H-atom abstraction from alcohols by alkyl radicals. Studies on the limiting stage

Vyazovkin, Vladimir L.; Большаков, Б. В.; Tolkatchev, V.A.

doi:10.1016/0301-0104(85)80151-8

Cited by 20 publications

(4 citation statements)

References 37 publications

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“…It follows from the data of Ref. 38 that the results of the experiments are in line with both expressions, (1)and (26). It is obvious that the decrease in the concentration of methyl radicals corresponds to a functional dependence, which decreases more slowly than the exponential one.…”

Section: The Effect O1" Matrix Inhomogeneity On the Kinetics Of Reactsupporting

confidence: 72%

“…The dependence of the rate constant of chemical reaction on temperature and the role of intermolecular vibrations that result in the encounter of reagents and thus in a decrease in the potential barrier 4~ were taken into account in deriving Eq. (26). As a result, the probability of tunneling of an atom increases as temperature increases.…”

Section: The Effect O1" Matrix Inhomogeneity On the Kinetics Of Reactmentioning

confidence: 95%

“…A comparison of tile results of the calculation carried out by formula (26) and experimental data zs.z6 for the reaction of atkyl radicals with alcohols made it possible to draw the following conclusions: a) the dependence of the concentration of radicals on time and on temperature is well described by Eq. (I); b) the width of distribution of the distances between the reagents becomes more narrow as temperature increases; and c) the center of gravity of distribution of the distances shifts toward shorter distances (i.e., to higher reaction rates) as temperature increases.…”

Section: The Effect O1" Matrix Inhomogeneity On the Kinetics Of Reactmentioning

confidence: 97%

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Polychronous kinetics with nonstationary rate constants. Effect of a medium

1997

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2450Characteristic features of the kinetics of solid-state cage reactions with distributed parameters of the relaxing matrix were considered. Depending on the ratio of the constants of the reaction rate and relaxation of environment, the kinetics of chemical conversions can be either exponential or nonexponential. Plausible reasons for the unsteady-state character of the kinetics of the processes of two types, viz., the reactions of alkyl radicals in amorphous alcohol matrices and conversions in biological systems, were discussed. The main reason for the unsteady-state character of the reactions of the first type is a dispersion of the equilibrium distances between the reagents. Kinetics of the reactions of the second type, such as rebinding of the ligands in the heine-containing proteins (e.g., in myoglobin), is determined by the distances in the pairs of reagents and the relaxation transitions.Key words: biological systems, intermolecular vibrations, nonstationary kinetics, relaxation of matrix, tunneling of atom.The kinetics of many solid-state chemical processes are inconsistent with conventional laws that the first-or second-order reactions obey. These include such radiochemical and photochemical processes as a stepwise recombination of radicals,t,z diffusion-controlled rearIions, 3.a the exothermic reactions of abstraction of the atoms from the molecules by radicals, 5,6 recombination of the ligands with the skeleton and relaxation of heinecontaining proteins. 7-1~ as well as electron transfer, ll,tz Certain stages of the processes mentioned above can be identical. Thus, for instance, diffusion of free valences is one of the stages of the stepwise recombination of radicals, while a diffusion-controlled reaction involves a chemical act, etc. Because of the multistage character of the complex reaction, the limiting stage responsible for the unsteady-state character of the kinetics (i.e., for the change in the rate constants with time) can hardly be distinguished. We restrict our attention to consideration of only one class of processes, the cage reactions with the atom transfer, in which only one stage plays a determining role.The kinetic cuwes of the hydrogen atom transfer from matrix molecules to a radical were found to differ from conventional kinetic curves, t3,14 This was explained by diffusion of radicals and superposition of the first-and second-order processes. In Ref. t5, an assumption was made on the basis of anaiysis of experimental data 16-18 that values of the rate constant in such processes can be dispersed due to various configurations in the pairs of re age n Is. * Nonstationary kinetics also were observed in studies of the low-temperature limit of the rate of the reactions of abstraction of the hydrogen atom in the methanol and ethanol matrices, zl,zz where the nonexponential character of kinetics was explained by inhomogeneity of the matrix. To calculate the rate constant, the initial segment of the kinetic curve was used under the assumption that the kinetics of the reaction can be describe...

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Section: The Effect O1" Matrix Inhomogeneity On the Kinetics Of Reactsupporting

confidence: 72%

Section: The Effect O1" Matrix Inhomogeneity On the Kinetics Of Reactmentioning

confidence: 95%

Section: The Effect O1" Matrix Inhomogeneity On the Kinetics Of Reactmentioning

confidence: 97%

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Polychronous kinetics with nonstationary rate constants. Effect of a medium

1997

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“…The treatment presented allows us to describe the kinetics of chemical processes accounting for the dispersion of equilibrium distances and matrix relaxation. Earlier experiments were conducted on the kinetics of low-temperature solid-phase reactions of the methyl radicals with hydrogen-containing alcohol molecules. ,− The kinetics of these reactions was explained − by dispersion of equilibrium distances between reagents. Of course, in these low-temperature processes there is no motion in the space of rate constants.…”

Section: Discussionmentioning

confidence: 99%

Polychronic Kinetics of Chemical Reactions with the Blending of Rate Constants

1997

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The peculiarities of the kinetics of solid-phase cage reactions with distributed parameters for the relaxing matrix are considered in this study. It is shown that the kinetics of chemical conversions can be either exponential or nonexponential depending on the ratio of the rate constants and relaxation constant. The different cases of the effect of the reaction system on the relaxation constant are considered. The possibility of the analysis of experimental data for reactions of alkyl radicals in amorphous alcohol matrixes and conversions in biological systems within the framework of the treatment developed is discussed.

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Isotopic effects in H‐atom abstraction from matrix molecules by methyl radicals in poly(methyl methacrylate). Determination of the scale of relative reagent motions

Bagryansky

Tolkatchev

1987

J. Polym. Sci. A Polym. Chem.

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SynopsisThe H-abstraction by CH, and CD, radicals from PMMA molecules is investigated at 77 and 120 K. The film samples are prepared from the mixture of normal and deuterated PMMA. The reaction is of the first order with respect to radical concentration. Its rate is retarded in time a s compared to the exponential law. An analysis of the molecular product ratio for the reaction allows the primary isotopic effect to be estimated. The initial reaction rate depends nonmonotonously on the proportion of the deuterated polymer in the matrix. Namely, there exists a range of isotopic compositions, with the reaction rate increasing with the portipn of a deuteropolymer in the mixture. At 77 K the reaction rate of CD, is lower than that of CH, in PMMA of a mixed isotopic composition. This results from the proximity of CD, with unreactive parental macromolecules, which reduces the local H concentrations around CD, as compared to the mean concentration. At 120 K the radical decay rates do not differ. This fact cannot be explained by unfrozen mobilities of radicals at high temperatures or by their migration from parental macromolecules. The phenomenon is interpreted in terms of thermal fluctuations involving a great area around a radical and ensuring its migration within this area. The electron spin echo method is used to estimate the distance between a radical and its parental macromolecule, a 2 9.6 A. This is the lower limit of the reaction accessibility.

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H-atom abstraction from alcohols by alkyl radicals. Studies on the limiting stage

Cited by 20 publications

References 37 publications

Polychronous kinetics with nonstationary rate constants. Effect of a medium

Polychronous kinetics with nonstationary rate constants. Effect of a medium

Polychronic Kinetics of Chemical Reactions with the Blending of Rate Constants

Isotopic effects in H‐atom abstraction from matrix molecules by methyl radicals in poly(methyl methacrylate). Determination of the scale of relative reagent motions

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