Rate Constants For Gas-phase Reactions Research Articles

Low-Temperature Gas-Phase Kinetics of Ethanol-Methanol Heterodimer Formation.

The structures of gas-phase noncovalently bound clusters have long been studied in supersonic expansions. This method of study, while providing a wealth of information about the nature of noncovalent bonds, precludes observation of the formation of the cluster, as the clusters form just after the orifice of the pulsed valve. Here, we directly observe formation of ethanol-methanol dimers via microwave spectroscopy in a controlled cryogenic environment. Time profiles of the concentration of reagents in the cell yielded gas-phase reaction rate constants of kMe-g = (2.8 ± 1.4) × 10-13 cm3 molecule-1 s-1 and kMe-t = (1.6 ± 0.8) × 10-13 cm3 molecule-1 s-1 for the pseudo-second-order ethanol-methanol dimerization reaction at 8 K. The relaxation cross section between the gauche and trans conformers of ethanol was also measured using the same technique. In addition, thermodynamic relaxation between conformers of ethanol over time allowed for selection of conformer stoichiometry in the ethanol-methanol dimerization reaction, but no change in the ratio of dimer conformers was observed with changing ethanol monomer stoichiometry.

Green Leaf Volatiles in the Atmosphere—Properties, Transformation, and Significance

This review thoroughly covers the research on green leaf volatiles (GLV) in the context of atmospheric chemistry. It briefly takes on the GLV sources, in-plant synthesis, and emission inventory data. The discussion of properties includes GLV solubility in aqueous systems, Henry’s constants, partition coefficients, and UV spectra. The mechanisms of gas-phase reactions of GLV with OH, NO3, and Cl radicals, and O3 are explained and accompanied by a catalog of products identified experimentally. The rate constants of gas-phase reactions are collected in tables with brief descriptions of corresponding experiments. A similar presentation covers the aqueous-phase reactions of GLV. The review of multiphase and heterogeneous transformations of GLV covers the smog-chamber experiments, products identified therein, along with their yields and the yields of secondary organic aerosols (SOA) formed, if any. The components of ambient SOA linked to GLV are briefly presented. This review recognized GLV as atmospheric trace compounds that reside primarily in the gas phase but did not exclude their transformation in atmospheric waters. GLV have a proven potential to be a source of SOA with a global burden of 0.6 to 1 Tg yr−1 (estimated jointly for (Z)-hexen-1-ol, (Z)-3-hexenal, and 2-methyl-3-buten-2-ol), 0.03 Tg yr−1 from switch grass cultivation for biofuels, and 0.05 Tg yr−1 from grass mowing.

Gas-Phase Reactions of Deprotonated Nucleobases with H, N, and O Atoms.

Complex organic molecules, the hallmark of terrestrial life, are increasingly detected in exotic environments throughout the universe. Our studies probe the ion chemistry of these biomolecules. We report gas-phase reaction rate constants for five deprotonated nucleobases (adenine, cytosine, guanine, thymine, and uracil) reacting with the atomic species H, N, and O. Hydrogen atoms react at moderate rates via associative electron detachment. Oxygen atom reactions occur more rapidly, generating complex product distributions; reaction pathways include associative electron detachment, substitution of the hydrogen atom by an oxygen atom, and generation of OCN-. Nitrogen atoms do not react with the nucleobase anions. The reaction thermodynamics were investigated computationally, and reported product channels are exothermic. Many of the proposed products have been observed in various astrochemical environments. These reactions provide insight into chemical processes that may occur at the boundaries between diffuse and dense interstellar clouds and in complex extraterrestrial ionospheres.

Prediction of Chain Propagation Rate Constants of Polymerization Reactions in Aqueous NIPAM/BIS and VCL/BIS Systems.

Microgels have a wide range of possible applications and are therefore studied with increasing interest. Nonetheless, the microgel synthesis process and some of the resulting properties of the microgels, such as the cross-linker distribution within the microgels, are not yet fully understood. An in-depth understanding of the synthesis process is crucial for designing tailored microgels with desired properties. In this work, rate constants and reaction enthalpies of chain propagation reactions in aqueous N-isopropylacrylamide/N,N'-methylenebisacrylamide and aqueous N-vinylcaprolactam/N,N'-methylenebisacrylamide systems are calculated to identify the possible sources of an inhomogeneous cross-linker distribution in the resulting microgels. Gas-phase reaction rate constants are calculated from B2PLYPD3/aug-cc-pVTZ energies and B3LYPD3/tzvp geometries and frequencies. Then, solvation effects based on COSMO-RS are incorporated into the rate constants to obtain the desired liquid-phase reaction rate constants. The rate constants agree with experiments within a factor of 2-10, and the reaction enthalpies deviate less than 5 kJ/mol. Further, the effect of rate constants on the microgel growth process is analyzed, and it is shown that differences in the magnitude of the reaction rate constants are a source of an inhomogeneous cross-linker distribution within the resulting microgel.

Estimation of Rate Constants for Gas-Phase Reactions of Chrysene, Benz[a]anthracene, and Benzanthrone with OH and NO3 Radicals via a Relative Rate Method in CCl4 Liquid Phase-System

ABSTRACTCCl4 liquid-phase reaction kinetics of polycyclic aromatic compounds (PACs), including chrysene (CHRY), benz[a]anthracene (BaA), and benzanthrone (BA), with NO3 radicals were investigated at 273 K using a relative rate method with naphthalene (NA) as the reference compound. The obtained relative reaction rates of CHRY, BaA, and BA to NA were 1.76 ± 0.08, 2.27 ± 0.20, 0.75 ± 0.01, respectively. CHRY, BaA, and BA are semi-volatile PACs with four aromatic rings. They are chemically modified into mutagenic nitropolycyclic aromatic compounds via gas-phase OH or NO3 radical-initiated reactions. Based on the PACs relative reactivity in the CCl4 liquid phase-system, the rate constants at 298 K for the gas-phase reaction of CHRY, BaA, and BA with OH radicals and those of CHRY, BaA, and BA with NO3 radicals were estimated to be (in cm3molecule−1s−1): CHRY, (4.4 ± 0.3) × 10−11 and (9.2 ± 3.2) × 10−28 [NO2]; BaA, (5.3 ± 0.5) × 10−11 and (1.3 ± 0.5) × 10−27 [NO2]; BA, (2.3 ± 0.1) × 10−11 and (3.1 ± 1.1) × 10−28 [NO2].

Kinetics of the gas-phase reactions of chlorine atoms with naphthalene, acenaphthene, and acenaphthylene.

Reactions of polycyclic aromatic hydrocarbons (PAHs) with chlorine atoms may occur in specific areas such as coastal regions and the marine boundary layer. In this work, rate constants for the gas-phase reactions of naphthalene, acenaphthene, and acenaphthylene with chlorine atoms have been measured using the relative rate technique. Experiments were performed at room temperature (293 ± 2 K) and atmospheric pressure in an atmospheric simulation chamber using a proton-transfer reaction mass spectrometer (PTR-MS) to monitor the concentrations of PAHs and the reference compounds (acetone, methanol, 1,3,5-trimethylbenzene, and isoprene) as a function of time. The rate constants obtained in this work were (in units of cm(3) molecule(-1) s(-1)) (4.22 ± 0.46) × 10(-12), (3.01 ± 0.82) × 10(-10), and (4.69 ± 0.82) × 10(-10) for naphthalene, acenaphthene, and acenaphthylene, respectively. These are the first measurements of the rate constants for gas-phase reactions of Cl atoms with acenaphthene and acenaphthylene. The rate constant determined in this study for the reaction of naphthalene with Cl atoms is not in agreement with the only other previously reported value in the literature. The results are used to assess the potential role of chlorine atom reactions in the atmospheric oxidation of PAHs.

Structure–activity relationship for the gas-phase reactions of NO3 radical with organic compounds: Update and extension to aldehydes

In this work is presented an extension and an update of a recently published structure–activity relationship (SAR) for the prediction of rate constants for gas-phase reactions of VOCs with NO3 radicals (See previous version in Kerdouci et al., 2011). Such predictive tools are very useful to describe the reactivity of organic compounds in atmospheric models. This SAR is constructed on the group-additivity method and is based on a wide kinetic database for 185 organic compounds. In addition to alkanes, alkenes and saturated and unsaturated oxygenated species already included in the first version of this SAR, this extended version comprises now aldehydes and unsaturated aldehydes. Indeed, since then, new kinetic studies have been performed for alkenals which allow the parameterization of this class of compounds. In addition, since new rate constants have been measured for unsaturated ethers, the parameterization of this family of compounds has been completed. Finally, this new version of the SAR shows very good predictive capabilities with more than 90% of the experimental rate coefficients being reproduced within a factor of two.

Estimation of gas-phase reaction rate constants of alkylnaphthalenes with chlorine, hydroxyl and nitrate radicals

Quantitative structure-property relationship/quantitative structure-activity relationship (QSPR/QSAR) models were developed for rate constants (k) of alkylnaphthalene reactions with chlorine (Cl), hydroxyl (*OH) and nitrate (NO(3)) radicals using partial least squares (PLS) regression. Quantum chemical descriptors computed by Parametric Method 3 (PM3) Hamiltonian were used as predictor variables. The cross-validated Q(cum)(2)values for the optimal QSPR/QSAR models of alkylnaphthalenes are 0.896, 0.728 and 0.774 for Cl, *OH and NO(3) radicals, respectively. Results from this study showed that rate constants with Cl, *OH and NO(3) are governed by different molecular structural descriptors. In the developed optimal QSPR/QSAR models, frontier molecular orbital energies and atomic charges are major descriptors that affect log k values. When the highest occupied molecular orbital (E(HOMO)) energy, the lowest unoccupied molecular orbital (E(LUMO)) energy, E(LUMO)+E(HOMO), and the average of net atomic charges on carbon atoms (Q(Cave)) are higher, the corresponding alkylnaphthalene reaction rate constants would be higher. In contrast, higher values of the most positive net atomic charges on hydrogen atoms (Q(H)(+)) could lead to the decrease of log k values. H-atom abstraction may occur on the hydrogen atom with the highest atomic charge (Q(H)). Radical addition reaction may also occur on the carbon (e.g. the carbon bonded to the alkyl group) with higher atomic charge (Q(C)) values. Other descriptors such as molecular weight (M(w)), standard heat of formation (DeltaH(f)), total energy (TE), electronic energy (EE), core-core repulsion energy (CCR), average molecular polarizability (a) and dipole moment (mu) were also important descriptors in the QSPR/QSAR models.

Rate constants of gas-phase reactions of trans-cyc-CF 2CF 2CHFCHF– and cyc-CF 2CF 2CH 2CHCl– with OH radicals at 253–328 K

Rate constants k 1 and k 2, for the reactions of trans-cyc-CF 2CF 2CHFCHF– and cyc-CF 2CF 2CH 2CHCl– with OH radicals at 253–328 K were determined to be (7.64 ± 2.96) × 10 −13 exp[−(1350 ± 110)/ T] and (5.25 ± 0.062) × 10 −13 exp[−(730 ± 40)/ T] cm 3 molecule −1 s −1, respectively, by means of a relative-rate method. OH radicals were generated by UV photolysis of ozone under 200 Torr of an O 3–H 2O–O 2–He gas mixture in an 11.5-dm 3 reaction chamber. The tropospheric lifetimes of trans-cyc-CF 2CF 2CHFCHF– and cyc-CF 2CF 2CH 2CHCl– were estimated at 6.7 and 1.0 years, respectively.

Linear free energy relationships on rate constants for the gas-phase reactions of hydroxyl radicals with PAHs and PCDD/Fs

Polyparameter linear free energy relationships (LFERs) on rate constants ( k OH) for gas-phase reactions of hydroxyl radicals with polycyclic aromatic hydrocarbons (PAHs) and polychlorinated dibenzo- p-dioxins and dibenzofurans (PCDD/Fs) were developed. Quantum chemical descriptors and partial least squares (PLS) regression were used for model development. Acenaphthylene was found to be an outlier and was excluded in the final model development. The cumulative variance of the dependent variable explained by the PLS components and determined by cross-validation ( Q cum 2 ), for the optimal model, is 0.97, indicating good robustness and predictive power of the model. The main molecular structural factor governing k OH values of PAHs and PCDD/Fs is molecular ability to donate electrons, as described by the energy of the highest occupied molecular orbital ( E HOMO), the average of net atomic charges on hydrogen atoms ( q H + ), and the average of net atomic charges on carbon atoms ( q C). PAH and PCDD/F molecules with high E HOMO and low q H + and q C values tend to have high log k OH values. The LFER model indicates the temperature dependence of log k OH is weak.

A QSAR for the hydroxyl radical reaction rate constant: validation, domain of application, and prediction

Abstract A large number of anthropogenic organic chemicals are emitted into the troposphere. Reactions with the hydroxyl radical are a dominant removal pathway for most organic compounds, but experimentally determined gas-phase reaction rate constants are only available for about 750 compounds. The lack of experimental data increases the importance of applying quantitative structure–activity relationships (QSAR) to evaluate and predict reactivities. It is generally acknowledged that these empirical relationships are valid only within the same domain for which they were developed. However, model validation is sometimes neglected and the application domain is not always well defined. The purpose of this paper is to outline how validation and domain definition can facilitate the modeling and prediction of the hydroxyl radical reaction rates for a large database. A substantial number of theoretical descriptors (867) were generated from 2D molecular structures for compounds present in the Syracuse Research Corporation's PhysProp Database. A QSAR model was developed for the hydroxyl radical reaction rate constant using a projection-based regression technique, partial least squares regression (PLSR). The PLSR model was subsequently validated with an external test set. The main factors of variation could be attributed to two reaction pathways, hydrogen atom abstraction and addition to double bonds or aromatic systems. A set of 17 293 compounds, drawn from the PhysProp Database, was projected onto the PLSR model and 74% were inside the applicability domain. The predicted hydroxyl reaction rates for 25% of these compounds were slow or negligible, with atmospheric half-lives in the range from days to years. Finally, the list of persistent organic compounds was matched against the OECD list of high production volume chemicals (HPVC). Together with the experimental data, nearly three hundred compounds were identified as both persistent and in high volume production.

Kinetics of the gas-phase reactions of OH radicals with alkanes and cycloalkanes

Abstract. The available database concerning rate constants for gas-phase reactions of the hydroxyl (OH) radical with alkanes through early 2003 is presented over the entire temperature range for which measurements have been made (~180-2000 K). Measurements made using relative rate methods are re-evaluated using recent rate data for the reference compound (generally recommendations from this review). In general, whenever more than one study has been carried out over an overlapping temperature range, recommended rate constants or temperature-dependent rate expressions are presented. The recommended 298 K rate constants, temperature-dependent parameters, and temperature ranges over which these recommendations are applicable are listed in Table 1.

CHIMERA: a software tool for reaction rate calculations and kinetics and thermodynamics analysis.

This article presents and overviews the CHIMERA program package, which provides a user-friendly graphical interface between quantum chemistry and chemical kinetics programs. CHIMERA facilitates calculations of rate constants for gas-phase reactions using transition state and Rice-Ramsperger-Kassel-Marcus theories. The program includes computational modules for simulation of gas-phase kinetics using simplified reactor models and for computation of chemical equilibria. The review includes a description of the theory implemented in the code, the program description, the general strategy of calculations using CHIMERA, and illustrative examples of the program application.

Rate Constants for the Gas-Phase Reactions of Silylene with Methanol, Deuterated Methanol, and Water

Gas-phase reaction rate constants for the reaction of silylene, SiH2, with deuterated methanol, CD3OD, have been determined over the temperature range 294−423 K and at total pressures over the range 100−800 Torr of the inert bath gas, Ar. Rate constants have also been measured for the reaction of SiH2 with CH3OH at 294 K over the range 100−800 Torr, also with Ar. Rate constants for the reaction of SiH2 with H2O over the range 50−200 Torr in Ar have been determined at 294 K. The second-order rate constants are pressure-dependent up to the maximum pressures investigated. For CD3OD, for which temperature-dependent data have been obtained, the rate constants decrease with increasing temperature, indicating that the reaction proceeds via the formation of a complex. At the highest temperature studied (423 K), the experimental decay curves indicate the system approaching equilibrium, providing direct experimental evidence for the formation of the complex. Analysis of the 423 K decay curves provides an experiment...

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Gas-phase reaction rate constants have been measured for the reactions of C (n = 2, 4-10) and CnH- (n = 2, 4, 6, 7) with atomic hydrogen in a flowing afterglow-selected ion flow tube at room temperature and 0.5 torr of helium pressure. In general, the rate constants are relatively fast but are less than the collisional rate (~10-9 cm3 s-1). All C and CnH- ions react by associative detachment (e.g., C + H → CnH + e-), and the C for n ≥ 7 also react by association (e.g., C + H → CnH-). Reaction of these carbon chain anions with molecular hydrogen was found to be too slow to measure in our instrument (k < 10-13 cm3 s-1).

Pressure and temperature dependence of the gas-phase reaction of silylene with dimethyl ether

Gas-phase reaction rate constants for the reaction of silylene, SiH2, with dimethyl ether, CH3OCH3 have been determined over the temperature range 294–441 K and at total pressures of inert bath gas (Ar or SF6) over the range 30–850 Torr. The second-order rate constants are pressure dependent, even up to the maximum pressure investigated of 850 Torr, and the rate constants decrease with increasing temperature, indicating that the reaction proceeds ia the formation of a complex. At the highest temperature studied (441 K), the experimental decay curves do not return to the baseline. This is attributed to the system reaching equilibrium, with SiH2 being produced by dissociation of the complex, and provides direct experimental evidence for the formation of the complex. Analysis of the decay curves provided an experimental determination of the equilibrium constant, Keq, at 441 K. The high-pressure rate constants, obtained by extrapolation of the experimental data using RRKM/master equation modelling, yield the Arrhenius parameters log (A/cm3 molecule−1 s−1) = − 7.6 ± 0.4 and Ea = 9.3 ± 2.8 kJ mol−1. The RRKM/master equation modelling gives a well depth for the SiH2–CH3OCH3 complex of 87 kJ mol−1. This compares with a value of 88.4 ± 1.7 kJ mol−1 determined from Keq at 441 K. Ab initio calculations, performed at the MP2/6-311 + G** level of theory, give a well-depth for the complex of 82.9 kJ mol−1, in excellent agreement with this value.

Temperature dependence of germylene reactions with acetylene, trimethylsilane, and phenylgermane

Gas-phase reaction rate constants have been determined over the temperature range 295–436 K for the reactions of germylene, GeH 2, with acetylene (GeH 2 addition across a triple bond), trimethylsilane (GeH 2 insertion into a Si–H bond), and phenylgermane. The room-temperature rate constant for germylene reacting with benzene has been measured and is found to be a factor of ∼300 smaller than that for phenylgermane, indicating that the latter reacts by GeH 2 insertion into the Ge–H bonds. A negative temperature dependence is observed in all cases. The activation energies, obtained from weighted linear fits to the data over the experimental temperature range, are −3.5±0.3, −11.0±0.4, and −3.6±0.3 kJ mol −1 for acetylene, trimethylsilane, and phenylgermane, respectively, while the respective frequency factors, log( A/cm 3 molecule −1 s −1), are −10.5±0.1, −11.8±0.1, −10.1±0.1.

Weighted Quasi-Newton and Variable-Order, Variable-Step Adams Algorithm for Determining Site-Specific Reaction Rate Constants

A weighted quasi-Newton algorithm for function minimization and a variable-order, variable-step Adams algorithm for ordinary differential equations were combined to solve site-specific gas-phase reaction rate constants. If the systems appear to be “stiff”, smaller steps were taken in Adams method to search for the solution. A user supplied error tolerance was used to determine the accuracy of the solution. Upon the return of each solution, a weighted algorithm is introduced into the calculation of Χ2 to minimize interference from noise. Lower and upper bounds for reagent fraction and rate constants were applied to ensure the validity of the results. The search direction is calculated by a quasi-Newton algorithm. When a saddle point is suspected, a local search is carried out with a view to moving away from the saddle point. The information obtained from site-specific rate constants may provide further insight into the reaction mechanism as well as gas-phase structure. It can be applied to gas-phase H/D ex...

Gas-Phase Oxidation Products of Biphenyl and Polychlorinated Biphenyls

Our laboratory recently measured the gas-phase reaction rate constants of polychlorinated biphenyls (PCBs) with the hydroxyl radical (OH) and concluded that OH reactions are the primary removal pathway of PCBs from the atmosphere. With the reaction system previously employed for kinetics, we have now investigated the products of these PCB−OH reactions. Experiments were carried out in either air or He as the diluent gas at approximately 1 atm in a 160-mL quartz chamber. Temperatures ranged between 318 and 363 K in order to enhance the vapor pressures of these less volatile compounds. OH was produced in situ by the photolysis of ozone in the presence of H2O. Reaction products from biphenyl, the three monochlorobiphenyls, and five dichlorobiphenyls were extracted from the chamber, derivatized by diazomethane, and analyzed by gas chromatographic mass spectrometry (GC/MS). Experiments gave benzoic acid and the appropriate chlorinated benzoic acids in significant product yields (8−17%).

Micro/macro modeling of CVD synthesis

We propose a CVD reaction scheme in which a source material undergoes a gas-phase reaction to produce an intermediate, and then the intermediate diffuses to the solid surface and changes into a solid film through a surface reaction. A series of simple Monte Carlo (SMC) codes has been developed to simulate the observed film shape on micro-trenches and holes. By using these codes, surface reaction rate constants were determined so as to reproduce the experimentally observed film shape. By means of a macro-scale reactive transport analysis of a hot wall tubular reactor, gas-phase reaction rate constants for single component systems were determined to simulate the experimental growth rate distributions. The composition and growth rate of Yttria stabilized Zirconia (YSZ) film, a solid solution of Yttria and Zirconia, were qualitatively explained by a sum of single component's growth rates. As an application of these reaction models, we simulated a rotating-disk CVD reactor under low pressure. The simulations based on a quasi three-dimensional model revealed that the susceptor rotation suppresses the buoyancy convection and forms steeper gradients in temperature and concentration near the susceptor uniformly over wide area. At higher temperatures, the growth rate increased with rotation speed, but at lower temperatures the growth rate decreased with increasing rotation speed because the reduced retention time in the high-temperature region suppressed the gas-phase reaction.