Reactivity hydrated electron

According to the Marcus theory [64] for outer-sphere reactions, there is good correlation between the heterogeneous (electrode) and homogeneous (solution) rate constants. This is the theoretical basis for the proposed use of hydrated-electron rate constants (ke) as a criterion for the reactivity of an electrolyte component towards lithium or any electrode at lithium potential. Table 1 shows rate-constant values for selected materials that are relevant to SE1 formation and to lithium batteries. Although many important materials are missing (such as PC, EC, diethyl carbonate (DEC), LiPF6, etc.), much can be learned from a careful study of this table (and its sources). 

Discovery of the hydrated electron and pulse-radiolytic measurement of specific rates (giving generally different values for different reactions) necessitated consideration of multiradical diffusion models, for which the pioneering efforts were made by Kuppermann (1967) and by Schwarz (1969). In Kuppermann s model, there are seven reactive species. The four primary radicals are eh, H, H30+, and OH. Two secondary species, OH- and H202, are products of primary reactions while these themselves undergo various secondary reactions. The seventh species, the O atom was included for material balance as suggested by Allen (1964). However, since its initial yield is taken to be only 4% of the ionization yield, its involvement is not evident in the calculation. 

Ionizing radiations (a, ft and y) react unselectively with all molecules and hence in the case of solutions they react mainly with the solvent. The changes induced in the solute due to radiolysis are consequences of the reactions of the solute with the intermediates formed by the radiolysis of the solvent. Radiolysis of water leads to formation of stable molecules H2 and H2O2, which mostly do not take part in further reactions, and to very reactive radicals the hydrated electron eaq, hydrogen atom H" and the hydroxyl radical OH" (equation 2). The first two radicals are reductants while the third one is an oxidant. However there are some reactions in which H atom reacts similarly to OH radical rather than to eaq, as e.g. abstraction of an hydrogen atom from alcohols, addition to a benzene ring or to an olefinic double bond, etc. 

How does structure determine organic reactivity, 35, 67 Hydrated electrons, reactions of, with organic compounds, 7,115 Hydration, reversible, of carbonyl compounds, 4, 1 Hydride shifts and transfers, 24, 57... 

The hydrated electron behaves as a nucleophile in its reactions with organic molecules and its reactivity is greatly enhanced by electron-withdrawing substituents attached to aromatic rings or adjacent to double bonds. Some of the features of the reactivity of are illustrated by the data in Table 3. 

Oxidative reactions of 2AP(-H) have been studied in oxygen-saturated solutions because 2AP(-H) radicals do not exhibit observable reactivities towards O2 [10]. In contrast, O2 rapidly reacts with hydrated electrons (rate constant of 1.9x10 ° s ) [51] and hydrated electrons do not interfere in... 

In the century since its discovery, much has been learned about the physical and chemical properties of the ammoniated electron and of solvated electrons in general. Although research on the structure of reaction products is well advanced, much of the work on chemical reactivity and kinetics is only qualitative in nature. Quite the opposite is true of research on the hydrated electron. Relatively little is known about the structure of products, but by utilizing the spectrum of the hydrated electron, the reaction rate constants of several hundred reactions are now known. This conference has been organized and arranged in order to combine the superior knowledge of the physical properties and chemical reactions of solvated electrons with the extensive research on chemical kinetics of the hydrated electron. 

The solvated electron has been studied in a number of organic liquids, among which are the aliphatic alcohols (27, 28, 3, 2d, 2, 27), some ethers (25, 5), and certain amines (9, 22, 2). Of these systems, it is only in the alcohols, to which this paper is principally but not exclusively directed, that both the chemical reactivity and the optical absorption spectrum of the solvated electron have been investigated in detail. The method used in these studies is that of pulse radiolysis (22, 22), developed some five years ago. The way was shown for such investigations of the solvated electron by the observation of the absorption spectrum of the hydrated electron (6, 28, 19) and by the subsequent kinetic studies (2d, 22, 20) which are being discussed in other papers in this symposium. 

The following review will summarize and systematize the available knowledge on the chemical reactivity of solvated electrons and the products of their reactions. Since most of the work was carried out with solvated electrons in aqueous solutions, we shall confine ourselves mainly to hydrated electrons. We do not intend to discuss the interaction of solvated electrons with their solvents since this will be covered in other chapters. 

Hydrated electrons are also formed as a product of the interaction of hydroxide ions with hydrogen atoms. This reaction was first established kinetically (4, 6, 81, 82, 99) and then corroborated spectro-photometrically using flash radiolysis (95, 96). It should be noted that the rate of the H + OH- - e aq reaction is only 1.8 X 107 M l sec.-1 (66) thus, this step may become rate determining in many reactions with reactive substrates. 

The Reactivity of Different Chemical Species toward Hydrated Electrons and the Products of these Reactions... 

Water, H20 + and Bronsted Acids. The most important reagent in the chemistry of e aq is obviously the solvent, water. Were it not for the relatively low reactivity of elq with H20 most of our information on hydrated electrons would be merely hypothetical. Fortunately the rate of the eaq + H20 - H + OH - reaction is slow enough to enable one to examine the kinetic behavior of any solute reacting with e aq at a rate over 106 Af-1 sec.-1... 

Comparing the reactivity of two metal ions of the same group in the periodic table invariably shows that the member of the higher series is more reactive. This is true for Cd+2 compared with Zn+2 Pd+2 compared with Ni+2 Pb+2 compared with Sn+2 as well as for Sb+6 compared with Asv (11). This effect is most probably caused by the higher availability of vacant electron orbitals, as well as by an increased polarizability of the molecule into which the hydrated electron is to be incorporated on encounter. 

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