High-pressure Limiting Rate Coefficients Research Articles

The gas-phase pyrolysis of cyclopropylamine. A computational study on the kinetics and reaction mechanism

Small amines represent an accessible model to understand the reactivity of the nitrogenous components of biomass during combustion and pyrolysis processes. In this work, the kinetics and mechanism of the gas-phase pyrolysis of cyclopropylamine, c-C3H5NH2, have been studied by ab initio and density functional calculations. From the derived information high-pressure limit rate coefficients were estimated using the transition state theory over the 400 – 1400 K temperature range. The results support a stepwise process, in which the initial ring-opening stage determines the rate through competitive direct, biradical and carbene mechanisms to give 1-aminopropene, CH3CH=CHNH2, and 1-propanimine, CH3CH2CH=NH, as reactive intermediates. The Arrhenius equation obtained at the G3(MP2)//B3LYP/6–311++G(3df,3pd) level of theory, log10 (koverall,∞/s-1) = (15.47 ± 0.11) – (57.68 ± 0.32) kcal mol-1 (2.303 RT)-1, agrees very well with the experimental expression determined by Parry and Robinson. According to these findings, the carbenic pathway is most important up to ∼500 K, above which the biradical mechanism predominates. Subsequent reaction steps include the addition of the CH3CH=CHNH2 and CH3CH2CH=NH species to another c-C3H5NH2 molecule to produce c-C3H5NHCH(NH2)CH2CH3, which afterwards decomposes into c-C3H5N=CHCH2CH3 and NH3. The results also reveal relatively high electronic barriers for the interconversion between the geometric isomers of CH3CH=CHNH2 and CH3CH2CH=NH, suggesting that the contribution of these processes is negligible. Further calculations of the title reaction in the presence of selected aliphatic amines RNH2 (R = Me, Et,n-Pr, i-Pr, allyl) confirm that the unimolecular fission of the ring is the rate determining step.

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.

Pressure Dependence of Rate Coefficients of Unimolecular and Chemical Activation Reactions Connected to the Potential Energy Wells of Si2H2Cl4, Si2Cl6, and Si2Cl4 via Rice-Ramsperger-Kassel-Marcus Calculations.

Rate coefficients for elementary reactions connected to the potential energy wells of Si2H2Cl4, Si2Cl6, and Si2Cl4, which are important Si2 species in chemical vapor deposition (CVD) processes that use chlorosilanes as silicon source gases, were determined through the Rice-Ramsperger-Kassel-Marcus theory under various conditions of temperature and pressure. The optimized structures and vibrational frequencies of the reactants, products, and transition state were obtained using (U)B3LYP/6-31+G(d,p), and the single-point energies of the optimized structures were recalculated using the coupled cluster method with single and double excitations plus triple perturbation (U)CCSD(T) with complete basis set extrapolation. Many of the unimolecular decomposition channels and chemical activation reactions investigated in this work were found to be in the fall-off regime under subatmospheric to moderately high-pressure conditions so that it is expected that accurate modeling of the gas phase in the chlorosilane CVD reactor requires careful determination of the rate coefficients as functions of temperature and pressure for the conditions of interest, instead of using high-pressure limit rate coefficients. The rate coefficients determined here were expressed through Chebyshev coefficients and also modified Arrhenius parameters to be used in simulations of systems under a wide range of temperature and pressure conditions.

Rate Coefficients for OH + NO (+N2) in the Fall-off Regime and the Impact of Water Vapor

The termolecular, association reaction between OH and NO is a source of nitrous acid (HONO), an important atmospheric trace gas. Rate coefficients for the title reaction as recommended by evaluation panels differ substantially at the temperatures and pressures that prevail in the Earth’s boundary layer where the reaction is in the fall-off regime between low- and high-pressure limiting rate coefficients. Using pulsed laser methods for generation and detection of OH, we have reinvestigated the kinetics of the title reaction at pressures of 22–743 Torr (1 Torr = 1.333 hPa) and temperatures (273, 298, and 333 K) in pure N2 and in N2–H2O bath gases. In situ optical absorption measurements were used to rule out any bias due to NO2 or HONO impurities. Our rate coefficients (k1) in N2 bath gas are parametrized in terms of low-pressure (k0) and high-pressure (k∞) rate coefficients and a fall-off parameter (FC) with k1,0N2 = 7.24 × 10–31 (T/300 K)−2.17 cm6 molecule–2 s–1, k1,∞ = 3.3 × 10–12 (T/300 K)−0.3 cm3 molecule–1 s–1, and FC = 0.53. Used with the “Troe” expression for termolecular reactions, these parameters accurately reproduce the current data in the fall-off regime and also capture literature rate coefficients at extrapolated temperatures. The presence of water vapor was found to enhance the rate coefficients of the title reaction significantly. The low-pressure limiting rate coefficient in H2O bath gas is a factor 5–6 larger than in N2, at room temperature (k1,0H2O = 4.55 × 10–30 (T/300 K)−4.85 cm6 molecule–2 s–1) indicating that H2O is much more efficient in quenching the association complex HONO* through collisional energy transfer. Based on measurements in N2–H2O mixtures, a parametrization of k1 including both N2 and H2O as third-body quenchers was derived. Neglecting the effect of H2O results, e.g., in an underestimation of k1 by >10% in the tropical boundary layer.

The merit of pressure dependent kinetic modelling in steam cracking.

Renewable cracking feedstocks from plastic waste and the need for novel reactor designs related to electrification of steam crackers drives the development of accurate and fundamental kinetic models for this process, despite its large scale implementation for more than half a century. Pressure dependent kinetics have mostly been omitted in fundamental steam cracking models, while they are crucial in combustion models. Therefore, we have assessed the importance of pressure dependent kinetics for steam cracking via in-depth modelling and experimental studies. In particular we have studied the influence of considering fall-off on the product yields for ethane and propane steam cracking. A high-pressure limit fundamental kinetic model is generated, based on quantum chemical data and group additive values, and supplemented with literature values for pressure dependent kinetic parameters for β-scission reactions and homolytic bond scissions of C2 and C3 species. Model simulations with high-pressure limit rate coefficients and pressure dependent kinetics are compared to new experimental measurements. Steam cracking experiments for pure ethane and propane feeds are performed on a tubular bench-scale reactor at 0.17 MPa and temperatures ranging from 1058 to 1178 K. All important product species are identified using a comprehensive GC × GC-FID/q-MS. For homolytic bond scissions, the inclusion of pressure dependent kinetics has a significant effect on the conversion profile for ethane steam cracking. On the other hand, pressure dependence of C2 β-scissions significantly influences conversion and product species profiles for both ethane and propane steam cracking. The C3 β-scissions pressure dependence has a negligible effect in ethane steam cracking, while for propane steam cracking the effect is non-negligible on the product species profiles.

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.

Thermal Decomposition of 2-Methyltetrahydrofuran behind Reflected Shock Waves over the Temperature Range of 1179–1361 K

The thermal unimolecular decomposition of 2-methyltetrahydrofuran (2-MTHF) was studied behind reflected shock waves in a single-pulse shock tube over the temperature range of 1179-1361 K and pressure range of 9-17 atm. Methane, ethylene, ethane, 1,3-butadiene, propylene, acetaldehyde, and acetylene were identified as products in the decomposition of 2-MTHF. A reaction scheme was proposed to explain the mechanism for the observed products. The experimentally determined rate coefficients were best fit to an Arrhenius expression for the overall decomposition and is represented as ktotalexp(1179-1361 K) = (3.23 ± 0.59) × 1011 s-1 exp(-51.3 ± 1.4 kcal mol-1/RT). Quantum chemistry methods were used to calculate the energetics and kinetics of various possible unimolecular dissociation pathways involved in the thermal decomposition of 2-MTHF. The initial decomposition of 2-MTHF occurs predominantly via ring-methyl (C-CH3) single bond fission, leading to the formation of tetrahydrofuran (C4H7O) radical, and methyl radical was found to be the major reaction compared to all the possible initial bond fission, ring opening, and molecular elimination channels. The temperature-dependent rate coefficients for the unimolecular dissociation of 2-MTHF were calculated using the RRKM (Rice-Ramsperger-Kassel-Marcus) theory in combination with the CCSD(T)/cc-pVTZ//B3LYP/cc-pVTZ level of electronic structure calculations over the temperature range of 800-1500 K. The computed high-pressure limiting rate coefficients for the initial decomposition of 2-MTHF through C-CH3 single bond fission channel were found to be ∼2 times higher in the temperatures between 800 and 900 K, and above this temperature, they agree well with the values reported in the literature.

An experimental and theoretical kinetic study of the reactions of hydroxyl radicals with tetrahydrofuran and two deuterated tetrahydrofurans

Rate coefficient expressions for the reactions of OH radicals with tetrahydrofuran (THF) (1), 2,2,5,5-tetradeutero-tetrahydrofuran (THF–d4) (2) and perdeuterated-tetrahydrofuran (THF–d8) (3) were determined from relative kinetic experiments over the temperature range T = 260–360 K displaying a small negative T-dependence. The following rate coefficients were determined at T = 298 K in 10–11 cm3 molecule–1 s−1: k1 = (1.73 ± 0.23), k2 = (0.85 ± 0.11) and k3 = (0.81 ± 0.12). The experimental k (T) values agree reasonably well with the high-pressure limit rate coefficients obtained from conventional transition state theory using input data from CCSD(T)/cc-pV(T,Q)Z//MP2/aug-cc-pVDZ computations. Standard enthalpies of formation for THF and two product furanyl radicals were also computed and compared with literature.

A high temperature shock tube study of phenyl recombination reaction using laser absorption spectroscopy

The chemistry of first aromatic ring, i.e., benzene (C6H6) and phenyl radial (C6H5), plays a key role in the growth of polycyclic aromatic hydrocarbons (PAHs) and ultimately soot formation. In this work, the self-recombination reaction of phenyl radicals was investigated over the temperature range of 950–1300 K and pressures near 1 atm by employing shock tube and laser absorption diagnostics. Phenyl radical was generated by the rapid thermal unimolecular dissociation of nitrosobenzene (C6H5NO), a clean precursor of C6H5 radical. The reaction progress was monitored by detecting C6H5 and NO simultaneously using visible laser absorption near 445 nm and mid-IR laser absorption near 5.517 µm, respectively. For the reaction C6H5NO → C6H5 + NO (R1), our data show an excellent agreement with earlier reports. The high-pressure limiting rate coefficient, by combining all available data, may be expressed as k1∞(T(K))=3.2×1066T−15.2e−37743/T s−1. This work reports the temperature dependence of the absorption cross-section of phenyl radical at 445 nm for first time. Our experiments indicate that the self-reaction of phenyl radicals yielding biphenyl, C6H5 + C6H5 → C6H5C6H5 (R2a), is a major channel. The rate coefficients of reaction (R2a) show a weak temperature dependence with an average value of k2a = (6.91 ± 0.42) × 1012 cm3 mol−1 s−1 in the temperature range of 950–1300 K. Our measured data, k2a(T, P = 1.1–1.5 atm), are found to be close to the high-pressure limiting rate coefficients. Combining with the literature low-temperature data, the self-recombination reaction of phenyl radicals may be expressed as k2a∞(T=300−1450K)=2.8×1017T−1.44e−540/T cm3 mol−1 s−1. The measurements of this study represent the first high-temperature direct experimental determination of the rate coefficients of this important prototype aromatic radical-radical reaction.

Thermal dissociation of alkyl nitrites and recombination of alkyl radicals

The thermal dissociations of three alkyl nitrites (C3H7ONO, n-C4H9ONO, and i-C4H9ONO) have been studied in a diaphragmless shock tube by laser schlieren densitometry over 700 < T < 1000 K and 60 < P < 250 Torr. The nitrites act as clean, thermal sources of alkyl radicals that create the radicals at much lower temperatures than are accessible with other precursors. The decomposition of the three nitrites proceeds via a common mechanism, which is discussed. Rate coefficients for dissociation of the alkyl nitrites to an alkoxy radical and NO, k1, were obtained with an uncertainty of 30%. Rice–Ramsperger–Kassel–Marcus/Master Equation (RRKM/ME) calculations for dissociation of the nitrites were performed and provide excellent simulations of the experimental results. The high-pressure limit rate coefficients from the RRKM/ME calculations are: k1,C3H7ONO=(5.69±1.71)×1021T−1.60exp(−21458/T)s−1; k1,n−C4H9ONO=(9.91±2.97)×1021T−1.59exp(−21588/T)s−1; k1,i−C4H9ONO=(3.92±1.18)×1021T−1.58exp(−21162/T)s−1. At the low temperatures of this work, the alkyl radicals are thermally stable and thus their recombination reactions were also studied. The recombination rate coefficients for ethyl, n-propyl and i-propyl radicals (uncertainties of 50%) are in excellent agreement with literature values, particularly recent theoretical results, and extend the range of experiments to higher temperatures.

On-the-fly ab initio calculations toward accurate rate coefficients

During automatic kinetic model generation a large number of reactions need to have kinetic data assigned to them. One of the main challenges is on how to account for the large scarcity of accurate data. The ever-growing computational power and availability of high performance computing solutions allows to envisage implementing on-the-fly ab initio calculations for a large number of reaction rate coefficients. Today extensive user knowledge and involvement is required to develop fast and effective ab initio based kinetic models. The present work introduces a series of automation procedures to minimize user involvement. It allows automated quantum chemical calculations for a large number of reactions for gas-phase processes with a wide variety in chemical species. 3D coordinates are calculated for all the reactants, products and transition states and automatically submitted to quantum chemistry packages. The latter yield accurate high-pressure limit rate coefficients. All steps are automated without the need for manual interventions, even for recognizing rotational frequencies and for verifying whether the saddle point searches lead to the appropriate transition state. A good agreement to literature data is found, i.e., most of the rate coefficients are within a factor of 2 of literature data.

Experimental and Kinetic Modeling Study of Cyclohexane Pyrolysis

The pyrolysis of undiluted cyclohexane has been studied in a continuous flow tubular reactor at temperatures from 913 to 1073 K and inlet feed flow rates in the range 288–304 g·h–1 at 0.17 MPa reactor pressure with average reactor residence time of 0.5 s calculated based on the pressure in the reactor, the temperature profile along the reactor, and the molar flow rate along the reactor estimated by the logarithmic average of the inlet and outlet molar flows. The reactions lead to conversions between 2% and 95%. Forty-nine products were identified and quantified using two-dimensional gas chromatography equipped with thermal conductivity and flame ionization detectors. The products with molecular weights between those of hydrogen and naphthalene constitute more than 99 mass % of the total products. A kinetic mechanism composed exclusively of elementary step reactions with high pressure limit rate coefficients has been generated with the automatic network generation tool “Genesys”. The kinetic parameters for the reactions originate either directly from high level ab initio calculations or from reported group additive values which were derived from ab initio calculations. The Genesys model performs well when compared to five models available in the literature, and its predictions agree well with the experimental data for 15 products without any adjustments of the kinetic parameters. Reaction path analysis shows that cyclohexane consumption is initiated by the unimolecular isomerization to 1-hexene but is overall dominated by hydrogen abstraction reactions by hydrogen atoms and methyl radicals. Dominant pathways to major products predicted with the new model are discussed and compared to other well performing models in the literature.

Pressure Dependence of Rate Coefficients for Formation and Dissociation of Pentachlorodisilane and Related Chemical Activation Reactions

Four entrance channels to the pentachlorodisilane (Si2HCl5) potential energy well, namely, SiCl2 + SiHCl3, SiCl4 + SiHCl, Cl3SiSiCl + HCl, and SiCl3 + SiHCl2, were analyzed in detail through transition state theory, Rice–Ramsperger–Kassel–Marcus (RRKM) theory, and solution of the multichannel master equation. The stationary points in the potential energy surface were optimized, and their vibrational frequencies and rotational constants calculated at the (U)B3LYP/6–31+G(d,p) level of theory; the (U)CCSD(T)/aug-cc-pVTZ level was then used for accurate estimation of activation energies. The pressure and temperature dependence of the rate coefficients of the channels related to Si2HCl5 stabilization/dissociation was determined along a wide range of conditions, for the first time. All channels showed strong pressure dependence in the four cases, at least at low-to-moderate pressure conditions. Each entrance channel leads to the formation of different products under different conditions, and the mechanism was analyzed in detail. The results indicated that at atmospheric pressure the reactions are in the falloff region, and therefore do not support the adoption of high-pressure limit rate coefficients in reaction models designed for simulation of systems at atmospheric or subatmospheric pressure conditions.

An Experimental and Master Equation Study of the Kinetics of OH/OD + SO2: The Limiting High-Pressure Rate Coefficients.

The kinetics of the reaction OH/OD + SO2 were studied using a laser flash photolysis/laser-induced fluorescence technique. Evidence for two-photon photolysis of SO2 at 248 nm is presented and quantified, and which appears to have been evident to some extent in most previous photolysis studies, potentially leading to values for the rate coefficient k1 that are too large. The kinetics of the reaction OH(v = 0) + SO2 (T = 295 K, p = 25-300 torr) were measured under conditions where SO2 photolysis was taken into account. These results, together with literature data, were modeled using a master equation analysis. This analysis highlighted problems with the literature data: the rate coefficients derived from flash photolysis data were generally too high and from the flow tube data too low. Our best estimate of the high-pressure limiting rate coefficient k1∞ was obtained from selected data and gives a value of (7.8 ± 2.2) × 10-13 cm3 molecule-1 s-1, which is lower than that recommended in the literature. A parametrized form of k1([N2],T) is provided. The OD(v = 0) + SO2 (T = 295 K, p = 25-300 torr) data are reported for the first time, and master equation analysis reinforces our assignment of k1∞.

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.

Role of the Recombination Channel in the Reaction between the HO and HO2 Radicals.

The kinetics of the gas phase recombination reaction HO + HO2 + He → HOOOH + He has been studied between 200 and 600 K by using the SACM/CT model and the unimolecular rate theory. The molecular properties of HOOOH were derived at the CCSD(T)/aug-cc-pVTZ ab initio level of theory, while relevant potential energy features of the reaction were calculated at the CCSD(T)/aug-cc-pVTZ//CCSD(T)/aug-cc-pVDZ level. The resulting high and low pressure limit rate coefficients are k∞ = 3.55 × 10-12 (T/300)0.20 cm3 molecule-1 s-1 and k0 = [He] 1.55 × 10-31 (T/300)-3.2 cm3 molecule-1 s-1. The rate coefficients calculated over the 6 × 10-4 - 400 bar range are smaller at least in a factor of about 60 than the consensus value determined for the main reaction channel HO + HO2 → H2O + O2, indicating that the recombination pathway is irrelevant.

A theoretical study of cyclic ether formation reactions

Cyclisation reactions of hydroperoxyl–alkyl radicals forming cyclic ethers and hydroxyl radicals play an important role in low temperature oxidation chemistry. These reactions contribute to the competition between radical chain propagation and chain branching reaction pathways which dominate the reactivity of alkanes at temperatures where negative temperature coefficient (NTC) behaviour is often observed. This work is motivated by previous experimental and modelling evidences that current literature rate coefficients for these reactions are in need of refinement and/or re-determination. In light of this, the current study presents quantum-chemically-derived high-pressure limit rate coefficients for all cyclisation reactions leading to cyclic ether formation in alkanes ranging in size from C2 to C5. Ro-vibrational properties of each stationary point were determined at the M06-2X/6-311++G(d,p) level of theory. Coupled cluster (CCSD(T)) and Møller–Plesset perturbation theory (MP2) methods were employed with various basis sets and complete basis set extrapolation techniques to compute the energies of the resulting geometries. These methods, combined with canonical transition state theory, have been used to determine 43 rate coefficients, with enough structural diversity within the reactions to allow for their application to larger species for which the use of the levels of theory employed herein would be computationally prohibitive. The validity of an alternative, and computationally less expensive, technique to approximate the complete basis set limit energies is also discussed, together with implications of this work for combustion modelling.

Theoretical kinetic study of the unimolecular decomposition of 2-bromopropene

The kinetics of the gas phase thermal decomposition of 2-bromopropene at 600–1400K has been studied by using the unimolecular rate theory combined with different formulations of the density functional theory and high level ab initio composite methods. This hydrogen bromide elimination reaction presents two dissociation channels, one forming propyne and another forming allene. High-pressure limit rate coefficients of (6.2±1.2)×1014exp [−(64.5±2kcalmol−1)/RT] and (1.1±0.1)×1014exp [−(63.6±2kcalmol−1)/RT]s−1 were obtained for these reaction pathways. The present results allow to elucidate reported contradictory experimental data.

Quantum Chemical and Kinetics Study of the Thermal Gas Phase Decomposition of 2-Chloropropene

A detailed theoretical study of the kinetics of the thermal decomposition of 2-chloropropene over the 600-1400 K temperature range has been done. The reaction takes place through the elimination of HCl with the concomitant formation of propyne or allene products. Relevant molecular properties of the reactant and transition states were calculated for each reaction channel at 14 levels of theory. From information provided by the BMK, MPWB1K, BB1K, M05-2X, and M06-2X functionals, specific for chemical kinetics studies, high-pressure limit rate coefficients of (5.8 ± 1.0) × 10(14) exp[-(67.8 ± 0.4 kcal mol(-1))/RT] s(-1) and (1.1 ± 0.2) × 10(14) exp[-(66.8 ± 0.5 kcal mol(-1))/RT] s(-1) were obtained for the propyne and allene channels, respectively. The pressure effect over the reaction was analyzed through the calculation of the low-pressure limit rate coefficients and falloff curves. An analysis of the branching ratio between the two channels as a function of pressure and temperature, based on these results and on computed specific rate coefficients, show that the propyne forming channel is predominant.

Mechanism of the gas-phase decomposition of trifluoro-, trichloro-, and tribromomethanols in the presence of hydrogen halides

Ab initio calculations at the G2 level were used in a theoretical analysis of the kinetics of the decomposition of trifluoro-, trichloro-, and tribromomethanols. The high-pressure limiting rate coefficients k(diss,∞) for the thermal dissociation of CF(3)OH, CCl(3)OH, and CBr(3)OH were calculated using the conventional transition state theory. The results of potential surface calculations show that in the presence of the hydrogen halides HX (X = F, Cl, and Br), considerably lower energy pathways are accessible for the decomposition of CF(3)OH, CCl(3)OH, and CBr(3)OH. The mechanism of the reactions appears to be complex and consists of three consecutive elementary processes with the formation of pre- and post-reaction adducts. The presence of hydrogen halides considerably decreases the energy barrier for the bimolecular decomposition of the alcohols CF(3)OH, CCl(3)OH, and CBr(3)OH. Results of this study indicate that hydrogen halides can considerably accelerate the homogeneous decomposition of perhalogenated methanols when they are present in the reaction area at sufficiently high concentrations. However, the atmospheric concentrations of hydrogen halides are too small for efficient removal of atmospheric CF(3)OH, CCl(3)OH, and CBr(3)OH.