Infrared signature of the hydroperoxyalkyl intermediate (·QOOH) in cyclohexane oxidation: An isomer-resolved spectroscopic study

Infrared (IR) action spectroscopy is utilized to characterize carbon-centered hydroperoxy-cyclohexyl radicals (·QOOH) transiently formed in cyclohexane oxidation. The oxidation pathway leads to three nearly degenerate ·QOOH isomers, β-, γ-, and δ-QOOH, which are generated in the laboratory by H-atom abstraction from the corresponding ring sites of the cyclohexyl hydroperoxide (CHHP) precursor. The IR spectral features of jet-cooled and stabilized ·QOOH radicals are observed from 3590 to 7010 cm-1 (∼10-20 kcal mol-1) at energies in the vicinity of the transition state (TS) barrier leading to OH radicals that are detected by ultraviolet laser-induced fluorescence. The experimental approach affords selective detection of β-QOOH, arising from its significantly lower TS barrier to OH products compared to γ and δ isomers, which results in rapid unimolecular decay and near unity branching to OH products. The observed IR spectrum of β-QOOH includes fundamental and overtone OH stretch transitions, overtone CH stretch transitions, and combination bands involving OH or CH stretch with lower frequency modes. The assignment of β-QOOH spectral features is guided by anharmonic frequencies and intensities computed using second-order vibrational perturbation theory. The overtone OH stretch (2νOH) of β-QOOH is shifted only a few wavenumbers from that observed for the CHHP precursor, yet they are readily distinguished by their prompt vs slow dissociation rates to OH products.

Dimeric product formation in the self-reaction of small peroxy radicals using synchrotron radiation vacuum ultraviolet photoionization mass spectrometry

Peroxy radicals (RO2) are key reactive intermediates in atmospheric oxidation processes and yet their chemistry is not fully unraveled. Little is known about their structures and the structures of the dimeric products (ROOR) in the self-reaction of small RO2, which are among the most abundant RO2 in the atmosphere. The product branching ratios of ROOR and their atmospheric roles are still in controversy. Here, the self-reaction of propyl peroxy radicals (C3H7O2), a typical small RO2 radical in the atmosphere, has been studied using synchrotron radiation vacuum ultraviolet photoionization mass spectrometry. Both radical (C3H7O) and closed-shell molecular (C3H6O, C3H7OH, C3H7OOC3H7) products in the self-reaction are observed in photoionization mass spectra and their elusive isomers are definitely identified in mass-selected photoionization spectra. Three isomers of the C3H7OOC3H7 dimeric products, R1OOR1, R1OOR2, and R2OOR2 (R1 and R2 represent 1-C3H7 and 2-C3H7, respectively), as well as their complex structures have been determined for the first time. Kinetic experiments are performed and compared with chemical simulations to reveal the sources of specific products. The branching ratio of the C3H7OOC3H7 dimeric channel is measured at 10 ± 5%. This work demonstrates that the dimeric product formation in the self-reaction of small RO2 radicals is non-negligible and should provide valuable new insight into atmospheric modelling.

1-Hexene Ozonolysis across Atmospheric and Combustion Temperatures via Synchrotron-Based Photoelectron Spectroscopy and Chemical Ionization Mass Spectrometry

This study investigates the complex interaction between ozone and the autoxidation of 1-hexene over a wide temperature range (300-800 K), overlapping atmospheric and combustion regimes. It is found that atmospheric molecular mechanisms initiate the oxidation of 1-hexene from room temperature up to combustion temperatures, leading to the formation of highly oxygenated organic molecules. As temperature rises, the highly oxygenated organic molecules contribute to radical-branching decomposition pathways inducing a high reactivity in the low-temperature combustion region, i.e., from 550 K. Above 650 K, the thermal decomposition of ozone into oxygen atoms becomes the dominant process, and a remarkable enhancement of the conversion is observed due to their diradical nature, counteracting the significant negative temperature coefficient behavior usually observed for 1-hexene. In order to better characterize the formation of heavy oxygenated organic molecules at the lowest temperatures, two analytical performance methods have been combined for the first time: synchrotron-based mass-selected photoelectron spectroscopy and orbitrap chemical ionization mass spectrometry. At the lowest studied temperatures (below 400 K), this analytical work has demonstrated the formation of the ketohydroperoxides usually found during the LTC oxidation of 1-hexene, as well as of molecules containing up to nine O atoms.

Direct Observation of Gas-Phase Hydroxymethylene: Photoionization and Kinetics Resulting from Methanol Photodissociation

Carbene species play an integral role in high-energy chemistry, transition-metal-carbene chemistry, catalysis, photolytic formation of carbohydrates, and possibly even the formation of interstellar sugars. In 1921, "reactive formaldehyde"─now known as hydroxymethylene (HCOH)─was first implicated as an intermediate in photocatalytic processes. However, due to its transient nature, direct observation of HCOH has predominantly been attained using cryogenic isolation methods. As a result, HCOH gas-phase reactivity measurements have been limited. We directly observed HCOH using photoionization spectroscopy following UV photodissociation of methanol. Our measurements show it reacts slowly with O2 at room temperature. This work provides evidence for the formation mechanism of HCOH from CH3OH and its subsequent reactivity under gas-phase reaction conditions.

Reaction dynamics of S(3P) with 1,3-butadiene and isoprene: crossed-beam scattering, low-temperature flow experiments, and high-level electronic structure calculations

Sulfur atoms serve as key players in diverse chemical processes, from astrochemistry at very low temperature to combustion at high temperature. Building upon our prior findings, showing cyclization to thiophenes following the reaction of ground-state sulfur atoms with dienes, we here extend this investigation to include many additional reaction products, guided by detailed theoretical predictions. The outcomes highlight the complex formation of products during intersystem crossing (ISC) to the singlet surfaces. Here, we employed crossed-beam velocity map imaging and high-level ab initio methods to explore the reaction of S(3P) with 1,3-butadiene and isoprene under single-collision conditions and in low-temperature flows. For the butadiene reaction, our experimental results show the formation of thiophene via H2 loss, a 2H-thiophenyl radical through H loss, and thioketene through ethene loss at a slightly higher collision energy compared to previous observations. Complementary Chirped-Pulse Fourier-Transform mmWave spectroscopy (CP-FTmmW) measurements in a uniform flow confirmed the formation of thioketene in the reaction at 20 K. For the isoprene reaction, we observed analogous products along with the 2H-thiophenyl radical arising from methyl loss and C3H4S (loss of ethene or H2 + acetylene). CP-FTmmW detected the formation of thioformaldehyde via loss of 1,3-butadiene, again in the 20 K flow. Coupled-cluster calculations on the pathways found by the automated kinetic workflow code KinBot support these findings and indicate ISC to the singlet surface, leading to the generation of various long-lived intermediates, including 5-membered heterocycles.

Vibrational spectroscopy and dissociation dynamics of cyclohexyl hydroperoxide

Organic hydroperoxides (ROOH) are ubiquitous in the atmospheric oxidation of volatile organic compounds (VOCs) as well as in low-temperature oxidation of hydrocarbon fuels. The present work focuses on a prototypical cyclic hydroperoxide, cyclohexyl hydroperoxide (CHHP). The overtone OH stretch (2νOH) spectrum of jet-cooled CHHP is recorded by IR multiphoton excitation with UV laser-induced fluorescence detection of the resulting OH products. A distinctive IR feature is observed at 7012.5 cm-1. Two conformers of CHHP are predicted to have similar stabilities (within 0.2 kcal mol-1) and overtone OH stretch transitions (2νOH), yet are separated by a significant interconversion barrier. The IR power dependence indicates that absorption of three or more IR photons is required for dissociation of CHHP to cyclohexoxy (RO) and OH radical products. Accompanying high-level single- and multi-reference electronic structure calculations quantitatively support the experimental results. Calculations are extended to a range of organic hydroperoxides to examine trends in bond dissociation energies associated with RO + OH formation and compared with prior theoretical results.

Isomer-resolved unimolecular dynamics of the hydroperoxyalkyl intermediate (•QOOH) in cyclohexane oxidation

The oxidation of cycloalkanes is important in the combustion of transportation fuels and in atmospheric secondary organic aerosol formation. A transient carbon-centered radical intermediate (•QOOH) in the oxidation of cyclohexane is identified through its infrared fingerprint and time- and energy-resolved unimolecular dissociation dynamics to hydroxyl (OH) radical and bicyclic ether products. Although the cyclohexyl ring structure leads to three nearly degenerate •QOOH isomers (β-, γ-, and δ-QOOH), their transition state (TS) barriers to OH products are predicted to differ considerably. Selective characterization of the β-QOOH isomer is achieved at excitation energies associated with the lowest TS barrier, resulting in rapid unimolecular decay to OH products that are detected. A benchmarking approach is employed for the calculation of high-accuracy stationary point energies, in particular TS barriers, for cyclohexane oxidation (C6H11O2), building on higher-level reference calculations for the smaller ethane oxidation (C2H5O2) system. The isomer-specific characterization of β-QOOH is validated by comparison of experimental OH product appearance rates with computed statistical microcanonical rates, including significant heavy-atom tunneling, at energies in the vicinity of the TS barrier. Master-equation modeling is utilized to extend the results to thermal unimolecular decay rate constants at temperatures and pressures relevant to cyclohexane combustion.

Pollution characteristics, source appointment and environmental effect of oxygenated volatile organic compounds in Guangdong-Hong Kong-Macao Greater Bay Area: Implication for air quality management

Same as other bay areas, the Guangdong-Hong Kong-Macao Greater Bay Area (GBA) is also suffering atmospheric composite pollution. Even a series of atmospheric environment management policies have been conducted to win the "blue sky defense battle", the atmospheric secondary pollutants (e.g., O3) originated from oxygenated volatile organic compounds (OVOCs) still threaten the air quality in GBA. However, there lacks a systematic summary on the emission, formation, pollution and environmental effects of OVOCs in this region for further air quality management. This review focused on the researches related to OVOCs in GBA, including their pollution characteristics, detection methods, source distributions, secondary formations, and impacts on the atmosphere. Pollution profile of OVOCs in GBA revealed that the concentration percentage among total VOCs from Guangzhou and Dongguan cities exceeded 50 %, while methanol, formaldehyde, acetone, and acetaldehyde were the top four highest concentrated OVOCs. The detection technique on regional atmospheric OVOCs (e.g., oxygenated organic molecules (OOMs)) underwent an evolution of off-line derivatization method, on-line spectroscopic method and on-line mass spectrometry method. The OVOCs in GBA were mainly from primary emissions (up to 80 %), including vehicle emissions and biomass combustion. The anthropogenic alkenes and aromatics in urban area, and natural isoprene in rural area also made a significant contribution to the secondary emission (e.g., photochemical formation) of OVOCs. About 20 % in average of ROx radicals was produced from photolysis of formaldehyde in comparison with O3, nitrous acid and rest OVOCs, while the reaction between OVOCs and free radical accelerated the NOx-O3 cycle, contributing to 15 %-60 % cumulative formation of O3 in GBA. Besides, the heterogeneous reactions of dicarbonyls generated 21 %-53 % of SOA. This review also provided suggestions for future research on OVOCs in terms of regional observation, analytical method and mechanistic study to support the development of a control and management strategy on OVOCs in GBA and China.

Automated Generation of a Compact Chemical Kinetic Model for n-Pentane Combustion

We have employed automated mechanism generation tools to construct a detailed chemical kinetic model for combustion of n-pentane, as a step toward the generation of compact kinetic models for larger alkanes. Pentane is of particular interest as a prototype for combustion of alkanes and as the smallest paraffin employed as a hybrid rocket fuel, albeit at cryogenic conditions. A reaction mechanism for pentane combustion thus provides a foundation for modeling combustion of extra-large alkanes (paraffins) that are of more practical interest as hybrid rocket fuels, for which manual construction becomes infeasible. Here, an n-pentane combustion kinetic model is developed using the open-source software package Reaction Mechanism Generator (RMG). The model was generated and tested across a range of temperatures (650 to 1350 K) and equivalence ratios (0.5, 1.0, 2.0) at pressures of 1 and 10 atm. Available thermodynamic and kinetic databases were incorporated wherever possible. Predictions using the mechanism were validated against the published laminar burning velocities (Su) and ignition delay times (IDT) of n-pentane. To improve the model performance, a comprehensive analysis, including reaction path and sensitivity analyses of n-pentane oxidation, was performed, leading us to modify the thermochemistry and rate parameters for a few key species and reactions. These were combined as a separate data set, an RMG library, that was then used during mechanism generation. The resulting model predicted IDTs as accurately as the best manually constructed mechanisms, while remaining much more compact. It predicted flame speeds to within 10% of published experimental results. The degree of success of automated mechanism generation for this case suggests that it can be extended to construct reliable and compact models for combustion of larger n-alkanes, particularly when using this mechanism as a seed submodel.

Alkylperoxy radicals are responsible for the formation of oxygenated primary organic aerosol

Organic aerosol (OA) is an air pollutant ubiquitous in urban atmospheres. Urban OA is usually apportioned into primary OA (POA), mostly emitted by mobile sources, and secondary OA (SOA), which forms in the atmosphere due to oxidation of gas-phase precursors from anthropogenic and biogenic sources. By performing coordinated measurements in the particle phase and the gas phase, we show that the alkylperoxy radical chemistry that is responsible for low-temperature ignition also leads to the formation of oxygenated POA (OxyPOA). OxyPOA is distinct from POA emitted during high-temperature ignition and is chemically similar to SOA. We present evidence for the prevalence of OxyPOA in emissions of a spark-ignition engine and a next-generation advanced compression-ignition engine, highlighting the importance of understanding OxyPOA for predicting urban air pollution patterns in current and future atmospheres.

Stereo-electronic factors influencing the stability of hydroperoxyalkyl radicals: transferability of chemical trends across hydrocarbons and ab initio methods

The hydroperoxyalkyl radicals (˙QOOH) are known to play a significant role in combustion and tropospheric processes, yet their direct spectroscopic detection remains challenging. In this study, we investigate molecular stereo-electronic effects influencing the kinetic and thermodynamic stability of a ˙QOOH along its formation path from the precursor, alkylperoxyl radical (ROO˙), and the depletion path resulting in the formation of cyclic ether + ˙OH. We focus on reactive intermediates encountered in the oxidation of acyclic hydrocarbon radicals: ethyl, isopropyl, isobutyl, tert-butyl, neopentyl, and their alicyclic counterparts: cyclohexyl, cyclohexenyl, and cyclohexadienyl. We report reaction energies and barriers calculated with the highly accurate method Weizmann-1 (W1) for the channels: ROO˙ ⇌ ˙QOOH, ROO˙ ⇌ alkene + ˙OOH, ˙QOOH ⇌ alkene + ˙OOH, and ˙QOOH ⇌ cyclic ether + ˙OH. Using W1 results as a reference, we have systematically benchmarked the accuracy of popular density functional theory (DFT), composite thermochemistry methods, and an explicitly correlated coupled-cluster method. We ascertain inductive, resonance, and steric effects on the overall stability of ˙QOOH and computationally investigate the possibility of forming more stable species. With new reactions as test cases, we probe the capacity of various ab initio methods to yield quantitative insights on the elementary steps of combustion.

Unimolecular Reactions of 2-Methyloxetanyl and 2-Methyloxetanylperoxy Radicals

Alkyl-substituted cyclic ethers are intermediates formed in abundance during the low-temperature oxidation of hydrocarbons and biofuels via a chain-propagating step with ȮH. Subsequent reactions of cyclic ether radicals involve a competition between ring opening and reaction with O2, the latter of which enables pathways mediated by hydroperoxy-substituted carbon-centered radicals (Q̇OOH). Due to the resultant implications of competing unimolecular and bimolecular reactions on overall populations of ȮH, detailed insight into the chemical kinetics of cyclic ethers remains critical to high-fidelity numerical modeling of combustion. Cl-initiated oxidation experiments were conducted on 2-methyloxetane (an intermediate of n-butane oxidation) using multiplexed photoionization mass spectrometry (MPIMS), in tandem with calculations of stationary point energies on potential energy surfaces for unimolecular reactions of 2-methyloxetanyl and 2-methyloxetanylperoxy isomers. The potential energy surfaces were computed using the KinBot algorithm with stationary points calculated at the CCSD(T)-F12/cc-pVDZ-F12 level of theory. The experiments were conducted at 6 Torr and two temperatures (650 K and 800 K) under pseudo-first-order conditions to facilitate Ṙ + O2 reactions. Photoionization spectra were measured from 8.5 eV to 11.0 eV in 50-meV steps, and relative yields were quantified for species consistent with Ṙ → products and Q̇OOH → products. Species detected in the MPIMS experiments are linked to specific radicals of 2-methyloxetane. Species from Ṙ → products include methyl, ethene, formaldehyde, propene, ketene, 1,3-butadiene, and acrolein. Ion signals consistent with products from alkyl radical oxidation were detected, including for Q̇OOH-mediated species, which are also low-lying channels on their respective potential energy surfaces. In addition to species common to alkyl oxidation pathways, ring-opening reactions of Q̇OOH radicals derived from 2-methyloxetane produced ketohydroperoxide species (performic acid and 2-hydroperoxyacetaldehyde), which may impart additional chain-branching potential, and dicarbonyl species (3-oxobutanal and 2-methylpropanedial), which often serve as proxies for modeling reaction rates of ketohydroperoxides. The experimental and computational results underscore that reactions of cyclic ethers are inherently more complex than currently prescribed in chemical kinetic models utilized for combustion.

The multichannel i-propyl + O2 reaction system: A model of secondary alkyl radical oxidation

The i-propyl + O2 reaction mechanism has been investigated by definitive quantum chemical methods to establish this system as a benchmark for the combustion of secondary alkyl radicals. Focal point analyses extrapolating to the ab initio limit were performed based on explicit computations with electron correlation treatments through coupled cluster single, double, triple, and quadruple excitations and basis sets up to cc-pV5Z. The rigorous coupled cluster single, double, and triple excitations/cc-pVTZ level of theory was used to fully optimize all reaction species and transition states, thus, removing some substantial flaws in reference geometries existing in the literature. The vital i-propylperoxy radical (MIN1) and its concerted elimination transition state (TS1) were found 34.8 and 4.4 kcal mol-1 below the reactants, respectively. Two β-hydrogen transfer transition states (TS2, TS2') lie above the reactants by (1.4, 2.5) kcal mol-1 and display large Born-Oppenheimer diagonal corrections indicative of nearby surface crossings. An α-hydrogen transfer transition state (TS5) is discovered 5.7 kcal mol-1 above the reactants that bifurcates into equivalent α-peroxy radical hanging wells (MIN3) prior to a highly exothermic dissociation into acetone + OH. The reverse TS5 → MIN1 intrinsic reaction path also displays fascinating features, including another bifurcation and a conical intersection of potential energy surfaces. An exhaustive conformational search of two hydroperoxypropyl (QOOH) intermediates (MIN2 and MIN3) of the i-propyl + O2 system located nine rotamers within 0.9 kcal mol-1 of the corresponding lowest-energy minima.

Unimolecular Reactions of 2,4-Dimethyloxetanyl Radicals

Alkyl-substituted oxetanes are cyclic ethers formed via unimolecular reactions of QOOH radicals produced via a six-membered transition state in the preceding isomerization step of organic peroxy radicals, ROO. Owing to radical isomer-specific formation pathways, cyclic ethers are unambiguous proxies for inferring QOOH reaction rates. Therefore, accounting for subsequent oxidation of cyclic ethers is important in order to accurately determine rates for QOOH → products. Cyclic ethers can react via unimolecular reaction (ring-opening) or via bimolecular reaction with O₂ to form cyclic ether-peroxy adducts. The computations herein provide reaction mechanisms and theoretical rate coefficients for the former type in order to determine competing pathways for the cyclic ether radicals. Rate coefficients of unimolecular reactions of 2,4-dimethyloxetanyl radicals were computed using master equation modeling from 0.01 to 100 atm and from 300 to 1000 K. Coupled-cluster methods were utilized for stationary-point energy calculations, and uncertainties in the computed rate coefficients were accounted for using variation in barrier heights and in well depths. The potential energy surfaces reveal accessible channels to several species via crossover reactions, such as 2-methyltetrahydrofuran-5-yl and pentanonyl isomers. For the range of temperature over which 2,4-dimethyloxetane forms during n-pentane oxidation, the following are the major channels: 2,4-dimethyloxetan-1-yl → acetaldehyde + allyl, 2,4-dimethyloxetan-2-yl → propene + acetyl, and 2,4-dimethyloxetan-3-yl → 3-butenal + methyl, or, 1-penten-3-yl-4-ol. Well-skipping reactions were significant in a number of channels and also exhibited a markedly different pressure dependence. The calculations show that rate coefficients for ring-opening are approximately an order of magnitude lower for the tertiary 2,4-dimethyloxetanyl radicals than for the primary and secondary 2,4-dimethyloxetanyl radicals. Unlike for reactions of the corresponding ROO radicals, however, unimolecular rate coefficients are independent of the stereochemistry. Moreover, rate coefficients of cyclic ether radical ring-opening are of the same order of magnitude as O₂ addition, underscoring the point that a competing network of reactions is necessary to include for accurate chemical kinetics modeling of species profiles for cyclic ethers.

Automated Reaction Kinetics of Gas-Phase Organic Species over Multiwell Potential Energy Surfaces

Automation of rate-coefficient calculations for gas-phase organic species became possible in recent years and has transformed how we explore these complicated systems computationally. Kinetics workflow tools bring rigor and speed and eliminate a large fraction of manual labor and related error sources. In this paper we give an overview of this quickly evolving field and illustrate, through five detailed examples, the capabilities of our own automated tool, KinBot. We bring examples from combustion and atmospheric chemistry of C-, H-, O-, and N-atom-containing species that are relevant to molecular weight growth and autoxidation processes. The examples shed light on the capabilities of automation and also highlight particular challenges associated with the various chemical systems that need to be addressed in future work.

Synthesis and Late-Stage Diversification of BN-Embedded Dibenzocorannulenes as Efficient Fluorescence Organic Light-Emitting Diode Emitters

We report the synthesis and late-stage diversification of a new class of hetero-buckybowl, BN-embedded dibenzocorannulenes (B₂ N₂ -DBCs). The synthesis is achieved via one-shot halogenative borylation, comprising the nitrogen-directed haloboration of alkyne and an intramolecular bora-Friedel-Crafts reaction, which provides BN-embedded dibenzocorannulene possessing two bromo substituents (B₂ N₂ -DBC-Br). B₂ N₂ -DBC-Br undergoes diversification via coupling reactions to provide a variety of arylated derivatives (B₂ N₂ -DBC-R), exhibiting strong blue fluorescence. An organic light-emitting diode (OLED) employing one of the derivatives as an emitter exhibited a high external quantum efficiency of 6.6 % and long operational lifetime of 907 h at an initial luminance of 1000 cd m^-2 , indicating the significant potential for the development of efficient and stable hetero-buckybowl-based OLED materials.

Ambient-pressure hydrogenation of CO2 into long-chain olefins

The conversion of CO₂ by renewable power-generated hydrogen is a promising approach to a sustainable production of long-chain olefins (C₄₊⁼) which are currently produced from petroleum resources. The decentralized small-scale electrolysis for hydrogen generation requires the operation of CO₂ hydrogenation in ambient-pressure units to match the manufacturing scales and flexible on-demand production. Herein, we report a Cu-Fe catalyst which is operated under ambient pressure with comparable C₄₊⁼ selectivity (66.9%) to that of the state-of-the-art catalysts (66.8%) optimized under high pressure (35 bar). The catalyst is composed of copper, iron oxides, and iron carbides. Iron oxides enable reverse-water-gas-shift to produce CO. The synergy of carbide path over iron carbides and CO insertion path over interfacial sites between copper and iron carbides leads to efficient C-C coupling into C₄₊⁼. This work contributes to the development of small-scale low-pressure devices for CO₂ hydrogenation compatible with sustainable hydrogen production.

The conversion of CO2 by renewable power-generated hydrogen is a promising approach to a sustainable production of long-chain olefins. Here the authors report a Cu-Fe catalyst which achieves the hydrogenation of CO2 into long-chain olefins under ambient pressure via the synergy of carbide mechanism and CO insertion mechanism.

High-Resolution Double Velocity Map Imaging Photoelectron Photoion Coincidence Spectrometer for Gas-Phase Reaction Kinetics

We present a new photoelectron photoion coincidence (PEPICO) spectrometer that combines high mass resolution of cations with independently adjustable velocity map imaging of both cations and electrons. We photoionize atoms and molecules using fixed-frequency vacuum ultraviolet radiation. Mass-resolved photoelectron spectra associated with each cation's mass-to-charge ratio can be obtained by inversion of the photoelectron image. The mass-resolved photoelectron spectra enable kinetic time-resolved probing of chemical reactions with isomeric resolution using fixed-frequency radiation sources amenable to small laboratory settings. The instrument accommodates a variety of sample delivery sources to explore a broad range of physical chemistry. To demonstrate the time-resolved capabilities of the instrument, we study the 193 nm photodissociation of SO₂ via the C̃(¹B₂) ← X̃(¹A₁) transition. In addition to the well-documented O(³P_j) + SO(³Σ^-) channel, we observe direct evidence for a small yield of S(³P_j) + O₂(³Σ_g^-) as a primary photodissociation product channel, which may impact sulfur mass-independent fractionation chemistry.

Infrared spectroscopic signature of a hydroperoxyalkyl radical (•QOOH)

Infrared (IR) action spectroscopy is utilized to characterize a prototypical carbon-centered hydroperoxyalkyl radical (•QOOH) transiently formed in the oxidation of volatile organic compounds. The •QOOH radical formed in isobutane oxidation, 2-hydroperoxy-2-methylprop-1-yl, •CH₂(CH₃)₂COOH, is generated in the laboratory by H-atom abstraction from tert-butyl hydroperoxide (TBHP). IR spectral features of jet-cooled and stabilized •QOOH radicals are observed from 2950 to 7050 cm^-1 at energies that lie below and above the transition state barrier leading to OH radical and cyclic ether products. The observed •QOOH features include overtone OH and CH stretch transitions, combination bands involving OH or CH stretch and a lower frequency mode, and fundamental OH and CH stretch transitions. Most features arise from a single vibrational transition with band contours well simulated at a rotational temperature of 10 K. In each case, the OH products resulting from unimolecular decay of vibrationally activated •QOOH are detected by UV laser-induced fluorescence. Assignments of observed •QOOH IR transitions are guided by anharmonic frequencies computed using second order vibrational perturbation theory, a 2 + 1 model that focuses on the coupling of the OH stretch with two low-frequency torsions, as well as recently predicted statistical •QOOH unimolecular decay rates that include heavy-atom tunneling. Most of the observed vibrational transitions of •QOOH are readily distinguished from those of the TBHP precursor. The distinctive IR transitions of •QOOH, including the strong fundamental OH stretch, provide a general means for detection of •QOOH under controlled laboratory and real-world conditions.

Stereoisomer-dependent unimolecular kinetics of 2,4-dimethyloxetane peroxy radicals

2,4,dimethyloxetane is an important cyclic ether intermediate that is produced from hydroperoxyalkyl (QOOH) radicals in low-temperature combustion of n-pentane. However, reaction mechanisms and rates of consumption pathways remain unclear. In...

Energy-resolved and time-dependent unimolecular dissociation of hydroperoxyalkyl radicals (•QOOH)

Hydroperoxyalkyl radicals (•QOOH) are transient intermediates in the atmospheric oxidation of volatile organic compounds and combustion of hydrocarbon fuels in low temperature (< 1000 K) environments. The carbon-centered •QOOH radicals...

Develop the low-temperature oxidation mechanism of cyclopentane: an experimental and theoretical study

At present, the reactivity of cyclic alkanes is estimated by analogy comparison with non-cyclic hydrocarbons. Due to the difference in the structure of cycloalkanes and non-cycloalkanes, the thermodynamic data obtained by analogy is not applicable. In this study, a molecular beam sampling vacuum ultraviolet photoionization time-of-flight mass spectrometer (MB-VUV-PI-TOFMS) was applied to study the low-temperature oxidation of cyclopentane (CPT) at a total pressure range from 1-3 atm and low-temperature range between 500 K and 800 K. The low-temperature reaction products including cyclic olefins, cyclic ethers, highly oxygenated intermediates (e.g., ketohydroperoxide KHP, keto-dihydroperoxide KDHP, olefinic hydroperoxides OHP and ketone structure products) were observed. Further investigation of the oxidation of CPT, the electronic structure calculations at the UCCSD(T)-F12a/aug-cc-pVDZ//B3LYP/6-31+ G(d,p) level were carried out to explore the reactivity of O2 molecules adding sequentially to cyclopentyl radicals. Experimental and theoretical observations showed that the dominant product channel in the reaction of CPT radicals with O2 is HO2 elimination, yielding cyclopentene. The pathways of second and third O2 addition, the dissociation of hydroperoxide were further confirmed. The results of this study will develop the low-temperature oxidation mechanism of CPT, which can be used for future research on accurately simulating the combustion process of CPT.

Chemistry of Functionalized Reactive Organic Intermediates in the Earth's Atmosphere: Impact, Challenges, and Progress

The gas-phase oxidation of organic compounds is an important chemical process in the Earth's atmosphere. It governs oxidant levels and controls the production of key secondary pollutants, and hence has major implications for air quality and climate. Organic oxidation is largely controlled by the chemistry of a few reactive intermediates, namely, alkyl (R) radicals, alkoxy (RO) radicals, peroxy (RO₂) radicals, and carbonyl oxides (R₁R₂COO), which may undergo a number of unimolecular and bimolecular reactions. Our understanding of these intermediates, and the reaction pathways available to them, is based largely on studies of unfunctionalized intermediates, formed in the first steps of hydrocarbon oxidation. However, it has become increasingly clear that intermediates with functional groups, which are generally formed later in the oxidation process, can exhibit fundamentally different reactivity than unfunctionalized ones. In this Perspective, we explore the unique chemistry available to functionalized organic intermediates in the Earth's atmosphere. After a brief review of the canonical chemistry available to unfunctionalized intermediates, we discuss how the addition of functional groups can introduce new reactions, either by changing the energetics or kinetics of a given reaction or by opening up new chemical pathways. We then provide examples of atmospheric reaction classes that are available only to functionalized intermediates. Some of these, such as unimolecular H-shift reactions of RO₂ radicals, have been elucidated only relatively recently, and can have important impacts on atmospheric chemistry (e.g., on radical cycling or organic aerosol formation); it seems likely that other, as-yet undiscovered reactions of (multi)functional intermediates may also exist. We discuss the challenges associated with the study of the chemistry of such intermediates and review novel experimental and theoretical approaches that have recently provided (or hold promise for providing) new insights into their atmospheric chemistry. The continued use and development of such techniques and the close collaboration between experimentalists and theoreticians are necessary for a complete, detailed understanding of the chemistry of functionalized intermediates and their impact on major atmospheric chemical processes.

Watching a hydroperoxyalkyl radical (•QOOH) dissociate

A prototypical hydroperoxyalkyl radical (•QOOH) intermediate, transiently formed in the oxidation of volatile organic compounds, was directly observed through its infrared fingerprint and energy-dependent unimolecular decay to hydroxyl radical and cyclic ether products. Direct time-domain measurements of •QOOH unimolecular dissociation rates over a wide range of energies were found to be in accord with those predicted theoretically using state-of-the-art electronic structure characterizations of the transition state barrier region. Unimolecular decay was enhanced by substantial heavy-atom tunneling involving O-O elongation and C-C-O angle contraction along the reaction pathway. Master equation modeling yielded a fully a priori prediction of the pressure-dependent thermal unimolecular dissociation rates for the •QOOH intermediate-again increased by heavy-atom tunneling-which are required for global models of atmospheric and combustion chemistry.

Quantitative Detection of Products and Radical Intermediates in Low-Temperature Oxidation of Cyclopentane

We present a combined experimental and theoretical investigation of the autoignition chemistry of a prototypical cyclic hydrocarbon, cyclopentane. Experiments using a high-pressure photolysis reactor coupled to time-resolved synchrotron VUV photoionization mass spectrometry directly probe the short-lived radical intermediates and products in cyclopentane oxidation reactions. We detect key peroxy radical intermediates ROO and OOQOOH, as well as several hydroperoxides, formed by second O₂ addition. Automated quantum chemical calculations map out the R + O₂ + O₂ reaction channels and demonstrate that the detected intermediates belong to the dominant radical chain-branching pathway: ROO (+ O₂) → γ-QOOH + O₂ → γ-OOQOOH → products. ROO, OOQOOH, and hydroperoxide products of second-O₂ addition undergo extensive dissociative ionization, making their experimental assignment challenging. We use photoionization dynamics calculations to aid in their characterization and report the absolute photoionization spectra of isomerically pure ROO and γ-OOQOOH. A global statistical fit of the observed kinetics enables reliable quantification of the time-resolved concentrations of these elusive, yet critical species, paving the way for detailed comparisons with theoretical predictions from master-equation-based models.

Trimethylamine Outruns Terpenes and Aromatics in Atmospheric Autoxidation

Autoxidation in the atmosphere has been realized in the last decade as an important process that forms highly oxidized products relevant for the formation of secondary organic aerosol and likely with detrimental human health effects. It is experimentally shown that the OH radical-initiated oxidation of trimethylamine, the most highly emitted amine in the atmosphere, proceeds via rapid autoxidation steps dominating its atmospheric oxidation process. All three methyl groups are functionalized within a timescale of 10 s following the reaction with OH radicals leading to highly oxidized products. The exceptionally large density of functional groups in the oxidized products is expected to define their chemical properties. A detailed reaction mechanism based on theoretical calculations is able to describe the experimental findings. The comparison with results of the reinvestigated OH radical- and ozone-initiated autoxidation of a series of terpenes and aromatics reveals the trimethylamine process as the most efficient one discovered up to now for atmospheric conditions.

Precisely spun super rotors

Improved optical control of molecular quantum states promises new applications including chemistry in the quantum regime, precision tests of fundamental physics, and quantum information processing. While much work has sought to prepare ground state molecules, excited states are also of interest. Here, we demonstrate a broadband optical approach to pump trapped SiO⁺ molecules into pure super rotor ensembles maintained for many minutes. Super rotor ensembles pumped up to rotational state N = 67, corresponding to the peak of a 9400 K distribution, had a narrow N spread comparable to that of a few-kelvin sample, and were used for spectroscopy of the previously unobserved C²Π state. Significant centrifugal distortion of super rotors pumped up to N = 230 allowed probing electronic structure of SiO⁺ stretched far from its equilibrium bond length.

Optical pulses can be useful to create and control molecules in higher quantum states. Here the authors use optical pumping to create rotationally excited states of SiO⁺ molecular ion into super rotor ensemble.

Ab initio rate coefficients for reactions of 2,5-dimethylhexyl isomers with O2: temperature- and pressure-dependent branching ratios

Chemical kinetics of O2-addition to alkyl radicals (R), termed first O2-addition in the oxidation mechanism of alkanes, are of central importance to next-generation combustion strategies designed for operations in the low- to intermediate-temperature region (<1000 K). In the present work, stationary points on potential energy surfaces (PES), temperature- and pressure-dependent rate coefficients, and branching fractions of product formation from R + O2 reactions initiated by the addition of molecular oxygen (3O2) to the three alkyl radicals of a branched alkane, 2,5-dimethylhexane, are reported. The stationary points were determined utilizing ab initio/DFT methods and the reaction energies were computed using the composite CBS-QB3 method. Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) calculations were employed to compute rate coefficients, from which branching fractions were determined over the pressure range of 10-3-20 atm and the temperature range of 400-900 K on three different surfaces. The quantum chemistry results reveal several distinct features. For the addition of O2 to the tertiary alkyl radical 2,5-dimethylhex-2-yl, the most energetically favorable channel leads to the formation of 2,2,5,5,-tetramethyl-tetrahydrofuran, a cyclic ether intermediate formed coincident with OH in a chain-propagating step from the decomposition of tertiary-tertiary hydroperoxyalkyl (QOOH). On the R + O2 surface of the secondary radical, 2,5-dimethylhex-3-yl, the pathways for the formation of methyl-propanal + iso-butene + OH via concerted C-C and O-O bond scission of tertiary QOOH and that of cyclic ether + OH are the most energetically favorable pathways. The R + O2 surface for the reaction of the primary radical, 2,5-dimethylhex-1-yl, reveals two competitive chain-propagation channels, leading to 2-iso-propyl-4-methyl-tetrahydrofuran + OH and 2,2,5-trimethyltetrahydropyran + OH. Below 100 Torr, the formation of the aforementioned species dominates the respective total R + O2 rate coefficient, while at pressures above 1 atm collisionally stabilized alkylperoxy (ROO) dominates at the temperatures considered here. The results of this study are in very good agreement with the experimentally measured intermediates and products of the 2,5-dimethylhexyl radical + O2 reaction.

Kinetics and mechanism of OH-mediated degradation of three pentanols in the atmosphere

Pentanols as potential biofuels have attracted considerable interest, and thus it is of great importance to gain insights into their combustion and atmospheric chemistry.

Direct time-resolved detection and quantification of key reactive intermediates in diethyl ether oxidation at T = 450-600 K

High-pressure multiplexed photoionization mass spectrometry (MPIMS) with tunable vacuum ultraviolet (VUV) ionization radiation from the Lawrence Berkeley Labs Advanced Light Source is used to investigate the oxidation of diethyl ether (DEE). Kinetics and photoionization (PI) spectra are simultaneously measured for the species formed. Several stable products from DEE oxidation are identified and quantified using reference PI cross-sections. In addition, we directly detect and quantify three key chemical intermediates: peroxy (ROO˙), hydroperoxyalkyl peroxy (˙OOQOOH), and ketohydroperoxide (HOOP[double bond, length as m-dash]O, KHP). These intermediates undergo dissociative ionization (DI) into smaller fragments, making their identification by mass spectrometry challenging. With the aid of quantum chemical calculations, we identify the DI channels of these key chemical species and quantify their time-resolved concentrations from the overall carbon atom balance at T = 450 K and P = 7500 torr. This allows the determination of the absolute PI cross-sections of ROO˙, ˙OOQOOH, and KHP into each DI channel directly from experiment. The PI cross-sections in turn enable the quantification of ROO˙, ˙OOQOOH, and KHP from DEE oxidation over a range of experimental conditions that reveal the effects of pressure, O₂ concentration, and temperature on the competition among radical decomposition and second O₂ addition pathways.

Combustion in the future: The importance of chemistry

Combustion involves chemical reactions that are often highly exothermic. Combustion systems utilize the energy of chemical compounds released during this reactive process for transportation, to generate electric power, or to provide heat for various applications. Chemistry and combustion are interlinked in several ways. The outcome of a combustion process in terms of its energy and material balance, regarding the delivery of useful work as well as the generation of harmful emissions, depends sensitively on the molecular nature of the respective fuel. The design of efficient, low-emission combustion processes in compliance with air quality and climate goals suggests a closer inspection of the molecular properties and reactions of conventional, bio-derived, and synthetic fuels. Information about flammability, reaction intensity, and potentially hazardous combustion by-products is important also for safety considerations. Moreover, some of the compounds that serve as fuels can assume important roles in chemical energy storage and conversion. Combustion processes can furthermore be used to synthesize materials with attractive properties. A systematic understanding of the combustion behavior thus demands chemical knowledge. Desirable information includes properties of the thermodynamic states before and after the combustion reactions and relevant details about the dynamic processes that occur during the reactive transformations from the fuel and oxidizer to the products under the given boundary conditions. Combustion systems can be described, tailored, and improved by taking chemical knowledge into account. Combining theory, experiment, model development, simulation, and a systematic analysis of uncertainties enables qualitative or even quantitative predictions for many combustion situations of practical relevance. This article can highlight only a few of the numerous investigations on chemical processes for combustion and combustion-related science and applications, with a main focus on gas-phase reaction systems. It attempts to provide a snapshot of recent progress and a guide to exciting opportunities that drive such research beyond fossil combustion.

An Intramolecular Hydrogen-Shift in a Peroxy Radical at Cryogenic Temperatures: The Reaction of 2-Hydroxyphenyl Radical with O2

Peroxy radical hydrogen-shifts are pivotal elementary reaction steps in the oxidation of small hydrocarbons in autoignition and the lower atmosphere. Although these reactions are typically associated with a substantial barrier, we demonstrate that the [1,5]H-shift in the peroxy species derived from the 2-hydroxyphenyl radical 1 is so facile that it even proceeds rapidly in an argon matrix at 35 K through a proton-coupled electron transfer mechanism. Hydrogen-bound complexes of o-benzoquinone are identified as the main reaction products by infrared spectroscopy although their formation through O-O bond scission is hampered by a barrier of 11.9 kcal mol^-1 at the ROCCSD(T)/cc-pVTZ/UB3LYP/6-311G(d,p) level of theory.

Online analysis of gas-phase radical reactions using vacuum ultraviolet lamp photoionization and time-of-flight mass spectrometry

A home-made vacuum ultraviolet photoionization time-of-flight mass spectrometer has been developed and coupled to an atmospheric simulation chamber operated at atmospheric pressure and to a fast flow tube at low pressure (1-10 Torr). Gas sampling from the simulation chamber is realized directly via a capillary effusive beam, and sampling from the flow tube is via a continuous molecular beam inlet. Both devices are connected simultaneously to the ionization chamber of the mass spectrometer and can be switched in-between within minutes to study gas-phase radical reactions of atmospheric interest in a large range of reaction conditions and reaction times (from milliseconds in the flow tube to hours in the simulation chamber). A cage-shaped photoionization source combined with a commercial 10.6 eV krypton lamp has been developed to provide a high ion collection efficiency along the long light path in the cage. This way, a multiplexed detection with high sensitivity down to the sub-parts per billion volume concentration range, e.g., a limit of detection of 0.3 ppbv with an accumulation time of 60 s for benzene and 1.3 ppbv for the methyl radical, is obtained. The performance and suitability of the setup are illustrated by the study of the chlorine-initiated oxidation reaction of toluene in the atmospheric simulation chamber and in the fast flow tube. Stable products and reactive intermediates have been well-determined and their reaction dynamics are discussed.

Energetics and mechanisms for the acetonyl radical + O2 reaction: An important system for atmospheric and combustion chemistry

The acetonyl radical (^•CH₂COCH₃) is relevant to atmospheric and combustion chemistry due to its prevalence in many important reaction mechanisms. One such reaction mechanism is the decomposition of Criegee intermediates in the atmosphere that can produce acetonyl radical and OH. In order to understand the fate of the acetonyl radical in these environments and to create more accurate kinetics models, we have examined the reaction system of the acetonyl radical with O₂ using highly reliable theoretical methods. Structures were optimized using coupled cluster theory with singles, doubles, and perturbative triples [CCSD(T)] with an atomic natural orbital (ANO0) basis set. Energetics were computed to chemical accuracy using the focal point approach involving perturbative treatment of quadruple excitations [CCSDT(Q)] and basis sets as large as cc-pV5Z. The addition of O₂ to the acetonyl radical produces the acetonylperoxy radical, and multireference computations on this reaction suggest it to be barrierless. No submerged pathways were found for the unimolecular isomerization of the acetonylperoxy radical. Besides dissociation to reactants, the lowest energy pathway available for the acetonylperoxy radical is a 1-5 H shift from the methyl group to the peroxy group through a transition state that is 3.3 kcal mol^-1 higher in energy than acetonyl radical + O₂. The ultimate products from this pathway are the enol tautomer of the acetonyl radical along with O₂. Multiple pathways that lead to OH formation are considered; however, all of these pathways are predicted to be energetically inaccessible, except at high temperatures.

Dihalogenated Methylperoxy Radicals: Spectroscopic Characterization and Photodecomposition by Release of HO

Two atmospherically relevant dihalogenated methylperoxy radicals CHX₂ OO^. (X=F and Cl) have been generated through O₂ -oxidation of the corresponding alkyl radicals CHX₂ ^. in the gas phase. The IR spectroscopic characterization of both radicals in cryogenic Ar- and N₂ -matrices (15 K) is supported by ¹⁸ O-labeling and ab initio calculations at the UCCSD(T)/aug-cc-pVTZ level. Upon 266 nm laser irradiation, both radicals decompose mainly by releasing hydroxyl radicals (→HO^. +X₂ CO) via the intermediacy of intriguing α-hydroperoxyalkyl radicals (^. CX₂ OOH), implying that the photooxidation of dihalogenated hydrocarbons might serve as important sources of HO^. radicals in the atmosphere.

Mechanisms and kinetics of the low-temperature oxidation of 2-methylfuran: insight from DFT calculations and kinetic simulations

The low-temperature oxidation (LTO) mechanisms of the 2-methylfuran (2-MF) biofuel and the corresponding thermodynamic and kinetic properties have been explored by density functional theory (DFT) and composite G4 methodologies as well as kinetic simulations. The O₂ addition to the main furylCH₂ radical from the methyl dehydrogenation in 2-MF forms three peroxide radicals PO1, PO2, and PO3 with the energy barriers of 15.1, 19.3, and 20.6 kcal mol^-1 and the reaction ΔG of -8.2, 5.7, and -0.1 kcal mol^-1 (298 K and 1 atm), respectively. Through hydrogen transfer followed by dehydroxylation, these nascent products evolve into stable aldehydes and cyclic ketones, which may further decompose into smaller species under the action of OH. Calculations and simulations show that the product P1 from the dehydroxylation of PO1 has a dominant population (higher than 96%) among the final products, although the temperature and pressure may influence the species profiles and rate constants to some extent. Based on the G4-calibrated thermodynamic parameters, the temperature and pressure dependence of the rate constants and the two- and three-parameter Arrhenius coefficients for all reactions considered here have been determined by using the transition state theory (TST) and Rice-Ramsperger-Kassel-Marcus (RRKM) methods. The present results provide a comprehensive understanding of the mechanisms and kinetics of the LTO process of the 2-MF biofuel.

Identifying isomers of peroxy radicals in the gas phase: 1-C3H7O2vs. 2-C3H7O2

The two isomers of propylperoxy radical 1-C₃H₇O₂ and 2-C₃H₇O₂, together with their rotamers, are individually identified and assigned.

Highly Oxygenated Organic Molecules (HOM) from Gas-Phase Autoxidation Involving Peroxy Radicals: A Key Contributor to Atmospheric Aerosol

Highly oxygenated organic molecules (HOM) are formed in the atmosphere via autoxidation involving peroxy radicals arising from volatile organic compounds (VOC). HOM condense on pre-existing particles and can be involved in new particle formation. HOM thus contribute to the formation of secondary organic aerosol (SOA), a significant and ubiquitous component of atmospheric aerosol known to affect the Earth’s radiation balance. HOM were discovered only very recently, but the interest in these compounds has grown rapidly. In this Review, we define HOM and describe the currently available techniques for their identification/quantification, followed by a summary of the current knowledge on their formation mechanisms and physicochemical properties. A main aim is to provide a common frame for the currently quite fragmented literature on HOM studies. Finally, we highlight the existing gaps in our understanding and suggest directions for future HOM research.

New insights into 3M3M1B: the role of water in ˙OH-initiated degradation and aerosol formation in the presence of NOX (X = 1, 2) and an alkali

Metal-free catalysis of the ˙OH-initiated degradation of 3M3M1B, nitrate aerosol formation, and peroxynitrate decomposition.

NO2 Suppression of Autoxidation-Inhibition of Gas-Phase Highly Oxidized Dimer Product Formation

Atmospheric autoxidation of volatile organic compounds (VOC) leads to prompt formation of highly oxidized multifunctional compounds (HOM) that have been found crucial in forming ambient secondary organic aerosol (SOA). As a radical chain reaction mediated by oxidized peroxy (RO2) and alkoxy (RO) radical intermediates, the formation pathways can be intercepted by suitable reaction partners, preventing the production of the highest oxidized reaction products, and thus the formation of the most condensable material. Commonly, NO is expected to have a detrimental effect on RO2 chemistry, and thus on autoxidation, whereas the influence of NO2 is mostly neglected. Here it is shown by dedicated flow tube experiments, how high concentration of NO2 suppresses cyclohexene ozonolysis initiated autoxidation chain reaction. Importantly, the addition of NO2 ceases covalently bound dimer production, indicating their production involving acylperoxy radical (RC(O)OO•) intermediates. In related experiments NO was also shown to strongly suppress the highly oxidized product formation, but due to possibility for chain propagating reactions (as with RO2 and HO2 too), the suppression is not as absolute as with NO2. Furthermore, it is shown how NO x reactions with oxidized peroxy radicals lead into indistinguishable product compositions, complicating mass spectral assignments in any RO2 + NO x system. The present work was conducted with atmospheric pressure chemical ionization mass spectrometry (CIMS) as the detection method for the highly oxidized end-products and peroxy radical intermediates, under ambient conditions and at short few second reaction times. Specifically, the insight was gained by addition of a large amount of NO2 (and NO) to the oxidation system, upon which acylperoxy radicals reacted in RC(O)O2 + NO2 → RC(O)O2NO2 reaction to form peroxyacylnitrates, consequently shutting down the oxidation sequence.

Inhibition of hydrocarbon autoxidation by nitroxide-catalyzed cross-dismutation of hydroperoxyl and alkylperoxyl radicals

Nitroxides are putative intermediates in the accepted reaction mechanisms of the diarylamine and hindered amine antioxidants that are universally added to preserve synthetic and natural hydrocarbon-based materials. New methodology which enables monitoring of hydrocarbon autoxidations at low rates of radical generation has revealed that diarylnitroxides and hindered nitroxides are far better inhibitors of unsaturated hydrocarbon autoxidation than their precursor amines, implying intervention of a different mechanism. Experimental and computational investigations suggest that the nitroxides catalyze the cross-dismutation of hydroperoxyl and alkylperoxyl radicals to yield O2 and a hydroperoxide, thereby halting the autoxidation chain reaction. The hydroperoxyl radicals - key players in hydrocarbon combustion, but essentially unknown in autoxidation - are proposed to derive from a tunneling-enhanced intramolecular (1,4-) hydrogen-atom transfer/elimination sequence from oxygenated radical addition intermediates. These insights suggest that nitroxides are preferred additives for the protection of hydrocarbon-based materials from autoxidation since they exhibit catalytic activity under conditions where their precursor amines are less effective and/or inefficiently converted to nitroxides in situ.

Thermochemistry and Kinetic Studies on the Autoignition of 2-Butanone: A Computational Study

Unimolecular reactions of alkylperoxy(ROO^•), hydroperoxyalkyl(^•QOOH), and hydroperoxyalkylperoxy(^•OOQOOH) radicals of 2-butanone, which is a potential biofuel molecule, have been studied computationally. These radicals are responsible for the chain branching at low temperature oxidation and play a significant role in modeling the autoignition. The composite CBS-QB3 method was used to study the thermochemistry and energetics of all the species involved. Intrinsic reaction coordinate (IRC) calculations were carried out for all the transition states along various reaction pathways. All the possible reactions like H-migration, ^•OH elimination, and HO^•₂ elimination reactions were studied for these radicals. It was found that, the isomerization of ^•OOQOOH to HOOQOO^• is the most favorable channel, which involves 8- and 9-membered cyclic transition states. However, the decomposition pathway involves the H-migration from carbon to oxygen. The mechanism for the decomposition of all ^•OOQOOH radicals with their potential energy level diagrams are reported. The temperature dependent rate coefficients were also studied using Canonical Variational Transition state theory (CVT) with small curvature tunneling (SCT) in the temperature range of 400-1500 K, which is relevant to the combustion. Thermodynamic parameters for all the reactions involved were calculated. The high barrier (1,3 H-migration) reactions were found to be exothermic and spontaneous, which is unexpected.