Pressure Dependence Of Rate Research Articles

Theoretical Study on the Kinetics of Secondary Oxygen Addition Reactions for N-Butyl Radicals.

Chemical kinetics for second oxygen addition reactions (·QOOH + O2) of long-chain alkanes are of great importance in low-temperature combustion technologies. However, kinetic data for key reactions of ·QOOH + O2 systems are often difficult to obtain experimentally and are primarily estimated or calculated by using theoretical methods. In this work, barrier heights (BHs), reaction energies (ΔEs), and relative energies (REs) of stationary points for key reactions of two representative ·QOOH + O2 systems in the low-temperature oxidation of n-butyl as well as pressure-dependent rate constants for the involved reactions are calculated with the high-level quantum chemical method CCSD(T)-F12b/CBS. These results can be employed in the development of low-temperature combustion mechanisms for n-butane and longer straight-chain alkanes. In addition, the performance of some quantum chemistry methods with a lower computational cost on BHs, ΔEs, and REs as well as rate constants is also investigated. Our results indicate that the maximum error on these energies with PNO-LCCSD(T)-F12a is within 1 kcal/mol, and rate constants with this method are in the best agreement with reference values, with a maximum relative error of about half the reference values. Due to its low computational cost and memory requirements, this method is strongly recommended for studying low-temperature combustion reactions involving larger hydrocarbon fuels.

Full-dimensional accurate potential energy surface and dynamics for the unimolecular isomerization reaction CH3NC ⇌ CH3CN.

The reaction CH3NC ⇌ CH3CN, a model reaction for the study of unimolecular isomerization, is important in astronomy and atmospheric chemistry and has long been studied by numerous experiments and theories. In this work, we report the first full-dimensional accurate potential energy surface (PES) of this reaction by the permutation invariant polynomial-neural network method based on 30 974 points, whose energies are calculated at the CCSD(T)-F12a/AVTZ level. Then, ring polymer molecular dynamics is used to derive the free energy barrier of the reaction at the experimental temperature range of 472.55-532.92K. Reaction kinetics are studied at the high-pressure limit and in the fall-off region by standard transition state theory and the master equation, respectively. The calculated temperature- and pressure-dependent rate coefficients are in good agreement with previous experimental and theoretical results. Furthermore, quasi-classical trajectory simulations are performed on this PES to study the intramolecular energy transfer dynamics at initial vibrational energies of 4.336, 5.204, and 6.505eV.

Pressure-dependent kinetic analysis of the N2H3 potential energy surface.

The pressure-dependent reactions on the N2H3 potential energy surface (PES) have been investigated using CCSD(T)-F12/aug-cc-pVTZ-F12//B2PLYP-D3/aug-cc-pVTZ. This study expands the N2H3 PES beyond the previous literature by incorporating a newly identified isomer, NH3N, along with additional bimolecular reaction channels associated with this isomer, namely NNH + H2 and H2NN(S) + H. Rate coefficients for all relevant pressure-dependent reactions, including well-skipping pathways, are predicted using a combination of ab initio transition state theory and master equation simulations. The dominant product of the NH2 + NH(T) recombination is N2H2 + H, while at high pressures and low temperatures, N2H3 formation becomes significant. Similarly, collisions involving H2NN(S) + H predominantly produce N2H2 + H. Secondary reactions such as H2NN(S) + H ⇌ NNH + H2 and H2NN(S) + H ⇌ NH2 + NH(T) are found to play a significant role at high temperatures across all examined pressures, while H2NN(S) + H ⇌ NH3N becomes prominent only at high pressures. Notably, none of these four H2NN(S) reactions have been included with pressure-dependent rate coefficients in previous NH3 oxidation models. The rate coefficients reported here provide valuable insights for modeling the combustion of ammonia, hydrazine, and their derivatives in diverse environments.

Association Kinetics for Perfluorinated n-Alkyl Radicals.

Radical-radical reaction channels are important in the pyrolysis and oxidation chemistry of perfluoroalkyl substances (PFAS). In particular, unimolecular dissociation reactions within unbranched n-perfluoroalkyl chains, and their corresponding reverse barrierless association reactions, are expected to be significant contributors to the gas-phase thermal decomposition of families of species such as perfluorinated carboxylic acids and perfluorinated sulfonic acids. Unfortunately, experimental data for these reactions are scarce and uncertain. Furthermore, obtaining reliable theoretical predictions for such reactions is a laborious and computationally intensive task. In this work, the chemical kinetics of the various association/decomposition reactions producing/decomposing the C2-C4 series of unbranched n-perfluoroalkanes (C2F6, C3F8, and C4F10) are examined using state-of-the-art ab initio transition-state-theory-based master-equation calculations. The variable-reaction-coordinate transition-state theory (VRC-TST) formalism is employed in computing the microcanonical and canonical rates for the association reactions. Reaction thermochemistry is obtained via composite quantum chemistry calculations and the laddering of error-canceling reaction schemes via a connectivity-based hierarchy approach employing ANL1/ANL0-style reference energies. Lennard-Jones collision model parameters for the considered systems were estimated by a direct dynamics approach, and collisional energy transfer parameters were obtained from analogies to systems of similar size and heavy-atom connectivity. A one-dimensional master equation approach was used to convert the microcanonical rate coefficients from the VRC-TST analysis into temperature- and pressure-dependent rate constants for the association reactions and the reverse dissociation reactions. The data are reported in standardized formats for usage in comprehensive chemical kinetic models for PFAS thermal destruction.

Kinetic study of hydrogen abstraction and unimolecular decomposition reactions of diethylamine during pyrolysis and oxidation

Diethylamine (DEA) represents a nitrogen-containing bio-oil model compound and an amine-model compound during the co-combustion of ammonia and hydrocarbons. Kinetics of the hydrogen abstraction reactions by H, O, OH, O2, HO2, CH3, and C2H5, and unimolecular decomposition reactions including 1,3-elimination, 1,2-hydrogen transfer, and C-N/C-C bond dissociations of DEA are theoretically investigated. Using the DLPNO-CCSD(T)/CBS(T-Q) method as a benchmark, cost-effective M06-2X/def2-TZVP method is selected for kinetic investigation. High-pressure-limit rate constants for the reactions with tight saddle points are determined by canonical variational transition-state theory with the multi-structural torsional anharmonicity and small-curvature tunneling. Hydrogen abstraction reactions at the C site connecting to the N site dominant in the low-to-mediate temperature range. Pressure dependent rate constants for the unimolecular decomposition reactions are determined by SS-QRRK/MSC-Dean approach. 1,3-elimination and C-C dissociation reactions dominant at the low-to-mediate and high temperature ranges, respectively.

A non-isothermal breakage-damage model for plastic-bonded granular materials incorporating temperature, pressure, and rate dependencies

Plastic-bonded granular materials (PBM) are widely used in industrial sectors, including building construction, abrasive applications, and defense applications such as plastic-bonded explosives. The mechanical behavior of PBM is highly nonlinear, irreversible, rate dependent, and temperature sensitive governed by various micromechanical attributions such as grain crushing and binder damage. This paper presents a thermodynamically consistent, microstructure-informed constitutive model to capture these characteristic behaviors of PBM. Key features of the model include a breakage internal variable to upscale the grain-scale information to the continuum level and to predict grain size evolution under mechanical loading. In addition, a damage internal state variable is introduced to account for the damage, deterioration, and debonding of the binder matrix upon loading. Temperature is taken as a fundamental external state variable to handle non-isothermal loading paths. The proposed model is able to capture with good accuracy several important aspects of the mechanical properties of PBM, such as pressure-dependent elasticity, pressure-dependent yield strength, brittle-to-ductile transition, temperature dependency, and rate dependency in the post-yielding regime. The model is validated against multiple published datasets obtained from confined and unconfined compression tests, covering various PBM compositions, confining pressures, temperatures, and strain rates.

Theoretical Kinetics of Radical-Radical Reaction NH2ṄH + ṄH2 and Its Implications for Monomethylhydrazine Pyrolysis Mechanism.

Significant discrepancies were observed between the experiments and the simulations for ṄH2 time-histories in monomethylhydrazine pyrolysis with the robust mechanism proposed by Pascal and Catoire. The rate of formation analyses for ṄH2 indicated the significance of the reaction NH2ṄH + ṄH2 = H2NN + NH3, which has not been well-defined. In this study, ab initio calculations were performed for the theoretical description of the NH2ṄH + ṄH2 chemistry. Most stationary points on the potential energy surface were quantified at the CCSD(T)/CBS//M06-2X/aug-cc-pVTZ level, and the multireference methods were employed for barrier-less reaction and some transition states. The temperature- and pressure-dependent rate coefficients were determined using classical and microcanonical variational transition state theories. Four primary reaction channels were identified as competitive: 1) The H atom abstraction reaction yielding N2H2(T) + NH3, dominating at 1350-3000 K across the 0.001-100 atm pressure range. 2) The H atom abstraction reaction forming N2H2(S) + NH3, dominating at 800-1350 K and competing with the processes of chemical activation and collisional stabilization below 800 K. 3) The chemical-activated reaction resulting in H2NN(S) + NH3, dominating below 800 K at 0.001 atm. 4) The collisional-stabilized recombination reaction leading to N3H5, becoming significant as pressure increases and dominating below 600 and 650 K at 1 and 100 atm, respectively. The implications of newly calculated NH2ṄH + ṄH2 kinetics for the monomethylhydrazine pyrolysis mechanism were evaluated, and updates were implemented. Sensitivity analyses indicated the necessity of additional research efforts to comprehend the dynamics of CH3NH2 unimolecular and N2H2 + ṄH2 reaction systems. The rate coefficients presented in this study can be employed to develop the chemical kinetic model of nitryl-containing systems.

Theoretical and experimental study on the pyrolysis of N-methylpyrrolidone

As a widely used organic solvent, N-methylpyrrolidone (NMP) may suffer thermal decomposition during large-scale industrial usage. The complex potential energy surface and corresponding pressure-dependent rate coefficients of NMP and NMP radicals were obtained at a high level of theory. The intermediates and products of NMP pyrolysis within 900–1200 K were identified and measured by synchrotron vacuum ultraviolet photoionization mass spectrometry. Theoretical calculations show that the Keto-enol tautomerization and direct ring-opening channel yielding C2H4, CO, and CH3NCH2 exhibit the highest rate coefficient among NMP unimolecular reactions at low and high temperatures, respectively. However, the fuel decomposition process cannot be exclusively dominated by single low-barrier channel. The ring-opening reaction forming imino aldehyde radical and following decarbonylation reactions are favored channels during the NMP radical decomposition, while the formation of the dihydropyrrolidone via H or CH3 loss competes with them at higher temperatures. Various smaller heterocycle radicals can be produced and act as intermediates for the multi-step isomerization of the ring-opening products with a relatively low energy barrier, changing the position of the functional group and radical site. A variety of nitrogenous and oxygenated species were observed in the experiment, such as ketene, methyl isocyanate, pyrrole, benzene, and different kinds of imines. Among them, C2H4, HCN, and CO are the major products for NMP decomposition. Potential reaction pathways from NMP to the major experimentally observed products were inferred based on the calculated results and existing knowledge on the pyrolysis of nitrogenous and oxygenated compounds. These results shed light on the complex C/H/O/N reaction system. The calculated rate coefficients and extensive mole fractions data could provide good support for the modeling of a detailed reaction mechanism in the future.

Reassessment of the High-Temperature Oxidation of Di Ethyl Ether through Ab Initio Calculations.

Recent investigations of diethyl ether (DEE) high-temperature pyrolysis and fuel-rich oxidation have highlighted the failure of existing kinetic models to describe experimental CO production. The DEE high-temperature pyrolysis and oxidation chemistry is thus investigated through ab initio calculations. Geometries, frequencies, and hindered-rotor potentials of reactants, products, and transition states of key reactions (fuel decomposition radical decomposition and H-abstraction reactions) are calculated with the B2PLYP-D3/def2-TZVPD method, whereas final energies are refined using CCSD(T)/aug-cc-pV(D,T)Z. Temperature- and pressure-dependent rate constants are then derived from either canonical transition state theory (CTST) or ME/RRKM analysis with the inclusion of tunneling effect and hindered-rotor corrections and compared to experimental measurements when available as well as to previously suggested values. This new information is then merged with a C0-C3 core chemistry model and a low-temperature chemistry DEE subset from the literature to propose a new kinetic model for the combustion of DEE. This model is tested successfully against a large database related to the high-temperature oxidation and pyrolysis chemistry of DEE, including ignition delay times, shock tube speciation data, time-resolved CO profiles, laminar flame speeds, flame structures, and jet-stirred reactor data.

Theoretical study of the second- and third-H-abstraction reactions of monomethylhydrazine and nitrogen dioxide and its application to hypergolic ignition modeling

The chemical kinetics of the second–H-abstraction and the third-H-abstraction reactions of monomethylhydrazine (MMH) and nitrogen dioxide (NO2) and its application to modeling hypergolic ignition of MMH/NO2 were investigated in this work. Potential energy surfaces (PESs) for the key reactions were obtained by employing the CCSD(T)/CBS//M06-2X/6-311++G(d,p) single-reference method and the MRCI(7e,6o)/CBS//M06-2X/6-311++G(d,p) multi-reference method. The RRKM/Master Equation approach was used to calculate the temperature- and pressure-dependent rate constants. For the second-H-abstraction reactions, the trans-CH3NNH and HNO2/HONO are the major products. For the third-H-abstraction reactions, the bimolecular reaction of cis-CH3NNH+NO2→CH3NN+HNO2 is more favorable. The calculated rate constants and thermodynamics data were fitted and incorporated into two existing mechanisms to investigate their influence on modeling MMH/NO2 hypergolic ignition. The modeling results show that the system reactivity of MMH/NO2 is promoted by the second- and third-H-abstraction reactions, as the ignition delay times decrease by a factor of two at 300–800 K. The production rate and sensitivity analyses further validate the significance of the second-H-abstraction reactions at low temperatures. The sensitivity analyses at 1000 K also indicate the necessity of further investigations on CH2O, N2H2/NO2, and CH3NH/NO2 sub-mechanisms for modeling the MMH/NO2 combustion at high temperatures. This work provides not only new kinetic and thermochemical data but also a better understanding of the hypergolic ignition of MMH/NO2.

Fulvenallenyl Radical (C7H5·)-Mediated Gas-Phase Synthesis of Bicyclic Aromatic C10H8 Isomers: Can Fulvenallenyl Efficiently React with Closed-Shell Hydrocarbons?

To understand the reactivity of resonantly stabilized radicals, often found in relevant concentrations in gaseous environments, it is important to determine their main reaction pathways. Here, it is investigated whether the fulvenallenyl radical (C7H5·) reacts preferentially with closed-shell molecules or radicals. Electronic structure calculations on the C10H9 potential energy surface accessed by the reactions of C7H5· with methylacetylene (CH3CCH) and allene (H2CCCH2) were combined with RRKM-ME calculations of temperature- and pressure-dependent rate constants using the automated EStokTP software suite and kinetic modeling to assess the reactivity of C7H5· with closed-shell unsaturated hydrocarbons. Experimentally, the reactions were attempted in a chemical microreactor heated to 998 ± 10 K by preparing fulvenallenyl radicals via pyrolysis of trichloromethylbenzene (C7H5Cl3) and seeding the radicals in methylacetylene or allene carrier gas, with product identification by means of photoionization mass spectrometry. The measured photoionization efficiency curve of m/z = 128 was assigned to a linear combination of the reference curves of two C10H8 isomers, azulene (minor) and naphthalene (major), presumably resulting from the C7H5· plus C3H4 reactions. However, the calculations demonstrated that these reactions are too slow, and kinetic modeling of processes in the reactor allowed us to conclude that the observation of naphthalene and azulene is due to the C7H5· plus C3H3· reaction, where propargyl is produced by direct hydrogen atom abstraction by chlorine (Cl) atoms from allene or methylacetylene and Cl stem from the pyrolysis of C7H5Cl3. Modeling results under the copyrolysis conditions of toluene and methylacetylene in high-temperature shock tube experiments confirmed the prevalence of the fulvenallenyl reaction with propargyl over its reactions with C3H4 even when the concentrations of allene and methylacetylene largely exceed that of propargyl. Overall, the reactions of fulvenallenyl with both allene and methylacetylene were found to be noncompetitive in the formation of naphthalene and azulene thus attesting the inefficiency of the fulvenallenyl radical reactions with the prototype closed-shell hydrocarbon species. In the meantime, the new reaction pathways revealed, including H-assisted isomerizations between C10H8 isomers and decomposition reactions of various C10H9 isomers, emerge as relevant and are recommended for inclusion in combustion kinetic models for naphthalene formation.

Unraveling the impact of acrylonitrile on naphthyl radical during the growth process of polycyclic aromatic hydrocarbons

The introduction of ammonia effectively inhibits soot formation through its chemical interactions, partly attributable to the effects exerted by CN species. This study aims to investigate the influence of larger CN species, specifically acrylonitrile (C3H3N), identified in NH3-doped flames, on the growth process of polycyclic aromatic hydrocarbons (PAHs). The potential energy surfaces (PESs) governing the reaction between C3H3N and the naphthyl radical were explored utilizing G3(MP2, CC)//B3LYP/6-311G++(d,p) level theory. The formed C3H3N adducts exhibit the potential to generate PAHs featuring CH=CHCN side chain or nitrogen-containing PAHs (N-PAHs). The Rice-Ramsperger-Kassel-Marcus (RRKM) theory was employed to calculate temperature- and pressure-dependent rate coefficients, enabling the assessment of yield distribution across the temperature range of 300–2500 K. In the 1-naphthyl + C3H3N reaction at 1 atm, the preferential formation of acenaphthylene-1-carbonitrile occurs at 300–1600 K, shifting towards PAHs with CH=CHCN side chains at temperatures exceeding 1600 K. The predominant production of PAHs with CH=CHCN side chains occurs in the 2-naphthyl + C3H3N reaction across all temperature ranges. These findings enhance our understanding of the mechanisms involving CN species and PAHs, significantly advancing the study of how intricate mechanisms influence PAH growth in NH3-doped flames.

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.

Kinetics and thermodynamics of unimolecular dissociation of n-C3H7I

Abstract The present work expands previous studies on the kinetics of the n-C3H7I unimolecular decomposition and the thermodynamic properties of n-C3H7I and i-C3H7I molecules, by providing combined experimental and theoretical data on the rate constant for reaction of n-C3H7I + Ar ⇌ n-C3H7 + I + Ar, as well as thermodynamic data for iodopropane isomers, calculated based on the density functional theory. The n-C3H7I dissociation rate constant has been precisely determined in shock-tube experiments by applying atomic resonance absorption spectrometry (ARAS) at the resonance transition wavelength of atomic iodine (183.0 nm) in a temperature range from 830 to 1230 K at a pressure of 3–4 bar. The resulting expression is presented in the Arrhenius form: k 1st = 1.17 × 1013exp(−191.4 kJ mol−1/RT) (s−1). Theoretical RRKM/ME calculation of the temperature- and pressure-dependent rate constant and channel branching ratio have been based on quantum chemical calculations and were performed over a wide range of thermodynamic conditions (T = 300–2000 K, p = 10−4 to 102 bar). Additionally, the thermochemistry of the reactions of n-C3H7I dissociation and isomerization has been calculated on B3LYP/cc-pVTZ-PP level of theory. Thermodynamic data, which are provided in NASA polynomial format, are in a better agreement with the available experimental data and previous theoretical estimates.

High-Pressure Limit and Pressure-Dependent Rate Rules for β-Scission Reaction Class of Hydroperoxyl Alkyl Hydroperoxyl Radicals (•P(OOH)2) in Normal-Alkyl Cyclohexanes Combustion.

Chemical kinetic studies of the β-scission reaction class of hydroperoxyl alkyl hydroperoxyl radicals (•P(OOH)2) from normal-alkyl cyclohexanes are carried out systematically through high-level ab initio calculations. Geometry optimizations and frequency calculations for all species involved in the reactions are performed at the B3LYP/CBSB7 level of theory. Electronic single-point energy calculations are calculated at the CBS-QB3 level of theory. Rate constants for the reactions of β-scission, in the temperature range of 500-1500 K and the pressure range of 0.01-100 atm, are calculated using transition state theory (TST) and Rice-Ramsberger-Kassel-Marcus/Master-Equation (RRKM/ME) theory taking asymmetric Eckart tunneling corrections and the one-dimensional hindered rotor approximation into consideration. The rate rules are obtained by averaging the rate constants of the representative reactions of this class. These rate rules can greatly assist in constructing more accurate low-temperature combustion mechanisms for normal-alkyl cyclohexanes.

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.