Estimates Of Rate Coefficients Research Articles

Can we estimate H$\mathsf{_{2}}$(j= 0) rate coefficients from He rate coefficients? Application to the SiS molecule

Context. Over the next few years, the ALMA and Herschel missions will perform high spatial and spectral resolution studies at infrared and sub-millimeter wavelengths. Modeling of molecular emission requires excitation calculations using radiative, as well as collisional rates, with the most abundant species. In the interstellar medium, the dominant collision partner is H2, but little data is available for collisions with H2. If data for collisions with He are available, it has often been proposed to use the more available rate coefficients for collision with He, with the appropriate reduced mass correction, as a first estimate of rate coefficients with H2(j = 0). The validity of this approximation is not known. Aims. The present paper focuses on the calculation of rate coefficients among the first rotational levels of the SiS molecule in its ground vibrational state in collision with para-H2 and compares these new data with recently published He ones to investigate the validity of using He rate coefficients to estimate H2(j = 0) rate coefficients. Methods. A new potential energy surface for the SiS-para-H2 system was obtained using highly correlated ab initio calculations. Dynamical calculations of pure rotational (de)excitation of SiS by para-H2 were performed for the first rotational levels within the coupled-states approximation. Results. Collisional cross sections among the 51 first rotational levels of SiS were calculated for kinetic energies up to 2500 cm −1 . State-to-state rate coefficients are calculated for temperatures ranging from 5 K up to 300 K. A propensity rule that favors even ∆j transitions is found and is explained by the near homonuclear symmetry of the SiS-para-H2 potential energy surface. A detailed comparison with recent SiS-He rate coefficients is also presented. We demonstrate that collision with He is a reasonable model for collisions with para-H2, although this approximation must be used with caution.

Predicting arene rate coefficients with respect to hydroxyl and other free radicals in the gas-phase: a simple and effective method using a single topological descriptor

Abstract. The reactivity of aromatic compounds is of great relevance to pure and applied chemical disciplines, yet existing methods for estimating gas-phase rate coefficients for their reactions with free radicals lack accuracy and universality. Here a novel approach is taken, whereby strong relationships between rate coefficients of aromatic hydrocarbons and a Randić-type topological index are investigated, optimized and developed into a method which requires no specialist software or computing power. Measured gas-phase rate coefficients for the reaction of aromatic hydrocarbons with OH radicals were correlated with a calculated Randić-type index, and optimized by including a term for side chain length. Although this method is exclusively for use with hydrocarbons, it is more diverse than any single existing methodology since it incorporates alkenylbenzenes into correlations, and can be extended towards other radical species such as O(3P) (and tentatively NO3, H and Cl). A comparison (with species common to both techniques) is made between the topological approach advocated here and a popular approach based on electrophilic subsituent constants, where it compares favourably. A modelling study was carried out to assess the impact of using estimated rate coefficients as opposed to measured data in an atmospheric model. The difference in model output was negligible for a range of NOx concentrations, which implies that this method has utility in complex chemical models. Strong relationships (e.g. for OH, R2=0.96) between seemingly diverse compounds including benzene, multisubstituted benzenes with saturated, unsaturated, aliphatic and cyclic substitutions and the nonbenzenoid aromatic, azulene suggests that the Randić-type index presented here represents a new and effective way of describing aromatic reactivity, based on a quantitative structure-activity relationship (QSAR).

Charge transfer reactions in Xe plasma expansion

Charge transfer reactions of fast Xe ions with hydrocarbons including methane (CH4), ethene (C2H4), and propane (C3H8) are studied by adding these hydrocarbon gases into a cross flowing Xe plasma expansion. Branching ratios and relative reaction rates for the charge transfers of fast Xe+ with each of the three hydrocarbon gases are measured under different rf powers of the inductively coupled Xe discharge. For CH4∕Xe system, we find that fast Xe+ reacts readily with CH4 generating CH4+ and CH3+ in a ratio of 1:0.56, with an estimated rate coefficient of (2.3±0.3)×10−10cm3∕s at 75W rf power which slowly increases to (2.9±0.3)×10−10cm3∕s at 250W (error bars reflect only the uncertainties due to the unknown extent of the ion recombination that follows the charge transfer reaction). These observed charge transfer reactions are made possible by the kinetically excited Xe ions produced by free expansion of the plasma. For the C2H4∕Xe system product ions C2H4+ and C2H2+ are observed, and for C3H8∕Xe, C2H4+ and C2H5+ and minor product ions including C2H2+ and C3H7+ are observed.

The Reaction of Nitrogen Atoms with Methyl Radicals: Are Spin-Forbidden Channels Important?

A computational study of the N(4S) + CH3 reaction has been carried out. The reactants approach through an attractive potential surface leading to an intermediate, H3CN, whose formation does not involve any barrier. In agreement with the experimental results, the dominant channel for this reaction is H2CN+H. The theoretically estimated rate coefficient for the overall process at 298 K is 9.1 x 10(-12) cm3 s(-1) molecule(-1), which is nearly 1 order of magnitude lower than the experimental result, but also much larger than those computed for the reactions of ground-state nitrogen atoms with halomethyl radicals. The analysis of the singlet potential energy surface, and the corresponding computational kinetic study, shows that for the reaction of excited nitrogen atoms with methyl radicals, the preferred product from the kinetic point of view is also H2CN+H, but in this case production of HCN is significant (with branching ratios around 0.185). According to our calculations, spin-forbidden processes are highly unlikely for the N(4S) + CH3 reaction. However, further evolution of the preferred products, H2CN+H, might explain the experimental observation of hydrogen cyanide as a minor product in this reaction.

Can topological indices be used to predict gas-phase rate coefficients of importance to tropospheric chemistry? Reactions of alkenes with OH, NO 3 and O 3

Rate coefficients for the gas-phase reactions of unsaturated hydrocarbons with OH, NO 3 and O 3 are of vital importance to atmospheric modelling. Many of these rate coefficients are unknown – possibly resulting from the variety of these compounds and the inherent expense of deriving these data experimentally – and a reliable method for their prediction would therefore be of value to such models. This study presents a method for estimating rate coefficients for C 2–C 10 olefins. Measured rate coefficients for the reaction of unsaturated hydrocarbons with OH and NO 3 radicals and O 3 are correlated with the Randić topological descriptor and an established correlation parameter, i.e. ionization potentials calculated using the frontier molecular orbital (FMO) approach. Although the ionization potential method produces better correlations in general, OH correlations of aliphatic species, subdivided into groups of varying total number of primary (1°), secondary (2°) and tertiary (3°) carbon atoms ( ∑ x ° x ) produced several new trends with the Randić index that were not present in the ionization potential correlations. Strong correlation was observed with a Randić-type index optimized to include a term for side chain length ( ∑ x ° x ) . The presence of strong relationships, made predictable by molecular constitution, makes topological descriptors a useful and accessible tool for estimating rate coefficients for the reactions of OH with aliphatic species. This study indicates that alkene reactivity is affected strongly by constitution and that abstraction as well as addition may be important for some classes of compound.

Propagation of uncertainty in chemically activated systems

AbstractChemically activated systems play an important role in combustion and atmospheric chemistry. The overall reaction paths exhibit complex pressure and temperature dependencies because each intermediate involves a coupled system of competing multistep isomerization, dissociation, and stabilization paths. A number of estimation techniques exist for deriving the required thermochemical and elementary kinetic input parameters for rate coefficient estimation. The availability of high‐level ab initio methods promises to reduce the inaccuracies associated with older empirical methods. The objective of this study is to evaluate the importance of the various thermochemical parameters entering the rate coefficient calculation. We describe and illustrate a computational framework to quantify the functional relationship between the thermochemical properties and the macroscopic observables through appropriate response surface methods. The approach is demonstrated by analyzing the impact of thermochemical properties in estimating autoignition delays in propane oxidation. © 2006 American Institute of Chemical Engineers AIChE J, 2006

Volume production of D− negative ions in low-pressure D2 plasmas—negative ion densities versus plasma parameters

Pure volume production of D− negative ions is studied in a rectangular arc chamber. Production and control of D2 plasmas are tested by varying the intensity of the magnetic filter MF. The values of electron temperature Te and electron density ne in D2 plasmas are slightly higher than those in H2 plasmas. Te in D2 plasmas cannot be decreased below 1 eV in the extraction region with the same MF conditions for optimizing H2 plasmas. A stronger MF field is required for controlling Te in D2 plasmas. Therefore, the transport in or the production of H2 and D2 plasmas is different, i.e. there is an isotope effect, a consequence of which is that the H− and D− densities have different spatial distributions. Extracted H− and D− currents are primarily determined by H− and D− densities in front of the extraction hole, respectively. According to the estimated rate coefficients and collision frequencies of main collision processes, it is reconfirmed that Te in the extraction region should be maintained below 1 eV with ne kept high for the enhancement of H− and D− production. For the enhancement of D− production, preliminary results on VUV emission measurement are presented and discussed briefly.

Can topological indices be used to predict gas-phase rate coefficients of importance to tropospheric chemistry? Free radical abstraction reactions of alkanes

As tropospheric chemical models become ever more complex, reliable rate coefficient data for gas-phase hydrogen abstraction reactions involving trace hydrocarbons becomes increasingly necessary. There are hundreds of non-methane hydrocarbons (NMHCs) of potential importance to tropospheric chemistry, many of which have not been subjected to experimental inquiry. This study—the first of its kind to investigate hydrocarbons ranging from C 2 to C 10 using topological indices—aims to provide a reliable and accessible method of estimating rate coefficients for these species. Rate coefficients of free radical abstraction reactions of NMHCs were correlated with the Randić and Balaban topological indices for radical moieties OH, Cl and NO 3. These correlations were compared with those of an established frontier orbital approach based on calculated ionization potentials (IPs). The Randić index was found to correlate better than IP for each of the radicals studied, and correlated particularly effectively for the Cl radical. The Balaban index did not correlate for branched alkanes except for NO 3. Where only unbranched alkanes were considered, the Balaban index proved most reliable, demonstrating a clear linear relationship. Topological indices present an accurate and diverse method for estimating free radical abstraction rate coefficients that does not require the computing power, specialist software packages, or complex mathematics inherent in ab initio calculations. A further advantage of using topological indices is that they are calculated in an unambiguous manner. More reliable tools for estimating rate coefficients have direct implications for improving models, and may also provide a direction for future laboratory work, either by highlighting particularly reactive species or identifying potentially spurious rate coefficient data.

Correlations for gas-phase reactions of NO 3, OH and O 3 with alkenes: An update

Methods are developed for predicting rate coefficients for reactions of initiators of tropospheric oxidation with unsaturated compounds that are abundant in the atmosphere; prognostic tools of this kind are essential for atmospheric chemists and modellers. To pursue the aim of exploring such tools, the kinetics of reactions of NO 3, OH and O 3 with a series of alkenes are examined for correlations relating the logarithms of the rate coefficients to the energies of the highest occupied molecular orbitals (HOMOs) of the alkenes. A comparison of the values predicted by the correlations with experimental data (where the latter exist) allowed us to assess the reliability of our method. We used a series of theoretical methods to calculate the HOMO energies, and found that higher computational effort improves the agreement of the predicted rate coefficients with experimental values, especially for reactions of NO 3 with alkenes that possess vinyllic halogen substituents. As a consequence, it is expedient to suggest new correlations to replace those presented by us and others that were based on the lower level of theory. We propose the following correlations for the reactions of NO 3, OH and O 3 with alkenes: ln( k NO 3 /cm 3 molecule −1 s −1)=6.40( E HOMO/eV)+31.69, ln( k OH/cm 3 molecule −1 s −1)=1.21( E HOMO/eV)−12.34 and ln( k O 3 /cm 3 molecule −1 s −1)=3.28( E HOMO/eV)−6.78. These new correlations have been developed using the larger experimental data sets now available, and the impact of the extended data on the quality of the correlations is examined in the paper. Atmospheric lifetimes have been calculated from both experimental and estimated rate coefficients to provide an overview of removal efficiencies for different classes of alkenes with respect to oxidative processes initiated by NO 3, OH and O 3. A figure is presented to show the spatial scales over which alkenes may survive transport in competition with attack by NO 3, OH and O 3. Removal by NO 3 or OH is always more important than removal by O 3, and reactions with NO 3 dominate for scales up to a few hundred metres.

Structure–activity relations (SARs) for gas-phase reactions of NO 3, OH and O 3 with alkenes: An update

We present updated structure–activity relations (SARs) for the prediction of rate coefficients for gas-phase reactions with alkenes of the major atmospheric oxidants NO 3, OH and O 3. Such SARs provide one way of incorporating essential information about reactivity into atmospheric models. Rate coefficients obtained from correlations relating the logarithms of the rate coefficients to the energies of the highest occupied molecular orbitals (HOMOs) of the alkenes were used to refine the SARs. SARs have an advantage for the user over the direct application of the correlations in that knowledge of the structure of the alkene of interest is sufficient to estimate rate coefficients, and no quantum-mechanical calculations need to be performed. A comparison of the values predicted by the SARs with experimental data where they exist allowed us to assess the reliability of our method.

Kinetics and mechanisms of gas phase elimination of ethyl 1‐piperidine carboxylate, ethyl pipecolinate, and (revisited) ethyl 1‐methyl pipecolinate

AbstractThe kinetics of the gas‐phase elimination of the title compounds has been determined in a static reaction system over the temperature range of 340–420°C and pressure range of 45–96 Torr. The reactions proved to be homogeneous, unimolecular, and obey a first‐order rate law. The estimated rate coefficients are represented by the following Arrhenius expressions: Ethyl 1‐piperidine carboxylate Ethyl pipecolinate Ethyl 1‐methyl pipecolinate The first step of decomposition of these esters is the formation of the corresponding carboxylic acids and ethylene. The acid intermediate undergoes a very fast decarboxylation process. The mechanism of this elimination reactions is suggested on the basis of the kinetic and thermodynamic parameters. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 383–389, 2005

Reaction coordinates and rates from transition paths.

The molecular mechanism of a reaction in solution is reflected in its transition-state ensemble and transition paths. We use a Bayesian formula relating the equilibrium and transition-path ensembles to identify transition states, rank reaction coordinates, and estimate rate coefficients. We also introduce a variational procedure to optimize reaction coordinates. The theory is illustrated with applications to protein folding and the dipole reorientation of an ordered water chain inside a carbon nanotube. To describe the folding of a simple model of a three-helix bundle protein, we variationally optimize the weights of a projection onto the matrix of native and nonnative amino acid contacts. The resulting one-dimensional reaction coordinate captures the folding transition state, with formation and packing of helix 2 and 3 constituting the bottleneck for folding.

Formation of Chloroform and Chlorinated Organics by Free-Chlorine-Mediated Oxidation of Triclosan

The widely used antimicrobial agent triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol) readily reacts with free chlorine under drinking water treatment conditions. Overall second-order kinetics were observed, first-order in free chlorine and first-order in triclosan. Over the pH range of 4-11.5, the kinetics were pH sensitive as a result of the pH dependent speciation of both triclosan and free chlorine. Using a Marquardt-Levenberg routine, it was determined that this pH effect indicates that the dominant reaction in this system is between the ionized phenolate form of triclosan and hypochlorous acid (HOCl). The overall second-order rate coefficient was determined to be kArO- = 5.40 (+/- 1.82) x 10(3) M(-1) s(-1). Three chlorophenoxyphenols and two chlorophenols were identified by gas chromatographic-mass spectroscopic analysis. The chlorophenoxyphenol compounds include two monochlorinated triclosan derivatives (5,6-dichloro-2-(2,4-dichlorophenoy)phenol and 4,5-dichloro-2-(2,4-dichlorophenoxy)phenol) and one dichlorinated derivative (4,5,6-trichloro-(2,4-dichlorophenoxy)phenol); these species form via bimolecular electrophilic substitution of triclosan. 2,4-Dichlorophenol was detected under all reaction conditions and forms via ether cleavage of triclosan. In experiments with excess free chlorine, 2,4,6-trichlorophenol was formed via electrophilic substitution of 2,4-dichlorophenol. Chloroform formation was observed when an excess of free chlorine was present. A Hammett-type linear free-energy relationship (LFER) using Brown-Okamoto parameters (sigma+) was established to correlate the reactivity of HOCI and the phenolate forms of triclosan and other chlorophenols (log kArO- = -(10.7 +/- 2.2)Sigmasigma(+)o,m,p + 4.43). This LFER was used to obtain estimates of rate coefficients describing the reactivity of the intermediates 5,6-dichloro-2-(2,4-dichlorophenoy)phenol (kArO- approximately equal to 6 x 10(2)), 4,5-dichloro-2-(2,4-dichlorophenoxy)phenol (kArO- approximately equal to 3 x 10(2)), and 4,5,6-trichloro-(2,4-dichlorophenoxy)phenol (kArO- approximately equal to 4 x 10(1)).

Computational modeling of the SiH 3+O 2 reaction and silane combustion

Recent theoretical and experimental studies have provided improved estimates of thermochemical and chemical kinetic data for the silicon–hydrogen–oxygen system. In particular, the SiH 3+O 2 reaction has been the subject of considerable interest. Estimates of rate coefficients for specific SiH 3+O 2 channels as a function of temperature and of pressure and branching fraction assignments are made in the current work using Quantum Rice–Ramsperger–Kassel (QRRK) analyses for k( E) and Master Equation (ME) analyses for fall-off. The QRRK/ME analyses were based on potential energy surface data provided by previous ab initio studies. The overall rate coefficient for SiH 3+O 2→ products shows a slight negative temperature dependence, which agrees with previous experimental studies. The results indicate significant pressure and temperature dependence for the product branching fractions and are in good agreement with experimental measurements of the SiH 3+O 2 product channels. In particular, the channel producing H atoms (SiH 3+O 2→cyclic-OSiH 2O+H) is dominant at high temperatures and/or low pressures, and the O-atom channel (SiH 3+O 2→H 3SiO+O) is not significant (>5% branching fraction) below 1000 K at any pressure examined in the QRRK study ( P=0.001–10 atm). A detailed chemical mechanism for silane combustion is presented based on the QRRK estimates and other evaluated rate coefficient and thermochemical parameters. The mechanism is validated at high temperatures by comparison of calculated ignition delay times with shock-tube data and by comparison of calculated OH radical profiles with atmospheric pressure burner data. The mechanism is validated at low temperatures by comparison of ignition times with constant volume explosion data.

Uncatalyzed and ruthenium(III)‐catalyzed reaction of acidic chlorite with methylene violet

AbstractThe kinetics and mechanism of the uncatalyzed and Ru(III)‐catalyzed oxidation of methylene violet (3‐amino‐7‐diethylamino‐5‐phenyl phenazinium chloride) (MV+) by acidic chlorite is reported. With excess concentrations of other reactants, both uncatalyzed and catalyzed reactions had pseudo‐first‐order kinetics with respect to MV+. The uncatalyzed reaction had first‐order dependence on chlorite and H+ concentrations, but the catalyzed reaction had first‐order dependence on both chlorite and catalyst, and a fractional order with respect to [H+]. The rate coefficient of the uncatalyzed reaction is (5.72 ± 0.19) M−2 s−1, while the catalytic constant for the catalyzed reaction is (22.4 ± 0.3) × 103 M−1 s−1. The basic stoichiometric equation is as follows: 2MV+ + 7ClO2− + 2H+ = 2P + CH3COOH + 4ClO2 + 3Cl−, where P+ = 3‐amino‐7‐ethylamino‐5‐phenyl phenazinium‐10‐N‐oxide. Stoichiometry is dependent on the initial concentration of chlorite present. Consistent with the experimental results, pertinent mechanisms are proposed. The proposed 15‐step mechanism is simulated using literature; experimental and estimated rate coefficients and the simulated plots agreed well with the experimental curves. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 294–303, 2003

Dependence of termination kinetics in methyl acrylate-dodecyl acrylate free-radical copolymerization on comonomer composition, pressure, temperature and conversion

The termination kinetics of the free-radical bulk copolymerisation of dodecyl acrylate (DA) and methyl acrylate (MA) has been investigated at various monomer mole fractions between 15 and 50°C and up to 2000 bar. The ratio of termination to propagation rate coefficients, (k t /k p ) copo , is measured via the single pulse-pulsed laser polymerisation (SP-PLP) technique. Chain-length averaged k t,copo are deduced from (k t /k p ) copo in conjunction with k t,copo data that are estimated via a simplified version of the implicit penultimate unit effect (IPUE) model. At low and moderate degrees of monomer conversion extended ranges of almost constant k t are observed where termination is controlled by segmental diffusion with important contributions of steric effects. These plateau k t values are significantly - by almost two orders of magnitude - different for MA and DA. The increase with MA content of k t,copo is adequately described by a penultimate unit model which uses the geometric mean approximation for estimating rate coefficients of cross-termination between radicals of different free-radical terminus. The model applies within the entire pressure and temperature range of the present study. At high degrees of monomer conversion, at and above 50%, homo-k t and k t,copo are almost insentive toward the composition of the monomer mixture. The termination rate under these conditions is essentially controlled by reaction diffusion.

Coupled production and transport of selenium vapor in unsaturated soil: evaluation by experiments and numerical simulation

Volatilization of selenium (Se) from soil to the atmosphere involves several sequential chemical reactions that form volatile Se species, followed by transport of the gaseous Se through the soil. This paper describes a numerical model that simulates the chemical and physical processes governing the production and transport of Se vapor in unsaturated soil. The model couples the four Se species involved in the production of Se vapor through chemical reactions, and allows each to migrate through the soil by advection, liquid or vapor diffusion depending on its affinity for the dissolved or vapor phase. The coupled transformations and transport of the four Se species, i.e., selenate, selenite, elemental and organic Se, and Se vapor, were calculated based on the Crank–Nicolson finite difference method. The model was used to analyze fluxes of Se vapor measured from a soil amended with inorganic Se in the form of selenate and covered with unamended clean soil of various thicknesses. Evolution of Se vapor from the soil was very fast, with measurable amounts of Se detected within 24 h. The peak of Se volatilization, detected at the 6th day, reached 3.31 Se μg/day for the uncovered soil, but was reduced to near the detection limit (0.05 μg/day) in the presence of a 8- or 16-cm clean soil cover. With two reaction rate coefficients fitted to the data, the model described Se volatilization very well. The estimated rate coefficient of Se methylation was unexpectedly high, with a value of 0.167/day. The net volatilization of Se, however, was severely inhibited by the fast demethylation, i.e., the reverse reaction which converted volatile Se species back into nonvolatile forms. As a result, Se vapor only penetrated a few centimeters in the soil. The demethylation rate coefficient, assessed by independent transport experiments using dimethyl selenide, was estimated as 186.8/day, corresponding to a half-life of only 5.3 min for Se vapor. Results of this study indicated that rapid demethylation of Se vapor during its diffusive transport through a soil is probably an important limiting factor in the volatilization of Se under natural conditions.

Kinetic study of OH and NO3 radical reactions with 14 aliphatic aldehydes

Rate coefficients for the reactions of NO3 and OH radicals with 14 aliphatic C2 to C6 aldehydes in purified air at 298 ± 2 K and 1.00 ± 0.01 atm have been determined by the relative rate method using a static reactor equipped with long-path FTIR detection. The aldehydes studied comprise: C2—acetaldehyde; C3—propanal; C4—butanal and 2-methylpropanal; C5—pentanal, 2-methylbutanal, 3-methylbutanal and 2,2-dimethyl-propanal; C6—hexanal, 2-methyl-pentanal, 3-methylpentanal, 4-methylpentanal, 3,3-dimethylbutanal, and 2-ethylbutanal. The new data establish that the gas-phase reactivity of aliphatic aldehydes towards the NO3 and OH radicals follow the linear free energy relationship typical of addition reactions although the net result of the reactions is a H-abstraction. The experiments also indicate that more than 95% of the room temperature NO3 reaction with aliphatic aldehydes proceeds through an abstraction of the aldehydic hydrogen. The structure activity relationship for estimation of rate coefficients of the NO3 radical reaction with saturated organics (J. Phys. Chem. Ref. Data, 1991, 20, 45) fails completely for aliphatic aldehydes.

Measurement of Absolute Unimolecular and Bimolecular Rate Constants for CH3CHOO Generated by thetrans-2-Butene Reaction with Ozone in the Gas Phase

Ozone−alkene reactions form vibrationally excited Criegee intermediates (of the form R1R2COO), some of which, once thermalized, are thought to react with SO2, H2O, NOx, aldehydes, and alcohols. Several studies using relative rate techniques or ab initio calculations have resulted in estimates for the rate coefficients of reactions of the thermalized biradicals. The ranges of measured and estimated rate coefficients span 2−6 orders of magnitude, depending on the reaction partner. Using an atmospheric pressure flow reactor, we have made the first absolute rate coefficient determinations for reactions of a thermalized Criegee intermediate, measuring rates for unimolecular decomposition and reaction with acetaldehyde. For the thermalized CH3CHOO formed in trans-2-butene ozonolysis, values for kdec = 76 s-1 and kald = 1.0 × 10-12 cm3 molecule-1 s-1, accurate to within a factor of 3 and 6, respectively, were obtained.

Kinetic Analysis of Complex Chemical Activation and Unimolecular Dissociation Reactions using QRRK Theory and the Modified Strong Collision Approximation

A method to predict temperature and pressure-dependent rate coefficients for complex bimolecular chemical activation and unimolecular dissociation reactions is described. A three-frequency version of QRRK theory is developed and collisional stabilization is estimated using the modified strong-collision approximation. The methodology permits analysis of reaction systems with an arbitrary degree of complexity in terms of the number of isomer or product channels. Specification of both high and low pressure limits is also provided. The chemically activated reaction of vinyl radical with molecular oxygen is used to demonstrate the approach. Subsequent dissociation of the stabilized vinyl peroxy radical is used to illustrate prediction of dissociation rate coefficients. These calculations confirm earlier results that the vinoxy + O channel is dominant under combustion conditions. The results are also consistent with RRKM results using the same input conditions. This approach provides a means to provide reasonably accurate predictions of the rate coefficients that are required in many detailed mechanisms. The major advantage is the ability to provide reasonable estimates of rate coefficients for many complex systems where detailed information about the transition states is not available. It is also shown that a simpler 1-frequency model appears adequate for high temperature conditions.