Strong collision rate coefficients in unimolecular reaction rate theory

The traditional and a recently proposed renormalized approximation to the rate coefficient obtained from a strong collision master equation are derived and compared. It is shown that they deviate significantly when applied to the unimolecular reactions of large molecules with low activation energies at elevated temperatures. The derivation is extended to yield a method whereby the exact rate coefficient can be calculated without increasing the level of numerical effort. An analysis of the exact rate coefficient in the high and low collision frequency limits is found to verify the validity of the traditional approximate rate coefficient in these limits. The renormalized rate coefficient fails in the low frequency limit. Both approximations can be significantly in error at intermediate collision frequencies and the use of the exact expression is recommended.     

Dynamical properties and thermal rate coefficients for the Na + HF reaction using genetic algorithm

The genetic algorithm optimization technique (GAOT) was used to build a new potential energy surface (PES) to the Na + HF → NaF + H reaction. Quasi‐Classical Trajectories and Transition State Theory methods were used to obtain the dynamical properties and thermal rate coefficients (TRCs), respectively, of this new PES. These features were compared with the dynamical properties and TRCs available in the literature. It was found that the GAOT PES agrees very well with other PESs, in which the maximum difference found is smaller than 1.0 Ų for the cross‐sections. These results endow the GAOT approach as a method to build PESs of reactive scattering processes. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Non-equilibrium ion-(polar) molecule reaction rate coefficients

Using the Fokker-Planck version of an approximate Boltzmann equation for the ion (translational) energy distribution function f_I we have calculated the deviation, k, of the non-equilibrium ion-(polar)molecule reaction rate coefficient k (based) on f_I from its equilibrium value k⁽⁰⁾. The _D (dipole moment)-dependence of the reaction cross section applied leads to a corresponding dependence of k on _D.

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- , k - ( f_I) k⁽⁰⁾. k _D.

     

The O + O2 reaction: quantum detailed probabilities and thermal rate coefficients

The detailed quantum probabilities of the O + O₂ reactive system have been computed at zero total angular momentum using the time-independent quantum program ABC thanks to the restructuring of the code and its implementation on the EGEE production Grid. Their main features are discussed and out of them J-shifting thermal rate coefficients have been computed to compare with the experiment and quasiclassical trajectory results over a wide range of temperatures.     

Relative sensitivity coefficients for the analysis of steel by spark-source mass-spectrometry

Quantitative analysis by spark-source mass-spectrometry requires the knowledge of socalled sensitivity coefficients for the elements being determined. Five series of analyses have been carried out on five different steel standard reference materials (NBS-SRM 661-665), using photoplate detection. The relative sensitivity coefficients (S(R)) of Ti, V, Cr, Mn, Co, Ni, Cu, As, Zr, Nb, Mo, Sn, Sb, La, Ta and W were determined vs. iron as an internal standard. The S(R) values were independent of the elemental concentration. A relative standard deviation of about 15% was obtained. The accuracy as confirmed by comparing the results for a pure iron sample with those obtained by neutron-activation analysis was within the same limits.     

High temperature reaction rate coefficients derived from N-atom ARAS measurements and excimer photolysis of NO

Mixtures of NO and NO/H₂ in Ar were shock-heated and photolyzed with an ArF excimer laser. Measurements in these experiments of N-atom profiles using atomic resonance absorption spectrophotometry (ARAS) permitted the determination of two rate coefficients. The rate coefficient for the reaction was found to be 4.29 × 10¹³ exp(?787/T) cm³ mol^?1 sec^?1 (±20% at 1400 K to ±10% at 3500 K). This is the first direct high temperature measurement of this rate coefficient in the exothermic direction. The rate coefficient for the reaction was found to be 1.60 × 10¹⁴ exp(?12650/T) (±35% from 1950 to 2850 K). To our knowledge, this is the first direct measurement of this rate coefficient. A study of the N-atom ARAS absorption behavior revealed a noticeable pressure dependence, as well as a weak temperature dependence, in the Beer-Lambert law absorption coefficient. Proper consideration of these effects is important when the N-atom ARAS diagnostic is used for absolute concentration measurements.     

Temperature dependence of the rate coefficients for the reaction of chlorine atoms with chloromethanes

Rate coefficients for the reaction of Cl atoms with CH₃Cl (k₁), CH₂Cl₂ (k₂), and CHCl₃ (k₃) have been determined over the temperature range 222–298 K using standard relative rate techniques. These data, when combined with evaluated data from previous studies, lead to the following Arrhenius expressions (all in units of cm³ molecule⁻¹ s⁻¹): k₁ = (2.8 ± 0.3) × 10⁻¹¹ exp(−1200 ± 150/T); k₂ = (1.5 ± 0.2) × 10⁻¹¹ exp(−1100 ± 150/T); k₃ = (0.48 ± 0.05) × 10⁻¹¹ exp(−1050 ± 150/T). Values for k₁ are in substantial agreement with previous measurements. However, while the room temperature values for k₂ and k₃ agree with most previous data, the activation energies for these rate coefficients are substantially lower than previously recommended values. In addition, the mechanism of the oxidation of CH₂Cl₂ has been studied. The dominant fate of the CHCl₂O radical is decomposition via Cl‐atom elimination, even at the lowest temperatures studied in this work (218 K). However, a small fraction of the CHCl₂O radicals are shown to react with O₂ at low temperatures. Using an estimated value for the rate coefficient of the reaction of CHCl₂O with O₂ (1 × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹), the decomposition rate coefficient for CHCl₂O is found to be about 4 × 10⁶ s⁻¹ at 218 K, with the barrier to its decomposition estimated at 6 kcal/mole. As part of this work, the rate coefficient for Cl atoms with HCOCl was also been determined, k₇ = 1.4 × 10⁻¹¹ exp(−885/T) cm³ molecule⁻¹ s⁻¹, in agreement with previous determinations. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 515–524, 1999     

Estimation of penetrant transport properties through fixed site carrier membranes using the RC circuit model and sensitivity analysis

Gas permeability through fixed site carrier membranes (FSCMs) is predicted by the RC circuit models. If only one permeability value is available, other permeability values can readily be estimated as a function of applied pressure for a FSCM with given backward reaction rate and reaction equilibrium constants between carrier and penetrant, carrier concentration, and matrix permeability. The results were compared with experimental oxygen permeabilities through a PMMA membrane containing metallophorphyrin. The agreement between them was exceptional. In addition, the backward reaction rate constant between carrier and penetrant and the pressure fluctuation are found to be most sensitive in determining permeability in FSCMs according to the sensitivity analysis.     

Site-specific rate coefficients for reaction of OH with ethanol from 298 to 900 K

The rate coefficients for reactions of OH with ethanol and partially deuterated ethanols have been measured by laser flash photolysis/laser-induced fluorescence over the temperature range 298-523 K and 5-100 Torr of helium bath gas. The rate coefficient, k(1.1), for reaction of OH with C(2)H(5)OH is given by the expression k(1.1) = 1.06 × 10(-22)T(3.58)?exp(1126/T) cm(3) molecule(-1) s(-1), and the values are in good agreement with previous literature. Site-specific rate coefficients were determined from the measured kinetic isotope effects. Over the temperature region 298-523 K abstraction from the hydroxyl site is a minor channel. The reaction is dominated by abstraction of the α hydrogens (92 ± 8)% at 298 K decreasing to (76 ± 9)% with the balance being abstraction at the β position where the errors are 2σ. At higher temperatures decomposition of the CH(2)CH(2)OH product from β abstraction complicates the kinetics. From 575 to 650 K, biexponential decays were observed, allowing estimates to be made for k(1.1) and the fractional production of CH(2)CH(2)OH. Above 650 K, decomposition of the CH(2)CH(2)OH product was fast on the time scale of the measured kinetics and removal of OH corresponds to reaction at the α and OH sites. The kinetics agree (within ±20%) with previous measurements. Evidence suggests that reaction at the OH site is significant at our higher temperatures: 47-53% at 865 K.     

Estimation of gas transport coefficients from differential permeation, integral permeation, and sorption rate data

Experimental methods for studying the transport of gases in polymers may be divided into three categories: integral permeation rate measurement, in which the cumulative amount of a penetrant that has passed through a membrane is determined; differential permeation rate measurement, in which the rate of penetration through a membrane is measured directly; and sorption rate measurement, or determination of the cumulative amount of a penetrant absorbed in a polymer sample. This paper reviews commonly used techniques for estimating diffusion coefficients from transport data of all three types. Several new estimation formulas are presented, and the relative merits of different measurement and estimation methods are discussed. A general relationship between the traditional time lag method for integral rate data analysis and a recently developed moment method for differential rate data analysis is established, extending the applicability of the moment approach to the analysis of non-ideal transport in membranes of arbitrary geometry and composition.     

A parametric study of electron energy distribution functions and rate and transport coefficients in nonequilibrium helium plasmas

Electron energy distribution functions in helium plasmas have been calculated by solving the Boltzmann equation at given values of reduced electric field, in the presence of superelastic and electron-electron collisions. Analytical expressions have been found connecting macroscopic coefficients to reduced electric field E/N, relative metastable concentration [He(2³S)]/N, and degree of ionization n_e/N.     

Adaptive approach for nonlinear sensitivity analysis of reaction kinetics

We present a unified approach for linear and nonlinear sensitivity analysis for models of reaction kinetics that are stated in terms of systems of ordinary differential equations (ODEs). The approach is based on the reformulation of the ODE problem as a density transport problem described by a Fokker-Planck equation. The resulting multidimensional partial differential equation is herein solved by extending the TRAIL algorithm originally introduced by Horenko and Weiser in the context of molecular dynamics (J. Comp. Chem. 2003, 24, 1921) and discussed it in comparison with Monte Carlo techniques. The extended TRAIL approach is fully adaptive and easily allows to study the influence of nonlinear dynamical effects. We illustrate the scheme in application to an enzyme-substrate model problem for sensitivity analysis w.r.t. to initial concentrations and parameter values.     

Measurements of thermal energy neon ion—neutral reaction rate coefficients and product ion distributions

Reaction rate coefficients and product ion distributions have been measured for the reaction of Ne⁺ with H₂, N₂, CO, CO₂, N₂O, CH₄, O₂, NO, NH₃, SO₂, CH₃Cl, COS, H₂O and C₂H₄ at 300 K using a selected ion flow tube (SIFT) apparatus. In most cases the major reaction channel involves dissociative ionization while for N₂, CO⁺, H₂O, CH₄ and CH₃Cl these reactions proceed mainly or exclusively by simple charge transfer. For H₂ the process is exclusively hydrogen atom abstraction. The measured rate coefficients are compared with the values given by the Langevin and average-dipole orientation theories of ion—molecule collisions. In general the reaction probability (ratio of measured rate to the Langevin or ADO rate) is greater for the dissociative ionizaton reactions, although H₂O is an exception with quite fast simple charge transfer.     

Theoretical rate coefficients for the exchange reaction OH + D--> OD + H

In this work quasiclassical trajectory calculations were carried out to determine directly the rate coefficients for the isotopic exchange reaction, OH + D-->OD + H, using a potential-energy surface that carefully accounts for the long-range interactions. The calculated thermal rate coefficients are in good agreement with the experimental results.     

Predicting the preexponential temperature dependence of bimolecular metathesis reaction rate coefficients using transition state theory

The thermochemical kinetics formulation of conventional transition state theory for bimolecular reactions allows for a separate contribution from each degree of freedom (translation, rotation, vibration, etc.) in the activated complex to the entropy and heat capacity of activation, and thus to the preexponential terms in the Arrhenius rate expression, k = ATⁿ exp(?B/T). The number of vibrations and (possibly hindred) internal rotations varies depending on the nature of the reaction: atom + diatom, diatom + linear polyatom, etc. The temperature exponent n can be evaluated explicitly for each type of reaction if the harmonic oscillator-rigid free rotor approximation is valid for the reagents and activated complex and if the contribution from tunneling is small. Various reaction types are examined successively, and n is evaluated for each case. The possible contributions of other factors (vibrational anharmonicity, hindered internal rotation, tunneling, “looseness” of activated complex) to the value of n are also considered.     

Rate sensitivity analysis of a model of the Briggs-Rauscher reaction

An updated mechanism for the Briggs-Rauscher reaction (also known as Iodine Clock reaction) has been investigated by the principal component analysis of the rate sensitivity matrix. The analysis revealed that five reactions of the 15-step model were redundant. The results of principal component and of rate-of-production analyses together gave an insight into the basic processes of the Iodine Clock reaction.

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- ( ) . , 5 15- . .

     

Communication: rate coefficients from quasiclassical trajectory calculations from the reverse reaction: The Mu + H2 reaction re-visited

In a previous paper [P. G. Jambrina et al., J. Chem. Phys. 135, 034310 (2011)] various calculations of the rate coefficient for the Mu + H(2) → MuH + H reaction were presented and compared to experiment. The widely used standard quasiclassical trajectory (QCT) method was shown to overestimate the rate coefficients by several orders of magnitude over the temperature range 200-1000 K. This was attributed to a major failure of that method to describe the correct threshold for the reaction owing to the large difference in zero-point energies (ZPE) of the reactant H(2) and product MuH (～0.32 eV). In this Communication we show that by performing standard QCT calculations for the reverse reaction and then applying detailed balance, the resulting rate coefficient is in very good agreement with the other computational results that respect the ZPE, (as well as with the experiment) but which are more demanding computationally.     

Low-temperature rate coefficients for the reaction of ethynyl radical (C2H) with benzene

The reaction of the C2H radical with benzene is studied at low temperature using a pulsed Laval nozzle apparatus. The C2H radical is prepared by 193-nm photolysis of acetylene, and the C2H concentration is monitored using CH(A2Delta) chemiluminescence from the C2H + O2 reaction. Measurements at very low photolysis energy are performed using CF3C2H as the C2H precursor to study the influence of benzene photodissociation on the rate coefficient. Rate coefficients are obtained over a temperature range between 105 and 298 K. The average rate coefficient is found to be five times greater than the estimated value presently used in the photochemical modeling of Titan's atmosphere. The reaction exhibits a slight negative temperature dependence which can be fitted to the expression k(cm3 molecule(-1) s(-1)) = 3.28(+/-1.0) x 10(-10) (T/298)(-0.18(+/-0.18)). The results show that this reaction has no barrier and may play an important role in the formation of large molecules and aerosols at low temperature. Our results are consistent with the formation of a short lifetime intermediate that decomposes to give the final products.     

Automatic estimation of pressure-dependent rate coefficients

A general framework is presented for accurately and efficiently estimating the phenomenological pressure-dependent rate coefficients for reaction networks of arbitrary size and complexity using only high-pressure-limit information. Two aspects of this framework are discussed in detail. First, two methods of estimating the density of states of the species in the network are presented, including a new method based on characteristic functional group frequencies. Second, three methods of simplifying the full master equation model of the network to a single set of phenomenological rates are discussed, including a new method based on the reservoir state and pseudo-steady state approximations. Both sets of methods are evaluated in the context of the chemically-activated reaction of acetyl with oxygen. All three simplifications of the master equation are usually accurate, but each fails in certain situations, which are discussed. The new methods usually provide good accuracy at a computational cost appropriate for automated reaction mechanism generation.