Pressure-dependent Rate Constants Research Articles

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

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

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

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

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

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

Temperature and pressure dependent rate constants of the reactions of OH• with cyclopentene from variational TST and SS-QRRK methods.

In this work, the pressure- and temperature-dependent reaction rate constants for the hydrogen abstraction and addition of hydroxyl radicals to the unsaturated cyclopentene were studied. Geometries and vibrational frequencies of reactants, products, and transition states were calculated using density functional theory, with single-point energy corrections determined at the domain-based local pair natural orbital-coupled-cluster single double triple/cc-pVTZ-F12 level. The high-pressure limit rate constants were calculated using the canonical variational transition state theory with the small-curvature tunneling approximation. The vibrational partition functions were corrected by the effects of torsional and ring-puckering anharmonicities of the transition states and cyclopentene, respectively. Variational effects are shown to be relevant for all the hydrogen abstraction reactions. The increasing of the rate constants by tunneling is significant at temperatures below 500K. The pressure dependence on the rate constants of the addition of OH• to cyclopentene was calculated using the system-specific quantum Rice-Ramsperger-Kassel model. The high-pressure limit rate constants decrease with increasing temperature in the range 250-1000K. The falloff behavior was studied at several temperatures with pressures varying between 10-3 and 103 bar. At temperatures below 500K, the effect of the pressure on the addition rate constant is very modest. However, at temperatures around and above 1000K, taking pressure into account is mandatory for an accurate rate constant calculation. Branching ratio analyses reveal that the addition reaction dominates at temperatures below 500K, decreasing rapidly at higher temperatures. Arrhenius parameters are provided for all reactions and pressure dependent Arrhenius parameters are given for the addition of OH• to cyclopentene.

Quantitative Kinetics of HO2 Reactions with Aldehydes in the Atmosphere: High-Order Dynamic Correlation, Anharmonicity, and Falloff Effects Are All Important.

Kinetics provides the fundamental parameters for elucidating sources and sinks of key atmospheric species and for atmospheric modeling more generally. Obtaining quantitative kinetics in the laboratory for the full range of atmospheric temperatures and pressures is quite difficult. Here, we use computational chemistry to obtain quantitative rate constants for the reactions of HO2 with HCHO, CH3CHO, and CF3CHO. First, we calculate the high-pressure-limit rate constants by using a dual-level strategy that combines conventional transition state theory using a high level of electronic structure wave function theory with canonical variational transition state theory including small-curvature tunneling using density functional theory. The wave-function level is beyond-CCSD(T) for HCHO and CCSD(T)-F12a (Level-A) for XCHO (X = CH3, CF3), and the density functional (Level-B) is specifically validated for these reactions. Then, we calculate the pressure-dependent rate constants by using system-specific quantum RRK theory (SS-QRRK) and also by an energy-grained master equation. The two treatments of the pressure dependence agree well. We find that the Level-A//Level-B method gives good agreement with CCSDTQ(P)/CBS. We also find that anharmonicity is an important factor that increases the rate constants of all three reactions. We find that the HO2 + HCHO reaction has a significant dependence on pressure, but the HO2 + CF3CHO reaction is almost independent of pressure. Our findings show that the HO2 + HCHO reaction makes important contribution to the sink for HCHO, and the HO2 + CF3CHO reaction is the dominant sink for CF3CHO in the atmosphere.

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.

The role of collisional energy transfer in the thermal and prompt dissociation of 1-methyl allyl

Collisional energy transfer plays a pivotal role in a-priori calculations of the pressure-dependent rate constants obtained from a master equation. However, accurate determinations of collisional energy transfer parameters are rare and so most kinetic studies rely on best-fits of these parameters to experimental measurements (if available) or estimates guided from literature studies. In this work, we have quantified the effect of the uncertainty in energy transfer parameters on the thermal and prompt dissociation kinetics of a resonance stabilized radical, 1-methyl allyl (1MA), of relevance to the combustion of 1- and 2-butene isomers. Simulations using literature kinetics models were performed to assess the impact of these uncertainties on flame propagation and speciation data in laminar flames of 1- and 2-butene. Analyses of the uncertainty propagated by the energy transfer parameters for 1MA dissociation to the flame simulations, in particular the laminar flame speed, indicate an intricate coupling between the kinetics of 1MA dissociation (chain propagation) and its reaction with H-atoms (chain-termination). Ab-initio based theoretical calculations were also performed to obtain pressure-dependent kinetics for the reaction of 1MA with H-atoms. Lastly, theoretically calculated energy transfer parameters were used to best characterize the kinetics and branching between the chain propagating 1MA dissociation and the chain-terminating reaction 1MA + H.

Temperature and Pressure-Dependent Rate Constants for the Reaction of the Propargyl Radical with Molecular Oxygen.

Ab initio CCSD(T)/CBS(T,Q,5)//B3LYP/6-311++G(3df,2p) calculations have been conducted to map the C3H3O2 potential energy surface. The temperature- and pressure-dependent reaction rate constants have been calculated using the Rice–Ramsperger–Kassel–Marcus Master Equation model. The calculated results indicate that the prevailing reaction channels lead to CH3CO + CO and CH2CO + HCO products. The branching ratios of CH3CO + CO and CH2CO + HCO increase both from 18 to 29% with reducing temperatures in the range of 300–2000 K, whereas CCCHO + H2O (0–10%) and CHCCO + H2O (0–17%) are significant minor products. The desirable products OH and H2O have been found for the first time. The individual rate constant of the C3H3 + O2 → CH2CO + HCO channel, 4.8 × 10–14 exp[(−2.92 kcal·mol–1)/(RT)], is pressure independent; however, the total rate constant, 2.05 × 10–14 T0.33 exp[(−2.8 ± 0.03 kcal·mol–1)/(RT)], of the C3H3 + O2 reaction leading to the bimolecular products strongly depends on pressure. At P = 0.7–5.56 Torr, the calculated rate constants of the reaction agree closely with the laboratory values measured by Slagle and Gutman [Symp. (Int.) Combust.1988, 21, 875−883] with the uncertainty being less than 7.8%. At T < 500 K, the C3H3 + O2 reaction proceeds by simple addition, making an equilibrium of C3H3 + O2 ⇌ C3H3O2. The calculated equilibrium constants, 2.60 × 10–16–8.52 × 10–16 cm3·molecule–1, were found to be in good agreement with the experimental data, being 2.48 × 10–16–8.36 × 10–16 cm3·molecule–1. The title reaction is concluded to play a substantial role in the oxidation of the five-member radicals and the present results corroborate the assertion that molecular oxygen is an efficient oxidizer of the propargyl radical.

Thermal decomposition of isopentanol: A theoretical calculation and kinetic modeling analysis

Isopentanol (3-methyl-1-butanol, 3M1B) is a prospective bioderived fuel with advantages over ethanol that make it a suitable substitute for gasoline. Although there have been studies on the combustion properties of 3M1B, reaction kinetics for its thermal decomposition is scarce. This study explores the reaction pathways for the thermal decomposition of 3M1B through theoretical calculations. Six reactions with tight transition states, and nine bond dissociation reactions (i.e., barrierless reactions) were considered. For reactions with tight transition states, the CCSD(T)/CBS//M06–2X-D3(0)/def2-TZVP method was used, and the MRCISD(+Q)/CBS//CASPT2/cc-pVDZ method was adopted for the bond dissociation reactions. The conventional transition state theory (CTST), including multi-structural torsional anharmonicity (MS-T) correction, addressed those reactions with tight transition states. The MS-T method thoroughly examined the co-effect of the multiple structures and torsional anharmonicity. Direct bond-breaking reactions were studied using the variational transition state theory (VTST). Subsequently, system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory and Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) were employed to obtain the rate constants for reactions with tight transition states and direct bond-breaking reactions at different temperatures and pressures (T/P) regimes (T = 298–2400 K, P = 0.01–100 atm), respectively. The results showed that water elimination and C-C bond dissociation play a significant role for 3M1B unimolecular decomposition, other molecular elimination and bond dissociation channels can be ignored. Combining the reaction pathways and the corresponding pressure dependent rate constants into the kinetic model resulted in good predictions of the speciation profiles during 3M1B pyrolysis. These data are valuable for the development of the combustion models of 3M1B.

The kinetic model of ethylcyclohexane combustion over a wide temperature range and its comprehensive validation

An experimental and modeling study is performed to develop a high accuracy combustion model of ethylcyclohexane (ECH) over a wide temperature range. The shock tube experiments are conducted to determine ignition delay times for ECH/air mixtures at 678–1287 K. Extensive quantum-chemical calculations concerning the low temperature kinetics of ECH including oxygen addition to hydroperoxyalkyl (·QOOH) and the following decomposition of hydroperoxyalkylperoxy radicals (·OOQOOH) have been investigated at the level of CBS-QB3//M06-2X/6-311++G(d,p). Due to the mutual influence of hydrogen bond and ring structure, the species configuration needs to be carefully determined in order to obtain thermodynamic data and then kinetic parameters. Different temperature- and pressure-dependent rate constants are computed for the side chain of ECH, the ring part, and the part where the ring and the side chain intersect. The rate constants of the hydrogen migration reaction are compared with the values estimated by analogizing with linear alkanes. Obvious deviations are observed especailly at low temperature. A detailed kinetic model is developed based on the existing low temperature model of ECH with the replacement of reaction calculated in present work and the core mechanism of AramcoMech 3.0. The model is validated comprehensively with experimental results of ignition delay time, laminar flame speed and species concentration profiles in jet-stirred reactor (JSR), which shows a good prediction over a wide temperature range. The results of sensitivity analyses show that the computed reactions of ·QOOH and ·OOQOOH have a great impact on the low temperature oxidation of ECH, which help to accurately reproduce the negative-temperature-coefficient behavior of ECH ignition. The kinetic parameters obtained in this work are conductive to improve the combustion models of substituted cycloalkanes with alkyl side chains, and the obtained kinetic model with high accuracy of ECH can be used as a basis to develop model of jet fuel.

Exploring the kinetics and thermochemistry effects on C2-C6 alkene combustion chemistry by ȮH radical; Implications for Combustion Modeling and Simulation

A systematic theoretical study of C2 to C6 alkenes reacting with ȮH radical to explore the kinetics and thermochemistry effects are carried out in this work. The geometries and frequencies of 15 reactants, 15 pre-reaction complexes, 93 transition states, and 93 products are optimized at M06-2X/6-311++G(d,p) level of theory. The hindrance potentials for low-frequency torsional modes are treated at M06-2X/6–31 G level of theory. Single-point energies for all species involved in this work are calculated at spin-unrestricted CCSD(T)/CBS level of theory with basis-set correction from MP2/aug-cc-pVnZ (n = T and Q). High-pressure-limit rate constants for H-atom abstraction reaction channels and pressure-dependent rate constants for addition reaction channels are calculated by solving the Rice-Ramsperger Kassel-Marcus/Master Equation (RRKM/ME) implemented in the Master Equation System Solver (MESS) program, coupled with variational transition-state theory (VTST). Temperature-dependent thermochemical properties for all species and related radicals have been computed using a combination of composite methods to provide adequate accuracy results for modeling application. For straight-chain alkenes, the temperature range, in which the addition channel dominates, shifts to lower temperatures as the alkene gets longer. Hydrogen-atom abstractions from the allylic site of alkenes are the dominant reaction channels at high temperatures. The H-atom abstraction reactions from secondary alkylic and primary alkylic CH sites become more important as substituent groups become larger. High-pressure-limit and pressure-dependent rate constants calculated in this work have been extensively compared with the experimental and theoretical results available in the literature, and they show overall good agreement. The simulation results show that the calculated rate constants in this work have positive influences to the accuracy of simulation results at most reaction conditions. However, at some of the reaction conditions, thermochemical data have much more important influence to the simulation results at low temperature ranges. Rate constants obtained in this work and detailed branching ratio for each reaction can serve as a guidance in alkene combustion model development.

Kinetic analysis of the pathways to naphthalene formation from phenyl + 1,3-Butadiyne reaction

Having a better understanding of polycyclic aromatic hydrocarbon (PAH) formation under flame conditions contributes to optimizing the fuel reforming process, where soot poisons the downstream catalyst. In this work, the phenyl + 1,3-Butadiyne reaction is systematically investigated to examine its contribution to naphthalene formation. The reaction potential energy surfaces were calculated using DFT/M06–2X/cc-pvtz and G4 methods. The temperature- and pressure-dependent reaction rate constants were calculated using RRKM theory with solving master equation. The results revealed that 2-naphthyl could be directly formed by phenyl + 1,3-Butadiyne reaction. With H assistance, naphthalene could be formed by the pathway of phenyl + 1,3-Butadiyne → C6H5CHCCCH (+H) → C6H5CHCHCCH (+H) →naphthalene +H. The proposed pathway is kinetically favorable, and featured by relatively low energy barrier. The importance of the proposed pathway reaction was confirmed in a premixed and a diffusion C2H4/O2/Ar flame simulations, where the enhancement of naphthalene by the investigated reactions is notable. The mole fraction of A2 is promoted by a factor of 10% in premix C2H4/O2/Ar flame and 30% in C2H4/O2/Ar counterflow flame, bringing the prediction results closer to the experimental results. The relative contribution of different reaction route to A2 formation is evaluated for HACA, cyclopentadienyl radical-cyclopentadienyl radical, phenyl-vinylacetylene[1], benzyl radical-propargyl radical, indene-CH2 and phenyl-1,3-Butadiyne routes in premixed and diffusion C2H4/O2/Ar flames. This work suggests that the PAH growth by 1,3-Butadiyne addition reaction is an effective pathway for A2 formation, which should be considered in future PAH mechanism.

Potential Nonstatistical Effects on the Unimolecular Decomposition of H2O2.

An attempt is made to evaluate the nonstatistical effects in the thermal decomposition of hydrogen peroxide (H2O2). Previous experimental studies on this reaction reported an unusual pressure dependence of the rate constant indicating broader falloff behavior than expected from conventional theory. In this work, the possibility that the rate constant is affected by nonstatistical effects is investigated based on classical trajectory calculations on the global potential energy surfaces of H2O2 and H2O2 + Ar. The emphasis is on the intramolecular energy redistribution from the K-rotor, that is, the external rotor for rotation around the principal axis of least moment of inertia. The calculations for the H2O2 molecules excited above the dissociation threshold suggest that the energy redistribution from the torsion and K-rotor to vibrations can be competitive with dissociation. In particular, the slow redistribution of the energy associated with the K-rotor significantly affects the dissociation rate. The successive trajectory calculations for collisions of H2O2 with Ar show that the energy associated with the K-rotor can be collisionally transferred more efficiently than the vibrational energy. On the basis of these results and several assumptions, a simple model is proposed to account for the nonstatistical effects on the pressure-dependent thermal rate constants. The model predicts significant broadening of the falloff curve of the rate constants but still cannot fully explain the experimental data.

Reaction of SO3 with HONO2 and Implications for Sulfur Partitioning in the Atmosphere.

Sulfur trioxide is a critical intermediate for the sulfur cycle and the formation of sulfuric acid in the atmosphere. The traditional view is that sulfur trioxide is removed by water vapor in the troposphere. However, the concentration of water vapor decreases significantly with increasing altitude, leading to longer atmospheric lifetimes of sulfur trioxide. Here, we utilize a dual-level strategy that combines transition state theory calculated at the W2X//DF-CCSD(T)-F12b/jun'-cc-pVDZ level, with variational transition state theory with small-curvature tunneling from direct dynamics calculations at the M08-HX/MG3S level. We also report the pressure-dependent rate constants calculated using the system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory. The present findings show that falloff effects in the SO3 + HONO2 reaction are pronounced below 1 bar. The SO3 + HONO2 reaction can be a potential removal reaction for SO3 in the stratosphere and for HONO2 in the troposphere, because the reaction can potentially compete well with the SO3 + 2H2O reaction between 25 and 35 km, as well as the OH + HONO2 reaction. The present findings also suggest an unexpected new product from the SO3 + HONO2 reaction, which, although very short-lived, would have broad implications for understanding the partitioning of sulfur in the stratosphere and the potential for the SO3 reaction with organic acids to generate organosulfates without the need for heterogeneous chemistry.

Research of kinetics and anharmonicity for related reactions of NOx + H2O and cracking NOx

Reaction mechanism and anharmonicity involving in NOx were researched, in which included decomposition (NOx), isomerization (N2O3 and N2O4) and bimolecular reactions (NOx + H2O). Specially, high-pressure limiting and pressure-dependent rate constant were respectively calculated by TS and RRKM-ME theory. And, high-pressure limiting data were in well agreement with corresponding calculating results via two kinds of validation. Moreover, N2O3 and N2O4 were not steady, and products of N2O3 or N2O4 with H2O were not steadier than that of N2O5. Therefore, N2O5 was the key specie for NOx to be absorbed by H2O. Furthermore, anharmonic effect was prominent and shouldn’t be overlooked.

Large Pressure Effects Caused by Internal Rotation in the s-cis-syn-Acrolein Stabilized Criegee Intermediate at Tropospheric Temperature and Pressure.

Criegee intermediates are important atmospheric oxidants, and quantitative kinetics for stabilized Criegee intermediates are key parameters for atmospheric modeling but are still limited. Here we report barriers and rate constants for unimolecular reactions of s-cis-syn-acrolein oxide (scsAO), in which the vinyl group makes it a prototype for Criegee intermediates produced in the ozonolysis of isoprene. We find that the MN15-L and M06-2X density functionals have CCSD(T)/CBS accuracy for the unimolecular cyclization and stereoisomerization of scsAO. We calculated high-pressure-limit rate constants by the dual-level strategy that combines (a) high-level wave function-based conventional transition-state theory (which includes coupled-cluster calculations with quasiperturbative inclusion of quadruple excitations because of the strongly multiconfigurational character of the electronic wave function) and (b) canonical variational transition-state theory with small-curvature tunneling based on a validated density functional. We calculated pressure-dependent rate constants both by system-specific quantum Rice-Ramsperger-Kassel theory and by solving the master equation. We report rate constants for unimolecular reactions of scsAO over the full range of atmospheric temperature and pressure. We found that the unimolecular reaction rates of this larger-than-previously studied Criegee intermediate depend significantly on pressure. Particularly, we found that falloff effects decrease the effective unimolecular cyclization rate constant of scsAO by about a factor of 3, but the unimolecular reaction is still the dominant atmospheric sink for scsAO at low altitudes. The large falloff caused by the inclusion of the stereoisomerization channel in the master equation calculations has broad implications for mechanistic analysis of reactions with competitive internal rotations that can produce stable rotamers.

Reaction mechanism and kinetics of the important but neglected reaction of Hg with NO2 at low temperature

Reaction of Hg and NO2 significantly contributes to mercury removal in CO2 compression and purification unit. However, the reaction mechanism remains unclear and the product species is difficult to collect and identify experimentally. To gain insight into this reaction, we studied it using quantum chemical and kinetic calculations. A two-step reaction mechanism of Hg and NO2 was investigated. Initially, Hg could react with NO2 without a barrier to form two isomeric HgNO2 complexes (i.e. HgO2N and HgNO2) that achieve equilibrium very quickly. Then both HgNO2 isomers can add another NO2 to form a variety of HgN2O4 compounds, among which the most stable is trans-syn, syn-Hg(ONO)2. Kinetics were investigated using variational transition state theory (VTST) and RRKM theory to calculate the rate constants with respect to both temperature and pressure. The only effective reaction pathway is the one described above leading to trans-syn, syn-Hg(ONO)2. The temperature- and pressure-dependent rate constants of these relevant reactions were presented. The calculated third-order rate constant is 7.4 × 10-35 cm6 molecule-2 s−1 at 298 K and 1 atm, which is of the same order of magnitude as the experimental value. This reaction pathway shows a negative temperature dependence from 250 to 400 K, which is in accordance with the existing experimental results. The gas phase reaction mechanism of Hg with NO2 could well explain the observations in the transformation of mercury in the CO2 compression and purification unit.

Dissociation-induced depletion of high-energy reactant molecules as a mechanism for pressure-dependent rate constants for bimolecular reactions.

In 1922, Lindemann proposed the now-well-known mechanism for pressure-dependent rate constants for unimolecular reactions: reactant molecules with sufficiently high energies dissociate more quickly than collisions can reestablish the Boltzmann distribution of the internal energies of the molecule during its dissociation at low pressures - yielding pressure-dependent rate constants for unimolecular reactions due to the preferential depletion of the high energy states capable of dissociation. In the last century, incredible progress has been made in achieving a far greater understanding of and quantitative predictions for unimolecular and association reactions. In the modern era, pressure-dependent phenomenological rate constants are now nearly universally used to describe the rates of unimolecular and associative reactions in phenomenological kinetic modeling. However, there is a second, more indirect, implication of Lindemann's mechanism that relates to how these dissociation-induced non-equilibrium distributions impact bimolecular reactions, including non-associative bimolecular reactions - which are generally not considered to have pressure-dependent rate constants. Yet, as we show herein, the same high energy states depleted due to dissociation would otherwise react most rapidly in high-activation-energy bimolecular reactions - yielding a mechanism for pressure-dependent rate constants for bimolecular reactions (including non-associative reactions). Here, we present results from a case study for CH2O dissociation, isomerization, and bimolecular reaction with O2 to explore this question. Results from our master equation calculations indicate that the effect of dissociation-induced non-equilibrium distributions on bimolecular reactions can be substantial - even when chemical timescales are well separated from internal energy relaxational timescales (i.e. when the traditional rate constant description would be thought to apply). This effect is found to be more pronounced - and more complex - for bimolecular reactions involving molecular entities whose chemical timescales are merged with the internal energy relaxational timescales. Finally, we present some ideas for discussion regarding what should be considered as "chemical species" in phenomenological kinetic models.

From electronic structure to combustion model application for acrolein chemistry part I: Acrolein + H reactions and related chemistry

Detailed reaction kinetics of acrolein + H and related chemistry and its influence on the ignition delay time prediction of acrolein has been studied theoretically in this work. The geometry optimization and vibrational frequency calculations for every stationary point were performed at the BH&HLYP/6–311++G(d,p) level of theory, with one-dimensional hindered rotation treatments applied for low-frequency torsional modes determined by using BH&HLYP/6–31G(d). Electronic energies for all species were calculated at the ROCCSD(T)/cc-pVQ,TZ levels of theory. The kinetics and thermochemistry data were calculated and compared with existing literature results, where good agreement was observed. The branching ratios of the crucial products vary in the pressure range of 0.01–100 atm and temperatures from 298 to 2000 K. Taking 1 atm as an example, at temperatures above 1000 K, the main addition reaction products of acrolein + H are ethylene + formyl radical, while at lower temperatures, the formation of the resonantly stabilized radical, CH3ĊHCHO, is important. The dominant H-atom abstraction reaction channel by H atom proceeds at the α carbon atom of the aldehyde group of acrolein. However, the H-atom abstraction reactions are overwhelmed by the addition reactions. Decomposition reactions of four C3H3O radicals were calculated and analysed. Temperature-dependent thermochemical properties for all species in the reaction system and the pressure-dependent rate constants for each reaction pathway were incorporated into two acrolein combustion kinetic mechanisms, as well as two widely used mechanisms, AramcoMech 3.0 and JetSurF 2.0, to test the influence of the newly calculated data on acrolein oxidation. Some critical reactions for acrolein oxidation were highlighted by performing sensitivity and flux analyses.

Experimental and kinetic modeling studies of 2-acetylfuran pyrolysis at atmospheric pressure

Furan-based derivatives are promising biomass fuels that may be used directly in internal combustion engines as alternative fuels or as additives to fossil fuels. In this work, the experimental studies of 2-acetylfuran (AF2) pyrolysis were carried out in a jet-stirred reactor in the temperature range of 770 – 1130 K and at the pressure of 760 Torr, using synchrotron vacuum ultraviolet photo-ionization mass spectrometry, coupled with gas chromatography. Key pyrolysis intermediates such as methane, ethylene, acetylene, ketene, vinylacetylene, cyclopentadiene, ethynylketene, 2-methylfuran, furfural, carbon monoxide and isomers such as allene/propyne, furan/vinylketene, benzene/fulvene, etc., were identified and quantified. The potential energy surfaces of AF2 unimolecular decomposition reactions were calculated at CBS-QB3 level. The temperature- and pressure-dependent rate constants of the relevant reactions were calculated by solving the master equation based on Rice-Ramsperger-Kassel-Marcus theory. A detailed pyrolysis kinetics of AF2 was developed based on the previous combustion models of 2-methylfuran and was validated against the current experimental results. Rate of production and sensitivity analysis showed that the main consumption pathways of AF2 pyrolysis under atmospheric pressure are unimolecular decomposition reactions, leading to 2-furyl + acetyl and furoyl + methyl, and hydrogen atom-addition reaction to C(5), leading to 2,3-dihydro-5-acetyl-3-furyl, accounting at 1070 K for 25.1%, 13.2% and 20.7% of AF2 consumption, respectively (the remaining 41.0% are due to minor reaction channels contributing < 10% each). At this temperature, the total AF2 consumption is 73%. The contributions of ipso-substitution reactions by methyl and by hydrogen atom to AF2 consumption were comparable.