Stereoselectivity in Atmospheric Autoxidation

Radical Stereochemistry: Accounting for Diastereomers in Kinetic Mechanism Development

Recent work in combustion and atmospheric chemistry has revealed cases in which diastereomers must be distinguished to accurately model a reacting flow. This paper presents an open-source framework for introducing such stereoisomer resolution into a kinetic mechanism. We detail our definitions and algorithms for labeling and enumerating the stereoisomers of a molecule and then generalize our system to describe the transition state (TS) of a reaction. This allows for the stereospecific enumeration of reactants and products while accounting for "fleeting" stereochemistry that is unique to the TS. We also present the AutoMech Chemical Identifier (AMChI), an InChI-like string identifier that accounts for stereocenters omitted by InChI. This identifier is extended to describe the TSs of reactions, providing a universal lookup key for specific reaction channels. The final piece of our methodology is an analytic formula to remove redundancy from a stereoresolved mechanism when its enantiomers exist as a racemic mixture, making it as compact as possible while fully accounting for the differences between diastereomers. In applying our methodology to two subsets of the NUIGMech1.1 mechanism, we find that our approach reduces the extra species added for large-fuel oxidation from 2231 (133%, full expansion) to 694 (41%, nonredundant expansion). We also find that for pyrolysis more than a quarter of the species in the expanded mechanism cannot be properly described by an InChI string, requiring an AMChI string to communicate their identity. Finally, we find that roughly one-quarter of the large-fuel oxidation reactions and one-third of the pyrolysis reactions include fleeting TS stereochemistry, which may have relevant effects on their kinetics.

Atmospheric Photo-Oxidation of 2-Ethoxyethanol: Autoxidation Chemistry of Glycol Ethers

We investigate the gas-phase photo-oxidation of 2-ethoxyethanol (2-EE) initiated by the OH radical with a focus on its autoxidation pathways. Gas-phase autoxidation─intramolecular H-shifts followed by O2 addition─has recently been recognized as a major atmospheric chemical pathway that leads to the formation of highly oxygenated organic molecules (HOMs), which are important precursors for secondary organic aerosols (SOAs). Here, we examine the gas-phase oxidation pathways of 2-EE, a model compound for glycol ethers, an important class of volatile organic compounds (VOCs) used in volatile chemical products (VCPs). Both experimental and computational techniques are applied to analyze the photochemistry of the compound. We identify oxidation products from both bimolecular and autoxidation reactions from chamber experiments at varied HO2 levels and provide estimations of rate coefficients and product branching ratios for key reaction pathways. The H-shift processes of 2-EE peroxy radicals (RO2) are found to be sufficiently fast to compete with bimolecular reactions under modest NO/HO2 conditions. More than 30% of the produced RO2 are expected to undergo at least one H-shift for conditions typical of modern summer urban atmosphere, where RO2 bimolecular lifetime is becoming >10 s, which implies the potential for glycol ether oxidation to produce considerable amounts of HOMs at reduced NOx levels and elevated temperature. Understanding the gas-phase autoxidation of glycol ethers can help fill the knowledge gap in the formation of SOA derived from oxygenated VOCs emitted from VCP sources.

Highly Efficient Autoxidation of Triethylamine

Autoxidation has been acknowledged as a major oxidation pathway in a broad range of atmospherically important compounds including isoprene and monoterpenes. More recently, autoxidation has also been identified as central and even dominant in the atmospheric oxidation of the rather small nonhydrocarbons dimethyl sulfide (DMS) and trimethylamine (TMA). Here, we find even faster autoxidation in the aliphatic amine triethylamine (TEA). The atmospherically dominating autoxidation leads to highly oxygenated and functionalized compounds. Products with as many as three hydroperoxy (OOH) groups and an O:C ratio larger than 1 are formed. We present theoretical multiconformer transition-state theory (MC-TST) calculations of the unimolecular reactions in the autoxidation following the OH + TEA reaction and calculate peroxy radical H-shift rate coefficients >20 s-1 for the first two generations of H-shifts. The efficient autoxidation in TEA is verified by the observation of the proposed highly oxidized products and radicals in flow-tube experiments. We find that the initial OH hydrogen abstraction at the α-carbon is strongly favored, with the β-carbon abstraction yield being less than a few percent.

Organic Peroxides in Aerosol: Key Reactive Intermediates for Multiphase Processes in the Atmosphere

Organic peroxides (POs) are organic molecules with one or more peroxide (-O-O-) functional groups. POs are commonly regarded as chemically labile termination products from gas-phase radical chemistry and therefore serve as temporary reservoirs for oxidative radicals (HOx and ROx) in the atmosphere. Owing to their ubiquity, active gas-particle partitioning behavior, and reactivity, POs are key reactive intermediates in atmospheric multiphase processes determining the life cycle (formation, growth, and aging), climate, and health impacts of aerosol. However, there remain substantial gaps in the origin, molecular diversity, and fate of POs due to their complex nature and dynamic behavior. Here, we summarize the current understanding on atmospheric POs, with a focus on their identification and quantification, state-of-the-art analytical developments, molecular-level formation mechanisms, multiphase chemical transformation pathways, as well as environmental and health impacts. We find that interactions with SO2 and transition metal ions are generally the fast PO transformation pathways in atmospheric liquid water, with lifetimes estimated to be minutes to hours, while hydrolysis is particularly important for α-substituted hydroperoxides. Meanwhile, photolysis and thermolysis are likely minor sinks for POs. These multiphase PO transformation pathways are distinctly different from their gas-phase fates, such as photolysis and reaction with OH radicals, which highlights the need to understand the multiphase partitioning of POs. By summarizing the current advances and remaining challenges for the investigation of POs, we propose future research priorities regarding their origin, fate, and impacts in the atmosphere.

Cost-Effective Implementation of Multiconformer Transition State Theory for Alkoxy Radical Unimolecular Reactions

Alkoxy radicals are important intermediates in the gas-phase oxidation of volatile organic compounds (VOCs) determining the nature of the first-generation products. An accurate description of their chemistry under atmospheric conditions is essential for understanding the atmospheric oxidation of VOCs. Unfortunately, experimental measurements of the rate coefficients of unimolecular alkoxy radical reactions are scarce, especially for larger systems. As has previously been done for peroxy radical hydrogen shift reactions, we present a cost-effective approach to the practical implementation of multiconformer transition state theory (MC-TST) for alkoxy radical unimolecular (H-shift and decomposition) reactions. Specifically, we test the optimal approach for the conformational sampling as well as the best value for a cutoff of high-energy conformers. In order to obtain accurate rate coefficients at a reduced computational cost, an energy cutoff is employed to reduce the required number of high-level calculations. The rate coefficients obtained with the developed theoretical approach are compared to available experimental rate coefficients for both 1,5 H-shifts and decomposition reactions. For all but one of the reactions tested, the calculated MC-TST rate coefficients agree with experimental results to within a factor of 7. The discrepancy for the final reaction is about a factor of 15, but part of the discrepancy is caused by pressure effects, which are not included in MC-TST. Thus, for the fastest alkoxy reactions, deviation from the high-pressure limit even at 1 bar should be considered.

Probing autoxidation of oleic acid at air-water interface: A neglected and significant pathway for secondary organic aerosols formation

Fatty acids have been proposed to be a potential source of precursors for SOAs, but the autoxidation process was neglected in the oxidation studies. Here, the autoxidation of oleic acid was explored using microdroplet mass spectrometry. Bulk solution, concentration and solvent composition experiments provided direct evidences for that the autoxidation occurred at or near the air-water interface. The kinetic data showed an acceleration at this interface and was comparable to ozonation, indicating that autoxidation is an important pathway for SOAs formation. In addition, intermediates/products were captured and identified using tandem mass spectrometry, spin-trapping and quenched agents. The autoxidation mechanism was divided into addition intermediates (AIs) and Criegee intermediates (CIs) pathways mediated by hydroxyl radicals (OH). The CI chemistry which is ubiquitous in gas phase was observed at the air-water interface, and this leaded to the mass/volume loss of aerosols. Inversely, the AI chemistry caused the increase of mass, density and hygroscopicity of aerosols. AI chemistry was dominated compared to CI chemistry, but varied by concerning aerosol sizes, ultraviolet light (UV) and charge. Moreover, the MS approach of selectively probing the interfacial substances at the scale of sub-seconds opens new opportunities to study heterogeneous chemistry in atmosphere.

Hydrotrioxide (ROOOH) formation in the atmosphere

Organic hydrotrioxides (ROOOH) are known to be strong oxidants used in organic synthesis. Previously, it has been speculated that they are formed in the atmosphere through the gas-phase reaction of organic peroxy radicals (RO₂) with hydroxyl radicals (OH). Here, we report direct observation of ROOOH formation from several atmospherically relevant RO₂ radicals. Kinetic analysis confirmed rapid RO₂ + OH reactions forming ROOOH, with rate coefficients close to the collision limit. For the OH-initiated degradation of isoprene, global modeling predicts molar hydrotrioxide formation yields of up to 1%, which represents an annual ROOOH formation of about 10 million metric tons. The atmospheric lifetime of ROOOH is estimated to be minutes to hours. Hydrotrioxides represent a previously omitted substance class in the atmosphere, the impact of which needs to be examined.

Spiers Memorial Lecture: theory of unimolecular reactions

One hundred years ago, at an earlier Faraday Discussion meeting, Lindemann presented a mechanism that provides the foundation for contemplating the pressure dependence of unimolecular reactions. Since that time, our...

Atmospheric Chemistry of CH3OCF2CHF2

Fourier transform infrared spectroscopy has been used to follow the reaction of CH₃OCF₂CHF₂ with either Cl or OH radicals within a photoreactor. Rate constants of k(OH + CH₃OCF₂CHF₂) = (2.25 ± 0.60) × 10^-14 cm³ molecule^-1 s^-1 and k(Cl + CH₃OCF₂CHF₂) = (2.50 ± 0.39) × 10^-13 cm³ molecule^-1 s^-1 were determined at 296 ± 2 K. Theoretical and experimental investigation of the Cl + CH₃OCF₂CHF₂ reaction identified the formation of two main products, HC(O)OCF₂CHF₂ and COF₂. Chlorine (and OH) radicals react with CH₃OCF₂CHF₂ by H-abstraction from either the -CH₃ or -CHF₂ site. Abstraction from the -CH₃ site was determined to constitute at least 60%, as determined from the formation of the primary product, HC(O)OCF₂CHF₂, which can only form from this abstraction site. At longer reaction times, HC(O)OCF₂CHF₂ further reacts and the yield of COF₂ approaches two, the maximum possible with the number of F atoms in the reactant. The atmospheric lifetime of CH₃OCF₂CHF₂ with OH radicals was determined to be 1.4 years. The global warming potentials over 20-, 100-, and 500-year time horizons were estimated to be 325, 88, and 25, respectively.

Anthropogenic Volatile Organic Compound (AVOC) Autoxidation as a Source of Highly Oxygenated Organic Molecules (HOM)

Gas-phase hydrocarbon autoxidation is a rapid pathway for the production of in situ aerosol precursor compounds. It is a highway to molecular growth and lowering of vapor pressure, and it produces hydrogen-bonding functional groups that allow a molecule to bind into a substrate. It is the crucial process in the formation and growth of atmospheric secondary organic aerosol (SOA). Recently, the rapid gas-phase autoxidation of several volatile organic compounds (VOC) has been shown to yield highly oxygenated organic molecules (HOM). Most of the details on HOM formation have been obtained from biogenic monoterpenes and their surrogates, with cyclic structures and double bonds both found to strongly facilitate HOM formation, especially in ozonolysis reactions. Similar structural features in common aromatic compounds have been observed to facilitate high HOM formation yields, despite the lack of appreciable O3 reaction rates. Similarly, the recently observed autoxidation and subsequent HOM formation in the oxidation of saturated hydrocarbons cannot be initiated by O3 and require different mechanistic steps for initiating and propagating the autoxidation sequence. This Perspective reflects on these recent findings in the context of the direct aerosol precursor formation in urban atmospheres.

Diastereomers and Low-Temperature Oxidation

Diastereomers have historically been ignored when building kinetic mechanisms for combustion. Low-temperature oxidation kinetics, which continues to gain interest in both combustion and atmospheric communities, may be affected by the inclusion of diastereomers in radical chain-branching pathways. In this work, key intermediates and transition states lacking stereochemical specification in an existing diethyl ether low-temperature oxidation mechanism were replaced with their diastereomeric counterparts. Rate coefficients for reactions involving diastereomers were computed with ab initio transition state theory master equation calculations. The presence of diastereomers increased rate coefficients by factors of 1.2-1.6 across various temperatures and pressures. Ignition delay simulations incorporating these revised rate coefficients indicate that the diastereomers enhanced the overall reactivity of the mechanism by almost 15% and increased the peak ketohydroperoxide concentration by 30% in the negative temperature coefficient region at combustion-relevant pressures. These results provide an illustrative indication of the important role of stereomeric effects in oxidation kinetics.

Structures and reactivity of peroxy radicals and dimeric products revealed by online tandem mass spectrometry

Organic peroxy radicals (RO₂) play a pivotal role in the degradation of hydrocarbons. The autoxidation of atmospheric RO₂ radicals produces highly oxygenated organic molecules (HOMs), including low-volatility ROOR dimers formed by bimolecular RO₂ + RO₂ reactions. HOMs can initiate and greatly contribute to the formation and growth of atmospheric particles. As a result, HOMs have far-reaching health and climate implications. Nevertheless, the structures and formation mechanism of RO₂ radicals and HOMs remain elusive. Here, we present the in-situ characterization of RO₂ and dimer structure in the gas-phase, using online tandem mass spectrometry analyses. In this study, we constrain the structures and formation pathway of several HOM-RO₂ radicals and dimers produced from monoterpene ozonolysis, a prominent atmospheric oxidation process. In addition to providing insights into atmospheric HOM chemistry, this study debuts online tandem MS analyses as a unique approach for the chemical characterization of reactive compounds, e.g., organic radicals.

Organic peroxy radicals play a pivotal role in producing highly oxygenated organic molecules but the formation mechanisms remain elusive. Here, the authors show in-situ characterization of peroxy radicals and dimer structures in the gas-phase, using online tandem mass spectrometry analyses.

Isomerization and Degradation of Levoglucosan via the Photo-Fenton Process: Insights from Aqueous-Phase Experiments and Atmospheric Particulate Matter

So far, studies on the conversion of stereochemistry under photo-Fenton conditions and their atmospheric implication are still rare. Here, we found that the biomass burning marker, the chiral compound levoglucosan (L), undergoes oxidative degradation under photo-Fenton conditions and can be isomerized into mannosan (M) and galactosan (G) simultaneously. Among the formic acid, acetic acid, and oxalic acid in the degradation products of levoglucosan, it was found that the yield of formation of formic acid in the photo-Fenton pathway can be as high as 86%. It is worth noting that both levoglucosan and its isomers are present in the atmosphere and their concentrations are strongly correlated. At the same time, the range of their concentration ratios, L/(G + M), measured in the photo-Fenton experiments in the laboratory was found to agree well with that measured in atmospheric PM_2.5 samples. However, the sources of L, G, and M in the atmosphere are complex, and the photo-Fenton reaction may be an essential pathway for the distribution of L, G, and M in the atmosphere.

Atmospheric Autoxidation of Amines

Autoxidation has been acknowledged as a major oxidation pathway in a broad range of atmospherically important compounds including isoprene, monoterpenes, and very recently, dimethyl sulfide. Here, we present a high-level theoretical multiconformer transition-state theory study of the atmospheric autoxidation in amines exemplified by the atmospherically important trimethylamine (TMA) and dimethylamine and generalized by the study of the larger diethylamine. Overall, we find that the initial hydrogen shift reactions have rate coefficients greater than 0.1 s^-1 and autoxidation is thus an important atmospheric pathway for amines. This autoxidation efficiently leads to the formation of hydroperoxy amides, a new type of atmospheric nitrogen-containing compounds, and for TMA, we experimentally confirm this. The conversion of amines to hydroperoxy amides may have important implications for nucleation and growth of atmospheric secondary organic aerosols and atmospheric OH recycling.