摘要:

AbstractThis article deals with the kinetics of decomposition of four isomers: NCl‐(L)‐2‐Aminohexanoic acid, NCl‐(L)‐Leucine, NCl‐(L)‐Isoleucine, and NCl‐(L)‐tert‐Leucine, in mild acid and basic aqueous solution. An alternative mechanism for this process which takes place through a nitrenium‐like ion transition state is proposed. This mechanism is in consonance with the experimental data and with those available from the literature

ISSN:0538-8066    

DOI:10.1002/kin.550250102

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractAtmospheric pressure absolute rate coefficients have been determined for the gas phase reaction of OH radicals with methyl chloride (k1), methylene chloride (k2), and chloroform (k3) over an extended temperature range using a laser photolysis/laser‐induced fluorescence technique. The rate coefficients are best described by the following modified Arrhenius equations:\documentclass{article}\pagestyle{empty}\begin{document}$$ k_1 (295 - 995) = (4.64 \pm 0.58) \times 10^{ - 12} ({\rm T/300)}^{{\rm 0}{\rm .89}} \exp \{ (- 1447 \pm 75)/T\} cm^3 molec^{ - 1} s^{ - 1} $$\end{document}\documentclass{article}\pagestyle{empty}\begin{document}$$ k_1 (295 - 995) = (2.01 \pm 0.17) \times 10^{ - 12} ({\rm T/300)}^{{\rm 1}{\rm .09}} \exp \{ (- 771 \pm 48)/T\} cm^3 molec^{ - 1} s^{ - 1} $$\end{document}\documentclass{article}\pagestyle{empty}\begin{document}$$ k_1 (295 - 774) = (2.71 \pm 0.23) \times 10^{ - 13} ({\rm T/300)}^{{\rm 1}{\rm .52}} \exp \{ (- 261 \pm 42)/T\} cm^3 molec^{ - 1} s^{ - 1} $$\end{document}Measurements were obtained as a function of excimer photolysis intensity and are compared with previous results and extended to higher temperatures. Photolysis intensities in excess of 12 mJ‐cm−2were found to measurably increase (up to a factor of 2) the rate coefficients fork3between 400–775 K, with the effect increasing with increasing temperature. A similar, yet much smaller (ca. 20–35%) increase was observed fork2between 675–955 K. No effect was observed fork1at any temperature. Relative absorption coefficient measurements at 193.3 nm indicated that chlorinated methane photolysis increases with both increasing temperature and increasing chlorine substitution. These measurements suggest that reactant photolysis may be responsible for the observed dependence ofk2andk3on photolysis intensity at elevated temperatures. The puzzling and disconcerting discrepancy between previously published high temperature measurements ofk3and transition state model predictions is reconciled with these latest measurements. © 1993 John Wil

ISSN:0538-8066    

DOI:10.1002/kin.550250103

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe Absolute rate constants for the gas‐phase reactions of NO3with HO2and OH have been determined using the discharge flow laser magnetic resonance method (DF‐LMR). Since OH was found to be produced in the reaction of HO2with NO3, C2F3Cl was used to scavenge it. The overall rate constant,k1, for the reaction, HO2+ NO3→ products, was measured to bek1=(3.0 ± 0.7)×10−12cm3molecule−1s−1at (297 ± 2) K and P=(1.4 – 1.9) torr. This result is in reasonable agreement with the previous studies. Direct detection of HO2and OH radicals and the use of three sources of NO3enabled us to confirm the existence of the channel producing OH:HO2+NO3→OH+NO2+O2(1a); the other possible channel is HO2+NO3→HNO3+O2(1b). From our measurements and the computer simulations, the branching ratio,k1a/(k1a+k1b), was estimated to be (1.0 −0.3+0.0). The rate coefficient for the reaction of OH with NO3was determined to be (2.1 ± 1.0) × 10−11cm3molecule−1s

ISSN:0538-8066    

DOI:10.1002/kin.550250104

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractOne of the important applications of chemical kinetics is the attempt to understand complex processes through kinetic modeling. This process frequently requires that rate constants be obtained by extrapolation of data either to higher or lower temperatures than the experimental, or by estimation or correlation with such data. Thermochemical kinetics combined with conventional transition state theory forms a framework from which this may be done. However, rate data for H transfer reactions may have a significant contribution from tunneling. In this work, a one dimensional approach to tunneling, consistent with conventional transition state theory, is taken to show how tunneling affects the extrapolation and correlation of rate constants in thermochemical kinetics. It is concluded that extrapolation and correlation are both quite good even when tunneling comprises 80% of the reaction. However, this is not without limitations, which are discussed. © 1993 John Wiley&Sons, Inc

ISSN:0538-8066    

DOI:10.1002/kin.550250105

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe stoichiometry and kinetics of the oxidation of thiourea (SC(NH2)2) by chlorine dioxide (ClO2) have been studied by uv‐vis spectrophotometry using conventional and stopped‐flow mixing techniques at 25.0 ± 0.1°C, pH 0.3–4.8. In high acid and initial 10:1 molar ratio of thiourea to chlorine dioxide, thiourea is oxidized relatively rapidly to dithiobisformamidine ion ((NH2)2CSSC(NH2)22+), which slowly decomposes to thiourea, sulfur, and cyanamide (NCNH2). In high acid and excess ClO2, thiourea is oxidized to relatively stable formamidine sulfinic acid ((NH) (NH2)CSO2H). In high acid and molar ratios of ClO2to thiourea of 5:1 and higher, some oxidation to formamidine sulfonic acid ((NH) (NH2)CSO3H) occurs. At lower acidity, along with Cl−, the major ClO2reduction product, byproduct sulfate is detected and, at pH<3, ClO2−, also, appears. Kinetics data were collected for high excess thiourea with varying pH. The [ClO2]‐time curves are straight lines with negative slopes that increase in magnitude with increasing [thiourea]. The dependence on [thiourea] is first‐order; the dependence on [ClO2] is zero‐order for 90% of reaction. With decreasing pH, the rate increases and the disappearance of ClO2becomes autocatalytic. Studies of the effects of reaction products on the rate of reaction lead to the conclusion that autocatalysis at low pH is due to the greater reactivity of HClO2compared with ClO2−. A 10‐step mechanism incorporating a slow one‐electron transfer from thiourea to ClO2to generate the (NH) (NH2)CS · radical and subsequent more rapid reactions has been constructed and implemented in a computer simulation which provides a reasonably accurate fit to the observed kinetics curves. ©

ISSN:0538-8066    

DOI:10.1002/kin.550250106

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

ISSN:0538-8066    

DOI:10.1002/kin.550250107

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

ISSN:0538-8066    

DOI:10.1002/kin.550250101

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY