Solvated electron reaction rates

It is clear that the study of solvated electron reaction rates, using metal-amine solutions as the source of electrons, is currently in the early stages of development, and the conclusions drawn must be very limited in scope. If the species present in metal solutions in ethylenediamine and other amines can be firmly identified, the techniques described in this paper should prove useful for studying reactions of solvated electrons with... 

Solvated Electron Reaction Rates. Purines and Pyrimidines. The reaction rates of e aq with purines and pyrimidines at neutral pH are shown in Table II. All are very reactive, the reaction rates being close to diffusion controlled. However, the pyrimidine cytosine, which has an amino group at the C-4 position, is somewhat less reactive than thymine and uracil which have carbonyl groups at this position. Adenine, which also has an amino group in this position, has a very high reactivity, but this is probably because of the presence of the positively charged imidazole ring. 

We should remember (1) that the activation energy of eh reactions is nearly constant at 3.5 0.5 Kcal/mole, although the rate of reaction varies by more than ten orders of magnitude and (2) that all eh reactions are exothermic. To some extent, other solvated electron reactions behave similarly. The theory of solvated electron reaction usually follows that of ETR in solution with some modifications. We will first describe these theories briefly. This will be followed by a critique by Hart and Anbar (1970), who favor a tunneling mechanism. Here we are only concerned with fe, the effect of diffusion having been eliminated by applying Eq. (6.18). Second, we only consider simple ETRs where no bonds are created or destroyed. However, the comparison of theory and experiment in this respect is appropriate, as one usually measures the rate of disappearance of es rather than the rate of formation of a product. 

Hart and Anbar [17] pointed out that effective rate coefficients of solvated electron reactions with many strong oxidants were larger than those implied by the encounter distances for the solvated electron and oxidant by a factor of 1.5—2.0 times. Some of these effective encounter distances are listed in Table 5, together with others from recent work. For... 

Jhe discovery by radiation chemists of solvated electrons in a variety of solvents (5, 16, 20, 22, 23) has renewed interest in stable solutions of solvated electrons produced by dissolving active metals in ammonia, amines, ethers, etc. The use of pulsed radiolysis has permitted workers to study the kinetics of fast reactions of solvated electrons with rate constants up to the diffusion-controlled limit (21). The study of slow reactions frequently is made difficult because the necessarily low concentrations of electrons magnify the problems caused by impurities, while higher concentrations frequently introduce complicating second-order processes (9). The upper time limit in such studies is set by the reaction with the solvent itself. 

Early studies showed tliat tire rates of ET are limited by solvation rates for certain barrierless electron transfer reactions. However, more recent studies showed tliat electron-transfer rates can far exceed tire rates of diffusional solvation, which indicate critical roles for intramolecular (high frequency) vibrational mode couplings and inertial solvation. The interiDlay between inter- and intramolecular degrees of freedom is particularly significant in tire Marcus inverted regime [45] (figure C3.2.12)). 

The one-electron reduction of thiazole in aqueous solution has been studied by the technique of pulse radiolysis and kinetic absorption spectrophotometry (514). The acetone ketyl radical (CH ljCOH and the solvated electron e were used as one-electron reducing agents. The reaction rate constant of with thiazole determined at pH 8.0 is fe = 2.1 X 10 mole sec in agreement with 2.5 x 10 mole sec" , the value given by the National Bureau of Standards (513). It is considerably higher than that for thiophene (6.5 x 10" mole" sec" ) (513) and pyrrole (6.0 X10 mole sec ) (513). The reaction rate constant of acetone ketyl radical with thiazolium ion determined at pH 0.8 is lc = 6.2=10 mole sec" . Relatively strong transient absorption spectra are observed from these one-electron reactions they show (nm) and e... 

It is now well established that in lithium batteries (including lithium-ion batteries) containing either liquid or polymer electrolytes, the anode is always covered by a passivating layer called the SEI. However, the chemical and electrochemical formation reactions and properties of this layer are as yet not well understood. In this section we discuss the electrode surface and SEI characterizations, film formation reactions (chemical and electrochemical), and other phenomena taking place at the lithium or lithium-alloy anode, and at the Li. C6 anode/electrolyte interface in both liquid and polymer-electrolyte batteries. We focus on the lithium anode but the theoretical considerations are common to all alkali-metal anodes. We address also the initial electrochemical formation steps of the SEI, the role of the solvated-electron rate constant in the selection of SEI-building materials (precursors), and the correlation between SEI properties and battery quality and performance. 

Section 3 deals with reactions in which at least one of the reactants is an inorganic compound. Many of the processes considered also involve organic compounds, but autocatalytic oxidations and flames, polymerisation and reactions of metals themselves and of certain unstable ionic species, e.g. the solvated electron, are discussed in later sections. Where appropriate, the effects of low and high energy radiation are considered, as are gas and condensed phase systems but not fully heterogeneous processes or solid reactions. Rate parameters of individual elementary steps, as well as of overall reactions, are given if available. 

Of all the solvated electrons, eh is the most reactive several thousand of its reactions have been measured and, in many cases, activation energies, the effects of pH, and so forth, determined (Anbar and Neta, 1965 Anbar et al, 1973 Ross, 1975 CRC Handbook, 1991). Relatively few reactions of eamhave been studied because of its low reactivity. Rates of reactions of the solvated electron with certain scavengers are also available in alcohols, amines, and ethers. 

TABLE 6.7 Rates of Reaction of the Solvated Electron (Continued)... 

The solvated electron is reactive in alcohols, both with solutes and solvents (Watson and Roy, 1972). With methanol, ethanol, and 1- and 2-propanols, somewhat different rates of e-solvent reactions have been measured by Freeman (1970) and by Baxendale and Wardman (1971). However, the (pseudo-first-order) rates... 

Watson, C., Jr., and Roy S. (1972), Selected Specific Rates of Reactions of the Solvated Electrons in Alcohols, NSRDS-NBS 42, U.S. Government Printing Office, Washington, D.C. 

Due to these reactions, hydrogen peroxide is an intermediate product of radiolysis of aerated water. Rate constants of free radical reactions with dioxygen and hydrogen peroxide are collected in Table 3.19. For the characteristics of solvated electron and information about its reactions, see monographs [219-223],... 

Rate Constants of Solvated Electron, H, and HO Reactions with Dioxygen and Hydrogen Peroxide in Water at Room Temperature [223-225]... 

Interfacial electron transfer is the critical process occurring in all electrochemical cells in which molecular species are oxidized or reduced. While transfer of an electron between an electrode and a solvated molecule or ion is conceptually a simple reaction, rates of heterogeneous electron transfer processes depend on a multitude of factors and can vary over many orders of magnitude. Since control of interfacial electron transfer rates is usually essential for successful operation of electrochemical devices, understanding the kinetics of these reactions has been and remains a challenging and technologically important goal. 

Electron and charge transfer reactions play an important role in many chemical and biochemical processes. Dynamic solvation effects, among other factors, can largely contribute to determine the reaction rate of these processes and can be studied either by quantum mechanical or simulation methods. 

Several methods have been employed to extract the rate constant of the addition of nucleophiles to the aryl radicals from the kinetics of Sr I reactions. Relative reactivities of two nucleophiles towards the same aryl radical have been obtained from the ratio of the two substitution products after preparative-scale reaction of the substrate with a mixture of the two nucleophiles under photochemical or solvated-electron induction (Galli and Bunnett, 1981). 

---