Showing papers in "International Journal of Chemical Kinetics in 1994"

The CSP method for simplifying kinetics

Abstract: The Computational Singular Perturbation (CSP) method of simplified kinetics modeling is presented with emphasis on its comparative merits versus conventional methodologies. A new “refinement” procedure for the basis vectors spanning the fast reaction subspace is presented. A simple example is first worked through using the conventional partial-equilibrium and quasi-steady approximations, and is then treated in some detail using CSP.

Modeling the thermal DENOx process in flow reactors. Surface effects and Nitrous Oxide formation

Abstract: We have investigated the impact of surface reactions such as NH3 decomposition and radical adsorption on quartz flow reactor data for Thermal DeNOx using a model that accounts for surface chemistry as well as molecular transport. Our calculations support experimental observations that surface effects are not important for experiments carried out in low surface to volume quartz reactors. The reaction mechanism for Thermal DeNOx has been revised in order to reflect recent experimental results. Among the important changes are a smaller chain branching ratio for the NH2 + NO reaction and a shorter NNH lifetime than previously used in modeling. The revised mechanism has been tested against a range of experimental flow reactor data for Thermal DeNOx with reasonable results. The formation of N2O in Thermal DeNOx has been modelled and calculations show good agreement with experimental data. The important reactions in formation and destruction of N2O have been identified. Our calculations indicate that N2O is formed primarily from the reaction between NH and NO, even though the NH2 + NO2 reaction possibly contributes at lower temperatures. At higher temperatures N2O concentrations are limited by thermal dissociation of N2O and by reaction with radicals, primarily OH. © 1994 John Wiley & Sons, Inc.

Kinetic and mechanistic aspects of the NO oxidation by O2 in aqueous phase

Abstract: The oxidation kinetics of NO by O2 in aqueous solution was observed using a stopped flow apparatus. The kinetics follows a third order rate law of the form k · [NO]2 · [O2] in analogy to gas-phase results. The rate constant at 296 K was measured as (6.4 ± 0.8) · 106 M−2 s−1 with an activation energy of 2.3 kcal/mol and a preexponential factor of (4.0 ± 0.5) · 108 M−2 s−1. The rate constant displays a very slight pH dependence corresponding to less than a factor of three over the range 0 to 12. The system NO/O2 in aqueous solution is an efficient nitrosating agent which has been tested using phenol as a substrate over the pH range 0 to 12. The rate limiting step leading to formation of 4-nitrosophenol is the formation of the reactive intermediate whose competitive hydrolysis yields HONO or NO2−. The absence of NO3− in the autoxidation of NO, the exclusive presence of NO2− as a product of the nitrosation reaction of phenol, and the kinetic results of the N3− trapping experiments point towards N2O3 as the reactive intermediate. © 1994 John Wiley & Sons, Inc.

A shock tube study of the OH + OH → H2O + O reaction

Abstract: The rate coefficient for the reaction has been determined in mixtures of nitric acid (HNO3) and argon in incident shock wave experiments. Quantitative OH time-histories were obtained by cw narrow-linewidth uv laser absorption of the R1(5) line of the A2 σ+ X2 Πi (0,0) transition at 32606.56 cm−1 (vacuum). The experiments were conducted over the temperature range 1050–2380 K and the pressure range 0.18–0.60 atm. The second-order rate coefficient was determined to be with overall uncertainties of +11%, −16% at high temperatures and +25%, −22% at low temperatures. By incorporating data from previous investigations in the temperature range 298–578 K, the following expression is determined for the temperature range 298–2380 K © 1994 John Wiley & Sons, Inc.

Rate constants for the gas‐phase reactions of Ozone with unsaturated Aliphatic Alcohols

Abstract: The gas-phase reaction of ozone with unsaturated alcohols in air has been investigated at atmospheric pressure and ambient temperature (288–291 K). Cyclohexane was added to scavenge the hydroxyl radical which forms as a product of the ozone–unsaturated alcohol reaction. The reaction rate constants, in units of 10−18 cm3 molecule−1 s−1, are 16.2 ± 0.7 for (±) 3-buten-2-ol, 17.9 ± 1.8 for 1-penten-3-ol, 10.0 ± 0.3 for 2-methyl-3-buten-2-ol, 169 ± 25 for cis-2 penten-1-ol, and 251 ± 41 for 2-buten-1-ol (mixture of isomers). Substituent effects on reactivity are discussed. The reactivity of unsaturated alcohols towards ozone is similar to that of their alkene structural homologues. Implications of these results with respect to the atmospheric persistence of unsaturated alcohols are briefly discussed. © 1994 John Wiley & Sons, Inc.

Kinetics and mechanisms of the reactions of OH with some oxygenated compounds of importance in tropospheric chemistry

Abstract: The kinetics of the reactions have been studied in a discharge flow system under pseudo-first-order conditions. The OH concentration was monitored by laser induced fluorescence and helium was used as the carrier gas. Values of k1 = (8.1 ± 1.7) × 10−13, k2 = (1.31 ± 0.26) × 10−11, k3 = (2.6 ± 0.5) × 10−11, and k4 = (2.5 ± 0.4) × 10−11 cm3 molecule−1 s−1, at 298 K and 1 torr total pressure, were obtained. To validate the newly constructed system the rate constant for the reaction was determined in a similar manner. The value of k5 = (6.7 ± 0.9) × 10−12 cm3 molecule−1 s−1 at 298 K and 1 torr total pressure is in very good agreement with other literature values. The mechanisms for the atmospheric degradation of these compounds have been constructed to allow their incorporation in a photochemical trajectory computer model, to assess their impact on photochemical ozone creation in the troposphere. © 1994 John Wiley & Sons, Inc.

Branching ratio of the C2H2 + O reaction at 290 K from kinetic modelling of relative methylene concentration versus time profiles in C2H2/O/H systems

Abstract: In earlier work on the room temperature oxidation of C2H2 by O atoms, two distinct sources of methylene radicals have been identified: (i) direct, primary production via channel 1b of the C2H2 + O reaction, and (ii) delayed formation via the secondary reaction 3 involving the products HCCO and H of the other primary channel 1a: Presently, it was confirmed by a detailed sensitivity analysis that the precise shapes of the resulting total methylene concentration-versus-time profiles in C2H2/O systems depend strongly on the k1a/k1b branching ratio. Along that line, the important parameter k1a/k1b was determined from relative CH2 concentration-versus-time profiles measured in a variety of C2H2/O/H systems using Discharge Flow-Molecular Beam sampling Mass Spectrometry techniques (DF-MBMS). The data analysis was carried out by deductive kinetic modelling; the method, as applied to profile shapes, is discussed at length. Via this novel, independent approach, the CH2(3B1) yield of the two-channel C2H2 + O reaction was determined to be k1b/k1 = 0.17 ± 0.08. The indicated 2σ error includes possible systematic errors due to uncertainties in the rate constants of other reactions that influence the shapes of the CH2 profiles. The present result, which translates to an HCCO yield k1a/k1 = 0.83 ± 0.08, is in excellent agreement with other recent determinations. The above mechanism, with the subsequent reactions that it initiates, also reproduces the measured absolute [C2H2], [O], and [H] profiles with an average accuracy of 5%, thus validating the consistency of the C2H2/O/H reaction model put forward here. © 1994 John Wiley & Sons, Inc.

Absolute rate constant for the reaction of Phenyl radical with Acetylene

Abstract: The absolute rate constant for the reaction of phenyl radical with acetylene has been measured at 20 torr total pressure in the temperature range of 297 to 523 K using the cavity-ring-down technique. These new kinetic data could be quantitatively correlated with the data obtained earlier with a relative rate method under low-pressure (10−3–10−2 torr) and high-temperature (1000–1330 K) conditions. These kinetic data were analyzed in terms of the RRKM theory employing the thermochemical and molecular structure data computed with the BAC-MP4 technique. The calculated results reveal that the total rate constant for the C6H5 + C2H2 reaction (kt) is pressure-independent, whereas those for the formation of C6H5C2H (kb) and the C6H5C2H2 adduct (kc) are strongly pressure-dependent. A least-squares analysis of the calculated values for 300–2000 K at the atmospheric pressure of N2 or Ar can be given by and all in units of cm3/s. The latter equation effectively represents the two sets of experimental data. © 1994 John Wiley & Sons, Inc.

Rates of processes initiated by pulsed laser production of F atoms in the presence of HCl, CH4, and CF3H

Abstract: Time-resolved vibrational chemiluminescence from HF has been recorded following the production of F atoms by the pulsed laser photolysis (λ = 266 nm) of F2 in the presence of HCl, CH4, and CF3H. In the first two cases, experiments have been conducted by observing emission from HF(ν = 3) at four temperatures from 295 to 139 K. Rate constants have been determined over this range of temperature for the reactions of F atoms with HCl and CH4 and of CH3 radicals with F2, and for the relaxation of HF(ν = 3) by HCl and CH4. The reaction of F atoms with CF3H is slower than those with HCl and CH4 and measurements on the emission from HF(ν = 2) have been used to infer rate constants for reaction and relaxation only at 295 K. © 1994 John Wiley & Sons, Inc.

A spectrokinetic study of CH2I and CH2IO2 radicals

Abstract: The UV absorption spectrum and kinetics of CHzI and CHzIOz radicals have been studied in the gasphase at 295 K using a pulse radiolysis W absorption spectroscopic technique. UV absorption spectra of CH2I and CHzIO2 radicals were quantified in the range 220-400 nm. The spectrum of CH2I has absorption maxima at 280 nm and 337.5 nm. The absorption cross-section for the CHzI radicals at 337.5 nm was (4.1 -C 0.9) x cm2 molecule-'. The UV spectrum of CHzIOz radicals is broad. The absorption cross-section at 370 nm was (2.1 5 0.5) X cm2 molecule-l. The rate constant for the self reaction of CH2I radicals, k = 4 X cm3 molecule-' s-' at 1000 mbar total pressure of SFs, was derived by kinetic modelling of experimental absorbance transients. The observed self-reaction rate constant for CH2102 radicals was estimated also by modelling to k = 9 X 10-l' cm3 molecule-' s-'. As part of this work a rate constant of (2.0 % 0.3) X 10-lo cm3 molecule-' s-' was measured for the reaction of F atoms with CH3I. The branching ratios of this reaction for abstraction of an I atom and a H atom were determined to (64 -t 6)% and (36 2 6)%, respectively. 0 1994 John Wiley & Sons, Ine.

Gas‐phase kinetic study of the silylene addition reaction to acetylene and acetylene‐d2 over the temperature range 291–613 K

Abstract: Time-resolved studies of silylene, SiH2, generated by laser-flash photolysis of phenylsilane, have been employed to obtain rate constants for its bimolecular reactions with C2H2 and C2D2. The reactions have been studied in the gas-phase, in the pressure range 1–100 torr (with SF6 bath gas) at five temperatures in the range 291–613 K. Reaction with C2H2 is pressure dependent, consistent with a third body assisted association reaction. However the lack of a simple fit to RRKM calculated values suggests a more complex process with another reaction channel. Reaction with C2D2 is faster than with C2H2, showing a pressure dependent isotope effect. The data are consistent with a rapid isotopic scrambling mechanism. Further RRKM modeling of a three-channel decomposition mechanism for the suggested silirene adduct (intermediate) provides a semi-quantitative fit to the data. Rate constants extracted from the modeling are shown to be consistent with a mechanism leading to formation of both ethynylsilane and vinylsilylene, as previously proposed by O'Neal, Ring et al. from higher temperature studies. An enthalpy surface is shown to be consistent with this mechanism. © 1994 John Wiley & Sons, Inc.

Kinetic model for hydrocarbon-assisted particulate boron combustion

Abstract: A kinetic model is presented to describe the high temperature (1800 K < T < 3000 K) surface oxidation of particulate boron in a hydrocarbon combustion environment. The model includes a homogeneous gas-phase B/O/H/C oxidation mechanism consisting of 19 chemical species and 58 forward and reverse elementary reactions, multi-component gas-phase diffusion, and a heterogeneous surface oxidation mechanism consisting of ‘elementary’ adsorption and desorption reaction steps. Thermochemical and kinetic parameters for the surface reactions are estimated from available experimental data and/or elementary transition state arguments. The kinetic processes are treated using a generalized kinetics code, with embedded sensitivity analysis, for the combustion of a one-dimensional (particle radius), spherical particle. Model results are presented for the oxidation of a 200 μm boron particle in a JP-4/air mixture at ambient temperatures of 1400 K and 2000 K. These results include temperature and gas-phase species profiles as a function of radial distance and particle burning rates. © 1994 John Wiley & Sons, Inc.

Kinetics of the reactions of OH with alkanes

Abstract: The rate coefficients for the reactions of OH with ethane (k1), propane (k2), n-butane (k3), iso-butane (k4), and n-pentane (k5) have been measured over the temperature range 212–380 K using the pulsed photolysis-laser induced fluorescence (PP-LIF) technique. The 298 K values are (2.43±0.20) × 10−13, (1.11 ± 0.08) × 10−12, (2.46 ± 0.15) × 10−12, (2.06 ± 0.14) × 10−12, and (4.10 ± 0.26) × 10−12 cm3 molecule−1 s−1 for k1, k2, k3, k4, and k5, respectively. The temperature dependence of k1 and k2 can be expressed in the Arrhenius form: k1 = (1.03 ± 0.07) × 10−11 exp[−(1110 ± 40)/T] and k2 = (1.01 ± 0.08) × 10−11 exp[−(660 ± 50)/T]. The Arrhenius plots for k3 – k5 were clearly curved and they were fit to three parameter expressions: k3 = (2.04 ± 0.05) × 10−17T2 exp[(85 ± 10)/T] k4 = (9.32 ± 0.26) × 10−18T2 exp[(275 ± 20)/T]; and k5 = (3.13 ± 0.25) × 10−17T2 exp[(115 ± 30)/T]. The units of all rate constants are cm3 molecule−1 s−1 and the quoted uncertainties are at the 95% confidence level and include estimated systematic errors. The present measurements are in excellent agreement with previous studies and the best values for atmospheric calculations are recommended. © 1994 John Wiley & Sons, Inc.

Study of the kinetics and equilibrium of the benzyl‐radical association reaction with molecular oxygen

Abstract: The forward rate constant, k1, and the equilibrium constant, Kp, for the association reaction of the benzyl radical with oxygen have been determined. The rate constant k1 was measured as a function of temperature (between 298 and 398 K) and pressure (at 20 and 760 torr of N2) by two different techniques, argon-lamp flash photolysis and excimer-laser flash photolysis, both of which employed UV absorption spectroscopy (at 253 nm and 305 nm, respectively) to monitor the benzyl radical concentration. Over the range of conditions studied, we find that the reaction is independent of pressure and is almost independent of temperature, which is in accord with two early studies of the reaction but in apparent disagreement with more recent work. For our results in 760 torr of N2 and for 298 < T/K < 398, a linear least-squares fitting of the data yield the expression: k1 = (7.6 ± 2.4) × 10−13 exp[(190 ± 160)K/T] cm3 molecule−1 s−1. With the flash-photolysis technique, we determined Kp over the temperature range 398–525 K. Experimental values were analyzed alone and combined with theoretically determined entropy values of the benzyl and benzylperoxy radicals to determine the enthalpy of reaction: ΔH = (−91.4 ± 4) kJ mol−1. Previous work on the benzyl radical enthalpy of formation allows us to calculate ΔH°f 298 (Benzylperoxy) = (117 ± 6) kJ mol−1. In addition, we carried out an RRKM calculation of k1 using as constraints the thermodynamic information gained by the study of Kp. We find that all the studies of the association reaction are in good agreement once a fall-off effect is taken into account for the most recent work conducted at pressures near 1 torr of helium. © 1994 John Wiley & Sons, Inc.

Study of the pseudo‐steady‐state kinetics of CO2 hydrate formation and stability

Abstract: This article describes a dynamic model for formation and stability of CO2-hydrate on the interface of liquid CO2(LCO2) and ocean water at large depths. Experimental results indicate that a thin film of hydrate naturally forms on the interfaces between LCO2 and water, and inhibits diffusion between the two phases. Experiments further shows that the flux of CO2 through the hydrate film is dependent of the CO2-concentration in the ambient sea water. The model proposed here explains these phenomena by introducing four major mechanisms; diffusion of water to the LCO2-phase, formation of hydrate in the LCO2-hydrate interface, decay of hydrate in the water-hydrate interface, and diffusion of CO2 through the water phase. The model explains the CO2 flux not by diffusion through the hydrate film, but suggest a mechanism of continuous hydrate formation and decay. The overall effect is a “moving,” pseudo-steady-state hydrate film due to transport of CO2 through the film. The film velocity is dependent of liquid-liquid diffusivity parameters and reaction constant, and lacking experimental values of these parameters, an order–of-magnitude analysis is done by fitting the model to experimentally obtained data for the overall film velocity. The motivation for this work is to elucidate options for CO2 depositions in deep oceans, of which liquid CO2 sequestration is believed to be one of the most feasible. Spreading of CO2 from a liquid CO2-lake and associated lowering of pH in the ecosystem surrounding the lake is of large concern. The work presented here concludes that diffusion of CO2 in the ocean is largely reduced by the hydrate film and suggests that hydrate formation may alleviate some of the environmental concerns regarding deep ocean sequestration of liquid CO2. © 1994 John Wiley & Sons, Inc.

Ozonolysis of trans- and cis-2-butenes in low parts-per-million concentration ranges

Abstract: Ozonolysis of 1–5 ppm concentrations of trans- and cis-2-C4H8 was carried out in a 580 l spherical glass reaction vessel at 730 ± 5 torr and 296 ± 2 K. The yields of CH3CHO, HCHO, CO, CO2, CH4, and CH3OH were determined by long-path FTIR spectroscopy. About 60% of C4H8 that reacted with O3 decomposed via the formation of the excited CH3CHO2* intermediates into the following pathways: (4a) CO2 + CH4, (4b) CO2 + H + CH3, (4c) CO + OH + CH3, and (4d) CO + CH3OH. The branching ratios for each channel, expressed as the percent of the total pathways, were determined for trans isomer: 20, 30, 40, and 10, and for cis isomer: 29, 35, 24, and 12, respectively. The conversion of C4H8 relative to the reacted O3 was about 1.6 and 1.4 for trans and cis isomers, respectively. These results were explained by the reactions of OH radicals formed in (4c) with C4H8, in which secondary OH radicals were generated: C4H8 + OH + O2 CH3CH(OH)CH(CH3)OO, followed by CH3-CH(OH)-CH(CH3)OO 2 CH3CHO + OH. About 40% of C4H8 that reacted with O3 yielded a mixture of a carbonyl and a noncarbonyl product, assigned as hydroxyethyl formate, CH3CH(OH)OCHO, and secondary butene ozonide, respectively. The addition of HCHO increased the formation of the former while the latter was unaffected. These results were consistent with the mechanism proposed by Cremer et al. [Chem. Phys. Lett., 187, 491 (1991)], where the primary ozonide rearranges, before dissociation, to the carbonyl oxide-aldehyde complex (the dipole complex) which is the precursor of the secondary ozonide. © 1994 John Wiley & Sons, Inc.

Thermal Decomposition of CF3C(O)O2NO2, CClF2C(O)O2NO2, CCl2FC(O)O2NO2, and CCl3C(O)O2NO2

Abstract: Haloacetyl, peroxynitrates are intermediates in the atmospheric degradation of a number of haloethanes. In this work, thermal decomposition rate constants of CF3C(O)O2NO2, CClF2C(O)O2NO2, CCl2FC(O)O2NO2, and CCl3C(O)O2NO2 have been determined in a temperature controlled 420 l reaction chamber. Peroxynitrates (RO2NO2) were prepared in situ by photolysis of RH/Cl2/O2/NO2/N2 mixtures (R = CF3CO, CClF2CO, CCl2FCO, and CCl3CO). Thermal decomposition was initiated by addition of NO, and relative RO2NO2 concentrations were measured as a function of time by long-path IR absorption using an FTIR spectrometer. First-order decomposition rate constants were determined at atmospheric pressure (M = N2) as a function of temperature and, in the case of CF3C(O)O2NO2 and CCl3C(O)O2NO2, also as a function of total pressure. Extrapolation of the measured rate constants to the temperatures and pressures of the upper troposphere yields thermal lifetimes of several thousands of years for all of these peroxynitrates. Thus, the chloro(fluoro)acetyl peroxynitrates may play a role as temporary reservoirs of Cl, their lifetimes in the upper troposphere being limited by their (unknown) photolysis rates. Results on the thermal decomposition of CClF2CH2O2NO2 and CCl2FCH2O2NO2 are also reported, showing that the atmospheric lifetimes of these peroxynitrates are very short in the lower troposphere and increase to a maximum of several days close to the tropopause. The ratio of the rate constants for the reactions of CF3C(O)O2 radicals with NO2 and NO was determined to be 0.64 ± 0.13 (2σ) at 315 K and a total pressure of 1000 mbar (M = N2). © 1994 John Wiley & Sons, Inc.

Reactions of NO3-naphthalene adducts with O2 and NO2

Abstract: The kinetics of the gas-phase reaction of the NO3 radical with naphthalene have been investigated at 150 torr O2 + 590 torr N2 and 600 torr O2 + 140 torr N2 at 298 ± 2 K. Relative rate measurements were carried out in reacting NO3N2O5-naphthalene-propene-O2N2 mixtures by longpath Fourier transform infrared absorption spectroscopy. A rate constant ratio for the reactions of O2 and NO2 with the NO3-naphthalene adduct of k/k < 4 × 10−7 was obtained from the competition between O2 and NO2 for reaction with the NO3-naphthalene adduct and thermal decomposition of the adduct back to reactants. Atmospheric pressure ionization MS/MS measurements of the nitronaphthalene products of the NO3 radical-initiated reaction of naphthalene are consistent with the proposed reaction mechanism, and the atmospheric implications of the data are discussed. © 1994 John Wiley & Sons, Inc.

High‐temperature ignition of propane with MTBE as an additive: Shock tube experiments and modeling

Abstract: Ignition of propane has been studied in a shock tube and by computational modeling to determine the effect of methyl tert-butyl ether (MTBE) as a fuel additive MTBE and isobutene were added in amounts up to 25% of the fuel to propane-oxygen-argon mixtures in shock-tube experiments covering a range of temperatures between 1450 and 1800 K Ignition delays were measured from chemiluminescence at 432 nm due to excited CH radicals The temperature dependence of the ignition rates was analyzed to yield Arrhenius parameters of E{sub a}{approximately}40 kcal/mol and log (A){approximately} 90 sec{sup {minus}1} for the overall reaction Reactions involving MTBE and its decomposition products were combined with an established propane mechanism in a numerical model to describe the kinetic interaction of this additive with a typical hydrocarbon fuel The experiments and the kinetic model both show that MTBE and isobutene retard propane ignition with nearly equal efficiency The kinetic model demonstrates that isobutene kinetics are responsible for inhibition by both MTBE and isobutene, and the specific elementary reactions which produce this behavior are identified

On the role of transport in the combustion kinetics of a steady‐state premixed laminar CO + H2 + O2 flame

Abstract: The role of mechanistic steps, diffusion, and their interrelation is explored in a steady-state premixed laminar CO + H2 + O2 flame using a numerical model. Sensitivity coefficients and Green's functions calculated for this system offer systematic characterization of the role of diffusion and exothermicity in carbon monoxide oxidation kinetics. The results reveal that the uncertainties in transport parameters are as important to the model predictions as those in the kinetic steps. The rate controlling steps of the CO + H2 + O2 reaction are found to be different for adiabatic and nonadiabatic premixed flames, and also for systems with and without transport. In particular, the reactions of the hydroperoxyl radical with hydrogen, oxygen, and hydroxyl radicals are found to be important at all temperatures in the fuel lean (40 torr) adiabatic flame studied here. The diffusive mixing of chemical species from the low and the high temperature portions of the flame and the larger heats of reaction associated with the hydroperoxyl radicals are found to be responsible for the increased importance of these reactions. © 1994 John Wiley & Sons, Inc.

Concerted mechanism of the aminolysis of 2,4,6‐trinitrophenyl o‐ethyl dithiocarbonate in aqueous ethanol

Abstract: The reactions of the title substrate with a series of secondary alicyclic amines are subjected to a kinetic study in 44 wt% aqueous ethanol, 25.0°C, and ionic strength 0.2 M (KCl). The Bronsted-type plot (log kN vs. pKa of the amine, where kN is the second-order rate coefficient) obtained is linear with slope β = 0.53, which indicates a concerted mechanism. The predicted Bronsted break for a hypothetical stepwise mechanism is pK = 8.7, which was not observed (pKa range of amines: 6–11). The same reaction in water is stepwise, which shows that the tetrahedral intermediate found in water is much destabilized by the change of solvent from water to aqueous ethanol. © 1994 John Wiley & Sons, Inc.

Kinetics and mechanisms of the gas‐phase reactions of the NO3 radical with aromatic compounds

Abstract: The kinetics and nitrated products of the gas-phase reactions of the NO3 radical with methoxybenzene, 1,2-, 1,3-, and 1,4-dimethoxybenzene, dibenzofuran and dibenzo-p-dioxin have been investigated at 297 ± 2 K and in the presence of one atmosphere of air. A relative rate method was used for the kinetic measurements. No reactions of methoxybenzene or dibenzofuran with the NO3 radical were observed. The dimethoxybenzenes were observed to react by H-atom abstraction and NO3 radical addition to the aromatic ring, while dibenzo-p-dioxin reacted by NO3 radical addition to the aromatic rings. For these compounds, the NO3 radical addition pathways were observed to be reversible. At the NO2 concentrations employed, the NO3-aromatic adducts reacted with NO2 and the observed rate constants increased with increasing NO2 concentration. However, for dibenzo-p-dioxin the observed rate constant became independent of the NO2 concentration for concentrations ≥ 4.8 × 1013 molecule cm−3, and under these conditions the rate constant of 6.8 × 10−14 cm3 molecule−1 s−1 was taken to be that for addition of the NO3 radical to the aromatic rings. The proposed NO3 radical reaction mechanisms are discussed. © 1994 John Wiley & Sons, Inc.

Rates of reaction between the nitrate radical and some aliphatic ethers

Abstract: Rate coefficients for the reaction of NO3 with dimethyl ether, diethyl ether, di-n-propyl ether, and methyl t-butyl ether (MTBE) have been determined. Absolute rates were measured at temperatures between 258 and 373 K using the fast flow-discharge technique. Relative rate experiments were also made at 295 K in a reactor equipped with White optics and using FTIR spectroscopy to follow the reactions. The measured rate coefficients (in units of 10−15 cm3 molecule−1 s−1) at 295 K are: 0.26 ± 0.11, 2.80 ± 0.23, 6.49 ± 0.65, and 0.64 ± 0.06 for dimethyl ether, diethyl ether, di-n-propyl ether, and methyl t-butyl ether, respectively. The corresponding activation energies are 21.0 ± 5.0, 17.2 ± 4.0, 15.5 ± 2.1, and 20.1 ± 1.7 kJ mole−1. The error limits correspond to the 95%-confidence interval. © 1994 John Wiley & Sons, Inc.

A kinetic analysis of the photolysis of mixtures of acetone and propylene

Abstract: The photolysis of mixtures of acetone and propylene at 308 nm has been studied kinetically from approximately 300 K to 580 K. Rate constants were calculated for the reactions at total pressures ranging from 110 torr to 750 torr and for [C3H6]/[(CH3)2CO] ratios in the range 0.03 to 3.3. No dependence of the rate constants on total pressure or on this concentration ratio could be detected within this range of conditions. The temperature dependence of the rate constants is given by © 1994 John Wiley & Sons, Inc.

Kinetics of the reactions of Cl atoms with CH3Br and CH2Br2

Abstract: The rate coefficients for the reactions of Cl atoms with CH3Br, (k1) and CH2Br2, (k2) were measured as functions of temperature by generating Cl atoms via 308 nm laser photolysis of Cl2 and measuring their temporal profiles via resonance fluorescence detection The measured rate coefficients were: k1 = (155 ± 018) × 10−11 exp{(−1070 ± 50)/T} and k2 = (637 ± 055) × 10−12 exp{(−810 ± 50)/T} cm3 molecule−1 s−1 The possible interference of the reaction of CH2Br product with Cl2 in the measurement of k1 was assessed from the temporal profiles of Cl at high concentrations of Cl2 at 298 K The rate coefficient at 298 K for the CH2Br + Cl2 reaction was derived to be (536 ± 056) × 10−13 cm3 molecule−1 s−1 Based on the values of k1 and k2, it is deduced that global atmospheric lifetimes for CH3Br and CH2Br2 are unlikely to be affected by loss via reaction with Cl atoms In the marine boundary layer, the loss via reaction (1) may be significant if the Cl concentrations are high If found to be true, the contribution from oceans to the overall CH3Br budget may be less than what is currently assumed © 1994 John Wiley & Sons, Inc

Investigation of the correlation between the energy of the highest occupied molecular orbital (HOMO) and the logarithm of the OH rate constant of hydrofluorocarbons and hydrofluoroethers

Abstract: A regression based model was developed to determine whether highest occupied molecular orbital (HOMO) energies, calculated using Kohn-Sham orbital density functional theory (DFT), could be used to estimate the OH rate constants of hydrofluorocarbons (HFCs) and hydrofluoroethers (HFEs), proposed substitutes for stratospheric O3 depleting chlorofluorocarbons. The goodness of fit of the DFT model was compared with a second regression model, derived using recently reported HOMO energies obtained from Hartree Fock theory (HFT). Both models were employed to predict OH rate constants for a number of HFCs and HFEs whose OH rate constants have not been measured, thus providing data on the types of chemical structures that may increase the OH reactivity of the substitute and hence decrease its contribution to global warming. The estimated percent standard errors in the OH rate constant HFT and DFT regression models were 72% and 78%, respectively. The goodness of fits were such that the models can differentiate between two rate constants only when their ratio exceeds about a factor of four. Model results suggest that (1) only a limited number of HFEs will have OH rate constants that are more than an order of magnitude greater than the value for their corresponding HFCs and (2) the strategy of introducing an ether linkage into an HFC to dramatically enhance its reactivity will be most effective for the least fluorinated HFCs. © 1994 John Wiley & Sons, Inc.1

Oxidation of Arginine by Manganese(III) in Pyrophosphate and Acetate media: A kinetic study

Abstract: Kinetics of manganese(III) oxidation of L-arginine has been studied in the presence of pyrophosphate and acetate ions in acidic media at 328 K and 323 K, respectively. The nature of the oxidizing species formed in manganese(III) solutions was determined by spectrophotometric and redox potential measurements. The reaction shows a first-order dependence on [manganese(III) pyrophosphate] in the pyrophosphate medium, pH 2–3, and a half-order on [manganese(III) acetate] in HOAc-acetate medium. In both media, the kinetic order is one with respect to [arginine]. The dependencies of the rate on the reduction product, manganese(II), concentration are zero- and inverse first-orders in acetate and pyrophosphate media, respectively. Effects of varying dielectric constant of the medium and of added anions such as acetate, pyrophosphate, fluoride, chloride, and perchlorate have been investigated, in both media. There is evidence for the existence of free radicals as transient species. Activation parameters have been evaluated using the Arrhenius and Eyring plots. Mechanisms consistent with the observed kinetic data have been proposed and discussed. Kinetic data for the oxidations of some α-amino acids by manganese(III) species of different forms are summarized and compared. © 1994 John Wiley & Sons, Inc.

Kinetics of the reaction of C6H5 with HBr and DBr

Abstract: The rate constants for the reaction of C6H5 with HBr and DBr have been measured with the cavity–ring–down method in the temperature range of 297 to 523 K and 297 to 500 K, respectively. These rate constants can be effectively represented, in units of cm3/s, by Both activation energies are similar and positive, contrary to those of alkyl radical reactions, all of which exhibit negative temperature dependencies. The difference, as pointed out before [1], could be accounted for by the electron-withdrawing effect of the phenyl vis-a-vis the electron-donating ability of the alkyls. © 1994 John Wiley & Sons, Inc.