Influence of Nature of Matrix on Reactivity of Electrons
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Influence of the Nature of a Matrix on the Reactivity of Electrons in Irradiated Systems A. K. Plkaev,* B. G. Ershov, and 1. E. Makarov lnstltute of Physical Chemistry of the Academy of Sciences of the USSR, Moscow, USSR (Received July 23, 1975)
In this paper experimental data on the reactivity of solvated and mobile electrons in different irradiated systems are considered and generalized. Special attention is paid to the reactivity of solvated electrons toward ions of transuranium elements in dilute and concentrated alkali and carbonate aqueous solutions, toward some compounds in alcohols and melted alkali halide salts, and to the reactivity of mobile electrons in some glassy systems. Also the results of a study of decay kinetics of F centers in alkali halide crystals a t different temperatures by pulse radiolysis method are described, These data are discussed from the point of view of the influence of dielectric constant, viscosity of the medium, and other parameters on the reaction rate of solvated electrons. The problem of a tunnel mechanism of electron reactions in irradiated systems is also considered.
1. Introduction
Immediately after the detection of hydrated electrons (eaq-) in irradiated liquid water a great number of works on the reactivity of this particle were performed. Reviews of these works are available, for example, in book form.lJ At that time the reactivity of solvated electrons (es-) in other liquids was studied occasionally. T h a t was a reason why in radiation chemistry the idea of similar reactivity of es-, in particular, in water and simple alcohols, was widespread. However, in a 1971 paper3 it was shown that the rate constants for the reactions between e,- and anions in methyl alcohol are considerably lower than the rate constants of the reactions of eaq- with the same anions in water. The rate constants of the reactions of e,- with electroneutral molecules, as was measured in cited work, are approximately the same in water and methyl alcohol. A decrease of the rate constant of the reaction of e,- with the NOS- ion during the transition from water to methyl alcohol was also ob~ e r v e d .Later ~ a similar phenomenon was observed for ethyl alcohol.6 Due to lower dielectric constants, rate constants for the reactions of e,- with cations in simple alcohols may be larger than in water; for example, it is true for k(e,Zn2+) in water-ethanol mixtures5 and for k(e,ROH2+) in methano16s7and ethanoL6 According to work8 in glycerine rate constants for all the studied e,- reactions are much lower than the rate constants of the same reactions of eaq-. This effect was explained by the high viscosity of glycerine. The above-mentioned variation of e,- reactivity during the transition from water to alcohols is illustrated in Table I. In this table literature data on absolute rate constants of e,- reactions in these liquids which were measured by pulse radiolysis method are listed. In addition to the reactivity of e,- in water and alcohols in the literature there are values of the rate constants of some reactions of e,- in other irradiated liquids: ammonia,'"J4 amines,l5J6 hexamethylphosphoric triamide,17J8 t e t r a h y d r o f ~ r a n ' ~and - ~ ~other ether^,*^^^^ water-ethan01,~ water-dioxane,$ and water-dimethyl sulfoxide24 mixtures, etc. These data also testify to the influence of the matrix nature on e,- reactivity. At sufficiently low temperatures the mobile solvated electron is stabilized; this is the trapped electron (et,-). In the case of et,- the question about far electron transfer by a tunnel mechanism is widely discussed in the literature (see,
+
+
for example, ref 25-29). The tunnel mechanism is also postulated by some aUthors2J8130 for the reactions of eaq- in liquid water. In the present work data on the reactivity of e,- toward ions of transuranium elements in dilute and concentrated alkali and carbonate aqueous solutions and toward some compounds in alcohols are presented and discussed. The choice of ions of transuranium elements is due to the variety of forms of their existence and, as a consequence, to the possibility of the investigation of the eaq- reactivity toward ions of these elements in different valence states. Experiments with alcohols were undertaken for two reasons. First there is the contradiction in literature data on the reactivity of e,- toward the NO3- ion (see Table I). Second, recent new information on the properties of e,- in alcohols has been published (for example, on the mobility of these species31J2). This makes possible more definite conclusions about the influence of solvent nature on e,- reactivity. Also in this work the results of the investigation of electron reactivity toward some acceptors during y radiolysis of glassy alcohols and water-alcohol mixtures a t 77 K are described. The purposes of this investigation are the comparison of the reactivity of electrons toward the same solutes in liquid and solid phases and the elucidation of the influence of electron trap depth on the probability of a tunnel transfer of an electron to the acceptor molecule. Another type of electron center is the F center. It is an electron localized in an anion vacancy. F centers are formed, for example, during the irradiation of alkali halide crystals. Many years ago the important role of tunnel processes in the behavior of F-centers was noted.j3 It is interesting to obtain experimental evidence for electron tunneling in these systems. Alkali halide crystals are solids and for these systems the diffusion processes, under ordinary conditions, are slightly characteristic in the comparison with liquids. This circumstance favors the search of such evidence. To obtain this evidence the kinetics of F-center decay a t different temperatures has been studied by the pulse radiolysis method. Melts of alkali halide compounds were also an object of the investigation in the present work. It has been ~ h o w n
