Solvated-electron reduction

Electron-Transfer Reduction of 02. Within aqueous solutions the most direct means to the electron-transfer reduction of dioxygen is by pulse radiolysis. Irradiation of an aqueous solution by an electron beam yields (almost instantly 10-12 s) solvated electrons [e (aq)], hydrogen atoms (H-), and hydroxyl radicals (HO-)- If the solution contains a large excess of sodium formate [Na+ 0(0)CH] and is saturated with 02, then the radiolytic electron flux efficiently and cleanly reduces 02 to superoxide ion (O ) 21-25... 

Among possible alternative isomers, the preferential formation of diene 11, with the indicated location of the double bonds, is determined by the structure of the initially formed, most stable intermediate, radical-anion 14. Thus, the reduction of a single bond of toluene, as is represented in equation 5, requires the presence of an electron source (sodium), a solvent capable of electron solvation (liquid ammonia), and a proton donor (alcohol). 

Another reduction is possible when one wishes to treat only one or a few degrees of freedom quantum-mechanically while the rest of the system can be treated still in a classical way. First pioneering studies along such lines treated the problem of electron solvation in molten salts and liquid ammonia. But, it must be noted that, when one studies the dynamics of quantum degrees of freedom coupled to a classical environment, particular care is required This mixed quantum-classical dynamics has subtle features, and is still an active area of research. 

A wealth of information on the reduction of metal ions in aqueous solutions has been obtained and a compilation was published in 1988 [20], However, alkali or alkaline earth metal ions such as Li Na or cannot be reduced by the hydrated electron in aqueous solution but can form an ion pair with the solvated electron in polar liquids. Among the various reactions of the solvated electron, the reduction of halogenated hydrocarbons is often used in radiation chemistry to produce well-defined radicals because of the selective cleavage of the carbon-halogen bond by the attack ofthe solvated electron. This reaction produces the halide ion and a carbon-centered radical, and is of great interest for environmental problems related to the destruction of halogenated organic contaminants in water and soil [21,22]. 

Alkali metal arene radical anion complexes are useful sources of solvated electrons for reductive desulfonylation reactions.14 Aromatic compounds such... 

Electron Acceptance Reduction and Adduct Formation. Acceptance of electrons at specific sites on amino acids and peptides depends on their reactivities and produces different chemical consequences. Among the sites of particular importance are the terminal amino and carboxyl groups, the ring groups, the peptide carbonyl, and the sulfur bonds. Reactivities of these are reflected in the rate constants for reaction of solvated electrons with individual amino acids in aqueous solutions, as shown in Table I and as discussed by Simic (53). More detailed information, however, regarding the stepwise progression from attachment to specific radical formation has been obtained from low temperature studies. 

Alternative methods for preparing solvated alkali-metal graphites include either secondary solvation of the binary compounds or reduction of graphite by solutions of the alkali metal in electron-solvating media such as hexamethylphosphorus triamide (HMPT) or polycyclic aromatic systems in ethers , leading, e.g., to HMPT- or ether-solvated compounds. 

The TEM patterns of the freshly prepared specimen point to particles S 1 nm in size to be present, which is confirmed by the SAXS method. The second examination of the sample kept in air did not allow to determine these particles on a support, which may be connected with a decrease of contrast of representation due to oxidation of silver particles in air. This is proved by the data of diffuse reflectance electron spectroscopy. The data of Table 1 indicate that the synthesis of Ag samples from silver cation reduction with electrons solvated in liquid ammonia allows to obtain stable colloidal solution of highly dispersed silver particles of 2-50 nm in size (dp = 3 nm), silver blacks with specific surface of 2.3 and 7.6 m /g, and supported silver samples with a great contribution of particles < 6 nm in size. 

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... 

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

The Birch reduction of a benzenoid compound involves the addition of two electrons and two protons to the ring. The order in which these additions occur has been the subject of both speculation and study. Several reviews of the subject are available and should be consulted for details. The present discussion is concerned with summarizing data that is relevant to understanding the reaction from the preparative point of view. For convenience, reaction intermediates are shown without indicating their solvation by liquid ammonia. This omission should not obscure the fact that such solvation is largely responsible for the occurrence of the Birch reduction. 

Alkali and alkaline-earth metals have the most negative standard reduction potentials these potentials are (at least in ammonia, amines, and ethers) more negative than that of the solvated-electron electrode. As a result, alkali metals (M) dissolve in these highly purified solvents [9, 12] following reactions (1) and (2) to give the well-known blue solutions of solvated electrons. 

OCV conditions, by a newly formed SEI is expected to be a slow process. The SEI is necessary in PE systems in order to prevent the entry of solvated electrons to the electrolyte and to minimize the direct reaction between the lithium anode and the electrolyte. SEI-free Li/PE batteries are not practical. The SEI cannot be a pure polymer, but must consist of thermodynamically stable inorganic reduction products of... 

On the other hand, Doblhofer218 has pointed out that since conducting polymer films are solvated and contain mobile ions, the potential drop occurs primarily at the metal/polymer interface. As with a redox polymer, electrons move across the film because of concentration gradients of oxidized and reduced sites, and redox processes involving solution species occur as bimolecular reactions with polymer redox sites at the polymer/solution interface. This model was found to be consistent with data for the reduction and oxidation of a variety of species at poly(7V-methylpyrrole). This polymer has a relatively low maximum conductivity (10-6 - 10 5 S cm"1) and was only partially oxidized in the mediation experiments, which may explain why it behaved more like a redox polymer than a typical conducting polymer. 

The oxidation or reduction of a substrate suffering from sluggish electron transfer kinetics at the electrode surface is mediated by a redox system that can exchange electrons rapidly with the electrode and the substrate. The situation is clear when the half-wave potential of the mediator is equal to or more positive than that of the substrate (for oxidations, and vice versa for reductions). The mediated reaction path is favored over direct electrochemistry of the substrate at the electrode because, by the diffusion/reaction layer of the redox mediator, the electron transfer step takes place in a three-dimensional reaction zone rather than at the surface Mediation can also occur when the half-wave potential of the mediator is on the thermodynamically less favorable side, in cases where the redox equilibrium between mediator and substrate is disturbed by an irreversible follow-up reaction of the latter. The requirement of sufficiently fast electron transfer reactions of the mediator is usually fulfilled by such revemible redox couples PjQ in which bond and solvate... 

Steady photoemission currents can be realized when acceptors (scavengers) for the solvated electrons are present in the solution. A good scavenger should be nonelectroactive at the potenhal of interest, should react quickly with solvated electrons, and the reaction products should be either nonelectroactive or reducible. A reachon with acceptors implies that the current of reoxidation of the solvated electrons becomes lower, and thus a steady photoemission current appears. The acceptors most often used are nitrous oxide, N2O, and hydroxonium ions, HjO. In the former case, OH radical is produced in the scavenging process, which undergoes further reduction on the electrode, thus doubling the photocurrent ... 

The first step is so fast that it can hardly be measured experimentally, while the second step is much slower (probably as a result of the repulsion of negatively charged species, R and R2-, in the negatively charged diffuse electric layer). The reduction of an isolated benzene ring is very difficult and can occur only indirectly with solvated electrons formed by emission from the electrode into solvents such as some amines (see Section 1.2.3). This is a completely different mechanism than the usual interaction of electrons from the electrode with an electroactive substance. 

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