Pressure-dependent Rate Constants Research Articles

Theoretical Study on Reactions of Alkylperoxy Radicals.

We carried out a theoretical study on geometries, relative energies of stationary points, and reaction rate constants for ethyl + O2, propyl + O2, and butyl + O2 reactions, which are important reactions in the low-temperature oxidation of corresponding alkanes. Geometries with CCSD(T)/aug-cc-pVTZ for the ethyl + O2 system are adopted as the benchmark to choose a proper exchange-correlation functional for geometry optimization. Our results show that B3LYP with 6-311+G(d,p) can provide reliable structures for this system, and structures of the other two systems are determined with this functional. The performances of the explicitly correlated CCSD(T)-F12a and the locally correlated DLPNO-CCSD(T) methods on barrier heights and reaction energies are evaluated by comparing their results with those of CCSD(T)/aug-cc-pVQZ for the ethyl + O2 system. Our results indicate that reliable energy differences for this system are achieved with CCSD(T)-F12a using the cc-pVDZ-F12 basis set, and this method is employed in calculating single-point energies for the other two systems. The single-reference equation-of-motion spin-flip coupled-cluster method is adopted to obtain the potential energy surface of the barrierless reaction C2H5· + O2 → CH3CH2OO·, and the results are compared with those using broken-symmetry density functional theory and the Morse potential. Differences between energies with these methods are <1.6 kcal/mol, but the difference in the rate constants could be sizable at temperatures <500 K, and rate constants obtained in this work are reliable only for temperatures >500 K. Pressure-dependent rate constants for these reactions are determined using the Rice-Ramsperger-Kassel-Marcus/Master equation method. The obtained reaction energies, barrier heights, and rate constants could be valuable for reactions between the large alkane radical and O2, which are important in the low-temperature combustion of fuels such as kerosene and gasoline.

Pressure-dependent rate rules for intramolecular H-migration reactions of normal-alkyl cyclohexylperoxy radicals

For hydrocarbon fuels, reliable chemical mechanisms play an essential role towards improving the simulations of combustion process. However, in most of the previous modeling studies for alkylcycloalkanes combustion, the kinetic parameters are taken from acyclic alkanes based on the similarities between the reaction classes of alkylcycloalkanes and acyclic alkanes. Among the reaction families for alkylcycloalkanes, the intramolecular H-migration reactions of normal-alkyl cyclohexylperoxy radicals are one of the most important reaction families in the low temperature oxidation mechanisms. High-pressure limit rate rules and pressure-dependent rate rules for this reaction family have been developed based on quantum chemical computations at the CBS-QB3 level in combination with the transition state theory (TST) and Rice–Ramsberger–Kassel–Marcus/Master-Equation (RRKM/ME) theory. The reactions in this family involve a bicyclic transition state and can be divided into classes depending upon the cycle size of the newly formed cycle in the transition states and the types of the carbons from which the H atoms are migrated and if the H-migration occurs on the alkyl side chain, or on the cycle, or from the side chain to the cycle, or from the cycle to the side chain. For each reaction class, a representative set of reactions are chosen from methylcyclohexane to n-butylcyclohexane and the energy barriers of all chosen reactions are calculated and the average value for each class is obtained and compared with the value of the similar reaction class in alkanes. The high-pressure limit rate constants and the pressure-dependent rate constants of all reactions at pressures varying from 0.01 to 100 atm are calculated and the high-pressure limit rate rules and pressure-dependent rate rules for each class are derived from the average rate constants of reactions within each class. All calculated rate constants are fitted by a nonlinear least-squares method to the form of a modified Arrhenius rate expression.It is shown that there are large differences of the rate constants between alkylcycloalkanes and alkanes no matter if the H-migration occurs on the alkyl side chain, on the cycle or between the side chain and the cycle. Therefore, the construction of rate rules for alkylcycloalkanes, instead of taking values from similar reactions in alkanes, is necessary and significant to the low-temperature combustion modeling of alkylcyclohexanes.

Theoretical ab-initio kinetics of the reactions between isobutene plus hydroxyl

The kinetic chemistry of H-abstraction reactions of isobutene by the OH radical and the channels of the C4H9O complex formed by addition of the hydroxyl to isobutene were investigated in current work. Their rate constants at high pressure limits were computed based on the canonical transition state theory with tunneling effect considered, following the construction of the potential energy surface (PES) based on a duel-level quantum chemistry method at CCSD(T)/CBS//BHandHLYP/6-311G(d,p) level. Comparison of reaction rate constants of H-abstractions by the OH radical for isobutene with previous experimental and calculated data showed acceptable agreements. Isomerization pathways of the hydroxybutyl radicals to each other and to the isobutoxy radical were discussed in details. It was observed that the rate constants for both H-abstraction and isomerization via H-transfer were improved by considering tunneling effect. Subsequently, the pressure-dependent rate constants of multiple channels for each hydroxybutyl radical and of the dominant channel for isobutoxy radical were calculated based on RRKM/ME theory. The effect of chemical activation and thermal initial distribution was discussed with the case of 2-hydroxymethyl-prop-2-yl (W1) radical.

Pressure-dependent rate rules for cycloaddition, intramolecular H-shift, and concerted elimination reactions of alkenyl peroxy radicals at low temperature.

The reactions of cycloaddition, intramolecular H-shift and concerted elimination of alkenyl peroxy radicals are three kinds of important reactions in the low temperature combustion of alkenes. In this study, the cycloaddition reactions are divided into classes considering endo-cycloaddition, exo-cycloaddition and the size of the transition states; the intramolecular H-shift reactions are divided into classes depending upon the ring size of the transition states and the type of C-H bonds from which the hydrogen atom is transferred; the concerted elimination reactions are divided into classes according to the type of H-CβCαOO bond that is broken. All geometry optimizations are performed at the B3LYP/6-31G(2df,p) level. With the electronic structure calculations being performed using the composite Gaussian-4 (G4) method, high pressure limit rate constants and pressure-dependent rate constants at pressures varying from 0.01 to 100 atm are calculated by using canonical transition state theory and the Rice-Ramsberger-Kassel-Marcus/master equation method, respectively. All rate constants are given in the form of the modified Arrhenius expression. The high pressure limit rate rules and the pressure-dependent rate rules are derived by averaging the rate constants of a representative set of reactions in each class. The results show that the rate rules for these three classes of reactions have a large uncertainty and the impact of the pressure on the rate constants increases as temperature increases.

EStokTP: Electronic Structure to Temperature- and Pressure-Dependent Rate Constants-A Code for Automatically Predicting the Thermal Kinetics of Reactions.

A priori rate predictions for gas phase reactions have undergone a gradual but dramatic transformation, with current predictions often rivaling the accuracy of the best available experimental data. The utility of such kinetic predictions would be greatly magnified if they could more readily be implemented for large numbers of systems. Here, we report the development of a new computational environment, namely, EStokTP, that reduces the human effort involved in the rate prediction for single channel reactions essentially to the specification of the methodology to be employed. The code can also be used to obtain all the necessary master equation building blocks for more complex reactions. In general, the prediction of rate constants involves two steps, with the first consisting of a set of electronic structure calculations and the second in the application of some form of kinetic solver, such as a transition state theory (TST)-based master equation solver. EStokTP provides a fully integrated treatment of both steps through calls to external codes to perform first the electronic structure and then the master equation calculations. It focuses on generating, extracting, and organizing the necessary structural properties from a sequence of calls to electronic structure codes, with robust automatic failure recovery options to limit human intervention. The code implements one or multidimensional hindered rotor treatments of internal torsional modes (with automated projection from the Hessian and with optional vibrationally adiabatic corrections), Eckart and multidimensional tunneling models (such as small curvature theory), and variational treatments (based on intrinsic reaction coordinate following). This focus on a robust implementation of high-level TST methods allows the code to be used in high accuracy studies of large sets of reactions, as illustrated here through sample studies of a few hundred reactions. At present, the following reaction types are implemented in EStokTP: abstraction, addition, isomerization, and beta-decomposition. Preliminary protocols for treating barrierless reactions and multiple-well and/or multiple-channel potential energy surfaces are also illustrated.

Reaction mechanism, rate constants, and product yields for the oxidation of embedded five-member ring radicals with atomic oxygen

Oxidation mechanism of an embedded five-member ring on a surface of a carbonaceous particle with atomic oxygen has been studied employing G3(MP2,CC)//B3LYP/6-311G* potential energy surface calculations for C15H9 + O reactions. Temperature- and pressure-dependent reaction rate constants have been evaluated using the RRKM – Master Equation approach. The mechanism involves barrierless oxygen atom addition to a carbon atom on a free edge of a five-member ring followed by migration of the H atom from the attacked C atom to an ortho position followed by the five-member ring opening and elimination of the CO group, with the five-member ring being fully destroyed. When the embedded ring has three common sides with surrounding six-member rings and one free-edge carbon atom, the only product is the 4-phenanthrenyl radical. When the embedded ring has two common sides with the six-member rings and two free-edge C atoms, two different decarbonylation products possessing a phenalene core can be formed depending on the direction of the 1,2-H shift.

A computational investigation into the kinetics of NO + CH2CCH and its effect on NO reduction

A computational investigation into the kinetics of the NO + CH2CCH reaction is presented. The stationary points on the C3H3N1O1 potential energy surface are analyzed using the compound method ANL0, with key regions of the potential energy surface computed using multi-reference methods. The temperature- and pressure-dependent rate constants are computed using the RRKM/Master Equation. The dominant bimolecular products are HCN + CH2CO, CH2CNH + CO, and CH3CN + CO. Additional calculations for the thermal decomposition of an unimolecular intermediate, isoxazole, are in excellent agreement with the available experimental data. The new rate constants are implemented in a detailed chemical kinetic mechanism for the oxidation of C2H4 by O2 + NO. Analysis of a constant temperature, constant pressure batch reaction suggests that NO + CH2CCH could be an important pathway for both NO reduction and CH2CCH oxidation in reburn chemistry.

Kinetics of C10H7Br Pyrolysis

Author(s): Zagidullin, MV; Kaiser, RI; Ahmed, M; Porfiriev, DP; Medvedkov, IA; Mebel, AM; Azyazov, VN | Abstract: © 2018, Allerton Press, Inc. The temperature and pressure-dependent rate constants for the process C10H7Br ↔ C10H7+Br were evaluated using the variable reaction coordinate transition state theory VRC-TST. The calculated rate constants and computational fluid dynamics (CFD) calculations were employed to estimate the pyrolysis efficiency of 2-bromonaphthalene in the resistively-heated SiC high-temperature “chemical reactor” at the temperature of about 1500 K. The observed 40% pyrolysis efficiency is reproduced by CFD calculations if the value of the calculated rate constant for the C10H7Br pyrolysis is increased by a factor of 2.

Predictive kinetics for the thermal decomposition of RDX

The application of Variable Reaction Coordinate Transition State Theory for an energetic material is presented. The homolysis of the N–N bond in RDX is characterized using an embedding methodology in which key atoms in the bond-dissociation process are computed using CASPT2(10e,7o)/jun-cc-pVTZ, while the rest of the molecule is computed using M06-2X/jun-cc-pVTZ. Microcanonical rate theory is used to quantify the temperature and pressure dependent rate constants. The cleavage of the N–N bond is by far the dominant channel, with HONO elimination a distant second. The predicted rate constants are in excellent agreement with the experimental data. The computational approach can be used to provide accurate models for the combustion properties of novel energetic materials.

On the relative importance of HONO versus HNO2 in low-temperature combustion

Abstract This work investigates whether both HONO and HNO2 are essential in describing the reactivity for NO2-doped ignition experiments or if a strategy could be developed that lumps the two isomers into a single species without adversely affecting the model fidelity. First, the possibility of different product branching fractions is considered; temperature- and pressure-dependent rate constants are computed for H and CH3 addition to the N=O bond in both HONO and HNO2. These results suggest that addition of a radical to HONO and HNO2 do indeed have different products, but that the results are not likely to have a significant effect. Next, two different approaches to simplifying the HONO submechanism are considered. In the first, HNO2 is removed from the mechanism. In the second, HNO2 is replaced with HONO. These two strategies are implemented in different literature mechanisms and then used to compute ignition delay times for H2 and CH4. The results show that removing HNO2 has a modest effect on the ignition delay time, whereas systematically replacing HNO2 with HONO decreases the predicted ignition delay by approximately a factor of two. The recommendation is that for larger fuels, both HONO and HNO2 should be included in the mechanism.

Mechanistic and Kinetic Investigations on the Thermal Unimolecular Reaction of Heptafluoroisobutyronitrile.

Heptafluoroisobutyronitrile (C4) has been utilized as a dielectric compound to replace sulfur hexafluoride for the sake of environmental concern. Energetic profiles of the potential energy surface for the unimolecular reaction of C4 were calculated using density functional theory (M06-2X), quadratic complete basis set (CBS-QB3), Gaussian-4, multireference Rayleigh-Schrodinger perturbation theory (RS2, RS2C), and state-averaged multiconfiguration self-consistent-field (SA-MCSCF) ab initio methods. Among 38 production channels, the most energetically accessible reaction path is the three-centered rearrangement of cyanide to isocyanide. The C-CF3 bond appears to be the weakest bond in C4(X1A') and the symmetry-breaking C-CF3 bond cleavage undergoes to form the ground-state CF3(X2A1) and CF3CFCN (X2A″) radicals. All the possible isomerization pathways involving F- and CF3-migration together with the concerted elimination and stepwise decomposition routes were revealed for C4 and i-C4. Various isomers and potential toxic byproducts including FCN, CF3CN, C2F5CN, CF2═CFCF3, CF2═CFCN, CF4, C2F6, and alkyne compounds have been identified for the first time. Master equation analysis has been carried out to obtain the temperature- and pressure-dependent thermal rate constants. Theoretical kinetics for the loss of C4 due to pyrolysis is in good agreement with the temperature-ramped flow-tube experiment. The present theoretical work provides new insights on the thermal stability and chemical reactivity of C4. Moreover, i-C4 and C2F6 are proposed to be the potential characteristic gas molecules to monitor the insulation breakdown of C4 in the electric equipment.

Hydrogen shift isomerizations in the kinetics of the second oxidation mechanism of alkane combustion. Reactions of the hydroperoxypentylperoxy OOQOOH radical

Hydroperoxyalkylperoxy species are important intermediates that are generated during the autoignition of transport fuels. In combustion, the fate of hydroperoxyalkylperoxy is important for the performance of advanced combustion engines, especially for autoignition. A key fate of the hydroperoxyalkylperoxy is a 1,5 H-shift, for which kinetics data are experimentally unavailable. In the present work, we study 1-hydroperoxypentan-3-yl)dioxidanyl (CH3CH2CH(OO)CH2CH2OOH) as a model compound to clarify the kinetics of 1,5 H-shift of hydroperoxyalkylperoxy species, in particular α-H isomerization and alternative competitive pathways. With a combination of electronic structure calculations, we determine previously missing thermochemical data, and with multipath variational transition state theory (MP-VTST), a multidimensional tunneling (MT) approximation, multiple-structure anharmonicity, and torsional potential anharmonicity, we obtained much more accurate rate constants than the ones that can computed by conventional single-structure harmonic transition state theory (TST) and than the empirically estimated rate constants that are currently used in combustion modeling. The roles of various factors in determining the rates are elucidated. The pressure-dependent rate constants for these competitive reactions are computed using system-specific quantum RRK theory. The calculated temperature range is 298–1500 K, and the pressure range is 0.01–100 atm. The accurate thermodynamic and kinetics data determined in this work are indispensable in the detailed understanding and prediction of ignition properties of hydrocarbons and alternative fuels.

Evaluating mixture rules and combustion implications for multi-component pressure dependence of allyl + HO2 reactions

Many of the reactions important to combustion proceed through rovibrationally excited complexes, whose evolution depends on energy-transferring collisions with molecules in the surrounding mixture—yielding a strong dependence of rate constants on pressure and collisional energy transfer characteristics of the mixture. In nearly all realistic combustion environments, a substantial fraction of multiple species of distinct collisional energy transfer characteristics are present—thus requiring a “mixture rule” to interpolate kinetic data from individual single-component bath gases. While mixture effects for reactions proceeding through a single potential well to a single channel have been extensively investigated, mixture effects and mixture rules for multi-well and/or multi-channel reactions are significantly less characterized. The present study presents an investigation of the mixture effects, suitability of mixture rules for representing them, and their impacts on combustion predictions for the allyl + HO2 reaction system. The performance of different mixture rules for representing multi-component pressure dependence of rate constants for allyl + HO2 was evaluated through comparisons against master equation calculations for the mixture. The comparisons revealed that the classic linear mixture rule (LMR,P) yields deviations reaching a factor of two for typical combustion mixtures, whereas newly proposed mixture rules based on reduced pressure exhibit lower deviations of generally less ∼ 3%. Simulations of shock tube ignition delay times and jet-stirred reactor species profiles for C3H6 combustion suggest that mixture effects for allyl + HO2 on combustion predictions can be significant and may serve as a plausible explanation of the rate parameter adjustments employed in modeling studies to reproduce combustion measurements. Newly proposed mixture rules based on the reduced pressure are shown to accurately capture these mixture effects within combustion simulations.

Theoretical Studies on Isomerization and Decomposition Reactions of 2-Methyl-1-butanol Radicals

2-Methyl-1-butanol (2M1B) is a favorable candidate of substitute fuels characterized with high energy density and low hygroscopicity. 2M1B radicals, which are the products of H-abstraction reactions of 2M1B, and their isomerization and decomposition reactions play a cardinal impact on the distribution of combustion products. In this work, the primary isomerization and decomposition reaction channels of 2M1B radicals were investigated by using QCISD(T)/CBS//M062x/cc-pVTZ and CBS-QB3 method, respectively. The accurate phenomenological temperature- and pressure-dependent rate constants covering temperatures of 250–2500 K and pressures from 1 × 10–3 to 1 × 103 bar along with high-pressure limit rate constants for these channels were computed by solving the RRKM/master equation. The calculations revealed that the isomerization reaction of RC2 → RC6 has the highest energy barrier among these reactions, while the decomposition reaction RC6 → CH3CH2CHCH3 + CH2O has the lowest energy barrier. Furthermore, the comp...

Potential Energy Surface for Large Barrierless Reaction Systems: Application to the Kinetic Calculations of the Dissociation of Alkanes and the Reverse Recombination Reactions.

The isodesmic reaction method is applied to calculate the potential energy surface (PES) along the reaction coordinates and the rate constants of the barrierless reactions for unimolecular dissociation reactions of alkanes to form two alkyl radicals and their reverse recombination reactions. The reaction class is divided into 10 subclasses depending upon the type of carbon atoms in the reaction centers. A correction scheme based on isodesmic reaction theory is proposed to correct the PESs at UB3LYP/6-31+G(d,p) level. To validate the accuracy of this scheme, a comparison of the PESs at B3LYP level and the corrected PESs with the PESs at CASPT2/aug-cc-pVTZ level is performed for 13 representative reactions, and it is found that the deviations of the PESs at B3LYP level are up to 35.18 kcal/mol and are reduced to within 2 kcal/mol after correction, indicating that the PESs for barrierless reactions in a subclass can be calculated meaningfully accurately at a low level of ab initio method using our correction scheme. High-pressure limit rate constants and pressure dependent rate constants of these reactions are calculated based on their corrected PESs and the results show the pressure dependence of the rate constants cannot be ignored, especially at high temperatures. Furthermore, the impact of molecular size on the pressure-dependent rate constants of decomposition reactions of alkanes and their reverse reactions has been studied. The present work provides an effective method to generate meaningfully accurate PESs for large molecular system.

Computational Study on the Unimolecular Decomposition of JP-8 Jet Fuel Surrogates III: Butylbenzene Isomers ( n-, s-, and t-C14H10).

Ab initio G3(CCSD,MP2)//B3LYP/6-311G(d,p) calculations of potential energy surfaces have been carried out to unravel the mechanism of the initial stages of pyrolysis of three C10H14 isomers: n-, s-, and t-butylbenzenes. The computed energy and molecular parameters have been utilized in RRKM-master equation calculations to predict temperature- and pressure-dependent rate constants and product branching ratios for the primary unimolecular decomposition of these molecules and for the secondary decomposition of their radical fragments. The results showed that the primary dissociation of n-butylbenzene produces mostly benzyl (C7H7) + propyl (C3H7) and 1-phenyl-2-ethyl (C6H5C2H4) + ethyl (C2H5), with their relative yields strongly dependent on temperature and pressure, together with a minor amount of 1-phenyl-prop-3-yl (C9H11) + methyl (CH3). Secondary decomposition reactions that are anticipated to occur on a nanosecond scale under typical combustion conditions split propyl (C3H7) into ethylene (C2H4) + methyl (CH3), ethyl (C2H5) into ethylene (C2H4) + hydrogen (H), 1-phenyl-2-ethyl (C6H5C2H4) into mostly styrene (C8H8) + hydrogen (H) and to a lesser extent phenyl (C6H5) + ethylene (C2H4), and 1-phenyl-prop-3-yl (C9H11) into predominantly benzyl (C7H7) + ethylene (C2H4). The primary decomposition of s-butylbenzene is predicted to produce 1-phenyl-1-ethyl (C6H5CHCH3) + ethyl (C2H5) and a minor amount of 1-phenyl-prop-1-yl (C9H11) + methyl (CH3), and then 1-phenyl-1-ethyl (C6H5CHCH3) and 1-phenyl-prop-1-yl (C9H11) rapidly dissociate to styrene (C8H8) + hydrogen (H) and styrene (C8H8) + methyl (CH3), respectively. t-Butylbenzene decomposes nearly exclusively to 2-phenyl-prop-2-yl (C9H11) + methyl (CH3), and further, 2-phenyl-prop-2-yl (C9H11) rapidly eliminates a hydrogen atom to form 2-phenylpropene (C9H10). If hydrogen atoms or other reactive radicals are available to make a direct hydrogen-atom abstraction from butylbenzenes possible, the C10H13 radicals (1-phenyl-but-1-yl, 2-phenyl-but-2-yl, and t-phenyl-isobutyl) can be formed as the primary products from n-, s-, and t-butylbenzene, respectively. The secondary decomposition of 1-phenyl-but-1-yl leads to styrene (C8H8) + ethyl (C2H5), whereas 2-phenyl-but-2-yl and t-phenyl-isobutyl dissociate to 2-phenylpropene (C9H10) + methyl (CH3). Thus, the three butylbenzene isomers produce distinct but overlapping nascent pyrolysis fragments, which likely affect the successive oxidation mechanism and combustion kinetics of these JP-8 fuel components. Temperature- and pressure-dependent rate constants generated for the initial stages of pyrolysis of butylbenzenes are recommended for kinetic modeling.

Collision Frequency for Energy Transfer in Unimolecular Reactions.

Pressure dependence of unimolecular reaction rates is governed by the energy transfer in collisions of reactants with bath gas molecules. Pressure-dependent rate constants can be theoretically determined by solving master equations for unimolecular reactions. In general, master equation formulations describe energy transfer processes using a collision frequency and a probability distribution model of the energy transferred per collision. The present study proposes a novel method for determining the collision frequency from the results of classical trajectory calculations. Classical trajectories for collisions of several polyatomic molecules (ethane, methane, tetrafluoromethane, and cyclohexane) with monatomic colliders (Ar, Kr, and Xe) were calculated on potential energy surfaces described by the third-order density-functional tight-binding method in combination with simple pairwise interaction potentials. Low-order (including non-integer-order) moments of the energy transferred in deactivating collisions were extracted from the trajectories and compared with those derived using some probability distribution models. The comparison demonstrates the inadequacy of the conventional Lennard-Jones collision model for representing the collision frequency and suggests a robust method for evaluating the collision frequency that is consistent with a given probability distribution model, such as the exponential-down model. The resulting collision frequencies for the exponential-down model are substantially higher than the Lennard-Jones collision frequencies and are close to the (hypothetical) capture rate constants for dispersion interactions. The practical adequacy of the exponential-down model is also briefly discussed.

Effect of energy dependence of the density of states on pressure-dependent rate constants.

The FE integral for the normalized Boltzmann-weighted number of unimolecular states above the threshold energy is the key quantity for computing the collision efficiency in the pressure-dependent unimolecular rate theory developed by Troe, who calls this the energy dependence factor of the density of states. By using the Whitten-Rabinovitch approximation and assuming that the Whitten-Rabinovitch a(E) function is independent of energy, FE can be approximated by an analytical formula; this approximate formula is widely used because of its convenience and computational efficiency. Here we test its validity by comparing the rate constants computed by using the approximate FE to the ones determined by using the numerically integrated FE. For small-sized molecules and for reactions with high threshold energies E0, the differences are negligible at all temperatures, but in other cases, the approximate formula tends to underestimate FE and thus overestimates the collision efficiency, and this leads to smaller pressure falloff. When a(E) at high energies differs appreciably from a(E0), we find that the underestimation of pressure-dependent rate constants by using the approximate formula can be greater than a factor of 5 at high temperatures. The physical insight we draw from this study is that, for reactions with threshold energies below about 30 kcal mol-1, the rate of collisional energy transfer can be appreciably slowed down by the increase in the density of states at higher energies, and this increases the falloff effect by which finite-pressure rate constants become lower than the high-pressure limit, especially at higher temperatures.

Degradation of Carbonyl Hydroperoxides in the Atmosphere and in Combustion.

Oxygenates with carbonyl and hydroperoxy functional groups are important intermediates that are generated during the autoxidation of organic compounds in the atmosphere and during the autoignition of transport fuels. In the troposphere, the degradation of carbonyl hydroperoxides leads to low-vapor-pressure polyfunctional species that may precipitate in clouds and fog droplets or to the formation of secondary organic aerosols (SOAs). In combustion, the fate of carbonyl hydroperoxides is important for the performance of advanced combustion engines, especially for autoignition. A key fate of the carbonyl hydroperoxides is reaction with OH radicals, for which kinetics data are experimentally unavailable. Here, we study 4-hydroperoxy-2-pentanone (CH3C(═O)CH2CH(OOH)CH3) as a model compound to clarify the kinetics of OH reactions with carbonyl hydroperoxides, in particular H atom abstraction and OH addition reactions. With a combination of electronic structure calculations, we determine previously missing thermochemical data, and with multipath variational transition state theory (MP-VTST), a multidimensional tunneling (MT) approximation, multiple-structure anharmonicity, and torsional potential anharmonicity, we obtained much more accurate rate constants than the ones that can computed by conventional single-structure harmonic transition state theory (TST) and than the empirically estimated rate constants that are currently used in atmospheric and combustion modeling. The roles of various factors in determining the rates are elucidated. The pressure-dependent rate constants for the addition reaction are computed using system-specific quantum RRK theory. The calculated temperature range is 298-2400 K, and the pressure range is 0.01-100 atm. The accurate thermodynamic and kinetics data determined in this work are indispensable in the global modeling of SOAs in atmospheric science and in the detailed understanding and prediction of ignition properties of hydrocarbons and alternative fuels.

Reaction Paths and Chemical Activation Reactions of 2-Methyl-5-Furanyl Radical with 3O2.

Interest in high-energy substituted furans has been increasing due to their occurrence in biofuel production and their versatility in conversion to other useful products. Methylfurans are the simplest substituted furans and understanding their reaction pathways, thermochemical properties, including intermediate species stability, and chemical kinetics would aid in the study of larger furans. Furan ring C-H bonds have been shown to be extremely strong, approximately 120 kcal mol-1, due in part to the placement of the oxygen atom and aromatic-like resonance, both within the ring. The thermochemistry and kinetics of the oxidation of 2-methyfuran radical at position 5 of the furan ring, 2-methyl-5-furanyl radical (2MF5j), is analyzed. The resulting chemically activated species, 2MF5OOj radical, has a well depth of 51 kcal mol-1 below the 2MF5j + O2 reactants; this is 4-5 kcal mol-1 deeper than that of phenyl and vinyl radical plus O2, with both of these reactions known to undergo chain branching. Important, low-energy reaction pathways include chain branching dissociations, intramolecular abstractions, group transfers, and radical oxygen additions. Enthalpies of formation, entropies, and heat capacities for the stable molecules, radicals, and transition-state species are analyzed using computational methods. Calculated ΔH°f298 values were determined using an isodesmic work reaction from the CBS-QB3 composite method. Elementary rate parameters are from saddle point transition-state structures and compared to variational transition-state analysis for the barrierless reactions. Temperature- and pressure-dependent rate constants which are calculated using QRRK and master equation analysis is used for falloff and stabilization.