Pressure-dependent Rate Coefficients Research Articles

Theoretical and experimental study on the pyrolysis of N-methylpyrrolidone

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

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

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

Theoretical investigation of the OH-initiated atmospheric degradation mechanism of CX2CHX (X = H, F, Cl) by advanced quantum chemical and transition state theory methods.

Halogenated olefins are anthropogenic compounds with many industrial applications but at the same time raising many environmental and health concerns. Gas-phase electrophilic addition of the OH radical to the olefinic CC bond represents the primary sink for these chemicals in the atmosphere, with the degree and type of halogenation playing a significant role in their overall reactivity. In this work, we present a theoretical investigation of the reaction mechanisms and kinetics for the reactions between the OH radical and CH2CH2 (ethylene, ETH), CF2CHF (trifluoroethylene, TFE) and CCl2CHCl (trichloroethylene, TCE), simulated by state-of-the-art protocols and methods, with the aim of providing a detailed interpretation of the available experimental results, as well as new data of relevance to tropospheric chemistry. Specifically, potential energy surfaces (PESs) are obtained using the jun-Cheap (jChS) composite scheme, whereas temperature and pressure dependent rate coefficients and product distributions in the 100-600 K temperature range are calculated within the Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) framework. The rates for barrierless channels are obtained from variable reaction coordinate-variational transition state theory (VRC-VTST) combined with the two transition state model. While the reactions with ETH and TFE proceed mainly via the formation of addition adducts at P = 1 atm and T = 298 K, the dominant channel for TCE is the Cl-elimination reaction. Global rate constants for the two halogenated olefins, TFE and TCE, are found to be pressure-independent, contrary to the case of ETH. The computed rate constants, as well as their temperature and pressure dependence, are in remarkable agreement with the available experimental data, and they are used to derive atmospheric lifetimes (τ) for both TFE and TCE as a function of altitude (h) in the atmosphere, by taking into account variations in the rate coefficients (k (T, P)) and [OH] concentration.

Experimental and modeling study of the N, N-dimethylformamide pyrolysis at atmospheric pressure

As a polar solvent, N, N-Dimethylformamide (DMF) is a potential source of volatile organic compounds during industrial processing. The potential energy surface and pressure-dependent rate coefficients of DMF primary and secondary thermal decomposition were theoretically investigated by automatic ab initio calculations. A singlet aminohydroxycarbene route generating CO is found to be the most energetically favored channel among the unimolecular initiation reactions with a tight transition state. A kinetic model was proposed for DMF pyrolysis prediction and validated against the speciation data newly obtained from synchrotron vacuum ultraviolet photoionization mass spectrometry in a flow reactor within 773 K–1048 K and literature data in a jet stirred reactor within 450 K–900 K at atmospheric pressure. Satisfactory agreements were obtained for the major intermediates and products, but the model failed to capture the formation of some oxygenated compounds, possibly due to the missing reaction pathway for bimolecular addition. During pyrolysis, DMF is mainly consumed by the hydrogen abstraction reaction as the relatively high CN bond energy hinders the barrierless homolytic cleavage. Unlike aldehydes where the carbonyl site is favored, H-abstraction from DMF methyl sites is prevalent, although the acylamino group readily undergoes decarbonylation and β-scission forming oxygenated and nitrogenous parts. Consequently, CO and HCN are the major pyrolysis products. Besides, by incorporating essential high temperature kinetics, extended validation was conducted against the available intermediate temperature autoignition delay times (1000–1350 K, 2 bar) and laminar burning velocity data, and the reason for the discrepancy between the model prediction and experiments was inferred according to the sensitivity analyses.

Reactions of Ethynyloxy Radical with Hydroperoxyl Radical: Bridging Theoretical Reaction Dynamics and Chemical Modeling of Combustion.

A detailed and accurate combustion reaction mechanism is crucial for understanding the nature of fuel combustion. In this work, a theoretical study of reaction HCCO+HO2 using M06-2X/6-311++G(d,p) for geometry optimization and combined methods based on spin-unrestricted CCSD(T)/CBS level of theory with basis set extrapolation from MP2/aug-cc-pVnZ (n=T and Q) for energy calculations were performed. The temperature- and pressure-dependent rate coefficients at 300-2000 K and 0.01-100 atm, suitable for combustion conditions, were derived using the Rice-Ramsberger-Kassel-Marcus/Master-Equation approach. Furthermore, temperature-dependent thermochemistry data of key species for the HCCO+HO2 system has also been studied. Finally, an updated ketene model is developed by supplementing the most recent theoretical work and the theoretical work in this paper. This updated model was tested to simulate the speciation of ketene oxidation in available experimental research. It is shown that the updated model for predicting ketene oxidation exhibits a high level of agreement with experimental data across a wide range of species profiles. An analysis was conducted to identify the crucial reactions that influence ketene ignition. This paper's research findings are essential for enhancing the combustion mechanism of ketene and other hydrocarbons and oxygenated hydrocarbon fuels.

Theoretical Study of Intramolecular H-Migration Reactions of Peroxyl Radicals of JP-10 (Exo-Tetrahydrodicyclopentadiene)

ABSTRACT Intramolecular H-migration reactions of peroxyl radicals of JP-10 (C10H15OO•) play an important role in developing the oxidation mechanism of JP-10 at low temperature. However, accurate rate coefficients for these reactions are lacking in the literature. Therefore, 33 H-migration reactions of C10H15OO•, which belong to six series of reactions, were investigated. Quantum chemical computations at the CBS-QB3 level reveal that C10H15OO• do not obey the rule that the energy barrier of 1,4-H migration reaction is higher than that of 1,5-H migration reaction because of the special steric cyclic structure of C10H15OO• and the ring strain in the transition state. The transition state theory and Rice-Ramsberger-Kassel-Marcus/Master-Equation theory were used to obtain high-pressure limit rate coefficients and pressure-dependent rate coefficients, respectively. The results show that H-migration reactions of C10H15OO• have obvious pressure-dependent effect. The most competitive reaction was obtained among 33 reactions. Moreover, branching ratios of each series of reactions were used as weight ratios to lump this series of reactions. Six series of reactions were lumped as six lumped reactions by the isomer lumping method. This will provide accurate rate coefficients for constructing a mechanism with the low number of species and suitability for computational fluid dynamics simulation.

Reaction kinetics and implications of the decomposition and formation of C2H4O isomers

Acetaldehyde, ethylene oxide, and vinyl alcohol are three C2H4O isomers that are ubiquitous in combustion over a wide temperature range. A comprehensive reaction kinetics study has been performed for reactions that occur on the same C2H4O potential energy surface, mainly including the unimolecular reactions of three C2H4O isomers and the recombination reaction of CH3 with HCO. Temperature- and pressure-dependent rate coefficients were updated or newly predicted by performing quantum chemical calculations coupled with RRKM/master equation simulations. The dominance of CC bond fission is reconfirmed for acetaldehyde decomposition at higher temperatures. The negligible contribution of CH4 + CO channel from usual tight intramolecular H-shift transition state provides indirect evidence for previously reported roaming mechanism that was proposed to explain the observation of CH4 + CO in acetaldehyde pyrolysis. For ethylene oxide pyrolysis, current experiment and kinetic prediction have a good consistency in branching ratios between the formation of acetaldehyde and vinyl alcohol products. The radicals of CH3 + HCO can also be directly generated from ethylene oxide through a well-skipping reaction mechanism especially at higher temperatures. For CH3 + HCO recombination, we highlight the importance of CH3CO + H channel that was often overlooked previously. Kinetic simulations show that the incorporation of updated rate coefficients and newly identified channels results in a measurable influence on C2 chemistry, indicating the possibility and necessity of further improvement of core mechanism in combustion kinetic models.

A workflow for automatic generation and efficient refinement of individual pressure-dependent networks

Manual chemical kinetic model construction often requires modelists to at least implicitly guess all possible decay pathways and their relative fluxes for all important species. We present a workflow and associated tool for automatic generation and refinement of pressure-dependent networks (multiwell potential energy surfaces) that should enable modelists to efficiently and comprehensively identify decay pathways and estimate respective parameters for chemical species. This tool within the Arkane software combines the capabilities of the Reaction Mechanism Generator (RMG) software to generate possible reaction paths, determine thermochemistry, approximate rate coefficients and estimate frequencies with the capabilities of Arkane to use quantum chemical parameters to compute thermochemistry and pressure-dependent rate coefficients. A flux-based algorithm is used to decide which isomers to add to the network. Isomers added to the network are reacted to form other channels. When enough of the flux is accounted for by the isomers and bimolecular product channels, the generation process terminates to yield a comprehensive network. Network sensitivity analysis is applied to identify the most important wells and barriers that can be refined using quantum chemistry calculations. Iterative refinement can then be used to achieve accuracy. The comprehensive network can be easily reduced using energy and flux-based algorithms to the most important channels and isomers.

Theoretical correction on the existing understanding for hydroper-oxymethyl formate dissociation in DME low temperature oxidation

Hydroperoxymethyl formate (HPMF), a carbonyl-hydroperoxide-like species, plays a vital role in the low temperature oxidation of DME because HPMF is formed with ȮH as a co-product, and it can also decompose to generate ȮH radicals. Unfortunately there is neither theoretical nor experimental evidence nor an established explanation of HPMF kinetics in the literature. A theoretical understanding of the potential energy surface of HPMF dissociation kinetics is evaluated at the CCSD(T)/CBS//M06-2X/6-311++G(d,p) level of theory. Microcanonical variational transition state theory and Rice-Ramsperger-Kassel-Marcus/Master Equation calculations were performed to obtain temperature- and pressure-dependent rate coefficients. Two main reaction pathways, (1) the widely recognized channel involving O–O bond fission to produce ȮH + ȮCH2OCHO radicals, and (2) a novel channel producing HOCHO + CH2OO, were considered to be the most competitive reaction channels. Calculation results reveal that there are significant differences between the current calculations and literature values, and the pressure-dependent behavior of HPMF decomposition cannot be ignored, particularly at low temperatures (T < 600 K). The orders of magnitude difference between the model recommendations and theoretical calculations make it reasonable to doubt the previous kinetic description of HPMF O–O bond fission, with its mechanism remaining unreliable. Generally, this study serves as theoretical evidence to provide new insights into HPMF kinetics but also questions our current understanding of the low temperature DME oxidation mechanism.

Theoretical Investigations for Kinetics of the Chemical Reactions: H + SiClx (x = 1, 2, 3).

Hydrogen atoms and SiClx (x = 1, 2, 3) radicals coexist during the hydrogenation of silicon tetrachloride (STC, SiCl4), an important process in the fabrication of industrial polysilicon. In this work, the mechanisms and kinetics of the reactions between H and SiClx (x = 1, 2, 3) were studied by theory. The structures and vibrational frequencies of reactants, products, intermediates, and transition states (TSs) were determined at the B2PLYP/may-cc-pVTZ level. The single-point energies of minima and saddle points were refined using the coupled-cluster single-double with triple perturbative (CCSD(T)) with the complete basis set extrapolation method. Some special treatments were designed to obtain reliable wave functions for unimolecular reactions without tight TSs by the density functional theory. Subsequently, Lennard-Jones (L-J) parameters between each intermediate (SiHClx) and bath gas (He) were obtained at the MP2/jul-cc-pVTZ level to derive reliable temperature- and pressure-dependent rate coefficients for unimolecular reactions according to the variational Rice-Ramsperger-Kassel-Marcus theory. For bimolecular reactions, rate coefficients were determined by the variational transition-state theory. The rate coefficients of barrierless reactions were derived based on the loose TSs with the maximum free energy. Finally, the master equation analysis was used to investigate the variation of the rate coefficients with pressure and temperature in the activated paths.

Examining the accuracy of methods for obtaining pressure dependent rate coefficients.

The full energy-grained master equation (ME) is too large to be conveniently used in kinetic modeling, so almost always it is replaced by a reduced model using phenomenological rate coefficients. The accuracy of several methods for obtaining these pressure-dependent phenomenological rate coefficients, and so for constructing a reduced model, is tested against direct numerical solutions of the full ME, and the deviations are sometimes quite large. An algebraic expression for the error between the popular chemically-significant eigenvalue (CSE) method and the exact ME solution is derived. An alternative way to compute phenomenological rate coefficients, simulation least-squares (SLS), is presented. SLS is often about as accurate as CSE, and sometimes has significant advantages over CSE. One particular variant of SLS, using the matrix exponential, is as fast as CSE, and seems to be more robust. However, all of the existing methods for constructing reduced models to approximate the ME, including CSE and SLS, are inaccurate under some conditions, and sometimes they fail dramatically due to numerical problems. The challenge of constructing useful reduced models that more reliably emulate the full ME solution is discussed.

Theoretical study on isomerization, decomposition and ring-closure reaction kinetics of methyl pentanoate radicals

A comprehensive study on the isomerization, decomposition and ring-closure reaction kinetics, as well as thermodynamic properties of methyl pentanoate radicals were carried out in this work. The M06–2X/cc-pVTZ level of theory was employed to optimize geometries and analyze frequencies for all the stationary points on the potential energy surfaces. The CCSD(T) method with two basis sets, cc-pVDZ and cc-pVTZ, was used to calculate single point energies, which were further extrapolated to the complete basis set (CBS) limit to construct the potential energy surfaces. The energy calculation results indicated that isomerization of δ-R to α-R, decomposition of γ-R to CH2C(=O)OCH3 and propene, and ring-closure reaction of δ-R to 1-methoxycyclopentanoxyl radical are the most energetically favored reactions among the studied isomerization, decomposition and ring-closure reaction channels, respectively. High pressure limit, and temperature and pressure dependent rate coefficients were determined by solving the one-dimensional energy-dependent master equations with Tsinghua University Minnesota Master Equation program (TUMME). The rate coefficients and branching ratios were found to be significantly affected by temperature and pressure. The decomposition reactions yielding small unsaturated esters or small ester radicals dominate at high temperatures, while the isomerization reactions proceeding via five- and six-membered ring saddle points, as well as the ring-closure reactions play more important roles at low temperatures. Significant discrepancies are observed between the theoretically calculated rate coefficients and estimated results. The thermochemical properties of methyl pentanoate radicals were calculated using isodesmic reaction method and atomization method together with the multistructural torsional anharmonicity partition functions. This study provides accurate kinetic and thermodynamic data on methyl pentanoate radicals, which are expected to advance our understanding of combustion chemistry of methyl pentanoate and larger methyl esters.

Rate coefficients for 1,2-dimethyl-allyl + HO2/O2 and the implications for 2-methyl-2-butene combustion

A detail investigation is presented on the reaction of 1,2-dimethyl-allyl with HO2, including bimolecular reactions of aC5H9-c with HO2, thermal decomposition reactions of aC5H9OOH, and the subsequent decomposition and isomerization reactions of C5H9O based on theoretical work. In addition, the reaction of aC5H9-c with O2 and the resulting peroxy-radical chemistry is presented. The majority of the stationary points are obtained at CCSD(T)-F12/cc-pVTZ-F12//M06–2X/6–311G(d,p) level, with barrierless reaction and some transition states using multireference method. Microcanonical rate theory and the master equation are used to determine the temperature- and pressure-dependent rate coefficients, as implemented in a RRKM/ME code. Uncertainty analysis for energy barrier and ∆Edown were conducted. The newly developed submechanisms for the reactions of 1,2-dimethyl-allyl with HO2 and O2 are added to a previous literature model for 2-methy-butene. The results show that at low pressure/high temperature, the reaction aC5H9-c with HO2 would directly decompose to aC5H9O + OH, which, though technically a chain propagating reaction, and significantly increases the reactivity of the system. The updated mechanism is in much better agreement with the available experimental data.

Thermal Decomposition Kinetics of the Indenyl Radical: A Theoretical Study.

Quantum chemistry and statistical reaction rate theory calculations have been performed to investigate the products and kinetics of indenyl radical decomposition. Three competitive product sets are identified, including formation of a cyclopentadienyl radical (c-C5H5) and diacetylene (C4H2), which has not been included in prior theoretical kinetics investigations. Rate coefficients for indenyl decomposition are determined from master equation simulations at 1800-2400 K and 0.01-100 atm, and temperature- and pressure-dependent rate coefficient expressions are incorporated into a detailed chemical kinetic model for indene pyrolysis. Indenyl is found to predominantly decompose to o-benzyne (o-C6H4) + propargyl (C3H3), with lesser amounts of fulvenallenyl (C7H5) + C2H2 and c-C5H5 + C4H2.

Ab initio rate coefficients for reactions of 2,5-dimethylhexyl isomers with O2: temperature- and pressure-dependent branching ratios

Chemical kinetics of O2-addition to alkyl radicals (R), termed first O2-addition in the oxidation mechanism of alkanes, are of central importance to next-generation combustion strategies designed for operations in the low- to intermediate-temperature region (<1000 K). In the present work, stationary points on potential energy surfaces (PES), temperature- and pressure-dependent rate coefficients, and branching fractions of product formation from R + O2 reactions initiated by the addition of molecular oxygen (3O2) to the three alkyl radicals of a branched alkane, 2,5-dimethylhexane, are reported. The stationary points were determined utilizing ab initio/DFT methods and the reaction energies were computed using the composite CBS-QB3 method. Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) calculations were employed to compute rate coefficients, from which branching fractions were determined over the pressure range of 10-3-20 atm and the temperature range of 400-900 K on three different surfaces. The quantum chemistry results reveal several distinct features. For the addition of O2 to the tertiary alkyl radical 2,5-dimethylhex-2-yl, the most energetically favorable channel leads to the formation of 2,2,5,5,-tetramethyl-tetrahydrofuran, a cyclic ether intermediate formed coincident with OH in a chain-propagating step from the decomposition of tertiary-tertiary hydroperoxyalkyl (QOOH). On the R + O2 surface of the secondary radical, 2,5-dimethylhex-3-yl, the pathways for the formation of methyl-propanal + iso-butene + OH via concerted C-C and O-O bond scission of tertiary QOOH and that of cyclic ether + OH are the most energetically favorable pathways. The R + O2 surface for the reaction of the primary radical, 2,5-dimethylhex-1-yl, reveals two competitive chain-propagation channels, leading to 2-iso-propyl-4-methyl-tetrahydrofuran + OH and 2,2,5-trimethyltetrahydropyran + OH. Below 100 Torr, the formation of the aforementioned species dominates the respective total R + O2 rate coefficient, while at pressures above 1 atm collisionally stabilized alkylperoxy (ROO) dominates at the temperatures considered here. The results of this study are in very good agreement with the experimentally measured intermediates and products of the 2,5-dimethylhexyl radical + O2 reaction.

Experimental and Theoretical Investigations of the Radical-Radical Reaction: N2H3 + NO2.

The N2H3 + NO2 reaction plays a key role during the early stages of hypergolic ignition between N2H4 and N2O4. Here for the first time, the reaction kinetics of N2H3 in excess NO2 was studied in 2.0 Torr of N2 and in the narrow temperature range 298-348 K in a pulsed photolysis flow-tube reactor coupled to a mass spectrometer. The temporal profile of the product, HONO, was determined by direct detection of the m/z +47 amu ion signal. For each chosen [NO2], the observed [HONO] trace was fitted to a biexponential kinetics expression, which yielded a value for the pseudo-first-order rate coefficient, k', for the reaction of N2H3 with NO2. The slope of the plot of k' versus [NO2] yielded a value for the observed bimolecular rate coefficient, kobs, which could be fitted to an Arrhenius expression of (2.36 ± 0.47) × 10-12 exp((520 ± 350)/T) cm3 molecule-1 s-1. The errors are 1σ and include estimated uncertainties in the NO2 concentration. The potential energy surface of N2H3 + NO2 was investigated by advanced ab initio quantum chemistry theories. It was found that the reaction occurs via a complex reaction mechanism, and all of the reaction channels have transition state energies below that of the entrance asymptote. The radical-radical addition forms the N2H3NO2 adducts, while roaming-mediated isomerization reactions yield the N2H3ONO isomers, which undergo rapid dissociation reactions to several sets of distinct products. The RRKM multiwell master equation simulations revealed that the major product channel involves the formation of trans-HONO and trans-N2H2 below 500 K and the formation of NO + NH2NHO above 500 K, which is nearly pressure independent. The pressure-dependent rate coefficients of the product channels were computed over a wide pressure-temperature range, which encompassed the experimental data.

Experimental and kinetic modeling of the ignition delays of cyclohexane, cyclohexene, and cyclohexadienes: Effect of unsaturation

Cyclic and aromatic hydrocarbons are important components of usual commercial fuels, with C6-rings being among the most abundant cyclic structures. The combustion chemistry of C6-rings involves different levels of unsaturation, either as initial fuels (aromatics, naphtenes, …) or as intermediates formed during their combustion. In this work the ignition delays of cyclohexane, cyclohexene, 1,3-cyclohexadiene and 1,4-cyclohexadiene are systematically studied using experiments and kinetic modeling. Shock tube experiments were performed at high-temperature (above 1200 K) and for mean pressures of 6 atm. A detailed chemical kinetic model was developed that includes the combustion chemistry of the four cyclo-C6 fuels. Electronic structure calculations were performed at the CCSD(T)/CBS//B2PLYP-D3 level of theory on the pericyclic reactions of the unsaturated fuels. Pressure-dependent rate coefficients were computed by solving the master equation, and included in the mechanism. The model was validated against the new ignition data and against data of the literature. It was able to reproduce the experimental ranking of reactivity: cyclohexene > 14-CHD > cyclohexane > benzene ≈13-CHD. Kinetic analyses were performed to explain this difference of reactivity. It is shown that pericyclic reactions play a major role in the initial decomposition of the unsaturated fuels.

Unimolecular reactions of the resonance-stabilized cyclopentadienyl radicals and their role in the polycyclic aromatic hydrocarbon formation

Resonance-stabilized cyclopentadienyl radicals are important intermediate species in the combustion of transportation fuels. It not only serves as precursors for polycyclic aromatic hydrocarbon (PAH) formation, but also involves in the formation of fundamental PAH precursors such as propargyl and acetylene. In this work, the unimolecular reactions of the cyclopentadienyl radicals are theoretically studied based on high-level quantum chemistry and RRKM/master equation calculations. Stationary points on the potential energy surface (PES) are calculated at the CCSD(T)/CBS//M06–2X/6–311++(d,p) level of theory. The branching ratios of unimolecular reactions of the cyclopentadienyl radicals are analyzed for a broad temperature range from 500 to 2500 K and pressures from 0.01 to 100 atm. It is found that the isomerization reaction of the cyclopentadienyl radical via 1,2-hydrogen transfer dominates at low temperatures and high pressures, while the well-skipping decomposition reaction which forms propargyl and acetylene is important at high temperatures and low pressures. Both the decomposition reaction of the cyclopentadienyl radicals and its reverse reaction show pronounced pressure dependence, and their reaction rate constants are compared against available low-pressure experimental measurements and theoretical studies. The temperature- and pressure-dependent rate coefficients for important reactions involved on the C5H5 PES are calculated and updated in a chemical kinetic model. Impacts of the unimolecular reactions of the cyclopentadienyl radicals on the PAH formation are explored by the numerical modeling of a low-pressure cyclopentene counterflow diffusion flame.

The role of resonance-stabilized radical chain reactions in polycyclic aromatic hydrocarbon growth: Theoretical calculation and kinetic modeling

Resonance-stabilized radical chain (RSRC) reactions were recently proposed as alternative formation and growth pathways of polycyclic aromatic hydrocarbons (PAHs). In the present study, the growth of indene from the resonance-stabilized cyclopentadienyl radical following a series of RSRC sequences is investigated. Temperature- and pressure-dependent rate coefficients for acetylene addition reactions to the cyclopentadienyl, vinylcyclopentadienyl, cycloheptatrienyl, and benzyl radicals are determined based on high-level quantum chemistry and RRKM/master equation calculations. The acetylene addition to the resonance-stabilized cyclopentadienyl radical can generate resonance-stabilized vinylcyclopentadienyl, cycloheptatrienyl, and benzyl radicals. The formation reaction of cycloheptatrienyl has the highest rate constant compared to those of vinylcyclopentadienyl and benzyl. The acetylene addition to the above three resonance-stabilized C7H7 isomers forms indene. It is found that the indene formation reaction from C2H2 addition to benzyl has the highest rate constant, while that from C2H2 addition to cycloheptatrienyl is the lowest. The impacts of the investigated reactions on indene formation are then numerically explored in both counterflow diffusion and laminar premixed flame configurations. Results indicate that the RSRC pathways via C2H2 addition to vinylcyclopentadienyl and cycloheptatrienyl radicals have marginal contributions to indene formation.

Theoretical Investigations on the Oxidation of Heptafluoro-iso-butyronitrile by Atomic Oxygen in Dielectric Breakdown.

Heptafluoro-iso-butyronitrile [(CF3)2CFCN, in abbr., C4] with CO2 gas mixture is a promising dielectric candidate to replace sulfur hexafluoride (SF6) for the sake of environmental concern. Detailed mechanisms for the reactions of C4 with atomic oxygen in both triplet and singlet due to dissociation of CO2 were proposed using high-level ab initio methods including density functional theory, quadratic complete basis set, multireference Rayleigh-Schrodinger perturbation theory, and state-averaged multiconfiguration self-consistent field. The reaction of C4 with O(3P) proceeds via the stepwise C-O and N-O addition/elimination and direct displacement mechanisms. The reaction paths are bifurated into cis and trans conformations. The predominant product channel is the C-O association with the barriers 6.1-6.8 kcal/mol to form the C3F7C(O)N intermediates and followed by the C-C bond cleavage to produce C3F7 and NCO radicals. On the singlet surface, the three-center intramolecular conversion to form the C3F7OCN isomer becomes competitive. The singlet-triplet intersystem crossing is favorable due to the significant spin-orbital coupling. Master equation calculations were carried out to obtain the temperature- and pressure-dependent rate coefficients and yields of the O(3P) + C4 reaction and were compared with the analogous reactions of nitriles. It was found that only the NCO + C3F7 product channel is significant. The overall rate coefficients are pressure-independent in the range 1-10 atm and can be expressed by k∞(T/K)=(6.41 ± 0.22) × 10-13(T/298)1.30±0.01e-(3241.5±24.5)/T cm3 molecule-1 s-1. The present theoretical work provides useful insights into the breakdown diagnostic of the C4/CO2 insulation gas.