Elucidating the photodissociation fingerprint and quantifying the determination of organic hydroperoxides in gas-phase autoxidation.

Hydroperoxides are formed in the atmospheric oxidation of volatile organic compounds, in the combustion autoxidation of fuel, in the cold environment of the interstellar medium, and also in some catalytic reactions. They play crucial roles in the formation and aging of secondary organic aerosols and in fuel autoignition. However, the concentration of organic hydroperoxides is seldom measured, and typical estimates have large uncertainties. In this work, we developed a mild and environmental-friendly method for the synthesis of alkyl hydroperoxides (ROOH) with various structures, and we systematically measured the absolute photoionization cross-sections (PICSs) of the ROOHs using synchrotron vacuum ultraviolet-photoionization mass spectrometry (SVUV-PIMS). A chemical titration method was combined with an SVUV-PIMS measurement to obtain the PICS of 4-hydroperoxy-2-pentanone, a typical molecule for combustion and atmospheric autoxidation ketohydroperoxides (KHPs). We found that organic hydroperoxide cations are largely dissociated by loss of OOH. This fingerprint was used for the identification and accurate quantification of the organic peroxides, and it can therefore be used to improve models for autoxidation chemistry. The synthesis method and photoionization dataset for organic hydroperoxides are useful for studying the chemistry of hydroperoxides and the reaction kinetics of the hydroperoxy radicals and for developing and evaluating kinetic models for the atmospheric autoxidation and combustion autoxidation of the organic compounds.

The effect of hydrogen bonding on the reactivity of OH radicals with prenol and isoprenol: a shock tube and multi-structural torsional variational transition state theory study.

The presence of two functional groups (OH and double bond) in C5 methyl-substituted enols (i.e., isopentenols), such as 3-methyl-2-buten-1-ol (prenol) and 3-methyl-3-buten-1-ol (isoprenol), makes them excellent biofuel candidates as fuel additives. As OH radicals are abundant in both combustion and atmospheric environments, OH-initiated oxidation of these isopentenols over wide ranges of temperatures and pressures needs to be investigated. In alkenes, OH addition to the double bond is prominent at low temperatures (i.e., below ∼700 K), and H-atom abstraction dominates at higher temperatures. However, we find that the OH-initiated oxidation of prenol and isoprenol displays a larger role for OH addition at higher temperatures. In this work, the reaction kinetics of prenol and isoprenol with OH radicals was investigated over the temperature range of 900-1290 K and pressure of 1-5 atm by utilizing a shock tube and OH laser diagnostic. To rationalize these chemical systems, variational transition state theory calculations with multi-structural torsional anharmonicity and small curvature tunneling corrections were run using a potential energy surface characterized at the UCCSD(T)/jun-cc-pVQZ//M06-2X/6-311++G(2df,2pd) level of theory. A good agreement was observed between the experiment and theory, with both predicting a non-Arrhenius behavior and negligible pressure effects. OH additions to the double bond of prenol and isoprenol were found to be important, with at least 50% contribution to the total rate constants even at temperatures as high as 700 and 2000 K, respectively. This behavior was attributed to the stabilizing effect induced by hydrogen bonding between the reacting OH radical and the OH functional group of isopentenols at the saddle points. These stabilizing intermolecular interactions help mitigate the entropic effects that hinder association reactions as temperature increases, thus extending the prominent role of addition pathways to high temperatures. The site-specific rate constants were also found to be slower than their analogous reactions of OH + n-alkenes.

Formation of Organic Acids and Carbonyl Compounds in n-Butane Oxidation via γ-Ketohydroperoxide Decomposition.

A crucial chain-branching step in autoignition is the decomposition of ketohydroperoxides (KHP) to form an oxy radical and OH. Other pathways compete with chain-branching, such as "Korcek" dissociation of γ-KHP to a carbonyl and an acid. Here we characterize the formation of a γ-KHP and its decomposition to formic acid+acetone products from observations of n-butane oxidation in two complementary experiments. In jet-stirred reactor measurements, KHP is observed above 590 K. The KHP concentration decreases with increasing temperature, whereas formic acid and acetone products increase. Observation of characteristic isotopologs acetone-d3 and formic acid-d0 in the oxidation of CH3 CD2 CD2 CH3 is consistent with a Korcek mechanism. In laser-initiated oxidation experiments of n-butane, formic acid and acetone are produced on the timescale of KHP removal. Modelling the time-resolved production of formic acid provides an estimated upper limit of 2 s-1 for the rate coefficient of KHP decomposition to formic acid+acetone.

Early Chemistry of Nicotine Degradation in Heat-Not-Burn Smoking Devices and Conventional Cigarettes: Implications for Users and Second- and Third-Hand Smokers.

Nicotine exposure results in health risks not only for smokers but also for second- and third-hand smokers. Unraveling nicotine's degradation mechanism and the harmful chemicals that are produced under different conditions is vital to assess exposure risks. We performed a theoretical study to describe the early chemistry of nicotine degradation by investigating two important reactions that nicotine can undergo: hydrogen abstraction by hydroxyl radicals and unimolecular dissociation. The former contributes to the control of the degradation mechanism below 800 K due to a non-Arrhenius kinetics, which implies an enhancement of reactivity as temperature decreases. The latter becomes important at higher temperatures due to its larger activation energy. This change in the degradation mechanism is expected to affect the composition of vapors inhaled by smokers and room occupants. Conventional cigarettes, which operate at temperatures higher than 1000 K, are more prone to yield harmful pyridinyl radicals via nicotine dissociation, while nicotine in electronic cigarettes and vaporizers, with operating temperatures below 600 K, will be more likely degraded by hydroxyl radicals, resulting in a vapor with a different composition. Although low-temperature nicotine delivery devices have been claimed to be less harmful due to their nonburning operating conditions, the non-Arrhenius kinetics that we observed for the degradation mechanism below 873 K suggests that nicotine degradation may be more rapidly initiated as temperature is reduced, indicating that these devices may be more harmful than it is commonly assumed.

Analyzing the solid soot particulates formed in a fuel-rich flame by solvent-free matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry.

RATIONALE: The compositional and structural information of soot particles is essential for a better understanding of the chemistry and mechanism during the combustion. The aim of the present study was to develop a method to analyze such soot particulate samples with high complexity and poor solubility. METHODS: The solvent-free sample preparation matrix-assisted laser desorption/ionization (MALDI) technique was combined with the ultrahigh-resolution Fourier transform ion cyclotron resonance (FTICR) mass spectrometry (MS) for the characterization of solid soot particulates. Moreover, a modified iso-abundance plot (Carbon Number vs. Hydrogen Number vs. Abundance) was introduced to visualize the distributions of various chemical species, and to examine the agreement between the hydrogen-abstraction-carbon-addition (HACA) mechanism and the polycyclic aromatic hydrocarbon growth in the investigated flame system. RESULTS: This solvent-free MALDI method enabled the effective ionization of the solid soot particulates without any dissolving procedure. With the accurate m/z ratios from FTICR-MS, a unique chemical formula was assigned to each of the recorded mass signals. The combustion products were proven to be mainly large polycyclic aromatic hydrocarbons (PAHs), together with a small amount (<5%) of oxidized hydrocarbons. CONCLUSIONS: The developed method provides a new approach for the molecular characterization of soot particulates like carbonaceous materials. The investigated soot particulates are mainly PAHs with no or very short aliphatic chains. The growth mechanism of PAHs during combustion can be examined against the classic HACA mechanism.

Kinetics of the benzyl + HO2 and benzoxyl + OH barrierless association reactions: fate of the benzyl hydroperoxide adduct under combustion and atmospheric conditions.

Radical-radical association reactions are challenging to address theoretically due to difficulties finding the bottleneck that variationally minimizes the reactive flux. For this purpose, the variable reaction coordinate (VRC) formulation of the variational transition state theory (VTST) represents an appropriate tool. In this work, we revisited the kinetics of two radical-radical association reactions of importance in combustion modelling and poly-aromatic hydrocarbon (PAH) chemistry by performing VRC calculations: benzyl + HO2 and benzoxyl + OH, both forming the adduct benzyl hydroperoxide. Our calculated rate constants are significantly lower than those previously reported based on VTST calculations, which results from a more efficient minimization of the reactive flux through the bottleneck achieved by the VRC formulation. Both reactions show different trends in the variation of their rate constants with temperature. We observed that if the pair of single occupied molecular orbitals (SOMOs) of the associating radicals show a similar nature, i.e. similar character, and thereby a small energy gap, a highly stabilized transition state structure is formed as the result of a very efficient SOMO-SOMO overlap, which may cancel out the free energy bottleneck for the formation of the adduct and result in large rate constants with a negative temperature dependence. This is the case of the benzoxyl and OH radical pair, whose SOMOs show O2p nature with an energy gap of 20.2 kcal mol-1. On the other hand, the benzyl and HO2 radical pair shows lower rate constants with a positive temperature dependence due to the larger difference between both SOMOs (a 28.9 kcal mol-1 energy gap) as a consequence of the contribution of the multiple resonance structures of the benzyl radical. The reverse dissociation rate constants were also calculated using multi-structural torsional anharmonicity partition functions, which were not included in previous work, and the results show a much slower dissociation of benzyl hydroperoxide. Our work may help to improve kinetic models of interest in combustion and PAH formation, as well as to gain further understanding of radical-radical association reactions, which are ubiquitous in different environments.

A Systematic Theoretical Kinetics Analysis for the Waddington Mechanism in the Low-Temperature Oxidation of Butene and Butanol Isomers.

The Waddington mechanism, or the Waddington-type reaction pathway, is crucial for low-temperature oxidation of both alkenes and alcohols. In this study, the Waddington mechanism in the oxidation chemistry of butene and butanol isomers was systematically investigated. Fundamental quantum chemical calculations were conducted for the rate constants and thermodynamic properties of the reactions and species in this mechanism. Calculations were performed using two different ab initio solvers: Gaussian 09 and Orca 4.0.0, and two different kinetic solvers: PAPR and MultiWell, comprehensively. Temperature- and pressure-dependent rate constants were performed based on the transition state theory, associated with the Rice Ramsperger Kassel Marcus and master equation theories. Temperature-dependent thermochemistry (enthalpies of formation, entropy, and heat capacity) of all major species was also conducted, based on the statistical thermodynamics. Of the two types of reaction, dissociation reactions were significantly faster than isomerization reactions, while the rate constants of both reactions converged toward higher temperatures. In comparison, between two ab initio solvers, the barrier height difference among all isomerization and dissociation reactions was about 2 and 0.5 kcal/mol, respectively, resulting in less than 50%, and a factor of 2-10 differences for the predicted rate coefficients of the two reaction types, respectively. Comparing the two kinetic solvers, the rate constants of the isomerization reactions showed less than a 32% difference, while the rate of one dissociation reaction (P1 ↔ WDT12) exhibited 1-2 orders of magnitude discrepancy. Compared with results from the literature, both reaction rate coefficients (R4 and R5 reaction systems) and species' thermochemistry (all closed shell molecules and open shell radicals R4 and R5) showed good agreement with the corresponding values obtained from the literature. All calculated results can be directly used for the chemical kinetic model development of butene and butanol isomer oxidation.

Gas-to-Liquid Phase Transition of PAH at Flame Temperatures.

Significant evidence has shown that soot can be formed from polycyclic aromatic hydrocarbon (PAH) in combustion environments, but the transition of high molecular PAH from the gas phase to soot in a liquid or solid state remains unclear. In this study, the relationships between the boiling points of various planar PAHs and their thermodynamic properties are systematically investigated, to find a satisfactory marker for the phase transition event. Temperature-dependent thermodynamic properties, including entropy, specific heat capacity, enthalpy, and Gibbs free energy, are simultaneously calculated for PAHs, using density functional theory and three composite compound methods. Comparison of the results indicates that the individual G3 method, plus an atomization reaction approach, produces the most accurate thermochemistry parameters. Compared to entropy, enthalpy, and Gibbs free energy, the specific heat capacity at 298 K is found to be a better marker for the boiling point of PAHs due to the observed linear correlation, predictable characteristics, and fidelity of accuracy as a function of temperature. The correlation equation Y = 10.996X + 122.111 is proposed (where Y is the boiling temperature (K) and X is Cp at 298 K (cal/K/mol)). The standard deviation is as low as 16.7 K when comparing the calculated boiling points and experimentally determined values for 25 different aromatic species ranging from benzene to ovalene (C32H14). The effects of carbon number, structural arrangement, and partial pressure on the boiling point of large planar PAH are discussed. The results reveal that the carbon number in large planar PAH is the dominant factor determining its boiling points. It is shown that PAHs containing about 60-65 carbon atoms are likely to exist as liquids in flames, although the partial pressure of such species is very low.

Collision Efficiency Parameter Influence on Pressure-Dependent Rate Constant Calculations Using the SS-QRRK Theory.

The system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory (J. Am. Chem. Soc. 2016, 138, 2690) is suitable to determine rate constants below the high-pressure limit. Its current implementation allows incorporating variational effects, multidimensional tunneling, and multistructural torsional anharmonicity in rate constant calculations. Master equation solvers offer a more rigorous approach to compute pressure-dependent rate constants, but several implementations available in the literature do not incorporate the aforementioned effects. However, the SS-QRRK theory coupled with a formulation of the modified strong collision model underestimates the value of unimolecular pressure-dependent rate constants in the high-temperature regime for reactions involving large molecules. This underestimation is a consequence of the definition for collision efficiency, which is part of the energy transfer model. Selection of the energy transfer model and its parameters constitutes a common issue in pressure-dependent calculations. To overcome this underestimation problem, we evaluated and implemented in a bespoke Python code two alternative definitions for the collision efficiency using the SS-QRRK theory and tested their performance by comparing the pressure-dependent rate constants with the Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) results. The modeled systems were the tautomerization of propen-2-ol and the decomposition of 1-propyl, 1-butyl, and 1-pentyl radicals. One of the tested definitions, which Dean et al. explicitly derived (Z. Phys. Chem. 2000, 214, 1533), corrected the underestimation of the pressure-dependent rate constants and, in addition, qualitatively reproduced the trend of RRKM/ME data. Therefore, the used SS-QRRK theory with accurate definitions for the collision efficiency can yield results that are in agreement with those from more sophisticated methodologies such as RRKM/ME.

Data Science Approach to Estimate Enthalpy of Formation of Cyclic Hydrocarbons.

In spite of increasing importance of cyclic hydrocarbons in various chemical systems, studies on the fundamental properties of these compounds, such as enthalpy of formation, are still scarce. One of the reasons for this is the fact that the estimation of the thermodynamic properties of cyclic hydrocarbon species via cost-effective computational approaches, such as group additivity (GA), has several limitations and challenges. In this study, a machine learning (ML) approach is proposed using a support vector regression (SVR) algorithm to predict the standard enthalpy of formation of cyclic hydrocarbon species. The model is developed based on a thoroughly selected dataset of accurate experimental values of 192 species collected from the literature. The molecular descriptors used as input to the SVR are calculated via alvaDesc software, which computes in total 5255 features classified into 30 categories. The developed SVR model has an average error of approximately 10 kJ/mol. In comparison, the SVR model outperforms the GA approach for complex molecules and can be therefore proposed as a novel data-driven approach to estimate enthalpy values for complex cyclic species. A sensitivity analysis is also conducted to examine the relevant features that play a role in affecting the standard enthalpy of formation of cyclic species. Our species dataset is expected to be updated and expanded as new data are available to develop a more accurate SVR model with broader applicability.

Identification of volatile constituents released from IQOS heat-not-burn tobacco HeatSticks using a direct sampling method.

OBJECTIVES: To identify the chemicals released in I Quit Ordinary Smoking (IQOS) heat-not-burn tobacco aerosol and to assess their potential human health toxicity. METHODS: The heating temperature window of the IQOS heat-not-burn device was determined using a thermographic camera over a period of 100 s. Qualitative studies were performed using a novel real-time gas chromatograph-mass spectrometer set-up. Aerosols from six tobacco-flavoured IQOS HeatSticks (Amber, Blue, Bronze, Sienna, Turquoise and Yellow) were collected in a 1 mL loop via a manual syringe attached to the sample-out port of the valve. The gas transport line was heated to 200°C in order to prevent the condensation of volatile species. Compound identification was performed using the NIST11 mass spectrometry database library (US National Institute of Standards and Technology), where only chemicals with a match of 70% and above were listed as identifiable. RESULTS: The temperature profile of the IQOS device revealed a non-combustive process employed in generating the tobacco aerosol. Real-time qualitative analysis revealed 62 compounds encompassing a broad spectrum of chemicals such as carbonyls, furans and phthalates, which are highly toxic. DISCUSSION: Our findings complement the qualitative studies previously performed by Philip Morris International and others via indirect sampling methods. By analysing the aerosols in real time, we have identified a total of 62 compounds, from which only 10 were in common with previous studies. Several identified species such as diacetyl, 2,3-pentanedione, hydroxymethylfurfural and diethylhexyl phthalate are classified as highly toxic, with the latter considered carcinogenic.

Unraveling the structure and chemical mechanisms of highly oxygenated intermediates in oxidation of organic compounds.

Decades of research on the autooxidation of organic compounds have provided fundamental and practical insights into these processes; however, the structure of many key autooxidation intermediates and the reactions leading to their formation still remain unclear. This work provides additional experimental evidence that highly oxygenated intermediates with one or more hydroperoxy groups are prevalent in the autooxidation of various oxygenated (e.g., alcohol, aldehyde, keto compounds, ether, and ester) and nonoxygenated (e.g., normal alkane, branched alkane, and cycloalkane) organic compounds. These findings improve our understanding of autooxidation reaction mechanisms that are routinely used to predict fuel ignition and oxidative stability of liquid hydrocarbons, while also providing insights relevant to the formation mechanisms of tropospheric aerosol building blocks. The direct observation of highly oxygenated intermediates for the autooxidation of alkanes at 500-600 K builds upon prior observations made in atmospheric conditions for the autooxidation of terpenes and other unsaturated hydrocarbons; it shows that highly oxygenated intermediates are stable at conditions above room temperature. These results further reveal that highly oxygenated intermediates are not only accessible by chemical activation but also by thermal activation. Theoretical calculations on H-atom migration reactions are presented to rationalize the relationship between the organic compound's molecular structure (n-alkane, branched alkane, and cycloalkane) and its propensity to produce highly oxygenated intermediates via extensive autooxidation of hydroperoxyalkylperoxy radicals. Finally, detailed chemical kinetic simulations demonstrate the influence of these additional reaction pathways on the ignition of practical fuels.

Machine Learning To Predict Standard Enthalpy of Formation of Hydrocarbons.

Thermodynamic properites of molecules are used widely in the study of reactive processes. Such properties are typically measured via experiments or calculated by a variety of computational chemistry methods. In this work, machine learning (ML) models for estimation of standard enthalpy of formation at 298.15 K are developed for three classes of acyclic and closed-shell hydrocarbons, viz. alkanes, alkenes, and alkynes. Initially, an extensive literature survey is performed to collect standard enthalpy data for training ML models. A commercial software (Dragon) is used to obtain a wide set of molecular descriptors by providing SMILES strings. The molecular descriptors are used as input features for the ML models. Support vector regression (SVR) and artificial neural networks are used with a two-level K-fold cross-validation (K-fold CV) workflow. The first level is for estimation of accuracy of both the ML models, and the second level is for generation of the final models. The SVR model is selected as the best model based on error estimates over 10-fold CV. The final SVR model is compared against conventional Benson's group additivity for a set of octene isomers from the database, illustrating the advantages of the proposed ML modeling approach.

Ab initio and transition state theory study of the OH + HO2 → H2O + O2(3Σg-)/O2(1Δg) reactions: yield and role of O2(1Δg) in H2O2 decomposition and in combustion of H2.

Reactions of hydroxyl (OH) and hydroperoxyl (HO2) are important for governing the reactivity of combustion systems. We performed post-CCSD(T) ab initio calculations at the W3X-L//CCSD = FC/cc-pVTZ level to explore the triplet ground-state and singlet excited-state potential energy surfaces of the OH + HO2 → H2O + O2(3Σg-)/O2(1Δg) reactions. Using microcanonical and multistructural canonical transition state theories, we calculated the rate constant for the triplet and singlet channels over the temperature range 200-2500 K, represented by k(T) = 3.08 × 1012T0.07 exp(1151/RT) + 8.00 × 1012T0.32 exp(-6896/RT) and k(T) = 2.14 × 106T1.65 exp(-2180/RT) in cm3 mol-1 s-1, respectively. The branching ratios show that the yield of singlet excited oxygen is small (<0.5% below 1000 K). To ascertain the importance of singlet oxygen channel, our new kinetic information was implemented into the kinetic model for hydrogen combustion recently updated by Konnov (Combust. Flame, 2015, 162, 3755-3772). The updated kinetic model was used to perform H2O2 thermal decomposition simulations for comparison against shock tube experiments performed by Hong et al. (Proc. Combust. Inst., 2013, 34, 565-571), and to estimate flame speeds and ignition delay times in H2 mixtures. The simulation predicted a larger amount of O2(1Δg) in H2O2 decomposition than that predicted by Konnov's original model. These differences in the O2(1Δg) yield are due to the use of a higher ab initio level and a more sophisticated methodology to compute the rate constant than those used in previous studies, thereby predicting a significantly larger rate constant. No effect was observed on the rate of the H2O2 decomposition and on the flame speeds and ignition delay times of different H2-oxidizer mixtures. However, if the oxidizer is seeded with O3, small differences appear in the flame speed. Given that O2(1Δg) is much more reactive than O2(3Σg-), we do not preclude an effect of the singlet channel of the titled reaction in other combustion systems, especially in systems where excited oxygen plays an important role.

Theoretical kinetic study of the formic acid catalyzed Criegee intermediate isomerization: multistructural anharmonicity and atmospheric implications.

We performed a theoretical study on the double hydrogen shift isomerization reaction of a six carbon atom Criegee intermediate (C6-CI), catalyzed by formic acid (HCOOH), to produce vinylhydroperoxide (VHP), C6-CI + HCOOH → VHP + HCOOH. This Criegee intermediate can serve as a surrogate for larger CIs derived from important volatile organic compounds like monoterpenes, whose reactivity is not well understood and which are difficult to handle computationally. The reactant HCOOH exerts a pronounced catalytic effect on the studied reaction by lowering the barrier height, but the kinetic enhancement is hindered by the multistructural anharmonicity. First, the rigid ring-structure adopted by the saddle point to facilitate simultaneous transfer of two atoms does not allow the formation of as many conformers as those formed by the reactant C6-CI. And second, the flexible carbon chain of C6-CI facilitates the formation of stabilizing intramolecular C-HO hydrogen bonds; this stabilizing effect is less pronounced in the saddle point structure due to its tightness and steric effects. Thus, the contribution of the reactant C6-CI conformers to the multistructural partition function is larger than that of the saddle point conformers. The resulting low multistructural anharmonicity factor partially cancels out the catalytic effect of the carboxylic acid, yielding in a moderately large rate coefficient, k(298 K) = 4.9 × 10-13 cm3 molecule-1 s-1. We show that carboxylic acids may promote the conversion of stabilized Criegee intermediates into vinylhydroperoxides in the atmosphere, which generates OH radicals and leads to secondary organic aerosols, thereby affecting the oxidative capacity of the atmosphere and ultimately the climate.

Ab Initio, Transition State Theory, and Kinetic Modeling Study of the HO2-Assisted Keto-Enol Tautomerism Propen-2-ol + HO2 ⇔ Acetone + HO2 under Combustion, Atmospheric, and Interstellar Conditions.

Keto-enol tautomerisms are important reactions in gaseous and liquid systems with implications in different chemical environments, but their kinetics have not been widely investigated. These reactions can proceed via a unimolecular process or may be catalyzed by another molecule. This work presents a theoretical study of the HO2-catalyzed tautomerism that converts propen-2-ol into acetone at conditions relevant to combustion, atmospheric and interstellar chemistry. We performed CCSD(T)/aug-cc-pVTZ//M06-2X/cc-pVTZ ab initio and multistructural torsional variational transition state theory calculations to compute the forward and reverse rate constants. These rate constants have not been investigated previously, and modelers approximate the kinetics by comparison to analogue reactions. Two features of the potential energy surface of the studied tautomerism are highlighted. First, the HO2 radical exhibits a pronounced catalytic effect by inducing a double hydrogen atom transfer reaction with a much lower barrier than that of the unimolecular process. Second, a prereactive complex is formed with a strong OH···π hydrogen bond. The role of the studied reaction under combustion conditions has been assessed via chemical kinetic modeling of 2-butanol (a potential alternative fuel) oxidation. The HO2-assisted process was found to not be competitive with the unimolecular and HCOOH-assisted tautomerisms. The rate constants for the formation of the prereactive complex were calculated with the variable reaction coordinate transition state theory, and pressure effects were estimated with the system-specific quantum Rice-Ramsperger-Kassel theory; this allowed us to investigate the role of the complex by using the canonical unified statistical model. The formation and equilibration of the prereactive complex, which is also important at low pressures, enhances the reactivity by inducing a large tunneling effect that leads to a significant increase of the rate constants at cold and ultracold temperatures. These findings may help to understand and model the fate of complex organic molecules in the interstellar medium, and suggest an alternative route for the high energy barrier keto-enol tautomerism which otherwise is not kinetically favored at low temperatures.

Theoretical Kinetic Study of the Unimolecular Keto-Enol Tautomerism Propen-2-ol ↔ Acetone. Pressure Effects and Implications in the Pyrolysis of tert- and 2-Butanol.

The need for renewable and cleaner sources of energy has made biofuels an interesting alternative to fossil fuels, especially in the case of butanol isomers, with its favorable blend properties and low hygroscopicity. Although C4 alcohols are prospective fuels, some key reactions governing their pyrolysis and combustion have not been adequately studied, leading to incomplete kinetic models. Enols are important intermediates in the combustion of C4 alcohols, as well as in atmospheric processes. Butanol reactions kinetics is poorly understood. Specifically, the unimolecular tautomerism of propen-2-ol ↔ acetone, which is included in butanol combustion kinetic models, is assigned rate parameters based on the tautomerism vinyl alcohol ↔ acetaldehyde as an analogy. In an attempt to update current kinetic models for tert- and 2-butanol, a theoretical kinetic study of the titled reaction was carried out by means of CCSD(T,FULL)/aug-cc-pVTZ//CCSD(T)/6-31+G(d,p) ab initio calculations, with multistructural torsional anharmonicity and variational transition state theory considerations in a wide temperature and pressure range (200-3000 K; 0.1-108 kPa). Results differ from vinyl alcohol ↔ acetaldehyde analogue reaction, which shows lower rate constant values. It was observed that decreasing pressure leads to a decrease in rate constants, describing the expected falloff behavior. Tunneling turned out to be important, especially at low temperatures. Accordingly, pyrolysis simulations in a batch reactor for tert- and 2-butanol with computed rate constants showed important differences in comparison with previous results, such as larger acetone yield and quicker propen-2-ol consumption.

Computational Kinetics of Hydroperoxybutylperoxy Isomerizations and Decompositions: A Study of the Effect of Hydrogen Bonding.

Hydroperoxyalkylperoxy (OOQOOH) radicals are important intermediates in combustion chemistry. The conventional isomerization of OOQOOH radicals to form ketohydroperoxides has been long believed to be the most important chain branching reaction under the low-temperature combustion conditions. In this work, the kinetics of competing pathways (alternative isomerization, concerted elimination, and H-exchange pathways) to the conventional isomerization of different ß-, γ- and Δ-OOQOOH butane isomers are investigated. Six- and seven-membered ring conventional isomerizations are found to be the dominant pathways, whereas alternative isomerizations are more important than conventional isomerization, when the latter proceeded via a more strained transition state ring. The oxygen atoms in OOQOOH radicals introduce intramolecular hydrogen bonding (HB) that significantly affects the energies of reacting species and transition states, ultimately influencing chemical kinetics. Conceptually, HB has a dual effect on the stability of chemical species, the first being the stabilizing effect of the actual intramolecular HB force, and the second being the destabilizing effect of ring strain imposed by the HB conformer. The overall effect can be quantified by determining the difference between the minimum energy conformers of a chemical species or transition state that have HB and that do not have HB (non-hydrogen bonding (NHB)). The stabilization effect of HB on the species and transition sates is assessed, and its effect on the calculated rate constants is also considered. Our results show that, for most species and transition states, HB stabilizes their energies by as much as 2.5 kcal/mol. However, NHB conformers are found to be more stable by up to 2.7 kcal/mol for a few of the considered species. To study the effect of HB on rate constants, reactions are categorized into two groups ( groups one and two) based on the structural similarity of the minimum energy conformers of the reactant and transition state, for a particular reaction. For cases where the reactant and transition state conformers are similar (i.e., both HB or NHB structures), group one, the effect of HB on reaction kinetics is major only if the magnitudes of the stabilization energy of the reactant and transition state are quite different. Meanwhile, for group two, where the reactant and transition state prefer different conformers (one HB and the other NHB), HB affects the kinetics when the stabilization energy of the reactant or transition state is significant or the entropy effect is important. This information is useful in determining corrections accounting for HB effects when assigning rate parameters for chemical reactions using estimation and/or analogy, where analogies usually result in inaccuracies when modeling atmospheric and combustion processes.

High-Pressure Limit Rate Rules for α-H Isomerization of Hydroperoxyalkylperoxy Radicals.

Hydroperoxyalkylperoxy (OOQOOH) radical isomerization is an important low-temperature chain branching reaction within the mechanism of hydrocarbon oxidation. This isomerization may proceed via the migration of the α-hydrogen to the hydroperoxide group. In this work, a combination of high level composite methods-CBS-QB3, G3, and G4-is used to determine the high-pressure-limit rate parameters for the title reaction. Rate rules for H-migration reactions proceeding through 5-, 6-, 7-, and 8-membered ring transitions states are determined. Migrations from primary, secondary and tertiary carbon sites to the peroxy group are considered. Chirality is also investigated by considering two diastereomers for reactants and transition states with two chiral centers. This is important since chirality may influence the energy barrier of the reaction as well as the rotational energy barriers of hindered rotors in chemical species and transition states. The effect of chirality and hydrogen bonding interactions in the investigated energies and rate constants is studied. The results show that while the energy difference between two diastereomers ranges from 0.1-3.2 kcal/mol, chirality hardly affects the kinetics, except at low temperatures (atmospheric conditions) or when two chiral centers are present in the reactant. Regarding the effect of the H-migration ring size, it is found that in most cases, the 1,5 and 1,6 H-migration reactions have similar rates at low temperatures (below ∼830 K) since the 1,6 H-migration proceeds via a cyclohexane-like transition state similar to that of the 1,5 H-migration.

Integrated In†Situ Characterization of a Molten Salt Catalyst Surface: Evidence of Sodium Peroxide and Hydroxyl Radical Formation.

Sodium-based catalysts (such as Na2 WO4 ) were proposed to selectively catalyze OH radical formation from H2 O and O2 at high temperatures. This reaction may proceed on molten salt state surfaces owing to the lower melting point of the used Na salts compared to the reaction temperature. This study provides direct evidence of the molten salt state of Na2 WO4 , which can form OH radicals, using in situ techniques including X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), laser induced fluorescence (LIF) spectrometry, and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). As a result, Na2 O2 species, which were hypothesized to be responsible for the formation of OH radicals, have been identified on the outer surfaces at temperatures of ≥800 °C, and these species are useful for various gas-phase hydrocarbon reactions, including the selective transformation of methane to ethane.