Solvated electron various species

Persulfate (41) reacts with transition metal ions (e.g. Ag, Fe21, Ti31) according to Scheme 3.42. Various other reduetants have been described. These include halide ions, thiols (e.g. 2-mercaptoethanol, thioglycolic acid, cysteine, thiourea), bisulfite, thiosulfate, amines (triethanolamine, tetramethylethylenediamine, hydrazine hydrate), ascorbic acid, and solvated electrons (e.g. in radiolysis). The mechanisms and the initiating species produced have not been fully elucidated for... 

Uribe et al.117 examined the reduction of CO in liquid NH3-0.1 M KI at -50°C, using various working electrodes such as Pt, Ni, C, and Hg. The reaction of CO with electrogenerated solvated electrons produced dimeric species, which precipitated as K2C202. Electrochemical reduction of CO in an aqueous solution at porous gas-diffusion and wet-proof electrodes of Co, Ni, and Fe was carried out,178 and Cj to C3 hydrocarbons and ethylene were reported to be the products. 

The reorganization energy term derives from the solvent being unable to reorient on the same timescale as the electron transfer takes place. Thus, at the instant of transfer, the bulk dielectric portion of the solvent reaction field is oriented to solvate charge on species A, and not B, and over the course of the electron transfer only the optical part of the solvent reaction field can relax to the change in tire position of the charge (see Section 14.6). If the Bom formula (Eq. (11.12)) is used to compute the solvation free energies of the various equilibrium and non-equilibrium species involved, one finds that... 

The number of solvents that have been used in SrnI reactions is somewhat limited in scope, but this causes no practical difficulties. Characteristics that are required of a solvent for use in SrnI reactions are that it should dissolve both the organic substrate and the ionic alkali metal salt (M+Nu ), not have hydrogen atoms that can be readily abstracted by aryl radicals (c/. equation 13), not have protons which can be ionized by the bases (e.g. Nth- or Bu O" ions), or the basic nucleophiles (Nu ) and radical ions (RX -or RNu- ) involved in the reaction, and not undergo electron transfer reactions with the various intermediates in the reaction. In addition to these characteristics, the solvent should not absorb significantly in the wavelength range normally used in photostimulated processes (300-400 nm), should not react with solvated electrons and/or alkali metals in reactions stimulated by these species, and should not undergo reduction at the potentials employed in electrochemically promoted reactions, but should be sufficiently polar to facilitate electron transfer processes. 

In 1984, Edwards and coworkers reported the formation of Na43+ in zeolites Y and A and the formation of K43 1 in K+-exchanged zeolite Y. Later on, they found that if the zeolite was Na-Y, the formed metal ion clusters are Na43+ no matter what (Na or K) the reaction vapor was whereas if the zeolite used was K-Y, the obtained metal clusters will be K43+. That is, the formed metal cluster species is not related with the vapor but simply depends on the type of cation in the zeolite used. In fact, through variation of the reaction condition, various M,(i ion clusters can be prepared. If M = Na, n = 2 6, whereas if M = K, n 3,4. After the alkali metals enter the channels or cages of zeolites to form metal ion clusters, the electrons on the original metal atoms will be released to be shared by more than one metal atom. It has been confirmed that these free electrons actually occupy the holes formed by the metal atoms (ions). Therefore, these electrons are also called solid solvated electrons (in analogy with the solvated electrons formed by alkali metals in solvents such as liquid ammonia),[7] and the formed compounds are called solid electrides. 

Figure 6 Decays ofthe solvated electron recorded at 575 nm in pulse-radiolysis of ethane-1,2-diol in the presence of silver cations, Ag, at various initial concentrations in mol f. In pure ethane-1,2-diol and at the lowest concentration of Ag, the decay is mostly due to reactions of the solvated electron with other species produced by radiolysis. By increasing the concentration, the decay becomes faster as the solvated electron reacts predominantly with theAg cation.

The third class of mechanisms, involving either direct ionization of the liquid or electron ejection via field-emission, has been used to study the behavior of quasifree, localized, and solvated electrons. In contrast to the photoselectivity of the previous two schemes, a cascade of events occurs when high-energy electrons impart energy to a liquid. The resulting ions, excited states, and excess electrons provide a complex spectrum to unravel. However, the temporal evolution of each of the various species differs significantly and we are able to focus on the primary picosecond event, electron localization, with little interference. 

As was mentioned in section 2.3.3, ytterbium metal dissolves in liquid ammonia to yield ammoniated electrons and Yb " ions. This solution was used by White et al. (1978) to perform reductions of various aromatic systems, similar to the Birch reactions which use lithium or sodium as the metal. The addition of benzoic acid or anisole dissolved in a 10 1 mixture of THF-tert-butyl alcohol, to an ytterbium-ammonia solution gives 1,4-dihydrobenzoic acid (56% yield). Triple bonds are cleanly reduced to trans olefins (i.e. PhC CPh traK5-PhCH=CHPh 75%). The C=C double bonds of conjugated ketones are also reduced by this system. Since the reaction medium initially contains both solvated electrons and Yb + ions it is likely that the above reactions are not directly connected with the presence of divalent ytterbium species. 

Figure 3.6 shows the various relationships between the energy levels of solids and liquids. In electrolytes three energy levels exist, Ep, redox, Eox and Ered- The energy levels of a redox couple in an electrolyte is controlled by the ionization energy of the reduced species Ered, and the electron affinity of the oxidized species Eox in solution in their most probable state of solvation due to varying interaction with the surrounding electrolyte, a considerable... 

On the other hand, M or M can be selectively generated in radiation chemical reactions of any M in solution via pulse radiolysis (PR) and y-radiolysis (y-R) techniques [1,36-41], which are used in the present study. Pulse radiolysis has been widely used for the kinetic study involving M . Various processes, such as ionization, excitation, electron transfer, solvation, relaxation, decomposition, etc., occur initially in the radiation chemical reaction in solutions. Chemical species generated from the initial processes react with M as a solute molecule to generate effectively and selectively M, M , or M in the triplet... 

This chapter deals with the fundamental aspects of redox reactions in non-aque-ous solutions. In Section 4.1, we discuss solvent effects on the potentials of various types of redox couples and on reaction mechanisms. Solvent effects on redox potentials are important in connection with the electrochemical studies of such basic problems as ion solvation and electronic properties of chemical species. We then consider solvent effects on reaction kinetics, paying attention to the role of dynamical solvent properties in electron transfer processes. In Section 4.2, we deal with the potential windows in various solvents, in order to show the advantages of non-aqueous solvents as media for redox reactions. In Section 4.3, we describe some examples of practical redox titrations in non-aqueous solvents. Because many of the redox reactions are realized as electrode reactions, the subjects covered in this chapter will also appear in Part II in connection with electrochemical measurements. 

---