Variable Reaction Coordinate Transition State Theory Research Articles

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.

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.

Kinetics of the reaction CH2CO + O (3P): Are the CH2 and CO2 the most favorable products?

Overall thermal rate coefficients for the CH2CO + O (3P) ⟶ CH2 + CO2 reaction were estimated by combining the canonical variational theory with multidimensional small-curvature tunneling corrections (CVT/SCT) and multifaceted variable-reaction-coordinate transition state theory (VRC-TST) results. The energies, equilibrium geometries, and harmonic vibrational frequencies were calculated at M06-2X/aug-cc-pVTZ level of theory. Our thermal rate constants show excellent agreement with experimental data confirming the CH2 + CO2 as the most favorable channel. Fitted total thermal rate coefficients into a modified Arrhenius equation shows a temperature-dependent activation energy.

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.

Kinetics of CN (v = 1) reactions with butadiene isomers at low temperature by cw-cavity ring-down in a pulsed Laval flow with theoretical modelling of rates and entrance channel branching.

We present an experimental and theoretical investigation of the reaction of vibrationally excited CN (v = 1) with isomers of butadiene at low temperature. The experiments were conducted using the newly built apparatus, UF-CRDS, which couples near-infrared cw-cavity ring-down spectroscopy with a pulsed Laval flow. The well-matched hydrodynamic time and long ring-down time decays allow measurement of the kinetics of the reactions within a single trace of a ring-down decay, termed Simultaneous Kinetics and Ring-down (SKaR). The pulsed experiments were carried out using a Laval nozzle designed for the 70 K uniform flow with nitrogen as the carrier gas. The measured bimolecular rates for the reactions of CN (v = 1) with 1,3-butadiene and 1,2-butadiene are (3.96 ± 0.28) × 10-10 and (3.06 ± 0.35) × 10-10 cm3 per molecule per s, respectively. The reaction rate measured for CN (v = 1) with the 1,3-butadiene isomer is in good agreement with the rate previously reported for the reaction with ground state CN (v = 0) under similar conditions. We report the rate of the reaction of CN (v = 1) with the 1,2-butadiene isomer here for the first time. The experimental results were interpreted with the aid of variable reaction-coordinate transition-state theory calculations to determine rates and branching of the addition channels based on a high-level multireference treatment of the potential energy surface. H-abstraction reaction rates were also theoretically determined. For the 1,2-butadiene system, theoretical estimates are then combined with literature values for the energy-dependent product yields from the initial adducts to predict overall temperature-dependent product branching. H loss giving 2-cyano-1,3-butadiene + H is the main product channel, exclusive of abstraction, at all energies, but methyl loss forming 1-cyano-prop-3-yne is 15% at low temperature growing to 35% at 500 K. Abstraction forming HCN and various radicals is important at 500 K and above. The astrochemical implications of these results are discussed.

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).

Theoretical Investigation of the Mechanisms and Kinetics of the Bimolecular and Unimolecular Reactions Involving in the C4H6 Species

A theoretical study of the mechanisms and kinetics for the C4H6 system was carried out using ab initio molecular orbital theory based on the CCSD(T)/CBS//B3LYP/6-311++G(3df,2p) method in conjunction with statistical theoretical variable reaction coordinate transition-state theory and RRKM/ME calculations. The calculated results indicate that buta-1,3-diene, but-1-yne, and C4H5 + H can be the major products of the C3 + C1 reaction, while CCH2 + C2H4 and C4H5 + H play an important role in the C2 + C2 reaction. In contrast, the C4H6 fragmentation giving rise to C3 + C1 and C4H5 + H becomes the key reaction paths under any temperature and pressure. The rate constants for the system have been calculated in the 300-2000 K temperature range at various pressures for which the C2 + C2 → C4H6 high-P limit rate constant, 10.24 × 1014T-0.51 cm3/mol/s, agrees well with the measured value of Hidaka et al., 9.64 × 1014T-0.5 cm3/mol/s. Also, the high-P limit rate constants of the channels but-2-yne → 2-C4H5 + H and C3 + C1 → C4H6, being 1.7 × 1014 exp(-351.5 kJ·mol-1/RT) s-1 and 5.07 × 1013 exp(0.694 kJ·mol-1/RT) cm3/mol/s, are in good agreement with the available literature data 5 × 1014 exp(-365.3 kJ·mol-1/RT) s-1 and 4.09 × 1013 exp(1.08 kJ·mol-1/RT) cm3/mol/s reported by Hidaka et al. and Knyazev and Slagle, respectively. Moreover, the 298 K/50 Torr branching ratios for the formation of buta-1,2-diene (0.43) and but-1-yne (0.57) as well as the total rate constant 5.18 × 1013 cm3/mol/s of the channels C3 + C1 → buta-1,2-diene and C3 + C1 → but-1-yne are in excellent accord with the laboratory values given by Fahr and Nayak, being 0.4, 0.6, and (9.03 ± 1.8) × 1013 cm3/mol/s, respectively. Last but not least, the rate constants and branching ratios for the C4H6 dissociation processes in the present study also agree closely with the theoretically and experimentally reported data.

Theoretical study on the kinetics of the CF3O + NO reaction

The rate constant for the CF3O + NO reaction has been studied with the variable reaction coordinate transition-state theory and multichannel RRKM calculation based on the potential energy surface obtained by the G3B3//UB3LYP/6-311+G(3df) level of theory. The total rate constant was found to exhibit weak negative temperature dependence, in line with most experimental results. The formation of FNO + CF2O is dominant at ambient temperature. The branching ratio of FNO + CF2O accounts for 0.922–0.618 at T = 150–3000 K peaking around 400 K. When temperature rises, the FON + CF2O channel becomes more and more appreciable, the branching ratio of FON + CF2O accounts for 0.001–0.382 at T = 150–3000 K increasing with temperature. At room temperature, the total rate constant is almost independent of the pressure, in agreement with previous experimental result.

A theoretical study on gas-phase reactions of acrylic acid with chlorine atoms: mechanism, kinetics, and insights.

Chlorine atoms initiated oxidation reactions are significant for the removal of typical volatile organic compounds (VOCs) in the atmosphere. The intrinsic mechanisms of CH2=CHCOOH + Cl reaction have been carried out at the CCSD(T)/cc-pVTZ//M06-2X/6-311++G(d,p) level. There are hydrogen abstraction and C-addition pathways on potential energy surfaces. By analyses, the addition intermediates of IM1(ClCH2CHCOOH) and IM2(CH2CHClCOOH) are found to be dominant. The secondary reactions of IM1 and IM2 have been discussed in the presence of O3, O2, NO, and NO2. And we have also investigated the degradation mechanisms of ClCH2CHO2COOH with NO, NO2, and self-reaction. Moreover, the atmospheric kinetics has been calculated by the variable reaction coordinate transition-state theory (VRC-TST). As a result, the rate constants show negative temperature and positive pressure dependence. The atmospheric lifetime and global warming potentials of acrylic acid have been calculated. Overall, the current study elucidates a new mechanism for the atmospheric reaction of chlorine atoms with acrylic acid.

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.

Kinetics of 1-butyl and 2-butyl radical reactions with molecular oxygen: Experiment and theory

The reaction of O2 with butyl radicals is a key early step in the oxidation of n-butane, which is a prototypical alkane fuel with combustion properties that mimic those of many larger alkanes. Current combustion mechanisms employ kinetic descriptions for such radical oxidations that are based on fairly limited information. The present work employs a combination of experiment and theory to probe the kinetics of O2 reacting with both 1- and 2-butyl radicals. The experiments employ laser photolysis to generate butyl radicals and thereby initiate the reaction kinetics. Photoionization mass spectrometric observations of the time-dependent butyl radical concentration yield rate coefficients for the overall reaction. The experiments cover temperatures ranging from 200 to 500 K and He bath gas pressures ranging from 0.3 to 6 Torr. Ab initio transition state theory (TST) based master equation calculations are used to predict the kinetics over a broad range of conditions. The calculations consider both the barrierless R + O2 entrance channel, treated with direct CASPT2 variable reaction coordinate TST, and the decomposition of the RO2 complex to HO2 + alkenes, treated with CCSD(T)/CBS based TST. Theory and experiment are in good agreement, with maximum discrepancies of about 30%, suggesting the appropriateness of the theory based predictions for conditions of greater relevance to combustion. The kinetic description arising from this work should be of considerable utility to combustion modeling of n-butane, as well as of other related saturated hydrocarbons.

Theoretical Study of the Ti–Cl Bond Cleavage Reaction in TiCl4

Abstract In this work the kinetics of the TiCl4 ⇌ TiCl3 + Cl reaction is studied theoretically. A variable-reaction coordinate transition-state theory (VRC-TST) is used to calculate the high-pressure limit rate coefficients. The interaction energy surface for the VRC-TST step is sampled directly at the CASPT2(6e,4o)/cc-pVDZ level of theory including an approximate treatment of the spin-orbit coupling. The pressure-dependence of the reaction in an argon bath gas is explored using the master equation in conjunction with the optimised VRC-TST transition-state number of states. The collisional energy transfer parameters for the TiCl4–Ar system are estimated via a “one-dimensional minimisation” method and classical trajectories. The Ti–Cl bond dissociation energy is computed using a complete basis set extrapolation technique with cc-pVQZ and cc-pV5Z basis sets. Good quantitative agreement between the estimated rate constants and available literature data is observed. However, the fall-off behaviour of the model results is not seen in the current experimental data. Sensitivity analysis shows that the fall-off effect is insensitive to the choice of model parameters and methods. More experimental work and development of higher-level theoretical methods are needed to further investigate this discrepancy.

Predictive kinetics on the formation and decomposition of ethylbenzene

The kinetics of reactions over the ethylbenzene surface were studied by using ab initio transition state theory (TST) based master equation methodologies. The variable reaction coordinate TST (VRC-TST) approach was employed to obtain high pressure limit rates for barrierless channels, including C7H7+CH3, C6H5+C2H5, C6H5CHCH3+H, and C6H5CH2CH2+H. Interaction potentials between each pair of fragments were simply simulated with CASPT2/cc-pVDZ calculations and scaled by reference to QCISD(T)/CBS energies. The larger basis set, cc-pVTZ, was tested to have only a very limited improvement on the final kinetics. For benzyl+CH3 recombination, addition to benzylic-site forming ethylbenzene dominates overwhelmingly over addition to ortho- and para-sites forming o-methylbenzyl and p-methylbenzyl, respectively. For C6H5CHCH3+H recombination, addition to benzylic-site also dominates over addition to sites at the aromatic ring. Present predictions, from a theoretical perspective, demonstrate strong preference for the CC dominance in ethylbenzene decomposition. The quite good agreement with earlier studies lend confidence to present predictions. More importantly, our kinetic predictions imply that it could probably introduce large uncertainties if simply using kinetic parameterizations of toluene in modeling study of larger alkylbenzenes. The ethylbenzene system should serve as a better and safer reference.

Theoretical kinetics studies on the unimolecular degradation of trifluoromethyl sulfur pentafluoride, SF5CF3

Abstract Thermal gas-phase unimolecular decomposition of the atmospherically important greenhouse molecule, SF 5 CF 3 , is studied theoretically. The density functional theory method M06-2X is used to explore the potential energy surface of the unimolecular reaction of SF 5 CF 3 . Molecular energies are refined by single-point energy calculations by the combination CBS-Q electronic structure method. Next, statistical rate theories are employed to compute the thermal rate coefficients for important product channels as a function of temperature and pressure. Variable reaction coordinate-transition state theory (VRC-TST) is employed to calculate the rate constants for the reaction channels involving bond dissociations. The pressure dependence of the thermal rate coefficients is accounted for by a master equation formalism. The major reaction channel for the dissociation of SF 5 CF 3 is found to be C S bond cleavage producing SF 5 and CF 3 radicals with the Arrhenius equation as k 1 = 2.0 y 10 16 s ⿿1 exp (⿿265.4 kJ mol ⿿1 /RT). The present theoretical work may inspire experimental works regarding the unimolecular decomposition of SF 5 CF 3 .

Temperature and Pressure-Dependent Rate Coefficients for the Reaction of Vinyl Radical with Molecular Oxygen.

State-of-the-art calculations of the C2H3O2 potential energy surface are presented. A new method is described for computing the interaction potential for R + O2 reactions. The method, which combines accurate determination of the quartet potential along the doublet minimum energy path with multireference calculations of the doublet/quartet splitting, decreases the uncertainty in the doublet potential and thence the rate constants by more than a factor of 2. The temperature- and pressure-dependent rate coefficients are computed using variable reaction coordinate transition-state theory, variational transition-state theory, and conventional transition-state theory, as implemented in a new RRKM/ME code. The main bimolecular product channels are CH2O + HCO at lower temperatures and CH2CHO + O at higher temperatures. Above 10 atm, the collisional stabilization of CH2CHOO directly competes with these two product channels. CH2CHOO decomposes primarily to CH2O + HCO. The next two most significant bimolecular products are OCHCHO + H and (3)CHCHO + OH, and not C2H2 + HO2. C2H3 + O2 will be predominantly chain branching above 1700 K. Uncertainty analysis is presented for the two most important transition states. The uncertainties in these two barrier heights result in a significant uncertainty in the temperature at which CH2CHO + O overtakes all other product channels.

Ab Initio Kinetics of Hydrogen Abstraction from Methyl Acetate by Hydrogen, Methyl, Oxygen, Hydroxyl, and Hydroperoxy Radicals.

The kinetics of hydrogen abstraction by five radicals (H, O((3)P), OH, CH3, and HO2) from methyl acetate (MA) is investigated theoretically in order to gain further understanding of certain aspects of the combustion chemistry of biodiesels, such as the effect of the ester moiety. We employ ab initio quantum chemistry methods, coupled cluster singles and doubles with perturbative triples correction (CCSD(T)) and multireference averaged coupled pair functional theory (MRACPF2), to predict chemically accurate reaction energetics. Overall, MRACPF2 predicts slightly higher barrier heights than CCSD(T) for MA + H/CH3/O/OH, but slightly lower barrier heights for hydrogen abstraction by HO2. Based on the obtained reaction energies, we also report high-pressure-limit rate constants using transition state theory (TST) in conjunction with the separable-hindered-rotor approximation, the variable reaction coordinate TST, and the multi-structure all-structure approach. The fitted modified Arrhenius expressions are provided over a temperature range of 250 to 2000 K. The predictions are in good agreement with available experimental results. Abstractions from both of the methyl groups in MA are expected to contribute to consumption of the fuel as they exhibit similar rate coefficients. The reactions involving the OH radical are predicted to have the highest rates among the five abstracting radicals, while those initiated by HO2 are expected to be the lowest.

Pressure-dependent kinetics of initial reactions in iso-octane pyrolysis.

This study focuses on the studies of the main pressure-dependent reaction types of iso-octane (iso-C8H18) pyrolysis, including initial C-C bond fission of iso-octane, isomerization, and β-scission reactions of the alkyl radicals produced by the C-C bond fission of iso-octane. For the C-C bond fission of iso-octane, the minimum energy potentials are calculated at the CASPT2(2e,2o)/6-31+G(d,p)//CAS(2e,2o)/6-31+G(d,p) level of theory. For the isomerization and the β-scission reactions of the alkyl radicals, the optimization of the geometries and the vibrational frequencies of the reactants, transition states, and products are performed at the B3LYP/CBSB7 level, and their single point energies are calculated by using the composite CBS-QB3 method. Variable reaction coordinate transition state theory (VRC-TST) is used for the high-pressure limit rate constant calculation and Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) is used to calculate the pressure-dependent rate constants of these channels with pressure varying from 0.01-100 atm. The rate constants obtained in this work are in good agreement with those available from literatures. We have updated the rate constants and thermodynamic parameters for species involved in these reactions into a current chemical kinetic mechanism and also have improved the concentration profiles of main products such as C3H6 and C4H6 in the shock tube pyrolysis of iso-octane. The results of this study provide insight into the pyrolysis of iso-octane and will be helpful in the future development of branched paraffin kinetic mechanisms.

DFT and theoretical kinetics studies on the reaction of nitrate radical with α-pinene and β-pinene

The potential energy surfaces for the reactions of α-pinene and β-pinene with NO3 radicals are explored by using M06-2X hybrid meta density functional theory (HMDFT) method. On the basis of the optimized geometries and energies of the intermediates and transition states, the mechanisms of the title reactions are explained and the importance of different product channels is discussed. It is found that pinonaldehyde and NO radical is the most important products of the α-pinene+NO3 reaction. The major products of the β-pinene+NO3 reaction are predicted to be 10-pinenyl radical, nitric acid, 6,6-dimethylbicyclo[3.1.1]heptan-2-one, NO radical and formaldehyde. Variable Reaction Coordinate-Transition State Theory (VRC-TST) is used to compute the overall rate coefficients as a function of temperature. The computed rate constants are compared with the available experimental data.

Pressure-dependent branching in the reaction of 1CH2 with C2H4 and other reactions on the C3H6 potential energy surface

A number of reactions on the C3H6 potential energy surface (PES) are of central importance to various combustion environments. High level theoretical methods were used to predict the temperature and pressure dependent kinetics for these reactions. The rovibrational properties of the key stationary points in the systems were determined with the CCSD(T)/cc-pVTZ method. High accuracy energies for these stationary points were obtained via the consideration of basis set, higher-order correlation, core–valence, anharmonic vibrational, relativistic, and diagonal Born–Oppenheimer corrections. Variable reaction coordinate transition state theory (VRC-TST) was employed to treat the barrierless channels on the PES, while, for channels possessing a distinct barrier, rate coefficients were instead obtained with conventional transition state theory employing rigid-rotor harmonic oscillator assumptions. For the VRC-TST calculations, the interaction energies required for the evaluation of the reactive flux were evaluated with the CASPT2/cc-pVTZ approach for the orientational sampling, coupled with one-dimensional corrections based on higher accuracy evaluations along the minimum energy path. A priori predictions for the temperature and pressure-dependent rate coefficients were obtained through master-equation calculations incorporating these TST predictions for the microcanonical rate coefficients coupled with a simple model for the collisional energy transfer rates. These theoretical predictions compare favorably with the limited experimental data. The analysis yields modified Arrhenius rate expressions for the 1CH2+C2H4, C2H3+CH3, CH2CHCH2+H, CH3CCH2+H, and CH3CHCH+H recombination reactions as well as for the dissociations and isomerizations of propene and cyclopropane.