Polychronous kinetics with nonstationary rate constants. Effect of a medium

Abstract

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...

4 Dispersive kinetics

“…Solution for this particular case was presented and discussed recently by Goldanskii et al 8,9 It was shown that for l ì k 1 ,k 2 , in our notation,…”

“…0 and from eqns. (7) and (8), respectively, g 1 = k 1 and g 2 = k 2 ; furthermore, from eqn. (12) h = a/(1 + a), which yields, cf.…”