Variable Reaction Coordinate Transition State Research Articles

Reconciling Accuracy and Feasibility for Barrierless Reaction Steps by the PCS/DDCI/MC-PDFT Protocol: Methane and Ethylene Dissociations as Case Studies.

Several enhancements have been introduced into state-of-the-art computational protocols for the treatment of barrierless reaction steps in the framework of variable reaction coordinate variational transition state theory. The first step is the synergistic integration of the Iterative Difference Dedicated Configuration Interaction (I-DDCI) and Pisa Composite Scheme, which defines a reduced cost, yet very accurate, computational workflow. This approach provides a near black box tool for obtaining 1D reference potentials. Then, a general strategy has been devised for tuning the level of theory used in Monte Carlo (MC) sampling, employing Multiconfiguration Pair Density Functional Theory (MC-PDFT) with dynamically adjusted Hartree-Fock exchange. Concurrently, partial geometry optimizations during the MC simulations account for the coupling between the reaction coordinates and conserved modes. The protocol closely approaches full size consistency and yields highly accurate results, with several test computations suggesting rapid convergence of the I-DDCI correction with the basis set dimensions. The capabilities of the new platform are illustrated by two case studies (the hydrogen dissociation from CH4 and C2H4), which highlight its flexibility in handling different carbon hybridizations (sp3 and sp2). The remarkable accuracy of the computed rate constants confirms the robustness of the proposed method. Together with their intrinsic interest, these results pave the way for systematic investigations of complex gas-phase reactions through a reliable, user-friendly tool accessible to specialists and nonspecialists alike.

Theoretical and kinetic study of the thermal decomposition mechanism of long chain aldehydes

The mechanism of decomposition of the butanal and pentanal long chain aldehydes was investigated using the ab initio transition state theory-based master equation approach. The rate constants of the homolytic bond scission reactions of butanal and pentanol leading to the formation of CHO+C3H7, CH2CHO+C2H5, C2H4CHO+CH3, C3H7CO+H, C2H5ĊHCHO+H and CH2CHO+C3H7, C2H4CHO+C2H5, C3H6CHO+CH3, C4H9CO+H, C3H7ĊHCHO+H, respectively, as well as of the reverse barrierless recombination channels, were determined using Variable Reaction Coordinate Transition State Theory. The potential energy surfaces were determined at the multireference CASPT2/aug-cc-pVTZ level on ωB97X-D/jun-cc-pVTZ structures, while stochastic samplings were performed at the CASPT2 level using a (4e,4o) active space. Rate laws for the alkyl+R-CHO, H+R-ĊHCHO, and H+R-ĊO classes of reactions were then determined. Rate constants of H-abstraction reactions and of the three body butanal →CO+H2+C3H6 channel were determined using ωB97X-D/jun-cc-pVTZ structures and Hessians and CCSD(T) energies, extrapolated to the complete basis set. It was found that the calculated homolytic bond scission reactions are faster by a factor of 2–3 with respect to literature previous estimates. The calculated reaction rates were inserted in the CRECK kinetic scheme and used to simulate literature speciation data of butanal and pentanal pyrolysis in shock tubes and ignition delay times, finding that their inclusion into the kinetic scheme enhances the predictive capabilities of the mechanism. The aldehyde decomposition mechanism and the rate constants determined in this study are suitable to be used to interpret and predict the reactivity of aldehydes in a wide range of temperatures and pressures.

New insights into the mechanism and kinetics of the addition reaction of unsaturated Criegee intermediates to CF3COOH and tropospheric implications.

In this work, we have investigated the mechanism, thermochemistry and kinetics of the reaction of syn-cis-CH2RzCRyCO+O- (where Rz, Ry = H, CH3-) unsaturated Criegee intermediates (CIs) with CF3COOH using quantum chemical methods. The rate coefficients for the barrierless reactions were calculated using variable reaction coordinate variational transition state theory (VRC-VTST). For the syn-cis-CH2RzCRyCO+O- conformation in which conjugated CC and CO double bonds are aligned with each other, we propose a new pathway for the unidirectional addition of an OC-OH molecule (CF3COOH) to the CC double bond of syn-cis-CH2RzCRyCO+O-. The rate coefficient for the 1,4-CC addition reaction at 298 K is ∼10-10 to 10-11 cm3 s-1, resulting in the formation of CF3C(O)OCH2CRzRyCOOH trifluoroacetate alkyl allyl hydroperoxide (TFAAAH) as a new transitory adduct. It can act as a precursor for the formation of secondary organic aerosols (SOAs). This novel TFAAAH hydroperoxide was identified through a detailed quantum chemical study of the 1,4-addition mechanism and will provide new insights into the significance of the 1,4-addition reaction of unsaturated Cls with trace tropospheric gases on -CRzCH2 vinyl carbon atoms.

Enhanced C2-CN sub-mechanism: Impact on NO/N2O and soot precursor yields during C2H2/HCN oxidation

The combustion of hydrocarbon-ammonia fuels poses challenges related to the formation of soot and nitrogen oxides (NOx) emissions. Various kinetic mechanisms have been developed to describe the pathways of soot and NOx formation, but limited attention has been paid to the role of the C2-CN species, such as HC3N and CH2CHCN, in influencing the yields of soot and NOx. To address this gap, we present a detailed C2-CN sub-mechanism integrated into a NOx and polycyclic aromatic hydrocarbons (PAHs) kinetic model. The rate constants of the barrierless association reactions of C2H+CN, C2H3+CN, C2H5+CN, and CH3+CH2CN were updated using the variable reaction coordinate transition state theory (VRC-TST). We numerically investigated the oxidation of C2H2 and HCN at equivalence ratios of 2.0 and 3.0, which are major precursors for NOx and PAHs, in a perfectly stirred reactor (PSR) employing two kinetic models (with/without the developed C2-CN sub-mechanism). By comparing the rates of production (ROPs) obtained via both models, we analyzed the formation pathways of NO/N2O and Benzo(ghi)fluoranthene (BGHIF). Our findings reveal temperature-dependent trends in the calculated rate constants of C2H5+CN and CH3+CH2CN associations (positive dependence) as well as C2H+CN and C2H3+CN associations (negative dependence). The peak mole fractions of BGHIF at equivalence ratio (φ) of 2.0 and 3.0 predicted by the updated mechanism were 18.4 % and 13.6 % lower than the base mechanism. This reduction can be attributed to an additional consumption channel of C2H2 (C2H2+CN=HC3N+H) predicted by the C2-CN sub-mechanism. Furthermore, the C2-CN sub-mechanism exhibited a stronger suppression effect on N2O formation compared with NO formation at φ=2.0 and 3.0. This limitation can be explained by an additional consumption channel of CN via CN+C2H2=HC3N+H, resulting in decreased NO/N2O through the reaction sequence CN→NO→N2O and CN→NCO→HNCO→NH2→HNO→NO→N2O.

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 Kinetics Predictions for Reactions on the NH2O Potential Energy Surface.

Recent modeling studies of ammonia oxidation, which are motivated by the prospective role of ammonia as a zero-carbon fuel, have indicated significant discrepancies among the existing literature mechanisms. In this study, high-level theoretical kinetics predictions have been obtained for reactions on the NH2O potential energy surface, including the NH2 + O, HNO + H, and NH + OH reactions. These reactions have previously been highlighted as important reactions in NH3 oxidation with high sensitivity and high uncertainty. The potential energy surface is explored with coupled cluster calculations, including large basis sets and high-level corrections to yield high-accuracy (∼0.2 kcal/mol 2σ uncertainty) estimates of the stationary point energies. Variational transition state theory is used to predict the microcanonical rate constants, which are then incorporated in master equation treatments of the temperature- and pressure-dependent kinetics. For radical-radical channels, the microcanonical rates are obtained from variable reaction coordinate transition state theory implementing directly evaluated multireference electronic energies. The analysis yields predictions for the total rate constants as well as the branching ratios. We find that the NO + H2 channel contributes 10% of the total NH2 + O flux at combustion temperatures, although this channel is not included in modern NH3 oxidation mechanisms. Modeling is used to illustrate the ramifications of these rate predictions on the kinetics of NH3 oxidation and NOx formation. The present results for NH2 + O are important for predicting the chain branching and formation of NO in the oxidation of NH3 and thermal DeNOx.

Radical–Radical Reactions in Molecular Weight Growth: The Phenyl + Propargyl Reaction

The mechanism for hydrocarbon ring growth in sooting environments is still the subject of considerable debate. The reaction of phenyl radical (C6H5) with propargyl radical (H2CCCH) provides an important prototype for radical-radical ring-growth pathways. We studied this reaction experimentally over the temperature range of 300-1000 K and pressure range of 4-10 Torr using time-resolved multiplexed photoionization mass spectrometry. We detect both the C9H8 and C9H7 + H product channels and report experimental isomer-resolved product branching fractions for the C9H8 product. We compare these experiments to theoretical kinetics predictions from a recently published study augmented by new calculations. These ab initio transition state theory-based master equation calculations employ high-quality potential energy surfaces, conventional transition state theory for the tight transition states, and direct CASPT2-based variable reaction coordinate transition state theory (VRC-TST) for the barrierless channels. At 300 K only the direct adducts from radical-radical addition are observed, with good agreement between experimental and theoretical branching fractions, supporting the VRC-TST calculations of the barrierless entrance channel. As the temperature is increased to 1000 K we observe two additional isomers, including indene, a two-ring polycyclic aromatic hydrocarbon, and a small amount of bimolecular products C9H7 + H. Our calculated branching fractions for the phenyl + propargyl reaction predict significantly less indene than observed experimentally. We present further calculations and experimental evidence that the most likely cause of this discrepancy is the contribution of H atom reactions, both H + indenyl (C9H7) recombination to indene and H-assisted isomerization that converts less stable C9H8 isomers into indene. Especially at low pressures typical of laboratory investigations, H-atom-assisted isomerization needs to be considered. Regardless, the experimental observation of indene demonstrates that the title reaction leads, either directly or indirectly, to the formation of the second ring in polycyclic aromatic hydrocarbons.

The P(4S) + NH(3Σ–) and N(4S) + PH(3Σ–)reactions as sources of interstellar phosphorus nitride

Abstract Phosphorus nitride (PN) is believed to be one of the major reservoirs of phosphorus in the interstellar medium (ISM). For this reason, understanding which reactions produce PN in space and predicting their rate coefficients is important for modelling the relative abundances of P-bearing species and clarifying the role of phosphorus in astrochemistry. In this work, we explore the potential energy surfaces of the $\textrm{P}(^4\textrm{S}) + \textrm{NH}(^3\Sigma^-)$ and $\textrm{N}(^4\textrm{S}) + \textrm{PH}(^3\Sigma^-)$ reactions and the formation of $\textrm{H}(^2\textrm{S}) + \textrm{PN}(^1\Sigma^+)$ through high accuracy ab initio calculations and the variable reaction coordinate transition state theory (VRC-TST). We found that both reactions proceed without an activation barrier and with similar rate coefficients that can be described by a modified Arrhenius equation ( $k(T)=\alpha\!\left( T/300 \right)^{\beta} \exp\!{(\!-\!\gamma/T)})$ with $\alpha=0.93\times 10^{-10}\rm cm^3\,s^{-1}$ , $\beta=-0.18$ and $\gamma=0.24\, \rm K$ for the $\textrm{P} + \textrm{NH} \longrightarrow \textrm{H} + \textrm{PN}$ reaction and $\alpha=0.88\times 10^{-10}\rm cm^3\,s^{-1}$ , $\beta=-0.18$ and $\gamma=1.01\, \rm K$ for the $\textrm{N} + \textrm{PH} \longrightarrow \textrm{H} + \textrm{PN}$ one. Both reactions are expected to be relevant for modelling PN abundances even in the cold environments of the ISM. Given the abundance of hydrogen in space, we have also predicted rate coefficients for the destruction of PN via H + PN collisions.

Theoretical kinetics of HO2 + C5H5: A missing piece in cyclopentadienyl radical oxidation reactions

The resonantly-stabilized cyclopentadienyl radical (C5H5) is a key species in the combustion and molecular growth kinetics of mono and poly-aromatic hydrocarbons (M/PAHs). At intermediate-to-low temperatures, the C5H5 reaction with the hydroperoxyl radical (HO2) strongly impacts the competition between oxidation to smaller products and growth to PAHs, precursors of soot. However, literature estimates for the HO2 + C5H5 reaction rate are inaccurate and inconsistent with recent theoretical calculations, thus generating discrepancies in global combustion kinetic models. In this work, we perform state-of-the-art theoretical calculations for the HO2 + C5H5 reaction including variable reaction coordinate transition state theory for barrierless channels, accurate thermochemistry, and multi-well master equation (ME) simulations. Contrary to previous studies, we predict that OH + 1,3-C5H5O is the main reaction channel. The new rate constants are introduced in two literature kinetic models exploiting our recently developed ME based lumping methodology and used to perform kinetic simulations of experimental data of MAHs oxidation. It is found that the resonantly-stabilized 1,3-C5H5O radical is the main C5H5O isomer, accumulating in relevant concentration in the system, and that the adopted lumping procedure is fully consistent with results obtained with detailed kinetics. The reactivity of C5H5O with OH and O2 radicals is included in the kinetic mechanisms based on analogy rules. As a result, C5H5O mostly reacts with O2 producing smaller C3/C4 species and large amounts of C5H4O, suggesting that further investigations of the reactivity of both C5H5O and C5H4O with oxygenated radicals is necessary. Overall, this work presents new reliable rate constants for the HO2 + C5H5 reaction and provides indications for future investigations of relevant reactions in the sub-mechanisms of cyclopentadiene and MAH oxidation.

Theoretical study on the mechanism and kinetics of the oxidation of allyl radical with atomic and molecular oxygen

The C3H5O and C3H5O2 potential energy surfaces accessed by the oxidation reactions of allyl radical with atomic and molecular oxygen have been mapped out at the CCSD(T)-F12b/cc-pVTZ-F12//ωB97XD/6-311G(d,p) level of electronic structure theory to unravel the reaction mechanism. Temperature- and pressure-dependent reaction rate constants have been evaluated using variable reaction coordinate transition state and RRKM-Master Equation theoretical kinetics methods. The C3H5 + O reaction is found to be fast in the 300-2500 K temperature interval, with the total rate constant of 1.8-1.0 × 10−10 cm3 molecules s−1 independent of pressure in the considered 30 Torr – 100 atm range. Acrolein + H formed by a simple O addition/H elimination mechanism via the CH2CHCH2O association complex are predicted to be the major reaction products with the branching ratio decreasing from 61% at 300 K to 44% at 2500 K. Other substantial bimolecular products include C2H3 + formaldehyde (32%-26%) formed through the C-C bond β-scission in CH2CHCH2O and two minor products C2H4 + formyl radical HCO and allene + OH, where the latter, formed via direct H abstraction by O from the central carbon atom of allyl, becomes significant only at high temperatures above ∼1500 K. The C3H5 + O2 reaction studied in the 200-2500 K temperature range shows a peculiar kinetic behavior characteristic for a bimolecular reaction with a low entrance barrier, shallow association well, and high ensuing isomerization and decomposition barriers to bimolecular products. This behavior is described in terms of three distinct temperature regimes: the low-temperature one with slightly negative dependence of the rate constant, the intermediate one where the rate constant sharply drops, and the high-temperature regime with an Arrhenius-like rate constant. The rate constant is relatively high (10−13-10−12 cm3 molecule−1 s−1) in the low-temperature regime when the reaction produces the peroxy association complex CH2CHCH2OO, but slow at higher temperatures when it leads to bimolecular products. Under combustion relevant conditions, the C3H5 + O2 rate constant is by 5 to 3 orders of magnitude lower than that for C3H5 + O, but since O2 concentrations in flames could be as much as 3 orders of magnitude larger than O concentrations, C3H5 + O2 cannot be ruled out as a significant allyl radical sink. Above 1500 K, the C3H5 + O2 reaction can form vinoxy radical + formaldehyde (40%-15%) via the five- or four-membered ring closure and opening mechanism starting from the initial peroxy intermediate, acrolein + OH (41%-49%) via the H shift from the attacked carbon to the terminal oxygen followed by the immediate O-O bond cleavage, and allene + OH (18-25%) via the H migration from the central CH group to the terminal O atom accompanied or followed by the C-O bond rupture. Modified Arrhenius expressions fitting the computed rate constants for both reactions have been generated and proposed for kinetic models.

Non-Boltzmann Effects in Chain Branching and Pathway Branching for Diethyl Ether Oxidation

Low-temperature (LT) engine applications have several potential benefits, including reduced emissions and increased efficiency. Attaining these benefits requires accurate kinetic modeling of LT chain branching, which depends heavily on ketohydroperoxide (KHP) decomposition. For diethyl ether (DEE), a promising biofuel, current estimates of the KHP decomposition rate constant are largely based on empirical fits to data. In this study, we investigate the most important KHP isomer in DEE LT oxidation by applying variable reaction coordinate transition state theory to the main pathway for KHP decomposition: OO bond fission to produce •OH and a keto-alkoxy radical, •OQ′O. We also use ab initio kinetics methods to investigate the decomposition of •OQ′O, where we find dominant branching to acetic acid, with the remaining flux going to CH3C(O)OCHO. Additionally, new time-resolved measurements of DEE and acetic acid concentrations during LT (450–600 K) DEE oxidation are obtained in a laser photolysis flow reactor coupled with multiplexed photoionization mass spectrometry. These new experimental data, along with jet-stirred reactor data in the literature, are compared with the predictions of a recent DEE mechanism (Tran et al. Proc. Comb. Inst. 2019, 37, 511−519) that was modified with the newly calculated ab initio rate constants for KHP and •OQ′O decomposition. The predictions of the modified mechanism are quite poor when compared to the experimental data; this is primarily due to the new KHP ⇄ •OQ′O + •OH rate constant, which is 1–2 orders of magnitude slower than empirical values employed in recent mechanisms. To reconcile the new KHP rate constant and the experimental data, we explore and quantify the possible role of non-Boltzmann (nB) reaction sequences. The nB reactions have a substantial effect on both the overall mechanism reactivity and the •OQ′O branching to acetic acid. We also provide guidance on the proper implementation of nB reactions in kinetic mechanisms.

Theoretical kinetics predictions for NH2 + HO2

Recent modeling studies of NH3 oxidation, which are motivated by the prospective role of ammonia as a zero-carbon fuel, have indicated significant discrepancies between existing literature mechanisms. In this study high level theoretical kinetics predictions have been obtained for the reaction of NH2 with HO2, which has previously been highlighted as an important reaction with high sensitivity and high uncertainty. The potential energy surface is explored with coupled cluster calculations including large basis sets and high-level corrections to yield high accuracy (∼0.2 kcal/mol) estimates of the stationary point energies. Variational transition state theory is used to predict the microcanonical rate constants, which are then incorporated in master equation treatments of the temperature and pressure dependent kinetics. For the radical-radical channels, the microcanonical rates are obtained from variable reaction coordinate transition state theory implementing directly evaluated multireference electronic energies. The analysis yields predictions for the total rate constant as well as the branching to the NH3 + O2, H2NO + OH, and HNO + H2O channels. Rate constants are also reported for the H2NO + OH reaction as they arise naturally from the analysis. The rate constant and branching fraction determined in this work for the NH2 + HO2 reaction deviate significantly from values used in most previous modeling studies. The fact that the main product channel is chain terminating, rather than propagating, has strong implications for modeling NH3 ignition and oxidation, in particular at intermediate temperatures and elevated pressure.

Quantum-Chemical and Theoretical Kinetics Studies on the Gas-Phase Unimolecular Decomposition Reaction of Sulfur Hexafluoride, SF6

The aim of this study is to calculate the unimolecular rate coefficients for the unimolecular decomposition reaction of the industrially important molecule, SF6. The energies of stationary-points involved in the title reaction are calculated by the combination W1 method. Two main reaction paths are considered: SF6 → SF5 + F (R1) and SF6 → SF4 + F2 (R2). Having information on energies and molecular properties of reactants and transition-states, RRKM statistical rate theory is used to compute the rate coefficients as a function of temperature and pressure. For the bond dissociation process R1, special version of RRKM theory, i. e., Variable reaction coordinate-transition state theory (VRC-TST) is employed. Although the reaction R1 is the dominant process over a wide range of pressure and temperature, but the reaction R2 could be significant at high temperatures. The following Arrhenius expressions are obtained for high-pressure limiting rate constants of reaction paths R1 and R2: k∞,1 = 5.71 × 1016 s−1 exp (−429.8 kJ mol−1 /RT) k∞,2 = 2.14 × 1016 s−1 exp (−590.6 kJ mol−1 /RT).

State-of-the-Art Quantum Chemistry Meets Variable Reaction Coordinate Transition State Theory to Solve the Puzzling Case of the H2S + Cl System.

The atmospheric reaction of H2S with Cl has been reinvestigated to check if, as previously suggested, only explicit dynamical computations can lead to an accurate evaluation of the reaction rate because of strong recrossing effects and the breakdown of the variational extension of transition state theory. For this reason, the corresponding potential energy surface has been thoroughly investigated, thus leading to an accurate characterization of all stationary points, whose energetics has been computed at the state of the art. To this end, coupled-cluster theory including up to quadruple excitations has been employed, together with the extrapolation to the complete basis set limit and also incorporating core–valence correlation, spin–orbit, and scalar relativistic effects as well as diagonal Born–Oppenheimer corrections. This highly accurate composite scheme has also been paralleled by less expensive yet promising computational approaches. Moving to kinetics, variational transition state theory and its variable reaction coordinate extension for barrierless steps have been exploited, thus obtaining a reaction rate constant (8.16 × 10–11 cm3 molecule–1 s–1 at 300 K and 1 atm) in remarkable agreement with the experimental counterpart. Therefore, contrary to previous claims, there is no need to invoke any failure of the transition state theory, provided that sufficiently accurate quantum-chemical computations are performed. The investigation of the puzzling case of the H2S + Cl system allowed us to present a robust approach for disclosing the thermochemistry and kinetics of reactions of atmospheric and astrophysical interest.

Accelerating Variational Transition State Theory via Artificial Neural Networks.

An application of atomistic machine learning for variational transition state theory is presented. The rate constants for various radical-radical reactions are computed using variable reaction coordinate transition state theory. In order to simplify the calculation process, artificial neural networks are trained on a structured grid of potential energy data. The resulting surrogate potential energy surface is used in classical phase space representations to describe the interaction between two radical species in the gas phase. When the artificial neural network is trained to potential energy alone, the number of explicit electronic structure energy calculations required to compute a rate constant decreases by at least a factor of 4. When forces are included in the training data, the reduction is more dramatic-at least an order of magnitude.

Theoretical Study of the Reaction Mechanism and Kinetics of the Phenyl + Allyl and Related Benzyl + Vinyl Associations.

Potential energy surfaces for the allyl + phenyl and benzyl + vinyl barrierless radical association reactions have been studied at the CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311G** level of theory. Variable reaction coordinate transition state theory (VRC-TST) has been employed to evaluate high-pressure limit rate constants for the barrierless channels. Then, Rice-Ramsperger-Kassel-Marcus master equation (RRKM-ME) calculations have been performed to assess phenomenological rate constants and product branching ratios of various reaction channels at different temperatures and pressures. The initial step of both radical association reactions produces 3-phenylpropene which can further dissociate into a variety of bimolecular products including the indene precursor 1-phenylallyl + H. The results showed that at typical combustion conditions the collisional stabilization of 3-phenylpropene dominates both the phenyl + allyl and benzyl + vinyl reactions at temperatures below 1000 K and remains important at high pressures up to 2500 K. The main bimolecular products of the two reactions at high temperatures are predicted to be benzyl + vinyl and phenyl + allyl, respectively. The well-skipping mechanism to form 1-phenylallyl directly in the allyl + phenyl and benzyl + vinyl reactions appeared to be not significant, however, the reactions can provide some contributions into the formation of the indene precursor via the 3-phenylpropene stabilization/dissociation sequence and most of all, via the formation of 3-phenylpropene itself, which then can undergo H-abstraction by available radicals to produce 1-phenylallyl. The allyl + phenyl reaction can also contribute to the formation of two-ring PAH by producing benzyl radical at high temperatures, either by the well-skipping or stabilization/dissociation mechanisms; in turn, benzyl can readily react with acetylene or propargyl radical to form indene or naphthalene precursors, respectively. Rate expressions for all important reaction channels in a broad range of temperatures and pressures have been generated for kinetic modeling.

Theoretical kinetic study of the reaction between dimethyl disulfide and OH radicals

ABSTRACTThe potential energy profile of the reaction between dimethyl disulfide and OH• radicals is explored by utilizing ab initio and hybrid meta density functional theory methods. Having the energies and structural data of the stationary points, statistical rate theories, such as transition state theory and variable reaction coordinate-transition state theory, are employed to compute the overall rate constants, and discuss the mechanism and product channels. On the basis of the calculations, the overall rate coefficient is predicted to be 2.49 × 10−10 cm3 molecule−1 s−1 at 298 K. It is found that in the most favorable pathway, the reaction proceeds via formation of the relatively unstable intermediate CH3S•(OH)SCH3 decomposing rapidly to yield CH3S•+CH3SOH.

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

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

Revisiting the chemical kinetics of CH3 + O2 and its impact on methane ignition

The reaction between the methyl radical and molecular oxygen is one of the most important combustion reactions—it plays a controlling role in methane ignition. Although the reaction kinetics of CH3 + O2 has been extensively studied (at least experimentally) in the literature, high-level theoretical studies are still desired to understand the chemistry of this “small but complicated” reaction system and provide rate constants over wide range of temperature and pressure. In the present study, the rate coefficients of three major reaction channels of CH3 + O2, i.e., CH3OO stabilization (R1), CH3O + O channel (R2), and CH2O + OH channel (R3), were computed with temperature ranging from 300 to 2500 K and pressure from 0.01 to 100 atm by combining high-level quantum chemical calculations (CCSD(T)-F12/cc-pVQZ-F12//QCISD/6-311++G(2df,2p)) and RRKM/master equation simulations (with variable reaction coordinate transition state theory simulation for the barrierless channels). The computational results were compared with previously reported rate constants based on a thorough literature review. They show satisfactory agreement with those most recent measurements over the limited experimental temperature ranges—the discrepancies are within a factor of two for the three dominant reactions. The rate constants of R2 and R3 have very high sensitivity coefficients to the ignition delay time of methane. The impact of this reaction kinetics on methane ignition was evaluated by putting our currently computed rate coefficients into two recently reported kinetic models for methane combustion. There was a significant difference on the simulated ignition delay for an RCM experiment by replacing the rate constants of R2 and R3 with the newly computed results. It indicates the risk of estimating rate constants by simple extrapolation from experimental measurements over limited temperature ranges. The modified Arrhenius representations of the reaction channels of both CH3 + O2 and CH3OO dissociation were provided for utility in combustion modeling.

Functional Relationships between Kinetic, Flow, and Geometrical Parameters in a High-Temperature Chemical Microreactor.

Computational fluid dynamics (CFD) simulations and isothermal approximation were applied for the interpretation of experimental measurements of the C10H7Br pyrolysis efficiency in the high-temperature microreactor and of the pressure drop in the flow tube of the reactor. Applying isothermal approximation allows the derivation of analytical relationships between the kinetic, gas flow, and geometrical parameters of the microreactor, which, along with CFD simulations, accurately predict the experimental observations. On the basis of the obtained analytical relationships, a clear strategy for measuring rate coefficients of (pseudo) first-order bimolecular and unimolecular reactions using the microreactor was proposed. The pressure- and temperature-dependent rate coefficients for the C10H7Br pyrolysis calculated using variable reaction coordinate transition state theory were invoked to interpret the experimental data on the pyrolysis efficiency.