Pressure-dependent Rate Constants Research Articles

Theoretical Study of the Temperature- and Pressure-Dependent Rate Constants for Nine Reactions between COn (n = 0-4), Om (m = 1-3), C2O, and C3O2 during the Radiolysis of Carbon Dioxide.

We investigate the reaction pathways of nine important CO2-related reactions using the revDSD-PBEP86-D3(BJ)/jun-cc-pV(T+d)Z level and simultaneously employ an accurate composite method (jun-Cheap) based on coupled-cluster (CC) theory. Subsequently, the Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) is solved to calculate the temperature- and pressure-dependent rate constants. This work investigates reactions involving transition states that have been overlooked in previous literature, including the dissociation of singlet-state C3O2, the triple channel formation of C2O + CO to form C3O2, and the formation of O3 + CO. The results show that CO3 is highly prone to dissociation at high temperatures. Finally, the kinetic data show that over a wide temperature range, our calculations are consistent with previous experimental measurements. The majority of the reaction rate constants studied exhibit significant pressure dependence, while the O3 + CO reaction is pressure-independent at low temperatures. These results are instrumental in the development of detailed kinetic models for the CO2 radiolysis reaction network.

Theoretical study of temperature and pressure-dependent rate constants for 8 reactions involving CO and CO2 with C+, C2O2+, CnO3, CnO3+ (n = 1, 2) in carbon dioxide radiolysis

CO2-related reactions and their species play an important role in atmospheric and radiolysis chemistry. However, there remains a lack of temperature and pressure dependent reaction rates for many chemical reactions that are necessary for constructing relevant reaction mechanisms. 8 important atmospheric and radiolysis chemical reactions are investigated theoretically at the rev-DSD/j3z, CCSD/ACCD and CCSD(T)/CBS levels. The temperature, pressure-dependent rate constants are obtained by the RRKM/ME method, which show a reasonable agreement with the reported values. Based on our results, the mechanisms of correlation reaction of CO2 reaction network are refined and provide guidance for studying the directed conversion of CO2.

Theoretical Study of the Decomposition Reactions of 2-Vinylfuran.

With the development of new synthetic methods, 2-vinylfuran (V2F) has become a potential renewable biofuel. In this work, the potential energy surfaces for the V2F unimolecular dissociation reaction, the H-addition reaction, and the H-abstraction reaction were constructed at the G4 level. The temperature- and pressure-dependent rate constants for the relevant reactions on the potential energy surfaces were calculated by solving the master equation based on the transition state theory and Rice-Ramsperger-Kassel-Marcus theory. The results show that the rate constant for the intramolecular H-transfer reaction of V2F with H atoms from the C(5) site to the C(4) site to form 2-vinylfuran-3(2H)-carbene, followed by the decomposition to form h145te3o, is the highest. The rate constants for the H-abstraction reaction of V2F with H atoms were the largest at C(6) on the branched chain, followed by C(7), and the rate constants for the H-abstraction reaction at C(3), C(4), and C(5) on the furan ring were not competitive. Negative temperature coefficient effects are observed for the rate constants of the addition reactions of V2F with H atoms at low pressures, with the H-addition rate constant at the C(5) site being the largest. This work not only provides the necessary rate constants for the reaction mechanism of V2F combustion but also provides theoretical guidance for the practical application of furan-based fuels.

Predicting pressure-dependent rate constants for the furan + OH reactions and their impact under tropospheric conditions.

In this work, we have evaluated the influence of temperature and pressure on the mechanism of furan oxidation by the OH radical. The stationary points on the potential energy surface were described at the M06-2X/aug-cc-pVTZ level of theory. In the kinetic treatment at the high-pressure limit (HPL), we have combined the multistructural canonical variational theory with multidimensional small-curvature tunneling corrections and long-range transition state theory. The system-specific quantum Rice-Ramsperger-Kassel theory was employed to estimate the pressure-dependent rate. In the HPL, the OH addition on the α carbon is the dominant pathway in the mechanism, producing a product via the ring-opening process, also confirmed by the product branching ratio calculations. The overall rate constant, obtained by a kinetic Monte Carlo simulation, reads the form koverall=5.22×10-13T/3001.10⁡exp1247(K/T) and indicates that the furan oxidation by OH radicals is a pressure-independent reaction under tropospheric conditions.

Theoretical Study on Ethylamine Dissociation Reactions Using VRC-VTST and SS-QRRK Methods.

Barrierless bond dissociation reactions play an important role in fuel combustion. In this work, the pressure-dependent dissociation rate constants of ethylamine (EA) are accurately determined using variable-reaction-coordinate variational transition-state theory combined with the system-specific quantum Rice-Ramsperger-Kassel method. Before the kinetics calculations, the performances of four density functional theory methods in describing the bond dissociation of EA are evaluated against the benchmark method, FIC-MRCISD(T)+Q/cc-pVTZ, and the MN15-L/cc-pVTZ method is the best choice. By comparison of the Gibbs free energies and the rate constants for the bond dissociation reactions of EA, ethanol, and propane, the influence of functional groups on the reaction kinetics is discussed. The kinetics calculations show that the dissociation rate constants of EA are sensitive to pressure at low pressures and high temperatures, and the dominant channel is the reaction that yields C2H5 and NH2 radicals. A literature combustion model of EA is updated with our calculations, and the satisfactory agreement between the model predictions and reported ignition delay times of EA suggests the reliability of our calculations.

A theoretical and experimental study of 2-ethylfuran + OH reaction

2-Ethylfuran (EF2), having similar thermophysical properties as gasoline, can be produced from biomass and has received increasing attention in recent years. The reaction of EF2 with OH radical plays a fundamental role in combustion chemistry of furan fuels. This work provides a comprehensive investigation on the reaction kinetics of the title reaction. The performance of several density functional theory methods is evaluated against the DLPNOCCSD(T)/CBS(T-Q) benchmark method. The results show that M08-HX/jun-cc-pVTZ is the best one for both abstraction and addition reactions with the lowest average unsigned deviations of 0.61 and 0.63 kcal mol−1, respectively. Multi-structural variational transition state theory combined with small-curvature tunneling approximation (MS-CVT/SCT) is employed to calculate the high-pressure limit (HPL) rate constants over a wide temperature range of 500−2000 K. The system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) method is used to obtain the pressure-dependent rate constants for addition reactions at several selected pressures from 0.01 to 100 atm. We have measured the rate constants of EF2 + OH reaction behind reflected shock wave at temperatures of 985−1342 K and pressure around 1 bar. The measured rate constants exhibit a positive temperature dependence and may be described by a three-parameter Arrhenius expression: k=1.28×10−10×T6.89exp(5318T) (cm3 mol−1 s–1). The calculated results are in good agreement with measurements and the total rate constants exhibit a non-monotonic temperature dependence due to the competition between the abstraction and addition channels. At the HPL, the addition reaction is dominant, and the branching ratio is larger than abstraction reaction at 500–1800 K and is 0.92 at 500 K. Finally, to evaluate the performance of our calculations on the ignition delay time of EF2, three available kinetic models (Somers et al., Xu et al. and Li et al.) of EF2 are updated and the results show that the predictions of Li's model are improved.

Experimental and kinetic study on 2-methoxyethanol part I: Molecular chemistry and pyrolysis investigation

Glycol ethers have been widely recognized as an alternative biofuels. 2-Methoxyethanol (ME2, HOCH2CH2OCH3) is the simplest and most representative glycol ether. Hence, the combustion chemistry of ME2 has aroused extensive interest. The ME2 pyrolysis has been investigated experimentally in a jet-stirred reactor (JSR) at atmospheric pressure within the temperature range of 650–1075 K for the first time. FTIR, GC–MS, and GC-FID were used to identify and quantify species profiles in ME2 pyrolysis. Sixteen intermediates were newly detected by the present work including acetaldehyde, hydroxyacetaldehyde, and methyl vinyl ether which are essential for mechanism development. The H abstraction reactions of ME2 by H, CH3, HO2, OH, O2, and the unimolecular reactions of four fuel-related radicals were first studied via high level ab initio calculations. The geometries optimization, vibrational frequencies, and intrinsic reaction coordinate (IRC) calculations were carried out at the M06–2X/6–311++G(d,p) level of theory. Conventional transition state theory (TST) and Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) were used to calculate the temperature- and pressure-dependent rate constants from 300 to 2000 K and 0.01 to 100 atm for the first time. It is found that the H abstraction reactions at the two middle carbon atoms are the most favored pathways, and b-scissions for fuel-related radicals have the lowest energy barriers. The thermochemical properties of ME2 and its derived radicals have been obtained via the combination of composite methods. A mechanism involving 596 species and 3123 reactions was developed for the first time to predict the ME2 pyrolysis characteristics based on experimental and calculated results obtained by the present work. Good agreements were achieved between the measured and simulated results. Rate of production (ROP), as well as sensitivity analysis, were performed to get a deeper insight into the ME2 pyrolysis. It has been revealed that the major ME2 decomposition channels are the H abstractions by H, CH3, and OH radicals, and unimolecular reactions have scarce contributions to fuel decay. The results, in terms of the measured data and of the calculated results, and the newly developed kinetic model as well as findings obtained within this study will assist and contribute to a better understanding of combustion characteristics of ME2 and of glycol ethers as well.

On the decomposition mechanism of propanal: rate constants evaluation and kinetic simulations

The reactivity of aldehydes has been the subject of considerable interest in chemical kinetics, with propanal often chosen as the representative species. Despite its relevance, the reactivity of propanal is currently estimated from analogy and fitting of experimental data measured in limited temperature and pressure ranges, while the few literature theoretical studies have focused more on the exploration the potential energy surface (PES) than on the estimation of rate constants. The purpose of this work is to reinvestigate the propanal decomposition kinetics using the ab initio transition state theory based master equation approach with the intent of: (1) Determining accurate rate constants of key reaction channels; (2) Updating and validating an existing kinetic model by simulating available experimental data on propanal pyrolysis. It is found that propanal decomposition at the initial stages of pyrolysis occurs through four unimolecular barrierless reactions to form CHO + C2H5, CH2CHO + CH3, CH3CHCHO + H, and CH3CH2CO + H, and a termolecular pathway leading to the formation of C2H4 + CO + H2. High pressure rate constants were determined for each barrierless reaction channel using Variable Reaction Coordinate Transition State Theory and used to estimate phenomenological temperature and pressure dependent channel specific rate constants integrating the 1 dimensional master equation over the whole PES. The decomposition rate constants so determined are in agreement with the few available experimental data and significantly faster than previous literature estimates. The estimated kinetic parameters were finally implemented into the CRECK kinetic mechanism, leading to an improved agreement with shock tube pyrolysis data from the literature.

Theoretical and Modeling Study about the Low-Temperature Reaction Mechanism Between Oxymethylene Ether-2 (OME2) Radicals and O2.

Oxymethylene ether-2 (CH3-O-CH2-O-CH2-O-CH3, OME2), a carbon-neutral fuel, was hydrogenated from CO2 captured in air or exhaust gases and reused for synthesis with renewable electricity. In the current work, two different potential energy surfaces (PESs) for the reaction of OME2 radicals with O2 were constructed at the CCSD(T)/CBS//M062X/6-311++G(d,p) level. Based on the Rice-Ramsperger-Kassel-Marcus (RRKM) theory and transition state theory, the temperature- and pressure-dependent rate constants for the relevant reactions on the PES were calculated by solving the master equation. The Arrhenius equation has been used to fit the temperature- and pressure-dependent reaction rate constants. The main reaction channels on the PES are discussed, showing that initial adduct generation and intramolecular H-transfer reactions are the key reaction channels for low-temperature combustion. Among them, the HO2 concerted elimination reaction channel needs to overcome higher energy barriers leading to uncompetitive HO2 concerted elimination reactions. The calculated rate constants were updated to the OME2 combustion model, and the updated model is in considerable agreement with experimentally measured data on the ignition delay time in the shock tube. The present work provides support for further studies on the oxidation reaction of long-chain OME..

Polyrate 2023: A computer program for the calculation of chemical reaction rates for polyatomics. New version announcement

Polyrate 2023: A computer program for the calculation of chemical reaction rates for polyatomics. New version announcement

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.

Kinetics for the reaction of Criegee intermediate CH2OO with n-butyraldehyde and its atmospheric implications

The Criegee intermediates (CIs), formed through ozone oxidation of unsaturated hydrocarbons, can undergo reactions with various carbonyl compounds in the atmosphere. In this study, we have investigated the 1,3-cycloaddition reaction of n-butyraldehyde with the simplest Criegee intermediate, CH2OO, using the OH laser-induced fluorescence (LIF) method. The experiments were conducted over a temperature and pressure range of 283–318 K and 5–100 Torr, respectively. Our results reveal that the bimolecular rate constant of the reaction is dependent on both temperature and pressure. Specifically, the measured rate constants at 100 Torr are (3.51 ± 0.63), (2.97 ± 0.53), (2.70 ± 0.49), and (2.52 ± 0.45) × 10−12 cm3 molecule−1 s−1 at temperatures of 283, 298, 308, and 318 K, respectively, with a total error of 18%. By fitting the pressure-dependent rate constants according to the Lindemann mechanism, we obtained a high-pressure limit rate constant of (3.05 ± 0.55) × 10−12 cm3 molecule−1 s−1 at 298 K. The Arrhenius plot of the temperature-dependent rate constants yields an activation energy of (−1.71 ± 0.30) kcal mol−1 and a pre-exponential factor of (1.66 ± 0.24) × 10−13 cm−3 s−1 (total error). Furthermore, our findings indicate that the title reaction exhibits a more pronounced temperature-dependent effect compared to that of CH2OO with water dimer and SO2, as well as that of OH with n-butyraldehyde. This observation suggests that the significance of title reaction in the atmosphere increases at lower temperature.

Temperature -and pressure-dependent branching ratios for 2,6-dimethylheptyl radicals (C9H19) + O2 reaction: An ab initio and RRKM/ME approach on a key component of bisabolane biofuel

Chemical kinetics mechanisms contain elementary reactions and associated Arrhenius rate parameters are necessary to understand the modeling of hydrocarbon autoignition chemistry. The complexity of such mechanisms is increased due to interest in operating next-generation combustion strategies in the low-temperature region (≤1000 K), which is governed by O2-addition to alkyl radicals (R), subsequent radical isomerization and decomposition steps thereafter. In this work, we report theoretically the reaction of molecular oxygen to the four isomers of 2,6-dimethylheptyl radicals (C9H19). The stationary points on potential energy surfaces (PES), rate constants, and branching ratios from C9H19 isomers initiated by O2 has been investigated by a combination of ab initio/density functional theory (CBS-QB3) and advanced statistical rate theory i.e., microcanonical variational transition state theory (μCVTST) and Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) simulations. The temperature- and pressure-dependent rate constants and branching ratios over temperature range 400 K–1000 K (with an interval of 50 K) and with different pressures 0.001, 0.01, 0.1, 1, 10 and 100 bar were computed. The RRKM/ME calculations reveal for addition of O2 to the primary radical, 2,6-dimethylhept-1-yl + O2 reaction, the formation of 2-iso-butyl-4-methyl-tetrahydrofuran (a five membered cyclic ether) + OH via concerted C–C and O–O bond scission of primary-secondary QOOH and formation of ROO-1 is the kinetically favorable pathway below 600 K. For 2,6-dimethylhept-2-yl + O2, three competitive favorable channels lead to the formation of 5-iso-propyl-2,2-dimethyl-tetrahydrofuran + OH, a five membered cyclic ether formed coincident with OH in a chain-propagating step from decomposition of tertiary-secondary hydroperoxyalkyl (QOOH), formation of a six membered cyclic ether i.e., 2,2,6,6-tetramethylpyran and ROO-2, however at higher temperature (>900 K) and 2–6-dimethyl-2-heptene is contributed equally over other products. For 2,6-dimethylhept-3-yl + O2 reaction, leading to 5-iso-propyl-2,2-dimethyltetrahydrofuran + OH (same products as in ROO-2) from decomposition of secondary–tertiary hydroperoxyalkyl (QOOH). For 2,6-dimethylhept-4-yl + O2 reaction, it reveals the formation of chain-propagation channels leading to iso-butene + methyl-isobutanal + OH. The results of this study reveals the importance of temperature- and pressure-dependent branching ratios, which is necessary to understand the low temperature internal combustion mechanism. The unimolecular rate constants were also compared with the previously reported values for 2,6-dimethylheptyl radical + O2 and similar reaction system i.e., 2,5-dimethylhexyl radical + O2. The updated rate constants and branching ratios may also serve as general prototypes for low-temperature oxidation of branched alkanes of next-generation biofuels with similar structural motifs, such as bisabolane and farnesane.

High-Pressure-Limit and Pressure-Dependent Rate Rules for Intramolecular Cyclization Reactions of α,β-Hydroxyl Hydroperoxyalkyl Radicals

ABSTRACT The cyclization reaction class of hydroxyl hydroperoxyalkyl radicals is one of the crucial reaction classes in low-temperature combustion mechanism of alcohols and alkenes. However, there is a lack of reports on accurate high-pressure-limit rate rules and pressure-dependent rate rules for this reaction class in literature. In this paper, 108 reactions are selected and divided into different classes according to the type of hydroxyl hydroperoxyalkyl radicals, the ring size of the transition states, and the position of the OH group. The energy barriers of the cyclization reactions of hydroxyl hydroperoxyalkyl radicals are calculated by the isodesmic reaction method, and the high-pressure-limit rate constants and pressure-dependent rate constants of all reactions are calculated using the TST and RRKM/ME theory, from which a high-pressure-limit rate rule and pressure-dependent rate rule for each subclass are established. It found that the isodesmic reaction method gives accurate energy barriers and rate constants, and the calculated rate constants of the cyclization reactions of hydroxyl hydroperoxyalkyl radicals are different from those of analogous reactions in alkanes, which indicates the necessity of establishing the rate rules for the cyclization reactions of hydroxyl hydroperoxyalkyl radical. There are large differences in energy barriers and rate constants among the whole reaction classes of the cyclization reactions of hydroxyl hydroperoxyalkyl radicals, so it is necessary to divide the cyclization reactions class into subclasses, and it is also shown that the pressure has a significant effect on the rate constants. In summary, accurate high-pressure-limit rate rules and pressure-dependent rate rules for the cyclization of hydroxyl hydroperoxyalkyl radicals have been developed in this study, which is helpful for the automatic construction of higher accuracy combustion mechanisms for alcohols and alkenes.

Chemical kinetic study of the low temperature oxidation of Alkanes with a new scheme

Leading candidates of advanced internal combustion engine concepts are low temperature compression ignition engines. However, kinetic data for low-temperature oxidation reactions from experimental measurements was limited. Theoretical investigations on kinetic of low-temperature oxidation reactions with high-level quantum chemistry methods are too expensive especially for large fuels. Reliable relative energies can possibly be calculated with proper density functional approximations (DFAs) to obtain reasonable kinetic data for reactions of large fuels. In this work, we investigate performance of nineteen DFAs on reaction energies (REs), barrier heights (BHs) and relative energies of stationary points in seven reaction systems between C2-C4 alkyl radicals and oxygen, which are important low-temperature combustion reactions. Our results show that rDSD-PBEP86-D3(BJ) can provide reliable REs with a mean absolute deviation (MAD) of 0.62 kcal/mol, while different DFAs are suggested for different type of reactions so that MAD on BHs are within 1 kcal/mol. A scheme is proposed where relative energies of stationary points in these reaction systems are determined with rDSD-PBEP86-D3(BJ) for intermediates and final products and with corresponding optimal DFAs for BHs. Pressure-dependent rate constants for reactions in these reaction systems with this scheme are in good agreement with those where relative energies are determined with CCSD(T)-F12a. Moreover, error of relative energies with this scheme is not related to system size. Reasonable kinetic data for low-temperature combustion reaction systems of large hydrocarbon and oxygenated hydrocarbon fuels can be achieved based on relative energies determined with this scheme, which can be used for the construction of low-temperature combustion mechanisms.

Theoretical Studies on the Reaction Kinetic of 2-Acetylfuran with Hydroxyl Radicals.

With the development of synthetic methods, 2-acetylfuran (AF2) has become a potential biomass fuel. The potential energy surfaces of AF2 and OH including OH-addition reactions and H-abstraction reactions were constructed by theoretical calculations at the CCSDT/CBS/M06-2x/cc-pVTZ level. The temperature- and pressure-dependent rate constants of the relevant reaction pathways were solved based on transition state theory and Rice-Ramsperger-Kassel-Marcus theory, as well as Eckart tunneling effect correction. The results showed that the H-abstraction reaction on CH3 on the branched chain and the OH-addition reaction at the C (2) and C (5) sites on the furan ring were the main reaction channels in the reaction system. At low temperatures, the AF2 and OH-addition reactions dominate, and the percentage decreases gradually to zero with increasing temperature, and at high temperatures, the H-abstraction reactions on the branched chains become the most dominant reaction channel. The rate coefficients calculated in the current work improve the combustion mechanism of AF2 and provide theoretical guidance for the practical application of AF2.

Experimental and kinetic modeling study of low-temperature oxidation of n-pentane

Attractiveness in advanced low-temperature combustion engines drives a constantly updated understanding of low-temperature oxidation chemistry. In this work, the low-temperature oxidation chemistry of n-pentane in two jet-stirred reactors at atmospheric pressure and in the temperature range of 500–825 K was investigated using combined analysis methods of synchrotron vacuum ultraviolet photoionization mass spectrometry, gas chromatography, and Fourier transform infrared spectroscopy. Furthermore, the gaseous mixture from JSR was collected in acetonitrile for subsequent product characterization using flow injection analysis, high-pressure and ultra-high-pressure liquid chromatography coupled to a Thermo Scientific™ Orbitrap® Q-Exactive high-resolution mass spectrometry. Numerous intermediate species were identified by these analytical methods, which contributed to unraveling the low-temperature oxidation reaction network of n-pentane. A detailed n-pentane model was tentatively developed to reduce deviations between experimental measurements and model predictions by updating the rate constants of C5 keto-hydroperoxide decomposition, C5 hydroperoxy cyclic ether decomposition, and Korcek reactions of C5 keto-hydroperoxide, and by introducing pressure-dependent rate constants for the reaction classes of Q˙OOH + O2, Q˙OOH decompositions, concerted HȮ2-elimination of RȮ2, C5 keto-hydroperoxide decomposition, C5 hydroperoxy cyclic ether decomposition, and Korcek reactions of C5 keto-hydroperoxide, and by adding more detailed sub-mechanisms for C5 cyclic ethers and C5 keto-hydroperoxides. This updated model was validated against a set of available experimental data, including jet-stirred reactor species data and ignition delay times. These exploratory updates of the kinetic model reveal the considerable influence of the rate constants of hydroperoxide decomposition and the pressure-dependent rate constants of key reaction classes on the kinetic model predictions, highlighting the future demands for high-precision quantum chemistry calculations of the pressure-dependent rate constants of the aforementioned reaction classes to reduce mechanism uncertainties and to develop accurate and robust chemical kinetic models.

Mechanistic and Kinetic Approach on the Propargyl Radical (C3H3) with the Criegee Intermediate (CH2OO).

The detailed reaction mechanism and kinetics of the C3H3 + CH2OO system have been thoroughly investigated. The CBS-QB3 method in conjunction with the ME/vRRKM theory has been applied to figure out the potential energy surface and rate constants for the C3H3 + CH2OO system. The C3H3 + CH2OO reaction leading to the CH2-[cyc-CCHCHOO] + H product dominates compared to the others. Rate constants of the reaction are dependent on temperatures (300-2000 K) and pressures (1-76,000 Torr), for which the rate constant of the channel C3H3 + CH2OO → CH2-[cyc-CCHCHOO] + H decreases at low pressures (1-76 Torr), but it increases with rising temperature if the pressure P ≥ 760 Torr. Rate constants of the three reaction channels C3H3 + CH2OO → CHCCH2CHO + OH, C3H3 + CH2OO → OCHCHCHCHO + H, and C3H3 + CH2OO → CHCHCHO + CH2O fluctuate with temperatures. The branching ratio of the C3H3 + CH2OO → CH2-[cyc-CCHCHOO] + H channel is the highest, accounting for 51-98.7% in the temperature range of 300-2000 K and 760 Torr pressure, while those of the channels forming the products PR10 (OCHCHCHCHO + H) and PR11 (CHCHCHO + CH2O) are the lowest, less than 0.1%, indicating that the contribution of these two reaction paths to the title reaction is insignificant. The proposed temperature- and pressure-dependent rate constants, together with the thermodynamic data of the species involved, can be confidently used for modeling CH2OO-related systems under atmospheric and combustion conditions.

Influence of Oxygen Atoms and Ring Strain on the Low-Temperature Oxidation Pathways of 1,3-Dioxolane

Biohybrid fuels are a promising solution for making the transportation sector more environmentally friendly. One such interesting fuel candidate is 1,3-dioxolane, which can be produced from inedible biomass. However, very little kinetics data are available for the low-temperature oxidation of this fuel molecule. To remedy this, we present the reaction kinetics of O2 addition to 1,3-dioxolanyl radicals in this work. All energies have been calculated at the DLPNO-CCSD(T)/CBS//B2PLYPD3BJ/6-311+g(d,p) level of theory. Temperature- and pressure-dependent reaction rate constants have been calculated with the RRKM/master equation method. The effects of heterocyclic oxygen atoms and ring strain on the low-temperature oxidation of 1,3-dioxolane are also compared to that of similar fuel molecules containing five heavy atoms: cyclopentane, tetrahydrofuran, and diethyl ether (DEE). The ring-opening β-scission reactions of the dioxolane hydroperoxy species are found to be the most dominant pathways following the oxidation of 1,3-dioxolanyl radicals. The heterocyclic oxygen atoms in 1,3-dioxolane weaken its C-O bonds, which leads to low barrier heights of the ring-opening reactions. Ring strain in 1,3-dioxolane increases the barriers for isomerization reactions of peroxy radicals compared to the similar reactions of DEE, which has a chain structure.

Theoretical Investigation of Rate Rules for H-Intermigration Reactions for Cyclic Alkylperoxy Radicals

As a starting channel, the H-intermigration reaction of alkylperoxy radicals (ROO radicals) that yields hydroperoxyl alkyl radicals (QOOH radicals) determines the low-temperature chemistry of alkanes. In this work, this type of reaction was investigated for typical cyclic alkanes, which are important fuel components and soot precursors, using theoretical ab initio methods. First, all the molecular geometries and vibrational frequencies were computed using the density functional theory method and the single point energies were refined using the post-Hartree fork method (M062X/6-311G(d,p)//DLPNO-CCSD(T)/CBS). Then, high-pressure limit rate constants were evaluated with tight transition state theory, with which tunneling effects were considered using the Eckart model and low-frequency torsion modes were modeled as hindered rotors. Pressure-dependent rate constants were also calculated for typical reaction channels. Rate expressions in the Arrhenius form for 91 reactions are proposed. All reactions were categorized into seven reaction types and the rate rule for each reaction type was estimated with uncertainty factors of three to six. These rules can be potentially used in the development of low-temperature kinetic mechanisms for cycloalkanes. A comparison between different reaction types was also performed and the favorable channels are discussed.