Role of O2 + QOOH in Low-Temperature Ignition of Propane. 1. Temperature and Pressure Dependent Rate Coefficients

High-Temperature Pyrolysis Study of 2-Methyl-2-butanol behind Reflected Shock Wave: Shock Tube and Computational Study

Pyrolysis of a branched alcohol, 2-methyl-2-butanol (2M2BOH), was carried out behind the reflected shock wave in the temperature range of 1011-1303 K and under pressures varying from 9.3 to 14.6 atm. Qualitative and quantitative analysis of the postshock mixture was performed using gas chromatography-mass spectrometry and gas chromatography with a flame ionization detector, respectively. The rate coefficients for the C-C and C-O bond cleavage reaction pathways were calculated using the variational transition-state theory. The Rice-Ramsperger-Kessel-Marcus/Master equation was employed to calculate the rate coefficients for the H2O-elimination reactions, unimolecular dissociation, and isomerization reaction pathways. The overall decomposition rate coefficient for the 2-methyl 2-butanol (2M2BOH) was estimated to be k t o t a l e x p t ( 1011 - 1303 K ) = ( 3.29 ± 0.73 ) × 10 11 × exp [ - ( 47.41 ± 0.53 T ) ] s - 1 , where activation energy is given in kcal mol-1. The reaction path analysis was performed, which gives information regarding the contribution of individual intermediate species toward the decomposition of 2M2BOH. A set of reactions was proposed and used to simulate the combustion chemistry of 2M2BOH, which consists of 48 reactions and 39 species. The experimentally measured and simulated mole fractions for the reactant and products showed reasonably good agreement. This work additionally investigates the effect of branching on the decomposition kinetics of long-chain alcohols.

Unveiling the chemical kinetics of aminomethanol (NH2CH2OH): insights into O . H and O2 photo-oxidation reactions and formamide dominance

Aminomethanol is released into the atmosphere through various sources, including biomass burning. In this study, we have expounded the chemical kinetics of aminomethanol in the reaction pathways initiated by the hydroxyl radical (O ˙ H) with the aid of ab initio//density functional theory (DFT) i.e., coupled-cluster theory (CCSD(T))//hybrid-DFT (M06-2X/6-311++G (3df, 3pd). We have explored various possible directions of theO ˙ H radical on aminomethanol, as well as the formation of distinct pre-reactive complexes. Our computational findings reveal that the H transfer necessitates activation energies ranging from 4.1 to 6.5 kcal/mol from the -CH2 group, 3.5-6.5 kcal/mol from the -NH2 group and 7-9.3 kcal/mol from the -OH group of three rotational conformers. The H transfer from -CH2, -NH2 and -OH exhibits an estimated total rate constant (k OH) of approximately 1.97 × 10-11 cm3 molecule-1 s-1 at 300 K. The branching fraction analysis indicates a pronounced dominance of C-centered NH2C ˙ HOH radicals with a favorability of 77%, surpassing the N-centeredN ˙ HCH2OH (20%) and O-centered NH2CH2O ˙ (3%) radicals. Moreover, our investigation delves into the oxidation of the prominently favored carbon-centered NH2C ˙ HOH radical through its interaction with atmospheric oxygen molecules. Intriguingly, our findings reveal that formamide (NH2CHO) emerges as the predominant product in the NH2C ˙ HOH + 3O2 reaction, eclipsing alternative outcomes such as amino formic acid (NH2COOH) and formimidic acid (HN = C(H)-OH). At atmospheric conditions pertinent to the troposphere, the branching fraction value for the formation of formamide is about 99%, coupled with a rate constant of 5.5 × 10-12 cm3 molecule-1 s-1. Finally, we have scrutinized the detrimental impact of formamide on the atmosphere. Interaction of formamide with atmospheric hydroxyl radicals could give rise to the production of potentially perilous compounds such as HNCO. Further, unreactedN ˙ HCH2OH radicals may initiate the formation of carcinogenic nitrosamines when reacting with trace N-oxides (namely, NO and NO2). This, in turn, escalates the environmental risk factors.

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.

Intramolecular Catalytic Hydrogen Atom Transfer (CHAT)

Intramolecular catalysis (IntraCat) is the acceleration of a process at one site of a molecule catalyzed by a functional group in the same molecule; an external agent such as a solvent typically facilitates it. Here, we report a general first-principles-based IntraCat mechanism, which strictly occurs within a single molecule with no coreagent being involved─we call it intramolecular catalytic transfer of hydrogen atoms (CHAT). A reactive part of a molecule (chat catalyst moiety or chat agent, represented by -OOH, -COOH, -SH, -CH2OH, -HPO4, or another bifunctional H-donor/acceptor group) catalyzes an interconversion process, such as keto-enol or amino-imino tautomerization, and cyclization in the same molecule, while being regenerated in the process. It can thus be regarded as an intramolecular version of the intermolecular H atom transfer processes mediated by an external molecular catalyst, e.g., dihydrogen, water, or a carboxylic acid. Earlier, we proposed a general mechanistic systematization of intermolecular processes, illustrated in the simplest case of the H2-mediated reactions classified as dihydrogen catalysis [Asatryan, R.; et al. Catal. Rev.: Sci. Eng., 2014, 56, 403-475]. Following this systematization, the CHAT catalysis belongs to the category of relay transfer of H atoms, albeit in an intramolecular manner. A broader class of intramolecular processes includes all types of H-transfer reactions stimulated by an H-migration, which we call self-catalyzed H atom transfer (SC-HAT). The CHAT mechanism comprises a subset of SC-HAT in which the catalytic moiety is regenerated (i.e., acts as a true catalyst and not a reagent). We provide several characteristic examples of CHAT mechanism based on detailed analysis of the corresponding potential energy surfaces. All such cases showed a dramatically reduced activation barrier relative to the corresponding uncatalyzed H-transfer reactions. For example, we show that CHAT can facilitate long-range H-migration in larger molecules and can occur multiple times in one molecule with multiple interconverting groups. It also facilitates amino-imino tautomerization of unsaturated GABA-analogues and peptides, as well as intramolecular cyclization processes to form heterocycles, e.g., oxygenated rings. CHAT pathways may also explain the pH-dependent increase of mutarotation rate of glucose-6-phosphate demonstrated in pioneering experiments that introduced the classical IntraCat concept. In addition, we identify a ground electronic state CHAT pathway as an alternative to the UV-promoted long-range molecular crane keto-enol conversion with a remarkably low activation energy. To initially assess the possible impact of the new keto-enol conversion pathway on combustion of n-alkanes, we present a detailed kinetic analysis of isomerization and decomposition of pentane-2,4-ketohydroperoxide (2,4-KHP). The results are compared with key alternative reactions, including direct dissociation and Korcek channels (for which a new alkyl group migration channel is also identified), revealing the competitiveness of the CHAT pathway across a range of conditions. Taken together, this work provides insight into a general class of reaction pathways that has not previously being systematically considered and that may occur in a broad range of contexts from combustion to atmospheric chemistry to biochemistry.

Can a single ammonia and water molecule enhance the formation of methanimine under tropospheric conditions?: kinetics of •CH2NH2 + O2 (+NH3/H2O)

The aminomethyl (•CH2NH2) radical is generated from the photo-oxidation of methylamine in the troposphere and is an important precursor for new particle formation. The effect of ammonia and water on the gas-phase formation of methanimine (CH2NH) from the •CH2NH2 + O2 reaction is not known. Therefore, in this study, the potential energy surfaces for •CH2NH2 + O2 (+NH3/H2O) were constructed using ab initio//DFT, i.e., coupled-cluster theory (CCSD(T))//hybrid-density functional theory, i.e., M06-2X with the 6-311++G (3df, 3pd) basis set. The Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) simulation with Eckart's asymmetric tunneling was used to calculate the rate coefficients and branching fractions relevant to the troposphere. The results show 40% formation of CH2NH at the low-pressure (<1 bar) and 100% formation of CH2NH2OO• at the high-pressure limit (HPL) condition. When an ammonia molecule is introduced into the reaction, there is a slight increase in the formation of CH2NH; however, when a water molecule is introduced into the reaction, the increase in the formation of CH2NH was from 40% to ∼80%. The calculated rate coefficient for •CH2NH2 + O2 (+NH3) [1.9 × 10-23 cm3 molecule-1 s-1] and for CH2NH2 + O2 (+H2O) [3.3 × 10-17 cm3 molecule-1 s-1] is at least twelve and six order magnitudes smaller than those for free •CH2NH2 + O2 (2 × 10-11 cm3 molecule-1 s-1 at 298 K) reactions, respectively. Our result is consistent with that of previous experimental and theoretical analysis and in good agreement with its isoelectronic analogous reaction. The work also provides a clear understanding of the formation of tropospheric carcinogenic compounds, i.e., hydrogen cyanide (HCN).

A Theoretical Kinetic Study on Concerted Elimination Reaction Class of Peroxyl-hydroperoxyl-alkyl Radicals (•OOQOOH) in Normal-alkyl Cyclohexanes

The concerted elimination reaction class of peroxyl-hydroperoxyl alkyl radicals (•OOQOOH) plays a crucial role in the low-temperature combustion of normal-alkyl cyclohexanes. The generation of the relatively unreactive HO2 radicals in this reaction is one of the factors leading to the negative temperature coefficient (NTC) behavior, which hinders the low-temperature oxidation of normal-alkyl cyclohexanes. In this study, 44 reactions are selected and divided into 4 different subclasses according to the nature of the carbon atom where the H atom is eliminated and the reaction center position. Utilizing the CBS-QB3 method, we compute the energy barriers for the concerted elimination reactions of peroxyl-hydroperoxyl alkyl radicals. Following this, we assess both the high-pressure limit and pressure-dependent rate constants for all reactions by applying TST and RRKM/ME theory. These calculations allow for the development of rate rules, which come to fruition through an averaging process involving the rate constants of representative reactions within each subclass. Our work provides accurate rate constants and rate rules for this reaction class, which can aid in constructing more accurate combustion mechanisms for normal-alkyl cyclohexanes.

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.

Measurement of the Intramolecular Hydrogen-Shift Rate Coefficient for the CH3SCH2OO Radical between 314 and 433 K

The intramolecular hydrogen-shift rate coefficient of the CH₃SCH₂O₂ (methylthiomethylperoxy, MSP) radical, a product formed in the oxidation of dimethyl sulfide (DMS), was measured using a pulsed laser photolysis flow tube reactor coupled to a high-resolution time-of-flight chemical ionization mass spectrometer that measured the formation of the DMS degradation end product HOOCH₂SCHO (hydroperoxymethyl thioformate). Measurements performed over the temperature range of 314-433 K yielded a hydrogen-shift rate coefficient of k₁(T) = (2.39 ± 0.7) × 10⁹ exp(-(7278 ± 99)/T) s^-1 Arrhenius expression and a value extrapolated to 298 K of 0.06 s^-1. The potential energy surface and the rate coefficient have also been theoretically investigated using density functional theory at the M06-2X/aug-cc-pVTZ level combined with approximate CCSD(T)/CBS energies yielding k₁(273-433 K) = 2.4 × 10¹¹ × exp(-8782/T) s^-1 and k₁(298 K) = 0.037 s^-1 in fair agreement with the experimental results. Present results are compared with the previously reported values of k₁(293-298 K).

Experimental and Updated Kinetic Modeling Study of Neopentane Low Temperature Oxidation

Neopentane is an ideal fuel model to study low-temperature oxidation chemistry. The significant discrepancies between experimental data and simulations using the existing neopentane models indicate that an updated study of neopentane oxidation is needed. In this work, neopentane oxidation experiments are carried out using two jet-stirred reactors (JSRs) at 1 atm, at a residence time of 3 s, and at three different equivalence ratios of 0.5, 0.9, and 1.62. Two different analytical methods (synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography) were used to investigate the species distributions. Numerous oxidation intermediates were detected and quantified, including acetone, 3,3-dimethyloxetane, methacrolein, isobutene, 2-methylpropanal, isobutyric acid, and peroxides, which are valuable for validating the kinetic model describing neopentane oxidation. In the model development, the pressure dependencies of the rate constants for the reaction classes Q̇OOH + O₂ and Q̇OOH decompositions are considered. This addition improves the prediction of the low-temperature oxidation reactivity of neopentane. Another focus of model development is to improve the prediction of carboxylic acids formed during the low-temperature oxidation of neopentane. The detection and identification of isobutyric acid indicates the existence of the Korcek mechanism during neopentane oxidation. Regarding the formation of acetic acid, the reaction channels are considered to be initiated from the reactions of ȮH radical addition to acetaldehyde/acetone. This updated kinetic model is validated extensively against the experimental data in this work and various experimental data available in the literature, including ignition delay times (IDTs) from both shock tubes (STs) and rapid compression machines (RCMs) and JSR speciation data at high temperatures.

Green synthesis of propylene oxide directly from propane

The chemical industry faces the challenge of bringing emissions of climate-damaging CO₂ to zero. However, the synthesis of important intermediates, such as olefins or epoxides, is still associated with the release of large amounts of greenhouse gases. This is due to both a high energy input for many process steps and insufficient selectivity of the underlying catalyzed reactions. Surprisingly, we find that in the oxidation of propane at elevated temperature over apparently inert materials such as boron nitride and silicon dioxide not only propylene but also significant amounts of propylene oxide are formed, with unexpectedly small amounts of CO₂. Process simulations reveal that the combined synthesis of these two important chemical building blocks is technologically feasible. Our discovery leads the ways towards an environmentally friendly production of propylene oxide and propylene in one step. We demonstrate that complex catalyst development is not necessary for this reaction.

Effects of Oxygen: Experimental and VTST/DFT Studies on Cumene Autoxidation with Air under Atmospheric Pressure

The mechanism of how oxygen affects cumene autoxidation related to temperature is still bewildering. Kinetic analysis of cumene autoxidation with air at a pressure of 1.0 atm was investigated by experiments and variational transition state theory/DFT. Oxygen was the limiting factor for cumene autoxidation above 100 °C, although it had negligible impacts on cumene autoxidation at 70-100 °C. The kinetic analysis by VTST coupled with DFT calculations proved that {k _6,reverse[ROO^•]}/{k _7,forward[RH]₀ [ROO^•]} > 10³ (70-120 °C), suggesting that ROO^• tended to decompose back to R^• and O₂ rapidly, whereas it was much slower for ROO^• abstracting a hydrogen atom from RH to form ROOH. When the concentration of oxygen was higher than the critical value ([O₂]_critical), it could not significantly affect the equilibrium concentration of ROO^•, which in turn could not affect the autoxidation rate significantly. Besides, the critical oxygen concentration ([O₂]_critical) was exponentially related to 1/T, which was consistent with Hattori's experimental results.

Helium Droplet Infrared Spectroscopy of the Butyl Radicals

Butyl radicals ( n-, s-, i-,} and tert-butyl) are formed from the pyrolysis of stable precursors (1-pentyl nitrite, 2-methyl-1-butyl nitrite, isopentyl nitrite, and azo- tert-butane, respectively). The radicals are doped into a beam of liquid helium droplets and probed with infrared action spectroscopy from 2700-3125 cm-1, allowing for a low temperature measurement of the CH stretching region. The presence of anharmonic resonance polyads in the 2800-3000 cm-1 region complicates its interpretation. To facilitate spectral assignment, the anharmonic resonances are modeled with two model Hamiltonian approaches that explicitly couple CH stretch fundamentals to HCH bend overtones and combinations: a VPT2+K normal mode model based on CCSD(T) quartic force fields and a semi-empirical local mode model. Both of these computational methods provide generally good agreement with the experimental spectra.

A Hierarchical Theoretical Study of the Hydrogen Abstraction Reactions of H2/C1-C4 Molecules by the Methyl Peroxy Radical and Implications for Kinetic Modeling

The hydrogen atom abstraction by the methyl peroxy radical (CH₃O₂) is an important reaction class in detailed chemical kinetic modeling of the autoignition properties of hydrocarbon fuels. Systematic theoretical studies are performed on this reaction class for H₂/C₁-C₄ fuels, which is critical in the development of a base model for large fuels. The molecules include hydrogen, alkanes, alkenes, and alkynes with a carbon number from 1 to 4. The B2PLYP-D3/cc-pVTZ level of theory is employed to optimize the geometries of all of the reactants, transition states, and products and also the treatments of hindered rotation for lower frequency modes. Accurate benchmark calculations for abstraction reactions of hydrogen, methane, and ethylene with CH₃O₂ are performed by using the coupled cluster method with explicit inclusion of single and double electron excitations and perturbative inclusion of triple electron excitations (CCSD(T)), the domain-based local pair-natural orbital coupled cluster method (DLPNO-CCSD(T)), and the explicitly correlated CCSD(T)-F12 method with large basis sets. Reaction rate constants are computed via conventional transition state theory with quantum tunneling corrections. The computed rate constants are compared with literature values and those employed in detailed chemical kinetic mechanisms. The calculated rate constants are implemented into the recently developed NUIGMECH1.1 base model for kinetic modeling of ignition properties.

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.

CH2 + O2: reaction mechanism, biradical and zwitterionic character, and formation of CH2OO, the simplest Criegee intermediate

The singlet and triplet potential surfaces for the title reaction were investigated using the CBS-QB3 level of theory. The wave functions for some species exhibited multireference character and required the CASPT2/6-31+G(d,p) and CASPT2/aug-cc-pVTZ levels of theory to obtain accurate relative energies. A Natural Bond Orbital Analysis showed that triplet ³CH₂OO (the simplest Criegee intermediate) and ³CH₂O₂ (dioxirane) have mostly polar biradical character, while singlet ¹CH₂OO has some zwitterionic character and a planar structure. Canonical variational transition state theory (CVTST) and master equation simulations were used to analyze the reaction system. CVTST predicts that the rate constant for reaction of ¹CH₂ + ³O₂ is more than ten times as fast as the reaction of ³CH₂ (X³B₁) + ³O₂ and the ratio remains almost independent of temperature from 900 K to 3000 K. The master equation simulations predict that at low pressures the ¹CH₂O + ³O product set is dominant at all temperatures and the primary yield of OH radicals is negligible below 600 K, due to competition with other primary reactions in this complex system.

Dissociation-Induced Depletion of High-Energy Reactant Molecules as a Mechanism for Pressure-Dependent Rate Constants for Bimolecular Reactions

In 1922, Lindemann proposed the now-well-known mechanism for pressure-dependent rate constants for unimolecular reactions: reactant molecules with sufficiently high energies dissociate more quickly than collisions can reestablish the Boltzmann distribution...

Chemistry of Simple Organic Peroxy Radicals under Atmospheric through Combustion Conditions: Role of Temperature, Pressure, and NOx Level

Organic peroxy radicals (RO₂) are key intermediates in the oxidation of organic compounds in both combustion systems and the atmosphere. While many studies have focused on reactions of RO₂ in specific applications, spanning a relatively limited range of reaction conditions, the generalized behavior of RO₂ radicals across the full range of reaction conditions (temperatures, pressures, and NO levels) has, to our knowledge, never been explored. In this work, two simple model systems, n-propyl peroxy radical and γ-isobutanol peroxy radical, are used to evaluate RO₂ fate using pressure-dependent kinetics. The fate of these radicals was modeled based on literature data over 250-1250 K, 0.01-100 bar, and 1 ppt to 100 ppm of NO, which spans the typical range of atmospheric and combustion conditions. Covering this entire range provides a broad overview of the reactivity of these species under both atmospheric and combustion conditions, as well as under conditions intermediate to the two. A particular focus is on the importance of reactions that were traditionally considered to occur in only one of the two sets of conditions: RO₂ unimolecular isomerization reactions (long known to occur in combustion systems but only recently appreciated in atmospheric systems) and RO₂ bimolecular reactions of RO₂ with NO (thought to occur mainly in atmospheric systems and rarely considered in combustion chemistry).

High-Pressure-Limit and Pressure-Dependent Rate Rules for Unimolecular Reactions Related to Hydroperoxy Alkyl Radicals in Normal Alkyl Cyclohexane Combustion. 1. Concerted HO2 Elimination Reaction Class and β-Scission Reaction Class

The reactions of the concerted HO₂ elimination from alkyl peroxy radicals and the β-scission of the C-OOH bond from hydroperoxy alkyl radicals, which lead to the formation of olefins and HO₂ radicals, are two important reaction classes that compete with the second O₂ addition step of hydroperoxy alkyl radicals, which are responsible for the chain branching in the low-temperature oxidation of normal alkyl cycloalkanes. These two reaction classes are also believed to be responsible for the negative temperature coefficient behavior due to the formation of the relatively unreactive HO₂ radical, which has the potential to inhibit ignition of normal alkyl cycloalkanes. In this work, the kinetics of the above two reaction classes in normal alkyl cycloalkanes are studied, where reactions in the concerted elimination class are divided into subclasses depending upon the types of carbons from which the H atom is eliminated and the positions of the reaction center (on the alkyl side chain or on the cycle), and the reactions in the β-scission reaction class are divided into subclasses depending upon the types of the carbons on which the radical is located and the positions of the reaction center. Energy barriers by using quantum chemical methods at the CBS-QB3 level, high-pressure-limit rate constants by using canonical transition state theory, and pressure-dependent rate constants at pressures from 0.01 to 100 atm by using Rice-Ramsberger-Kassel-Marcus/Master Equation theory are calculated for a representative set of reactions from methyl cyclohexane to n-butyl cyclohexane in each subclass, from which high-pressure-limit rate rules and pressure-dependent rate rules for each subclass are derived from the average rate constants of reactions within each subclass. A comparison of the rate constants for the reactions in the two reaction classes calculated in this work is made with the rate constants of the same reactions from available mechanisms published in the literature, where most of the rate constants are approximately estimated from analogous reactions in alkanes or small alkyl cyclohexanes, and it is found that a large difference may exist between them, indicating that the present work, which provides more accurate kinetic parameters and reasonable rate rules for these reaction classes, can be helpful to construct higher-accuracy mechanism models for normal alkyl cyclohexane combustion.

Theoretical Study on Reactions of α-Site Hydroxyethyl and Hydroxypropyl Radicals with O2

α-Site alcohol radicals are the most important products of H-abstract reactions from alcohols since the hydroxyl moiety weakens the α-site C-H bond. Reactions between α-site alcohol radicals and O2 play an important role in combustion of alcohols, especially at relatively low temperatures. However, reliable reaction pathways and rate constants for these reactions are still lacking. Theoretical studies on reactions in α-hydroxyethyl radical (CH3C•HOH) + O2 and α-hydroxypropyl radical (C2H5C•HOH and CH3C•OHCH3) + O2 reaction systems are performed in this work. Pressure-dependent rate constants for the involved reactions in a wide range of temperatures are determined using the Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) method. Our results show that rate constants for reactions in the α-hydroxypropyl radical + O2 system are quite different from those in the CH3C•HOH + O2 system. Detailed reaction pathways for these reaction systems are clarified, although combustion characteristics of ethanol and propanol do not change much with the obtained rate constants for these reactions. Important reaction channels in producing enols, especially in the combustion of propanol, are also provided. The obtained rate constants for these reactions over a wide range of temperatures and pressures are helpful in developing combustion mechanisms for ethanol and propanol.

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.

Mechanistic insights of the degradation of an O-anisidine carcinogenic pollutant initiated by OH radical attack: theoretical investigations

O-Anisidine (O-AND) is one of the amino organic compounds that harm human health, and is considered as a carcinogenic chemical.

Hindered rotor benchmarks for the transition states of free radical additions to unsaturated hydrocarbons

The hindered internal rotors of 32 transition states (TSs) formed through four free radicals, namely methyl, vinyl, ethyl, methoxy (ĊH3, Ċ2H3, Ċ2H5, CH3) additions to acetylene, ethylene, allene, propyne, and propene (C2H2/C2H4/C3H4-a/C3H4-p/C3H6) are studied. To validate the uncertainties of rate constants that stem from the use of different electronic structure methods to treat hindered rotors, the rotations of the newly formed C-C and/or C-O rotors in the transition states are calculated using commonly used DFT methods (B3LYP, M06-2X, ωB97X-D and B2PLYP-D3 with two Pople basis sets (6-31+G(d,p), 6-311++G(d,p)) and cc-pVTZ). The hindrance potential energies V(χ) calculated using the M06-2X/6-311++G(d,p) method are benchmarked at the CCSD(T), CCSD(T)-F12, DLPNO-CCSD(T) levels of theory with cc-pVTZ-F12 and cc-pVXZ (X = T, Q) basis sets and are extrapolated to the complete basis set (CBS) limit. The DLPNO-CCSD(T)/CBS method is proven to reproduce the CCSD(T)/CBS energies within 0.5 kJ mol-1 and this method is selected as the benchmark for all of the rotors in this study. Rotational constants B(χ) are computed for each method based on the optimized geometries for the hindrance potential via the I(2,3) approximation. Thereafter, the V(χ) and B(χ) values are used to compute hindered internal rotation partition functions, QHR, as a function of temperature. The uncertainties in the V(χ), B(χ) and QHR calculations stem from the use of different DFT methods for the internal rotor treatment are discussed for these newly formed rotors. For rotors formed by + C2 alkenes/alkynes, the V(χ) and QHR values calculated using DFT methods are compared with the DLPNO-CCSD(T)/CBS results and analysed according to reaction types. Based on comparisons of the DFT methods with the benchmarking method, reliable DFT methods are recommended for the treatment of internal rotors for different reaction types considering both accuracy and computational cost. This work, to the authors' knowledge, is the first to systematically benchmark hindrance potentials which can be used to estimate uncertainties in theoretically derived rate constants arising from the choice of different electronic structure methods.

Pressure-dependent kinetics of peroxy radicals formed in isobutanol combustion

Bio-derived isobutanol has been approved as a gasoline additive in the US, but our understanding of its combustion chemistry still has significant uncertainties. Detailed quantum calculations could improve model accuracy leading to better estimation of isobutanol's combustion properties and its environmental impacts. This work examines 47 molecules and 38 reactions involved in the first oxygen addition to isobutanol's three alkyl radicals located α, β, and γ to the hydroxide. Quantum calculations are mostly done at CCSD(T)-F12/cc-pVTZ-F12//B3LYP/CBSB7, with 1-D hindered rotor corrections obtained at B3LYP/6-31G(d). The resulting potential energy surfaces are the most comprehensive isobutanol peroxy networks published to date. Canonical transition state theory and a 1-D microcanonical master equation are used to derive high-pressure-limit and pressure-dependent rate coefficients, respectively. At all conditions studied, the recombination of γ-isobutanol radical with O₂ forms HO₂ + isobutanal. The recombination of β-isobutanol radical with O₂ forms a stabilized hydroperoxy alkyl radical below 400 K, water + an alkoxy radical at higher temperatures, and HO₂ + an alkene above 1200 K. The recombination of β-isobutanol radical with O₂ results in a mixture of products between 700-1100 K, forming acetone + formaldehyde + OH at lower temperatures and forming HO₂ + alkenes at higher temperatures. The barrier heights, high-pressure-limit rates, and pressure-dependent kinetics generally agree with the results from previous quantum chemistry calculations. Six reaction rates in this work deviate by over three orders of magnitude from kinetics in detailed models of isobutanol combustion, suggesting the rates calculated here can help improve modeling of isobutanol combustion and its environmental fate.

Ab initio kinetics predictions for the role of pre-reaction complexes in hydrogen abstraction from 2-butanone by OH radicals

The existence of pre- and post-reaction complexes has been proposed to influence hydrogen abstraction reaction kinetics, but the significance still remains controversial. A theoretical study is presented to discuss the effects of complexes on hydrogen abstraction from 2-butanone by OH radicals based on the detailed PESs at the DLPNO-CCSD(T)/aug-cc-pVTZ//M06-2x-D3/may-cc-pVTZ level with five pre-reaction complexes at the entrance of the channels and four post-reaction complexes at the exit. The hydrogen bond interactions, steric effects, and contributions to the bonding orbital of the OH radical species and 2-butanone species in the complex structures were visualized and investigated by wavefunction analyses. Three kinds of mechanisms-the general bimolecular reaction, the reaction with the complexes considered, and the well-skipping reaction-were compared based on high-pressure-limit rate constants, predicted branching ratios, and fractional populations of reactants and products in the temperature range of 250-2000 K. The existence of complexes was proved to be crucial in the kinetics and mechanisms of the hydrogen abstraction from 2-butanone molecules by OH radicals.

Computational studies on the gas phase reaction of methylenimine (CH2NH) with water molecules

In this work, we used quantum chemical methods and chemical kinetic models to answer the question of whether or not formaldehyde (CH2O) and ammonia (NH3) can be produced from gas phase hydration of methylenimine (CH2NH). The potential energy surfaces (PESs) of CH2NH + H2O → CH2O + NH3 and CH2NH + 2H2O → CH2O + NH3 + H2O reactions were computed using CCSD(T)/6-311++G(3d,3pd)//M06-2X/6-311++G(3d,3pd) level. The temperature-and pressure-dependent rate constants were calculated using variational transition state theory (VTST), microcanonical variational transition state theory [Formula: see text] and Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) simulations. The PES along the reaction path forming a weakly bound complex (CH2NH⋯H2O) was located using VTST and [Formula: see text]VTST, however, the PES along the tight transition state was characterized by VTST with small curvature tunneling (SCT) approach. The results show that the formation of CH2NH + H2O → CH2NH⋯H2O is pressure -and temperature-dependent. The calculated atmospheric lifetimes of CH2NH⋯H2O (~ 8 min) are too short to undergo secondary bimolecular reactions with other atmospheric species. Our results suggest that the formation of CH2O and NH3 likely to occur in the combustion of biomass burning but the rate of formation CH2O and NH3 is predicted to be negligible under atmospheric conditions. When a second water molecule is added to the reaction, the results suggest that the rates of formation of CH2O and NH3 remain negligible.

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.

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.

Computational study on the mechanism and kinetics for the reaction between HO2 and n-propyl peroxy radical

The n-propyl peroxy radical (n-C₃H₇O₂) is the key intermediate during atmospheric oxidation of propane (C₃H₈) which plays an important role in the carbon and nitrogen cycles in the troposphere. In this paper, a comprehensive theoretical study on the reaction mechanism and kinetics of the reaction between HO₂ and n-C₃H₇O₂ was performed at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-311G(d,p) level of theory. Computational results show that the HO₂ + n-C₃H₇O₂ reaction proceeds on both singlet and triplet potential energy surfaces (PESs). From an energetic point of view, the formation of C₃H₇O₂H and ³O₂via triplet hydrogen abstraction is the most favorable channel while other product channels are negligible. In addition, the calculated rate constants for the title reaction over the temperature range of 238–398 K were calculated by the multiconformer transition state theory (MC-TST), and the calculated rate constants show a negative temperature dependence. The contributions of the other four reaction channels to the total rate constant are negligible.

The negative temperature dependence for the HO₂ + n-C₃H₇O₂ reaction in lower temperature regime.

Theoretical study on the gas phase reaction of CH2O + NH3: the formation of CH2ONH3, NH2CH2OH, or CH2NH + H2O

The gas phase reaction between CH₂O and NH₃ is an important reaction in cold interstellar clouds, combustion chemistry and organic chemistry. In this study, the stationary point on the potential energy surfaces (PESs) for the CH₂O + NH₃ reaction was computed at the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd) level. The temperature- and pressure-dependent rate constants were computed using advanced kinetic models, including microcanonical variational transition state theory and Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) techniques. Our result predicts that the CH₂O + NH₃ reaction forms a collisionally thermalized CH₂ONH₃ complex with respect to thermal unimolecular dissociation and the other products, i.e., NH₂CH₂OH and CH₂NH + H₂O, are negligible under atmospheric conditions. The calculated atmospheric lifetime of the CH₂ONH₃ complex is ∼17 min, which suggests that the CH₂ONH₃ complex can react with other atmospheric species. The results also suggest that the formation of CH₂NH and H₂O from the Strecker's process is negligibly small under all the conditions studied here. The decay rate of CH₂O + NH₃ (5.1 × 10^-4 s^-1 at 1500 K) suggests that aminomethanol (NH₂CH₂OH) is likely to occur in the high-temperature combustion of biomass burning, but the rate of formation of NH₂CH₂OH is negligible under atmospheric conditions. The predicted atmospheric lifetime (∼4 days) of NH₂CH₂OH in the presence of the OH radical suggests that further reactions with other atmospheric species are possible. The formation of the NH₂CHOH radical from the reaction OH + NH₂CH₂OH can lead to carcinogenic products, such as nitrosamines, acetamide, hydrocycnic acid, NH₂ and CO₂.

Oxidation Kinetics and Thermodynamics of Resonance-Stabilized Radicals: The Pent-1-en-3-yl + O2 Reaction

The kinetics and thermochemistry of the pent-1-en-3-yl radical reaction with molecular oxygen (CH2CHCHCH2CH3 + O2) has been studied by both experimental and computational methods. The bimolecular rate coefficient of the reaction was measured as a function of temperature (198-370 K) and pressure (0.2-4.5 Torr) using laser photolysis-photoionization mass-spectrometry. Quantum chemical calculations were used to explore the potential energy surface of the reaction, after which Rice-Ramsperger-Kassel-Marcus theory/master equation simulations were performed to investigate the reaction. The experimental data were used to adjust key parameters, such as well depths, in the master equation model within methodological uncertainties. The master equation simulations suggest that the formation rates of the two potential RO2 adducts are equal and that the reaction to QOOH is slower than for saturated hydrocarbons. The initial addition reaction, CH2CHCHCH2CH3 + O2, is found to be barrierless when accounting for multireference effects. This is in agreement with the current experimental data, as well as with past experimental data for the allyl + O2 reaction. Finally, we conducted numerical simulations of the pent-1-en-3-yl + O2 reaction system and observed significant amounts of penta-1,3-diene being formed under engine-relevant conditions.

Chemical Kinetics of Hydrogen Atom Abstraction from Propargyl Sites by Hydrogen and Hydroxy Radicals

Hydrogen atom abstraction from propargyl C-H sites of alkynes plays a critical role in determining the reactivity of alkyne molecules and understanding the formation of soot precursors. This work reports a systematic theoretical study on the reaction mechanisms and rate constants for hydrogen abstraction reactions by hydrogen and hydroxy radicals from a series of alkyne molecules with different structural propargyl C-H atoms. Geometry optimizations and frequency calculations for all species are performed at M06-2X/cc-pVTZ level of theory and the hindered internal rotations are also treated at this level. The high-level W1BD and CCSD(T)/CBS theoretical calculations are used as a benchmark for a series of DFT calculations toward the selection of accurate DFT functionals for large reaction systems in this work. Based on the quantum chemistry calculations, rate constants are computed using the canonical transition state theory with tunneling correction and the treatment of internal rotations. The effects of the structure and reaction site on the energy barriers and rate constants are examined systematically. To the best of our knowledge, this work provides the first systematic study for one of the key initiation abstraction reactions for compounds containing propargyl hydrogen atoms.

Temperature and Pressure Dependent Rate Coefficients for the Reaction of Ketene with Hydroxyl Radical

The reaction of ketene with hydroxyl radical is drawing growing attention, for it is found to constitute an important step during the combustion of hydrocarbon and oxygenated hydrocarbon fuels, e.g., acetylene, propyne, allene, acetone, gasoline, diesel, jet fuels, and biofuels. We studied the potential energy surface (PES) of this reaction using B2PLYP-D3/cc-PVTZ for geometry optimization and composite methods based on CCSD(T)-F12/cc-PVTZ-F12 for energy calculations. From this PES, temperature- and pressure-dependent rate coefficients and branching ratios at 200-3000 K and 0.01-100 atm were derived using the RRKM/ME approach. The reaction is dominated by four product channels: (i) OH addition on the olefinic carbon of ketene to form CH₂OH + CO, which is the most dominant under all conditions; (ii) H abstraction producing HCCO + H₂O, which is favored at high temperatures; (iii) OH addition on the carbonyl carbon to form CH₃ + CO₂, which is favored at low pressures and high temperatures; and (iv) collisional stabilization of CH₂COOH, which is favored at high pressures and low temperatures. With increasing temperatures, the overall rate constant k_overall exhibit first negative but then positive temperature dependency, with its switching point (also the minimum point) at ∼400 K. Both product channel CH₂OH + CO and HCCO + H₂O are independent of pressure, whereas formation of CH₃ + CO₂ and collisional stabilization of CH₂COOH are highly pressure dependent. Fitted modified Arrhenius expressions of the calculated rate constants are provided for the purpose of combustion modeling.

Reaction Mechanisms and Kinetics of the Hydrogen Abstraction Reactions of C₄⁻C₆ Alkenes with Hydroxyl Radical: A Theoretical Exploration

The reaction of alkenes with hydroxyl (OH) radical is of great importance to atmospheric and combustion chemistry. This work used a combined ab initio/transition state theory (TST) method to study the reaction mechanisms and kinetics for hydrogen abstraction reactions by OH radical on C₄–C₆ alkenes. The elementary abstraction reactions involved were divided into 10 reaction classes depending upon the type of carbon atoms in the reaction center. Geometry optimization was performed by using DFT M06-2X functional with the 6-311+G(d,p) basis set. The energies were computed at the high-level CCSD(T)/CBS level of theory. Linear correlation for the computed reaction barriers and enthalpies between M06-2X/6-311+G(d,p) and CCSD(T)/CBS methods were found. It was shown that the C=C double bond in long alkenes not only affected the related allylic reaction site, but also exhibited a large influence on the reaction sites nearby the allylic site due to steric effects. TST in conjunction with tunneling effects were employed to determine high-pressure limit rate constants of these abstraction reactions and the computed overall rate constants were compared with the available literature data.

Decomposition kinetics for HONO and HNO2

This work presents a detailed investigation into the isomerization and decomposition of HONO and HNO₂.

Sparsity Facilitates Chemical-Reaction Selection for Engine Simulations

Analysis of large-scale, realistic models incorporating detailed chemistry can be challenging because each simulation is computationally expensive, and a complete analysis may require many simulations. This paper addresses one such problem of this type, chemical-reaction selection in engine simulations. In this computationally challenging case, it is demonstrated how the important concept of sparsity can facilitate chemical-reaction selection, which is the process of finding the most important chemical reactions for modeling a chemical process. It is difficult to perform accurate reaction selection for engine simulations using realistic models of the chemistry, as each simulation takes processor weeks to complete. We developed a procedure to efficiently accomplish this selection process with a relatively small number of simulations using a form of global sensitivity analysis based on sparse regression. The chemical-reaction selection leads to an analysis of the ignition chemistry as it evolves within the compression-ignition engine simulations and allows for the spatial development of the selected chemical reactions to be studied in detail.

First kinetic study of the atmospherically important reactions BrHg˙ + NO2 and BrHg˙ + HOO

We use computational chemistry to determine the rate constants and product yields for the reactions of BrHg˙ with the atmospherically abundant radicals NO₂ and HOO. The reactants, products, and well-defined transition states are characterized using CCSD(T) with large basis sets. The potential energy profiles for the barrierless addition of HOO and NO₂ to BrHg˙ are characterized using CASPT2 and RHF-CCSDT, and the rate constants are computed as a function of temperature and pressure using variational transition state theory and master equation simulations. The calculated rate constant for the addition of NO₂ to BrHg˙ is larger than that for the addition of HOO by a factor of up to two under atmospheric conditions. For the reaction of HOO with BrHg˙ the addition reaction entirely dominates competing HOO + BrHg˙ reaction channels. The addition of NO₂ to BrHg˙ initially produces both BrHgNO₂ and BrHgONO, but after a few seconds under atmospheric conditions the sole product is syn-BrHgONO. A previously unsuspected reaction channel for BrHg˙ + NO₂ competes with the addition to yield Hg + BrNO₂. This reaction reduces the mercury oxidation state in BrHg˙ from Hg(i) to Hg(0) and slows the atmospheric oxidation of Hg(0). While the rate constant for this reduction channel is not well-constrained by the present calculations, it may be as much as 18% as large as the oxidation channel under some atmospheric conditions. As no experimental kinetic or product yield data are available for the reactions studied here, this work will provide guidance for atmospheric modelers and experimental kineticists.

Assessing the Molecular Basis of the Fuel Octane Scale: A Detailed Investigation on the Rate Controlling Steps of the Autoignition of Heptane and Isooctane

N-Heptane and 2,2,4-trimethylpentane (isooctane) are the key species in the modeling of ignition of hydrocarbon-based fuel formulations. Isooctane is knock-resistant whereas n-heptane is a very knock-prone hydrocarbon. It has been suggested that interconversion of their associated alkylperoxy and hydroperoxyalkyl species via hydrogen-transfer isomerization reaction is the key step to understand their different knocking behavior. In this work, the kinetics of unimolecular hydrogen-transfer reactions of n-heptylperoxy and isooctylperoxy are determined using canonical variational transition-state theory and multidimensional small curvature tunneling. Internal rotation of involved molecules is taken explicitly into account in the molecular partition function. The rate coefficients are calculated in the temperature range 300-900 K, relevant to low-temperature autoignition. The concerted HO₂ elimination is an important reaction that competes with some H-transfer and is associated with chain termination. Thus, the branching ratio between these reaction channels is analyzed. We show that variational and multidimensional tunneling effects cannot be neglected for the H-transfer reaction. In particular, the pre-exponential Arrhenius fitting parameter derived from our rate constants shows a strong dependence on the temperature, because tunneling increases quickly at temperatures below 500 K. On the basis of our results, the existing qualitative model for the reasons for different knock behavior observed for n-heptane and isooctane is quantitatively validated at the molecular level.

High temperature pyrolysis of 2-methyl furan

Experiments and theory reveal the complex dissociation of 2-methylfuran and the surprising importance of H-atom loss.