chemistry also generates OH or HO2 radical coproducts.

Perfluorooctanoic acid, PFOA, is one of the many concerning pollutants in our atmosphere; it is highly resistant to environmental degradation processes, which enables it to accumulate biologically. With direct routes of this chemical to the environment decreasing, as a consequence of the industrial phase out of PFOA, it has become more important to accurately model the effects of indirect production routes, such as environmental degradation of precursors; e.g., fluorotelomer alcohols (FTOHs). The study reported here investigates the chemistry, physical loss and transport of PFOA and its precursors, FTOHs, throughout the troposphere using a 3D global chemical transport model, STOCHEM-CRI. Moreover, this investigation includes an important loss process of PFOA in the atmosphere via the addition of the stabilised Criegee intermediates, hereby referred to as the "Criegee Field. " Whilst reaction with Criegee intermediates is a significant atmospheric loss process of PFOA, it does not result in its permanent removal from the atmosphere. The atmospheric fate of the resultant hydroperoxide product from the reaction of PFOA and Criegee intermediates resulted in a ≈0.04 Gg year-1 increase in the production flux of PFOA. Furthermore, the physical loss of the hydroperoxide product from the atmosphere (i.e., deposition), whilst decreasing the atmospheric concentration, is also likely to result in the reformation of PFOA in environmental aqueous phases, such as clouds, precipitation, oceans and lakes. As such, removal facilitated by the "Criegee Field" is likely to simply result in the acceleration of PFOA transfer to the surface (with an expected decrease in PFOA atmospheric lifetime of ≈10 h, on average from ca. ≈80 h without Criegee loss to 70 h with Criegee loss).

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The reaction of perfluorooctanoic acid with the smallest carbonyl oxide Criegee intermediate, CH 2 OO, has been measured and is very rapid, with a rate coefficient of (4.9 ± 0.8) × 10 -10 cm 3 s -1 , similar to that for reactions of Criegee intermediates with other organic acids. Evidence is shown for the formation of hydroperoxymethyl perfluorooctanoate as a product. With such a large rate coefficient, reaction with Criegee intermediates can be a substantial contributor to atmospheric removal of perfluorocarboxylic acids. However, the atmospheric fates of the ester product largely regenerate the initial acid reactant. Wet deposition regenerates the perfluorocarboxylic acid via condensed-phase hydrolysis. Gas-phase reaction with OH is expected principally to result in formation of the acid anhydride, which also hydrolyzes to regenerate the acid, although a minor channel could lead to destruction of the perfluorinated backbone.

Methanol is a benchmark for understanding tropospheric oxidation, but is underpredicted by up to 100% in atmospheric models. Recent work has suggested this discrepancy can be reconciled by the rapid reaction of hydroxyl and methylperoxy radicals with a methanol branching fraction of 30%. However, for fractions below 15%, methanol underprediction is exacerbated. Theoretical investigations of this reaction are challenging because of intersystem crossing between singlet and triplet surfaces – ∼45% of reaction products are obtained via intersystem crossing of a pre-product complex – which demands experimental determinations of product branching. Here we report direct measurements of methanol from this reaction. A branching fraction below 15% is established, consequently highlighting a large gap in the understanding of global methanol sources. These results support the recent high-level theoretical work and substantially reduce its uncertainties.

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Acetic acid (CH3COOH) is one of the most abundant carboxylic acids in the troposphere. In the study, the tropospheric chemistry of CH3COOH is investigated using the 3-D global chemistry transport model, STOCHEM-CRI. The highest mixing ratios of surface CH3COOH are found in the tropics by as much as 1.6 ppb in South America. The model predicts the seasonality of CH3COOH reasonably well and correlates with some surface and flight measurement sites, but the model drastically underpredicts levels in urban and midlatitudinal regions. The possible reasons for the underprediction are discussed. The simulations show that the lifetime and global burden of CH3COOH are 1.6–1.8 days and 0.45–0.61 Tg, respectively. The reactions of the peroxyacetyl radical (CH3CO3) with the hydroperoxyl radical (HO2) and other organic peroxy radicals (RO2) are found to be the principal sources of tropospheric CH3COOH in the model, but the model-measurement discrepancies suggest the possible unknown or underestimated sources which can contribute large fractions of the CH3COOH burden. The major sinks of CH3COOH in the troposphere are wet deposition, dry deposition, and OH loss. However, the reaction of CH3COOH with Criegee intermediates is proposed to be a potentially significant chemical loss process of tropospheric CH3COOH that has not been previously accounted for in global modeling studies. Inclusion of this loss process reduces the tropospheric CH3COOH level significantly which can give even larger discrepancies between model and measurement data, suggesting that the emissions inventory and the chemical production sources of CH3COOH are underpredicted even more so in current global models.

Criegee intermediates (CIs), carbonyl oxides formed in ozonolysis of alkenes, play key roles in the troposphere. The decomposition of CIs can be a significant source of OH to the tropospheric oxidation cycle especially during nighttime and winter months. A variety of model-measurement studies have estimated surface-level stabilized Criegee intermediate (sCI) concentrations on the order of 1 × 104 cm-3 to 1 × 105 cm-3, which makes a non-negligible contribution to the oxidising capacity in the terrestrial boundary layer. The reactions of sCI with the water monomer and the water dimer have been found to be the most important bimolecular reactions to the tropospheric sCI loss rate, at least for the smallest carbonyl oxides; the products from these reactions (e.g. hydroxymethyl hydroperoxide, HMHP) are also of importance to the atmospheric oxidation cycle. The sCI can oxidise SO2 to form SO3, which can go on to form a significant amount of H2SO4 which is a key atmospheric nucleation species and therefore vital to the formation of clouds. The sCI can also react with carboxylic acids, carbonyl compounds, alcohols, peroxy radicals and hydroperoxides, and the products of these reactions are likely to be highly oxygenated species, with low vapour pressures, that can lead to nucleation and SOA formation over terrestrial regions.

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Methyl vinyl ketone (MVK) and methacrolein (MACR) are important intermediate products in atmospheric degradation of volatile organic compounds, especially of isoprene. This work investigates the reactions of the smallest Criegee intermediate, CH2OO, with its co-products from isoprene ozonolysis, MVK and MACR, using multiplexed photoionization mass spectrometry (MPIMS), with either tunable synchrotron radiation from the Advanced Light Source or Lyman-α (10.2 eV) radiation for photoionization. CH2OO was produced via pulsed laser photolysis of CH2I2 in the presence of excess O2. Time-resolved measurements of reactant disappearance and of product formation were performed to monitor reaction progress; first order rate coefficients were obtained from exponential fits to the CH2OO decays. The bimolecular reaction rate coefficients at 300 K and 4 Torr are k(CH2OO + MVK) = (5.0 ± 0.4) × 10-13 cm3 s-1 and k(CH2OO + MACR) = (4.4 ± 1.0) × 10-13 cm3 s-1, where the stated ±2σ uncertainties are statistical uncertainties. Adduct formation is observed for both reactions and is attributed to the formation of a secondary ozonides (1,2,4-trioxolanes), supported by master equation calculations of the kinetics and the agreement between measured and calculated adiabatic ionization energies. Kinetics measurements were also performed for a possible bimolecular CH2OO + CO reaction and for the reaction of CH2OO with CF3CHCH2 at 300 K and 4 Torr. For CH2OO + CO, no reaction is observed and an upper limit is determined: k(CH2OO + CO) < 2 × 10-16 cm3 s-1. For CH2OO + CF3CHCH2, an upper limit of k(CH2OO + CF3CHCH2) < 2 × 10-14 cm3 s-1 is obtained.

Following measurements in the winter of 2012, formic acid (HCOOH) and nitric acid (HNO3) were measured using a chemical ionization mass spectrometer (CIMS) during the Summer Clean Air for London (ClearfLo) campaign in London, 2012. Consequently, the seasonal dependence of formic acid sources could be better understood. A mean formic acid concentration of 1.3 ppb and a maximum of 12.7 ppb was measured which is significantly greater than that measured during the winter campaign (0.63 ppb and 6.7 ppb, respectively). Daily calibrations of formic acid during the summer campaign gave sensitivities of 1.2 ion counts s−1 parts per trillion (ppt) by volume−1 and a limit of detection of 34 ppt. During the summer campaign, there was no correlation between formic acid and anthropogenic emissions such as NOx and CO or peaks associated with the rush hour as was identified in the winter. Rather, peaks in formic acid were observed that correlated with solar irradiance. Analysis using a photochemical trajectory model has been conducted to determine the source of this formic acid. The contribution of formic acid formation through ozonolysis of alkenes is important but the secondary production from biogenic VOCs could be the most dominant source of formic acid at this measurement site during the summer.

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The Criegee intermediate acetone oxide, (CH3)2COO, is formed by laser photolysis of 2,2-diiodopropane in the presence of O2 and characterized by synchrotron photoionization mass spectrometry and by cavity ring-down ultraviolet absorption spectroscopy. The rate coefficient of the reaction of the Criegee intermediate with SO2 was measured using photoionization mass spectrometry and pseudo-first-order methods to be (7.3 ± 0.5) × 10-11 cm3 s-1 at 298 K and 4 Torr and (1.5 ± 0.5) × 10-10 cm3 s-1 at 298 K and 10 Torr (He buffer). These values are similar to directly measured rate coefficients of anti-CH3CHOO with SO2, and in good agreement with recent UV absorption measurements. The measurement of this reaction at 293 K and slightly higher pressures (between 10 and 100 Torr) in N2 from cavity ring-down decay of the ultraviolet absorption of (CH3)2COO yielded even larger rate coefficients, in the range (1.84 ± 0.12) × 10-10 to (2.29 ± 0.08) × 10-10 cm3 s-1. Photoionization mass spectrometry measurements with deuterated acetone oxide at 4 Torr show an inverse deuterium kinetic isotope effect, kH/kD = (0.53 ± 0.06), for reactions with SO2, which may be consistent with recent suggestions that the formation of an association complex affects the rate coefficient. The reaction of (CD3)2COO with NO2 has a rate coefficient at 298 K and 4 Torr of (2.1 ± 0.5) × 10-12 cm3 s-1 (measured with photoionization mass spectrometry), again similar to rate for the reaction of anti-CH3CHOO with NO2. Cavity ring-down measurements of the acetone oxide removal without added reagents display a combination of first- and second-order decay kinetics, which can be deconvolved to derive values for both the self-reaction of (CH3)2COO and its unimolecular thermal decay. The inferred unimolecular decay rate coefficient at 293 K, (305 ± 70) s-1, is similar to determinations from ozonolysis. The present measurements confirm the large rate coefficient for reaction of (CH3)2COO with SO2 and the small rate coefficient for its reaction with water. Product measurements of the reactions of (CH3)2COO with NO2 and with SO2 suggest that these reactions may facilitate isomerization to 2-hydroperoxypropene, possibly by subsequent reactions of association products.

Hydroxyacetone (CH3C(O)CH2OH) is observed as a stable end product from reactions of the (CH3)2COO Criegee intermediate, acetone oxide, in a flow tube coupled with multiplexed photoionization mass spectrometer detection. In the experiment, the isomers at m/z = 74 are distinguished by their different photoionization spectra and reaction times. Hydroxyacetone is observed as a persistent signal at longer reaction times at a higher photoionization threshold of ca. 9.7 eV than Criegee intermediate and definitively identified by comparison with the known photoionization spectrum. Complementary electronic structure calculations reveal multiple possible reaction pathways for hydroxyacetone formation, including unimolecular isomerization via hydrogen atom transfer and -OH group migration as well as self-reaction of Criegee intermediates. Varying the concentration of Criegee intermediates suggests contributions from both unimolecular and self-reaction pathways to hydroxyacetone. The hydroxyacetone end product can provide an effective, stable marker for the production of transient Criegee intermediates in future studies of alkene ozonolysis.

Product formation from the low-temperature oxidation of two isotopologues of the proposed biofuel butanone was studied via multiplexed photoionization mass spectrometry (MPIMS) at 500 and 700 K to elucidate product branching ratios for R and QOOH pathways. Products were identified and branching ratios quantified for a number of species, with the aid of ab initio calculations. Chain-inhibiting C-C β-scission of R and select chain-propagating channels are discussed. Whilst methyl vinyl ketone and HOO, (from chain-inhibiting pathways) were found to be major products, chain propagation pathways leading to carbonyl and cyclic ether species following OH-elimination from QOOH were found to be pertinent at both temperatures. At 700 K, R C-C β-scission was significantly enhanced, as evident in the branching ratios, however the formation of QOOH-derived chain-propagation products remained relevant.

Lignocellulosic-derived biofuels represent an important part of sustainable transportation en- ergy and often contain oxygenated functional groups due to the mono- and polysaccharide content in cellulose and hemicellulose. The yields of conjugate alkene + HO2 formation in low-temperature tetrahydropyran oxidation were studied and the influence of oxygen heteroatoms in cyclic hydrocarbons on the associated chain-termination pathways stemming from R + O2 were examined. Relative to the initial radical concentration the trend in conjugate alkene branching fraction showed monotonic positive temperature dependence in both cyclohexane and tetrahydropyran except for tetrahydropyran at 10 torr where increasing the temperature to 700 K caused a decrease. Conjugate alkene branching fractions measured at 1520 torr for cyclohexane and tetrahydropyran followed monotonic positive temperature dependence. In contrast to the results at higher temperature where ring-opening of tetrahydropyranyl radicals interrupted R + O2chemistry and reduces the formation of conjugate alkenes branching fractions measured below 700 K were higher in tetrahydropyran compared to cyclohexane at 10 torr.

The rapid reaction of the smallest Criegee intermediate, CH2OO, with water dimers is the dominant removal mechanism for CH2OO in the Earth's atmosphere, but its products are not well understood. This reaction was recently suggested as a significant source of the most abundant tropospheric organic acid, formic acid (HCOOH), which is consistently underpredicted by atmospheric models. However, using time-resolved measurements of reaction kinetics by UV absorption and product analysis by photoionization mass spectrometry, we show that the primary products of this reaction are formaldehyde and hydroxymethyl hydroperoxide (HMHP), with direct HCOOH yields of less than 10%. Incorporating our results into a global chemistry-transport model further reduces HCOOH levels by 10-90%, relative to previous modeling assumptions, which indicates that the reaction CH2OO + water dimer by itself cannot resolve the discrepancy between the measured and predicted HCOOH levels.

The pulsed photolytic chlorine-initiated oxidation of diethyl ketone [DEK; (CH3CH2)2CO], 2,2,4,4-d4-DEK [d4-DEK; (CH3CD2)2CO], and 1,1,1,5,5,5-d6-DEK [d6-DEK; (CD3CH2)2CO] is studied at 8 torr and 1−2 atm and from 400−625 K. Cl atoms produced by laser photolysis react with diethyl ketone to form either primary (3-pentan-on-1-yl, RP) or secondary (3-pentan-on-2-yl, RS) radicals, which in turn react with O2. Multiplexed time-of-flight mass spectrometry, coupled to either a hydrogen discharge lamp or tunable synchrotron photoionizing radiation, is used to detect products as a function of mass, time, and photon energy. At 8 torr, the nature of the chain propagating cyclic ether + OH channel changes as a function of temperature. At 450 K, the production of OH is mainly in conjunction with formation of 2,4-dimethyloxetan-3-one, resulting from reaction of the resonance-stabilized secondary RS with O2. In contrast, at 550 K and 8 torr, 2-methyl-tetrahydrofuran-3-one, originating from oxidation of the primary radical (RP), is observed as the dominant cyclic ether product. Formation of both of these cyclic ether production channels proceeds via a resonance-stabilized hydroperoxy alkyl (QOOH) intermediate. Little or no ketohydroperoxide (KHP) is observed under the low-pressure conditions. At higher O2 concentrations and higher pressures (1−2 atm), a strong KHP signal appears as the temperature is increased above 450 K. Definitive isomeric identification from measurements on the deuterated DEK isotopologues indicates the favored pathway produces a γ-KHP via resonance-stabilized alkyl, QOOH, and HOOPOOH radicals. Time-resolved measurements reveal the KHP formation becomes faster and signal more intense upon increasing temperature from 450 to 575 K before intensity drops significantly at 625 K. The KHP time profile also shows a peak followed by a gradual depletion for the extent of experiment. Several tertiary products exhibit a slow accumulation in coincidence with the observed KHP decay. These products can be associated with decomposition of KHP by β-scission pathways or via isomerization of a γ-KHP into a cyclic peroxide intermediate (Korcek mechanism). The oxidation of d4-DEK, where kinetic isotope effects disfavor γ-KHP formation, shows greatly reduced KHP formation and associated signatures from KHP decomposition products.

This work provides new temperature-dependent mole fractions of elusive intermediates relevant to the low-temperature oxidation of dimethyl ether (DME). It extends the previous study of Moshammer et al. [J. Phys. Chem. A 2015, 119, 7361-7374 ] in which a combination of a jet-stirred reactor and molecular beam mass spectrometry with single-photon ionization via tunable synchrotron-generated vacuum-ultraviolet radiation was used to identify (but not quantify) several highly oxygenated species. Here, temperature-dependent concentration profiles of 17 components were determined in the range of 450-1000 K and compared to up-to-date kinetic modeling results. Special emphasis is paid toward the validation and application of a theoretical method for predicting photoionization cross sections that are hard to obtain experimentally but essential to turn mass spectral data into mole fraction profiles. The presented approach enabled the quantification of the hydroperoxymethyl formate (HOOCH2OCH2O), which is a key intermediate in the low-temperature oxidation of DME. The quantification of this keto-hydroperoxide together with the temperature-dependent concentration profiles of other intermediates including H2O2, HCOOH, CH3OCHO, and CH3OOH reveals new opportunities for the development of a next-generation DME combustion chemistry mechanism.

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We report a combined experimental and quantum chemistry study of the initial reactions in low-temperature oxidation of tetrahydrofuran (THF). Using synchrotron-based time-resolved VUV photoionization mass spectrometry, we probe numerous transient intermediates and products at P = 10-2000 Torr and T = 400-700 K. A key reaction sequence, revealed by our experiments, is the conversion of THF-yl peroxy to hydroperoxy-THF-yl radicals (QOOH), followed by a second O2 addition and subsequent decomposition to dihydrofuranyl hydroperoxide + HO2 or to γ-butyrolactone hydroperoxide + OH. The competition between these two pathways affects the degree of radical chain-branching and is likely of central importance in modeling the autoignition of THF. We interpret our data with the aid of quantum chemical calculations of the THF-yl + O2 and QOOH + O2 potential energy surfaces. On the basis of our results, we propose a simplified THF oxidation mechanism below 700 K, which involves the competition among unimolecular decomposition and oxidation pathways of QOOH.

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Cl-initiated oxidation reactions of three small-chain methyl esters, methyl propanoate (CH3CH2COOCH3; MP), methyl butanoate (CH3CH2CH2COOCH3; MB), and methyl valerate (CH3CH2CH2CH2COOCH3; MV), are studied at 1 or 8 Torr and 550 and 650 K. Products are monitored as a function of mass, time, and photoionization energy using multiplexed photoionization mass spectrometry coupled to tunable synchrotron photoionization radiation. Pulsed photolysis of molecular chlorine is the source of Cl radicals, which remove an H atom from the ester, forming a free radical. In each case, after addition of O2 to the initial radicals, chain-terminating HO2-elimination reactions are observed to be important. Branching ratios among competing HO2-elimination channels are determined via absolute photoionization spectra of the unsaturated methyl ester coproducts. At 550 K, HO2-elimination is observed to be selective, resulting in nearly exclusive production of the conjugated methyl ester coproducts, methyl propenoate, methyl-2-butenoate, and methyl-2-pentenoate, respectively. However, in MV, upon raising the temperature to 650 K, other HO2-elimination pathways are observed that yield methyl-3-pentenoate and methyl-4-pentenoate. In each methyl ester oxidation reaction, a peak is observed at a mass consistent with cyclic ether formation, indicating chain-propagating OH loss/ring formation pathways via QOOH intermediates. Evidence is observed for the participation of resonance-stabilized QOOH in the most prominent cyclic ether pathways. Stationary point energies for HO2-elimination pathways and select cyclic ether formation channels are calculated at the CBS-QB3 level of theory and assist in the assignment of reaction pathways and final products.

Chain-branching reactions represent a general motif in chemistry, encountered in atmospheric chemistry, combustion, polymerization, and photochemistry; the nature and amount of radicals generated by chain-branching are decisive for the reaction progress, its energy signature, and the time towards its completion. In this study, experimental evidence for two new types of chain-branching reactions is presented, based upon detection of highly oxidized multifunctional molecules (HOM) formed during the gas-phase low-temperature oxidation of a branched alkane under conditions relevant to combustion. The oxidation of 2,5-dimethylhexane (DMH) in a jet-stirred reactor (JSR) was studied using synchrotron vacuum ultra-violet photoionization molecular beam mass spectrometry (SVUV-PI-MBMS). Specifically, species with four and five oxygen atoms were probed, having molecular formulas of C8H14O4 (e.g., diketo-hydroperoxide/keto-hydroperoxy cyclic ether) and C8H16O5 (e.g., keto-dihydroperoxide/dihydroperoxy cyclic ether), respectively. The formation of C8H16O5 species involves alternative isomerization of OOQOOH radicals via intramolecular H-atom migration, followed by third O2 addition, intramolecular isomerization, and OH release; C8H14O4 species are proposed to result from subsequent reactions of C8H16O5 species. The mechanistic pathways involving these species are related to those proposed as a source of low-volatility highly oxygenated species in Earth's troposphere. At the higher temperatures relevant to auto-ignition, they can result in a net increase of hydroxyl radical production, so these are additional radical chain-branching pathways for ignition. The results presented herein extend the conceptual basis of reaction mechanisms used to predict the reaction behavior of ignition, and have implications on atmospheric gas-phase chemistry and the oxidative stability of organic substances.