Reactions of the Solvated Electron

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. 

Ionization of solute molecules by photolysis or high-energy radiolysis produces electrons which may recombine with their geminate radical (ion), react with an impurity (usually an oxidant) or be trapped in a solvent vacancy. The properties of the solvated electron resemble those of anions (e.g. a diffusion coefficient of 4.9 x 10-9 m2 s-1 in water). It is 

In Chaps. 3 and 4, estimates of encounter distances and mutual diffusion coefficients from similar experiments to those of Buxton et al. [18] are discussed. The complications to the analysis of diffusion-controlled rate processes in solution when the reactants interact strongly with one another or the reaction can occur over distances much larger than typical encounter distances do not lead to markedly different time-dependent rate coefficient expressions to the Smoluchowski form. Indeed, replacing R in eqn. (29) by an effective encounter distance, Reff, allows the compactness of the Smoluchowski rate coefficient to be extended to other situations. Means of estimating Reff are discussed in Chaps. 3, 4, 5 (Sect. 4.3), 8 (Sect. 2.6) and 9 (Sects. 4 and 6). 

---

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

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. 

The remainder of this section considers several experimental studies of reactions to which the Smoluchowski theory of diffusion-controlled chemical reaction rates may be applied. These are fluorescence quenching of aromatic molecules by the heavy atom effect or electron transfer, reactions of the solvated electron with oxidants (where no longe-range transfer is implicated), the recombination of photolytically generated radicals and the reaction of carbon monoxide with microperoxidase. 

REACTIONS OF THE SOLVATED ELECTRON IN CONCENTRATED IONIC SOLUTIONS... 

Many of the fast chemical reactions discussed in the preceding sections involve at least one reactant which is of low symmetry. The reactions of the solvated electron with nitrate, naphthalene or pyrene are instances where the oxidant has a mirror plane (in the molecular plane) in the accepting orbital. Hence, reaction of the solvated electron with such a scavenger when both are contained in this plane should be slower than in other configurations. Similarly, the contact quenching of fluorescence from naphthalene or 1,2-benzanthracene by carbon tetrabromide [7], or... 

Hummel and Luthjens [398] formed electron—cation pairs in cyclohexane by pulse radiolysis. With biphenyl added to the solvent, biphenyl cations and anions were formed rapidly on radiolysis as deduced from the optical spectra of the solutions. The optical absorption of these species decreased approximately as t 1/2 during the 500 ns or so after an 11ns pulse of electrons. The much lower mobility of the molecular biphenyl anion (or cation) than the solvated electron, es, (solvent or cation) increases the timescale over which ion recombination occurs. Reaction of the solvated electron with biphenyl (present in a large excess over the ions) produces a biphenyl anion near to the site of the solvated electron localisation. The biphenyl anion can recombine with the solvent cation or a biphenyl cation. From the relative rates of ion-pair reactions (electron-cation, electron—biphenyl cation, cation—biphenyl anion etc.), Hummel and Luthjens deduced that the cation (or hole) in cyclohexane was more mobile than the solvated electron (cf. Sect. 2.2 [352, 353]). 

In suggesting an increased effort on the experimental study of reaction rates, it is to be hoped that the systems studied will be those whose properties are rather better defined than many have been. By far and away more information is known about the rate of reactions of the solvated electron in various solvents from hydrocarbons to water. Yet of all reactants, few can be so poorly understood. The radius and solvent structure are certainly not well known, and even its energetics are imprecisely known. The mobility and importance of long-range electron transfer are not always well characterised, either. Iodine atom recombination is probably the next most frequently studied reaction. Not only are the excited states and electronic relaxation processes of iodine molecules complex [266, 293], but also the vibrational relaxation rate of vibrationally excited recombined iodine molecules may be at least as slow as the recombination rate [57], Again, the iodine atom recombination process is hardly ideal. 

The absolute rate constants were determined for a variety of reactions of the solvated electron in ethanol and methanol. Three categories of reaction were investigated (a) ion-electron combination, (b) electron attachment, and (c) dissociative electron attachment. These bimolecular rate constants (3, 27, 28) are listed in Table III. The rate constants of four of these reactions have also been obtained for the hydrated electron in water. These are also listed in the table so that a comparison may be made for the four rate constants in the solvents ethanol, methanol, and water. 

Table III. Absolute Rate Constants for Some Reactions of the Solvated Electron at 23° C.

Unlike many of the conventional electron transfers, many reactions of the solvated electron are diffusion controlled. 

The value for X2 is the same as that for this same reactant in an ordinary homogeneous or electrochemical electron transfer occurring at the same R and can be estimated from them, as described later (6). AF0/int is known for many reactions of the solvated electron, and w can be estimated approximately. Accordingly, a theoretical value of AF can be calculated from Equation 7 once X/ is known. Either X/ can be calculated from other sources (it depends on the model of the solvated electron) or a value can be used which best fits data on k t for several reactions, or both. In making such calculations it should be noted that AF is not highly accurately given by Equation 7, because of the various... 

While the reaction of the solvated electron with hydrogen ions is near the diffusion-controlled limit in aqueous and alcoholic systems (24), the reaction with hydrogen chloride in our system, which presumably gives ethylenediammonium ions, is much slower with k = 1.7 X 106 M l sec."1 Interestingly, the reaction with ammonium ions is at least an order of magnitude faster than this, indicating that appreciable proton transfer from ammonium ion to ethylenediamine does not occur. 

Simple olefins do not react with eaq at an appreciable rate, but compounds with an extended 7t-system such as butadiene can also accommodate an additional electron (k = 8 x 109 dm3 mol-1 s 1 Hart et al. 1964). However, as in the case of benzene, the rate is often below diffusion controlled [reaction (23) k = 7.2 x 106 dm3 mol 1 s 1 (Gordon et al. 1977) in THF, the reaction of the solvated electron with benzene is even reversible (Marasas et al. 2003)], and the resulting radical anion is rapidly protonated by water [reaction (24)]. 

Wagner BO, Klever H, Schulte-Frohlinde D (1974) Conductometric pulse radiolysis study on the reaction of the solvated electron with 5-bromouracil in aqueous solutions at different pH values. Z Naturforsch 29b 86-88... 

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.

Many reactions of the solvated electron with different solutes, such as aliphatic, aromatic or heterocyclic compounds, and also anions and cations, have been studied. 

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]. 

---