摘要:

AbstractThe reaction between chromium(VI) and L‐ascorbic acid has been studied by spectrophotometry in the presence of aqueous citrate buffers in the pH range 5.69–7.21. The reaction is slowed down by an increase of the ionic strength. At constant ionic strength, manganese(II) ion does not exert any appreciable inhibition effect on the reaction rate. The rate law found is\documentclass{article}\pagestyle{empty}\begin{document}$$ r = k_p k_r [Cr({\rm VI})][{\rm L - ascorbic\,acid][H}^{\rm + }]/(1 + k_p [H^ +]) $$\end{document}whereKpis the equilibrium constant for protonation of chromate ion andkris the rate constant for the redox reaction between the active forms of the oxidant (hydrogenchromate ion) and the reductant (L‐hydrogenascorbate ion). The activation parameters associated with rate constantkrare Ea= 20.4 ± 0.9 kJ mol−1, ΔH≠= 17.9 ± 0.9 kJ mol−1, and ΔS≠=−152 ± 3 J K−1mol−1. The reaction thermodynamic magnitudes associated with equilibrium constantKpare ΔH0= 16.5 ± 1.1 kJ mol−1and ΔS0= 167 ± 4 J K−1mol−1. A mechanism in accordance with the experimental data is proposed for the rea

ISSN:0538-8066    

DOI:10.1002/kin.550250402

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe kinetic isotope effect for the abstraction of hydrogen/deuterium from dimethylnitramine and dimethylnitramine‐d6by chlorine atoms has been studied in the temperature range 273–353 K. The rate constant ratiokH0/kDis given by the Arrhenius expression,kH/kD=(0.92 ± 0.07)exp(286 ± 250/RT), whereRis expressed in cal mol−1K−1. The absolute rate constant for the deuterium abstraction reaction is extrapolated askD=(1.50 ± 0.90) × 10−10exp(−1,486 ± 370/RT) cm3molecule−1s−1. The temperature dependence of the kinetic isotope effect was calculated using the conventional transition‐state theory, and the obtained values forkH/kDand ΔEH,Dare in good agreement with the experimental value for a bent transition state geometry, with two new vibrational frequencies of 340 cm−1(272 cm−1) corresponding to the in‐plane and out‐of‐plane motions of hydrogen (deuterium) atoms in the Cl…H…C arr

ISSN:0538-8066    

DOI:10.1002/kin.550250403

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe kinetics of the reaction between 1,2,3‐trihydroxybenzene (pyrogallol) and O2(autoxidation) have been determined by monitoring the concentration of dissolved dioxygen with a polarographic oxygen electrode. The reaction is carried out in pseudo‐first‐order excess pyrogallol, 25°C, 0.08 M NaCl, and 0.04 M phosphate buffer in the pH range 6.9–10.5. Data collection precedes reaction initiation, but only the data recorded after the estimated 3.2 s dead time are used in kinetics calculations. Observed rate constants are corrected for incomplete mixing, which is treated as a first‐order process that has an experimentally determined mixing rate constant of 4.0 s−1. The rate law for the reaction is −d[O2]/dt=kapp[PYR]tot[O2], in which [PYR]totis the total stoichiometric pyrogallol concentration. A mechanism is presented which explains the increase in rate with increasing [OH−] by postulating that H2PYR−(k2) has greater reactivity with dissolved dioxygen than does H3PYR (k1). The data best fit the equationkapp=(k1+k2KH[OH−])/(1 +KH[OH−]) when the value of the hydrolysis constantKH(the quotient of the pyrogallol acid dissociation and water autoprotolysis constants) for this medium equals 3.1×104M−1. The resulting values ofk1andk2, respectively, equal (0.13 + 0.01) M−1s−1and (3.5 plusmn; 0.1) M−1s−1. This reaction is recommended as a test reaction for calibrating the dynamic response of an O2‐ele

ISSN:0538-8066    

DOI:10.1002/kin.550250404

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractAbsolute rate constants for the addition of the 2‐cyano‐2‐propyl radical to 26 alkenes CH2=CXY at 315 K were determined in solution by time‐resolved electron‐spin‐resonance spectroscopy. They vary with the alkene substituents from 30 M−1s−1to 7′010 M−1s−1. For styrene the temperature dependence is given by log k/M−1s−1= 7.7 − 26.1/Θ where Θ = 2.303RTin kJ/mol. An analysis of the substituent effects in terms of polar and enthalpic factors reveals a dominant influence of the overall reaction enthalpy

ISSN:0538-8066    

DOI:10.1002/kin.550250405

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe kinetics of the reaction of OH radicals witht‐amyl methyl ether (TAME) have been reinvestigated using both absolute (flash photolysis resonance fluorescence) and relative rate techniques. Relative rate experiments were conducted at 295 K in 99 kPa (740 torr) of synthetic air using ethylt‐butyl ether, cyclohexane, and di‐isopropyl ether as reference compounds. Absolute rate experiments were performed over the temperature range 240–400 K at a total pressure of 4.7 kPa (35 torr) of argon. Rate constant determinations from both techniques are in good agreement and can be represented byk1=(6.32 ± 0.72) × 10−12exp[(−40 ± 70)/T] cm3molecule−1s−1.Quoted errors represent 2σ from the least squares analysis and do not include any estimate of systematic errors. We show that results from the previous kinetic study of reaction (1) are in error due to the presence of a reactive impurity. Results are discussed in terms of the atmospheric chemistry of TAME. © 1993

ISSN:0538-8066    

DOI:10.1002/kin.550250406

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractRate constants for the reactions of Cl atoms and OH radicals with haloalkanes were measured using the relative rate technique. From these values the atmospheric lifetimes of the organics with respect to Cl atoms and OH radicals were calculated. Cl atoms were produced by the photolysis of chlorine gas, and photolysis of methyl nitrite was the source of OH radicals. The rate constants were measured for a series of brominated and chlorinated alkanes for which measurements have not yet been reported excepting:k(Cl + 1‐chloropropane) andk(OH + 1‐chloropropane, 2‐chloropropane, and bromoethane). The organics studied were 1‐chloropropane, 2‐chloropropane, 1,3 dichloropropane, 2‐chloro 2methylpropane, bromoethane, 1‐bromopropane, 2‐bromopropane, 1‐bromobutane, 1‐bromopentane, and 1‐bromohexane. Cl atom reactions were measured at 298 K, the OH radical reactions were measured at temperatures between 298–308 K. © 1

ISSN:0538-8066    

DOI:10.1002/kin.550250407

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractA kinetic mechanism for the chain decomposition of formaldehyde consistent with recent theoretical and experimental results is presented. This includes new calculations and measurements of the rate constant for the abstraction reaction\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm CH}_{\rm 2} {\rm O + H} \to {\rm HCO + H}_{\rm 2} $$\end{document}The calculation uses a multi‐reference configuration interaction wavefunction to construct the potential energy surface which is used in a tunneling‐corrected TST calculation of the rate constant. The rate constant for the bond fission\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm CH}_{\rm 2} {\rm O + M} \to {\rm HCO + H + M} $$\end{document}at high temperatures was determined by an RRKM extrapolation of direct low temperature measurements. This mechanism has been successfully tested against laser‐schlieren measurements covering the temperature range 2200–3200 K. These measurements are insensitive to all but the above two reactions and they confirm the large, non‐Arrhenius rate for the abstraction reaction derived here from theory. Modeling of previous experiments using IR emission, ARAS, and CO laser absorption with this mechanism is quite satisfactory. The branching ratio of the rate of the faster molecular dissociation (CH2O + (M) → CO + H2+ (M)), to that of the bond fission reaction, was estimated to be no more than 2 or 3 over 2000 to 3000 K. Such a ratio is consistent with one recent theoretical estimate and most of the experimental observations. © 1993 John Wil

ISSN:0538-8066    

DOI:10.1002/kin.550250408

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractThermal decomposition of formaldehyde diluted with Ar was studied behind reflected shock waves in the temperature range of 1200–2000 K at total pressures between 1.3 and 3.0 atm. The study was carried out for compositions from the concentrated mixture, 4% CH2O, to the highly dilute mixture, 0.01% CH2O by using time‐resolve IR‐laser absorption and IR‐emission, and a single‐pulse technique. From a computer‐simulation study, the mechanism and the rate‐constant expressions that could explain all of our data and previously reported ARAS data were discussed. This data obtained over a wide concentration range from 50 ppm CH2O to 4% CH2O were satisfactorily modeled by a five‐reaction mechanism. © 1993 John

ISSN:0538-8066    

DOI:10.1002/kin.550250409

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe multiple photon absorption and decomposition of ethanol irradiated by pulsed 9P18 infrared radiation (1048.7 cm−1) from a TEA CO2laser has been studied in the fluence range 15 to 5 J cm−2. The absorption cross‐section is pressure‐dependent due to rapid collisional rotational hole‐filling. At low pressures the only important decomposition channel following absorption is molecular dehydration of ethanol to yield ethene, but at higher pressures hydrogen, methane, carbon monoxide, ethane, and ethyne are also produced. In the irradiation of pure ethanol under ‘collision‐free’ conditions, thermal decomposition following collisional redistribution of energy makes only a small contribution to the overall decomposition yield at fluences above 3.5 J cm−2but may become more significant at lower fluences. Modelling using RRKM calculations has been employed to link measured absorbed energies to extents of decomposition of ethanol. Both these calculations and the absorption measurements indicate that at low pressures only a fraction of the irradiated ethanol molecules absorb the 9P18 radiation. © 1993 Jo

ISSN:0538-8066    

DOI:10.1002/kin.550250410

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

ISSN:0538-8066    

DOI:10.1002/kin.550250401

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY