Fall-off Regime Research Articles

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.

Computational studies on thermo-kinetics aspects of pyrolysis of isopropyl acetate and its methyl, bromide and hydroxyl derivatives

The gas-phase decomposition kinetics of isopropyl acetate (IPA) and its methyl, bromide and hydroxyl derivatives into the corresponding acid and propene were investigated using density functional theory (DFT) with the ωB97XD and M06–2x functionals, as well as the benchmark CBS-QB3 composite method. Transition state theory (TST) and RRKM theory calculations of rate constants under atmospheric pressure and in the fall-off regime were used to supplement the measured energy profiles. The results show that the formation of propene and bromoacetic acid is the most dominant pathway at the CBS-QB3 composite method, both kinetically and thermodynamically. There was a good agreement with experimental results. Pressures greater than 0.01 bar, corresponding to larger barrier heights are insufficient to ensure saturation of the measured rate coefficient when compared to the RRKM kinetic rates.Natural bond orbitals (NBO) charges, bond orders, bond indices, and synchronicity parameters all point to the considered pathways taking place via a homogenous, first-order concerted, as well as an asynchronous mechanism involving a non-planar cyclic six-membered transition state. The calculated data exhibit that the elongation of the Cα−O bond length and subsequent polarization of the Cα+δ…O−δ bond is the rate-determining step of the considered reactions in the cyclic transition state, which appears to be involved in this type of reaction.

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.

Kinetics of OH + SO<sub>2</sub> + M: temperature-dependent rate coefficients in the fall-off regime and the influence of water vapour

Abstract. The OH-initiated oxidation of SO2 is the dominant, first step in the transformation of this atmospherically important trace gas to particulate sulfate, and accurate rate coefficients for the title reaction under all atmospheric conditions (pressures, temperatures, and humidity) are required to assess its role in, for example, new particle formation. Prior to this study, no temperature-dependent data were available in the fall-off regime for atmospherically relevant bath gases. We thus address an important omission in the kinetic database for this reaction and highlight significant discrepancies in recommended parameterizations. In this work, generation of OH via pulsed laser photolysis at 248 and 351 nm was coupled to its detection by laser-induced fluorescence to obtain rate coefficients (k1) for the title reaction at pressures of 14–742 Torr (1 Torr = 1.333 hPa) and temperatures of 220–333 K in N2 bath gas. In situ SO2 concentrations, central to accurate kinetic measurements under pseudo-first-order conditions, were measured by optical absorption. Under the conditions of the present study, the termolecular reaction between OH and SO2 is in the fall-off regime, and we parameterized the rate coefficients in N2 in terms of low- (k1,0) and high-pressure (k1,∞) limiting rate coefficients and a broadening factor (FC) to obtain k1,0N2=3.03×10-31 (T/300 K)−4.10 cm6 molecule−2 s−1, k1,∞=2.00×10-12 cm3 molecule−1 s−1, and FC=0.58. The effects of water vapour on the title reaction were explored through measurements in N2–H2O mixtures at 273, 298, and 333 K using the same experimental methods. The rate coefficients are significantly enhanced by the presence of water vapour with k1,0H2O=1.65×10-30 (T/300 K)−4.90 cm6 molecule−2 s−1, which indicates that H2O is a factor >5 more efficient in quenching the HOSO2* association complex than N2. A model-based comparison of our rate coefficients and parameterization with previous literature measurements and recommendations of evaluation panels are presented and discussed. The use of the new parameterization instead of the IUPAC or NASA evaluations, particularly after including H2O as a third-body quencher, leads to a significant (10 %–30 %) reduction in the lifetime of SO2 in some parts of the atmosphere and increases the H2SO4/SO2 ratio concomitantly.

A computational study on the kinetics of pyrolysis of isopropyl propionate as a biodiesel model: DFT and ab initio investigation

This work reports a computational kinetic study on the pyrolysis of isopropyl propionate (IPP) as a biodiesel model using density functional theory (DFT), namely ωB97XD and M06-2X levels. The obtained data are compared with the benchmark CBS-QB3 results. The calculated energy profiles have been supplemented with calculations of rate coefficients and branching ratios over the temperature range 563–651 K and under atmospheric pressure and in the fall-off regime was determined using transition state theory (TST) and statistical Rice-Ramsperger-Kassel-Marcus (RRKM). The obtained results reveal that the formation of propionic acid and propene is the most predominant path both thermodynamically and kinetically. However, production of other species is unlikely to occur except at elevated temperatures, with the hemolytic bond cleavage reactions aren't competitive under the applied temperature range. Comparison with linear methyl and ethyl esters indicates faster reactions for IPP. A good agreement with the available experimental findings has been found. Consistent with higher barrier heights, pressures P > 10−4 bar are necessary to achieve the high-pressure (HP) limit.

Oxidation reaction mechanism and kinetics between OH radicals and alkyl-substituted aliphatic thiols: OH-addition pathways

Kinetic rate constants for the oxidation reactions of OH radicals with CH3SH (1), C2H5SH (2), n-C3H7SH (3) and iso-C3H7SH (4) under inert conditions (Ar) over the temperature range 252−430 K have been studied using the CBS-QB3 composite method. Kinetic rate constants under atmospheric pressure and in the fall-off regime have been estimated using transition state theory (TST) and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory. Comparison with experiment confirms that in the OH-addition pathways 1−4 leading to the related products, the first bimolecular reaction step has effective negative activation energies around −2.61 to 3.70 kcal mol−1. Effective rate coefficients have been calculated according to a steady-state analysis of a two-step model reaction mechanism. As a result of the negative activation energies, pressures larger than 104 bar would be required to restore to some extent the validity of this approximation for all the channels. By comparison with experimental data, all our calculations for both the OH-addition and H-abstraction reaction pathways indicate that from a kinetic viewpoint and in line with the computed reaction energy barriers, the most favourable process is the OH-addition pathway to n-C3H7SH to yield the [ n-C3H7SH−OH]• species, whereas under thermodynamic control of the bimolecular reactions (R−SH+OH•), the most abundant product derived from the H-abstraction pathway will be the [ n-C3H7 S•+H2O] species.

Theoretical study on the mechanisms and kinetics of the β-elimination of 2,2-dihaloethyltrihalosilanes (X = F, Cl, Br) compounds: a DFT study along with a natural bond orbital analysis

The β-elimination kinetics of 2,2-dihaloethyltrihalosilanes in the gas phase has been studied computationally using density functional theory (DFT) along with the M06-2x exchange–correlation functional and the aug-cc-pVTZ basis set. The calculated energy profiles have been supplemented with calculations of rate constants under atmospheric pressure and in the fall-off regime, by means of transition state theory (TST), variational transition state theory (VTST), and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory. Activation energies and rate constants obtained using the M06-2x/aug-cc-pVTZ approaches are in good agreement with the available experimental data. Analysis of bond order, natural bond orbitals, and synchronicity parameters suggests that the β-elimination of the studied compounds can be described as concerted and slightly asynchronous. The transition states of these reactions correspond to four-membered cyclic structures. Based on the optimized ground state geometries, a natural bond orbital (NBO) analysis of donor–acceptor interactions also show that the resonance energies related to the electronic delocalization from $$\sigma_{{{\text{C}}_{ 1} {-}{\text{C}}_{ 2} }}$$ bonding orbitals to $$\sigma^{*}_{{{\text{C}}_{ 2} - {\text{Si}}_{ 3} }}$$ antibonding orbitals, increase from 2,2-difluoroethyltrifluorosilane to 2,2-dichloroethyltrichlorosilane and then to 2,2-dibromoethyltriboromosilane. The decrease of $$\sigma_{{{\text{C}}_{ 1} {-}{\text{C}}_{ 2} }}$$ bonding orbitals occupancies and increase of the $$\sigma^{*}_{{{\text{C}}_{ 2} - {\text{Si}}_{ 3} }}$$ antibonding orbitals occupancies through $$\sigma_{{{\text{C}}_{ 1} - {\text{C}}_{ 2} }} \to \sigma^{*}_{{{\text{C}}_{ 2} - {\text{Si}}_{ 3} }}$$ delocalizations could facilitate the β-elimination of the 2,2-difluoroethyltrifluorosilane compound, compared to 2,2-dichloroethyltrichlorosilane and 2,2-dibromoethyltriboromosilane.

Reaction mechanisms and kinetics of the elimination processes of 2-chloroethylsilane and derivatives: A DFT study using CTST, RRKM, and BET theories

The thermal decomposition kinetics of 2-chloroethylsilane and derivatives in the gas phase has been studied computationally using density functional theory, along with various exchange-correlation functionals (UM06-2x and ωB97XD) and the aug-cc-pVTZ basis set. The calculated energy profile has been supplemented with calculations of kinetic rate constants under atmospheric pressure and in the fall-off regime, using transition state theory (TST) and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory. Activation energies and rate constants obtained using the UM06-2x/aug-cc-pVTZ approach are in good agreement with the experimental data. The decomposition of 2-chloroethyltriethylsilane species into the related products [C2H4+Et3SiCl] is characterized by 6 successive structural stability domains associated to the sequence of catastrophes C8H19SiCl: 6-C†FCC†[FF]-0: C6H15SiCl+C2H4. Breaking of Si–C bonds and formation of Si–Cl bonds occur in the vicinity of the transition state.

Reaction Mechanisms and Kinetics of the O2Addition Pathways to the Main Thiophene-OH Adduct: A Theoretical Study

Density functional theory, along with the ΩB97XD and UM06-2x exchange-correlation functional, has been used to study the reaction mechanisms and kinetics of the atmospheric oxidation of the main (kinetically dominant) thiophene-OH adduct [C4H4S-OH]• (R1) by molecular oxygen in its triplet electronic ground state. Kinetic rate constants and branching ratios under atmospheric pressure and in the fall-off regime have been calculated by means of transition state theory (TST), variational transition state theory (VTST) and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory. In line with the computed energy profiles, the dominant process under both the thermodynamic and kinetic control of the reaction is O2addition at the C5position in syn mode. The computed branching ratios indicate that the regioselectivity of the reaction decreases with increasing temperature and decreasing pressure.

A shock tube laser schlieren study of cyclopentane pyrolysis

The dissociation of cyclopentane has been investigated in a diaphragmless shock tube with laser schlieren densitometry at nominal post-shock pressures, P2, of 35, 70, 150, and 300Torr and temperatures, T2, of 1472–2074K. These are the first experimental data reported in the high temperature fall-off regime for cyclopentane. The experimental density gradients were simulated using a chemical-kinetic model with good agreement between simulations and experiments. Rate coefficients for dissociation of cyclopentane were obtained and mechanistic details were elucidated. An RRKM model was developed to best fit the experimental results, and the rate coefficients calculated are: k1(35Torr)=(7.93±3.96)×1084 T−19.815 exp(−64366/T) s−1, k1(70Torr)=(4.79±2.39)×1077 T−17.705 exp(−62316/T) s−1, k1(150Torr)=(2.42±1.21)×1069 T−15.295 exp(−59862/T) s−1, k1 (300Torr)=(4.66±2.33)×1061 T−13.065 exp(−57493/T) s−1, and k1∞=(1.69±0.85)×1016 T−0.005 exp(−42983/T). Results show that cyclopentane dissociation is similar to that of cyclohexane, and involves formation of a biradical intermediate. The high-pressure limit rates for cyclopentane dissociation are also compared with previous results in the literature and show good agreement with a study by Tsang (1978) [21].

Reaction mechanisms and kinetics of the isomerization processes of naphthalene peroxy radicals

The isomerization processes of naphthalene peroxy radicals [C10H8–OH]–O2 into bicyclic peroxy or oxy hydroperoxide radicals via ring closure and intramolecular hydrogen transfers have been studied computationally using density functional theory, along with various exchange–correlation functionals and an extremely large basis set. The calculated energy profiles have been supplemented with calculations of kinetic rate constants under atmospheric pressure and in the fall-off regime, using transition state theory (TST) and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The cyclization of the R1-2OO-syn peroxy radical into the R1-2,9OO-syn bicyclic peroxy radical through formation of an O–O bridge is endothermic and reversible. Both from a thermodynamic and kinetic view points, the two most favorable processes for the R1-2OO-syn peroxy radical are ring closure into the R1-2,9OO-syn bicyclic peroxy radical species, and conversion through hydrogen transfer into the R1-P2O1-syn oxy hydroperoxide radical. Among all studied reaction channels, the latter process is the kinetically most competitive one. Also, in view of the computed rate constants, the R1-2OO-syn peroxy radical appears to be chemically much more reactive than the R1-4OO-syn species. All in all, the atmospheric oxidation mechanisms of naphthalene appear at this reaction stage to be quite different from that of benzene and its derivatives.

Dissociation of ortho-benzyne radicals in the high temperature fall-off regime

The decomposition of ortho-benzyne radicals (o-C6H4), generated from the dissociation of a new precursor, fluorobenzene (C6H5F), has been investigated in a diaphragmless shock tube in a combined laser schlieren densitometry, LS, (P2=30±2, 59±3, 121±5Torr, 2050<T2<2980)/time-of-flight mass spectrometry, TOF-MS (P5=1150±200Torr, 2300<T5<2800K) study. The LS density gradient profiles were simulated, and excellent agreement was found between simulations and experimental profiles. Rate coefficients for C6H5F→o-C6H4+HF, o-C6H4→C4H2+C2H2, and o-C6H4→C6H3+H were obtained. Good agreement with Xu et al. [Proc. Combust. Inst., 31(2007) 231-239] with respect to the o-benzyne dissociation branching ratio was found. However a strong pressure dependence was also observed in o-benzyne dissociation which was not seen by Xu et al. For o-C6H4→C4H2+C2H2: k2a,120Torr=(1.2±0.4)×1063T−14.27 exp(−52,710/T) k2a,60Torr=(1.4±0.5)×1060T−13.595 exp(−50,538/T), k2a,30Torr=(4.0±1.3)×1057T−13.015 exp(−48,628/T) s−1. The inclusion of high temperature o-benzyne abstraction reactions is necessary to simulate the reacting system, and we report estimates of the rate of o-C6H4+C6H5F.

A shock tube laser schlieren study of methyl acetate dissociation in the fall-off regime

The pyrolysis of methyl acetate, 2% and 4% dilute in krypton, was investigated in a diaphragmless shock tube (DFST) using laser schlieren densitometry (LS). Experiments were performed at 122 ± 3 and 63 ± 2 Torr over the temperature range of 1492-2266 K. Master equation models for the four main dissociation paths of methyl acetate based on a prior study by Peukert et al. [S. Peukert, R. Sivaramakrishnan, M. Su and J. Michael, Combust. Flame, 2012, 159, 2312-2323] were refined and formed the basis for simulating the LS experiments. The density gradient profiles from the LS experiments indicate that the initial dissociation proceeds predominantly by breakage of the C-O bond leading ultimately to two methyl radicals and CO2, accounting for 83-88% of the methyl acetate loss over this temperature range. Rate coefficients for dissociation of methyl acetate were satisfactorily simulated with a master equation model, with modelled rate coefficients of k120 Torr = 9.06 × 10(81) × T(-19.07) × exp(-61 600K/T) s(-1), k60 Torr = 3.71 × 10(82) × T(-19.34) × exp(-61 200K/T) s(-1), and of k∞ = 1.97 × 10(30) × T(-3.80) × exp(-47 900K/T) s(-1) for the major channel, based on fitting to 120 Torr and 60 Torr data taken in this study. The model also captures the pressure dependency of methyl acetate dissociation and resolves an earlier discrepancy concerning the mechanism of dissociation of methyl acetate.

Recombination of Allyl Radicals in the High Temperature Fall-Off Regime

The recombination of allyl radicals (C3H5), generated from the dissociation of 1,5-hexadiene or allyl iodide dilute in krypton, has been investigated in a diaphragmless shock tube using laser schlieren densitometry, LS, (900-1700 K, 10 ± 1, 29 ± 3, 57 ± 3, and 120 ± 4 Torr). The LS density gradient profiles were simulated and excellent agreement was found between simulations and experimental profiles. Rate coefficients for C3H5I → C3H5 + I and C3H5 + C3H5 → C6H10 were obtained and showed strong fall-off. Second order rate coefficients for allyl radical recombination were determined as k(1a,124Torr) = (2.6 ± 0.8) × 10(55)T( -12.995) exp(-8426/T), k(1a,57Torr) = (1.7 ± 0.5) × 10(60)T( -14.49) exp(-9344/T), and k(1a,30Torr) = (7.5 ± 2.3) × 10(66)T( -15.935) exp(-10192/T) cm(3) mol(-1)s(-1). The contribution of a disproportionation channel in allyl radical reactions was assessed, and the best agreement was obtained with no more than 5% disproportionation. Notably, because both the forward and back reactions of C6H10 ⇌ C3H5 + C3H5 were measured, utilizing two different precursors, the equilibrium constant of this reaction could be found, suggesting an entropy of formation of 1,5-hexadiene, 87.3 cal mol(-1 )K(-1), which is significantly smaller than that group additivity predicts, but larger than other reference literature values.

Reaction of Hydroxyl Radicals with C4H5N (Pyrrole): Temperature and Pressure Dependent Rate Coefficients

Absolute (pulsed laser photolysis, 4-639 Torr N(2) or air, 240-357 K) and relative rate methods (50 and 760 Torr air, 296 K) were used to measure rate coefficients k(1) for the title reaction, OH + C(4)H(5)N → products (R1). Although the pressure and temperature dependent rate coefficient is adequately represented by a falloff parametrization, calculations of the potential energy surface indicate a complex reaction system with multiple reaction paths (addition only) in the falloff regime. At 298 K and 760 Torr (1 Torr = 1.33 mbar) the rate coefficient obtained from the parametrization is k(1) = (1.28 ± 0.1) × 10(-10) cm(3) molecule(-1) s(-1), in good agreement with the value of (1.10 ± 0.27) × 10(-10) cm(3) molecule(-1) s(-1) obtained in the relative rate study (relative to C(5)H(8), isoprene) at this temperature and pressure. The accuracy of the absolute rate coefficient determination was enhanced by online optical absorption measurements of the C(4)H(5)N concentration at 184.95 nm using a value σ(184.95nm) = (1.26 ± 0.02) × 10(-17) cm(2) molecule(-1), which was determined in this work.

The Importance of NO+(H2O)4 in the Conversion of NO+(H2O)n to H3O+(H2O)n: I. Kinetics Measurements and Statistical Rate Modeling

The kinetics for conversion of NO(+)(H(2)O)(n) to H(3)O(+)(H(2)O)(n) has been investigated as a function of temperature from 150 to 400 K. In contrast to previous studies, which show that the conversion goes completely through a reaction of NO(+)(H(2)O)(3), the present results show that NO(+)(H(2)O)(4) plays an increasing role in the conversion as the temperature is lowered. Rate constants are derived for the clustering of H(2)O to NO(+)(H(2)O)(1-3) and the reactions of NO(+)(H(2)O)(3,4) with H(2)O to form H(3)O(+)(H(2)O)(2,3), respectively. In addition, thermal dissociation of NO(+)(H(2)O)(4) to lose HNO(2) was also found to be important. The rate constants for the clustering increase substantially with the lowering of the temperature. Flux calculations show that NO(+)(H(2)O)(4) accounts for over 99% of the conversion at 150 K and even 20% at 300 K, although it is too small to be detectable. The experimental data are complimented by modeling of the falloff curves for the clustering reactions. The modeling shows that, for many of the conditions, the data correspond to the falloff regime of third body association.

High-Temperature Shock Tube Measurements of Dimethyl Ether Decomposition and the Reaction of Dimethyl Ether with OH

We measured the first high-temperature rate measurements of two dimethyl ether (DME) reactions, (1) DME + Ar --> CH3O + CH3 + Ar and (2) DME + OH --> CH3OCH2 + H2O, in a shock tube by monitoring OH radicals. OH was measured with a narrow-line width laser absorption diagnostic using the well-known R1(5) line of the A-X(0,0) transition at 306.7 nm. The rate k1 is in the falloff regime at high temperatures, so it was measured at several pressures from 0.6 to 11.5 atm and temperatures from 1349 to 1790 K. OH radicals were formed by shock-heating mixtures of DME and O2 in Ar. These mixtures take advantage of the rapid decomposition of the product CH3O, forming H-atoms, which react with O2 to form OH. In carefully chosen mixtures, OH concentration is primarily sensitive to k1 and the well-known rate of H + O2 --> OH + O. Uncertainty in the k1 measurements was estimated to be +/-35%. The rate measurements were then modeled using RRKM theory, which describes the data quite well. Both the rate measurements and the RRKM model were fit from 1000 to 1800 K using the Troe falloff form: k(1,infinity)(T) = (4.38 x 10(21))T(-1.57) exp(-42,220 K/T) s(-1), k(1,o) = 7.52 x 10(15) exp(-21,537 K/T) cm3 mol(-1) s(-1), and F(cent) = 0.454 exp(-T/2510). The rate of k2 was measured at pressures near 1.6 atm and temperatures from 923 to 1423 K. OH radicals were generated by the thermal decomposition of the OH precursor tert-butyl hydroperoxide (TBHP), and k2 was inferred from the observed decay of OH with an estimated uncertainty of +/-40%. The high-temperature measurements were compared with several rate evaluations and previous low-temperature measurements. The rate evaluation by Curran et al. of k2 = (6.32 x 10(6))T2 exp(328 K/T) (cm3 mol(-1) s(-1)) was found to be an excellent fit to both the previous low-temperature measurements and this work.

An ab initio Rice-Ramsperger-Kassel-Marcus/master equation investigation of SiH4 decomposition kinetics using a kinetic Monte Carlo approach

The unimolecular reaction of decomposition of SiH(4) to SiH(2) and H(2) and the bimolecular reaction between SiH(3) and H were investigated by solving the master equation using a stochastic kinetic Monte Carlo (KMC) approach. Rice-Ramsperger-Kassel-Marcus (RRKM) microcanonical kinetic constants were determined using classic transition state theory for the reaction of decomposition to SiH(2) and H(2) and microcanonical J-resolved variational transition state theory for decomposition to SiH(3) and H. Structures of reactants and transition states were determined at the B3LYP/aug-cc-pVTZ level, while energies were calculated at the CCSD(T) level and extended to the complete basis set limit. Unimolecular kinetic constants were directly computed from the results of KMC simulations using a new algorithm while bimolecular rate constants were calculated from stochastic reaction probabilities. The simulation results are in good agreement with experimental data for the unimolecular decomposition of SiH(4), which is in the falloff regime in the temperature (1100-1700 K) and pressure (10(-3)-10(1) bar) range investigated. The calculated high and low pressure limit kinetic constants for SiH(4) decomposition to SiH(2) and H(2) are k(infinity)=1.2x10(13)T(0.477) exp(-28 988/T) and k(0)=1.4x10(42)T(-7.245) exp(-33 153/T). The calculated Troe falloff parameter is F(cent)=0.979 exp(-T/1427)+0.021 exp(T/1489). The rate of the bimolecular reaction between SiH(3) and H to give SiH(2) and H(2) is pressure independent between 10(-3) and 100 bar and slightly temperature dependent between 300 and 2000 K. The kinetic constant interpolated in this temperature and pressure range is 6.9x10(11)T(0.736) exp(134.8/T(K)) cm(3) mol(-1) s(-1), which is among the highest values proposed in the literature for this process.

Determination of the rate of H + O2 + M → HO2 + M (M = N2, Ar, H2O) from ignition of syngas at practical conditions

In H2 and H2/CO oxidation, the H+O2+M termination step is one of the most important reactions at elevated pressures. With the recent, increased interest in synthetic fuels, an accurate assessment of its rate coefficient becomes increasingly important, especially for real fuel/air mixtures. Ignition delay times in shock-tube experiments at the conditions selected in this study are only sensitive to the rates of the title reaction and the branching reaction H+O2=OH+O, the rate of which is known to a high level of accuracy. The rate coefficient of the title reaction for M=N2, Ar, and H2O was determined by adjusting its value in a detailed chemical kinetics model to match ignition delay times for H2/CO/O2/N2, H2/CO/O2/Ar, and H2/CO/O2/N2/H2O mixtures with fuel/air equivalence ratios of ϕ=0.5, 0.9, and 1.0. The rate of H+O2+N2=HO2+N2 was measured to be 2.7 (−0.7/+0.8)×1015cm6/mol2s for T=916–1265K and P=1–17atm. The present determination agrees well with the recent study of Bates et al. [R.W. Bates, D.M. Golden, R.K. Hanson, C.T. Bowman, Phys. Chem. Chem. Phys. 3 (2001) 2337–2342], whose rate expressions are suggested herein for modeling the falloff regime. The rate of H+O2+Ar=HO2+Ar was measured to be 1.9×1015cm6/mol2s for T=932–965K and P=1.4atm. The rate of H+O2+H2O=HO2+H2O was measured to be 3.3×1016cm6/mol2s for T=1071–1161K and P=1.3atm. These are the first experimental measurements of the rates of the title reactions in practical combustion fuel/air mixtures.

High-energy string scatterings of compactified open string

We calculate high-energy massive string scattering amplitudes of compactified open string. We derive infinite linear relations, or stringy symmetries, among soft high-energy string scattering amplitudes of different string states in the Gross kinematic regime (GR). In addition, we systematically analyze all hard power-law and soft exponential fall-off regimes of high-energy compactified open string scatterings by comparing the scatterings with their 26D noncompactified counterparts. In particular, we discover the existence of a power-law regime at fixed angle and an exponential fall-off regime at small angle for high-energy compactified open string scatterings. The linear relations break down as expected in all power-law regimes. The analysis can be extended to the high-energy scatterings of the compactified closed string, which corrects and extends the previous results in [J.C. Lee, Y. Yang, Linear relations and their breakdown in high energy massive string scatterings in compact spaces, Nucl. Phys. B 784 (2007) 22].