Solvated electron reactions hydrated electrons

Hickel B, Sehested K (1985) Activation energy for the reaction H + OH" —> eaq. Kinetic determination of the enthalpy and entropy of solvation of the hydrated electron. J Phys Chem 89 5271-5274 Hoffman MZ, Hayon E (1973) Pulse radiolysis study of sulfhydryl compounds in aqueous solution. J Phys Chem 77 990-996... 

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). 

Electron photoemission from an electrode into an electrolyte solution, which yields solvated (hydrated) electrons, can be regarded as a particular case of reactions with photoexcitation of the electrode (Section 29.2). 

The gas-phase lifetime of N20- is 10-3 s in alkaline solutions, it is still >10-8 s. Under suitable conditions, N20- may react with solutes, including N20. The hydrated electron reacts very quickly with NO (see Table 6.6). The rate is about three times that of diffusion control, suggesting some faster process such as tunneling. NO has an electron affinity in the gas phase enhanced upon solvation. The free energy change of the reaction NO + eh (NO-)aq is estimated to be --50 Kcal/mole. Both N02- and N03- react with eh at a nearly diffusion-controlled rate. The intermediate product in the first reaction, N02-, generates NO and... 

Upon ejection from an ion or molecule by photoionization or high energy radiolysis, the electron can be captured in the solvent to form an anionic species. This species is called the solvated electron and has properties reminiscent of molecular anions redox potential of —2.75eV and diffusion coefficient of 4.5 x 10-9 m2 s-1 (Hart and Anbar [17]) in water. Reactions between this very strong reductant and an oxidising agent are usually very fast. The agreement between experimental results and the Smoluchowski theoretical rate coefficients [3] is often close and within experimental error. For instance, the rate coefficient for reaction of the solvated (hydrated) electron in water with nitrobenzene has a value 3.3 x 10+1° dm3 mol-1 s-1. 

Encounter radii for reactions of the solvated and hydrated electrons with various electron scavengers, corrected for electrolyte screening... 

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 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. 

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. 

The discovery of the hydrated electron in irradiated aqueous solutions has made it necessary to re-examine the mechanisms proposed for the irradiation of aqueous solutions of substances which are biologically important. The new technique of pulse radiolysis has provided a breakthrough in many ways, particularly in determining absolute rate constants. These advances have made it possible to begin working out the reactivity of solvated electrons in vivo, although it is not yet possible to specify the precise role of the reactions in radiation biology. 

The hydrated electron is characterized by its strong absorption at 720 nm (e = 1.9 x 104 dm3 mol-1 cm-1 (Hug 1981) the majority of the oscillator strength is derived from optical transitions from the equilibrated s state to the p-like excited state (cf. Kimura et al. 1994 Assel et al. 2000). The 720-nm absorption is used for the determination of its reaction rate constants by pulse radiolysis (for the dynamics of solvation see, e.g Silva et al. 1998 for its energetics see, e.g Zhan et al. 2003). IP only absorbs in the UV (Hug 1981), and rate constants have largely been determined by EPR (Neta et al. 1971 Neta and Schuler 1972 Mezyk and Bartels 1995) and competition techniques (for a compilation, see Buxton et al. 1988). In many aspects, H and eaq behave very similarly, which made their distinction and the identification of eaq" difficult (for early reviews, see Hart 1964 Eiben 1970 Hart and Anbar 1970), and final proof of the existence of the... 

Sherman WV (1967b) Light-induced and radiation-induced reactions in methanol. I. y-Radiolysis of solutions containing nitrous oxide. J Phys Chem 71 4245-4255 Sherman WV (1967c) The y-radiolysis of liquid 2-propanol. III. Chain reactions in alkaline solutions containing nitrous oxide. J Phys Chem 71 1695-1702 Silva C, Walhout PK, Yokoyama K, Barbara PF (1998) Femtosecond solvation dynamics of the hydrated electron. Phys Rev Lett 80 1086-1089... 

Ye M, Schuler RH (1986) The reaction of e aq with H2P04" as a source of hydrogen atoms for pulse radiolysis studies in neutral solutions. Radiat Phys Chem 28 223-228 Zhang C-G, Dixon DA (2003) The nature and absolute hydration free energy of the solvated electron in water. J Phys Chem B 107 4403-4417... 

More recently still the electron produced in this and in other ways has been shown to have a finite independent existence as a hydrated species136. This is very similar to solvated electrons in liquid ammonia known since the early work of Franklin and Kraus137. The solvated electron in liquid ammonia disappears very slowly in the absence of a catalyst, but the life of the hydrated electron is much shorter if they undergo no other reactions, they are removed as follows... 

SCHEME 4.1 Schematics of radiolysis and reducing species. As a result of ionization of the water molecule, hydroxyl radicals and hydrated electrons are formed. The final radiolytic yield depends on the secondary reactions in spurs and on the presence of other compounds. See Refs 25,26,190, and 191 for the detailed discussion and references. Solvated electrons are mobile enough to escape spurs and to react with the heme protein complexes even at 77K. All other reactive products of radiolysis are immobilized in the solid solvent matrix, or trapped by radical quenchers. 

It is suggested, therefore, that hydrated electrons are not likely to be formed in the intracellular fluid and that the formation of solvated electrons is also of low probability in the presence of solutes that are capable of accommodating electrons. On the other hand, it should be remembered that electrons are being formed in radiolysed living systems and are finally incorporated in certain functional groups of the molecules involved. A qualitative, and perhaps a semiquantitative, correspondence is expected between the tendency of the constituents of the living cell to incorporate an electron and their reactivity towards hydrated electrons in dilute solutions. From this standpoint only, it may be beneficial to acquire qualitative as well as quantitative information on the reactions of biopolymers and their functional groups with hydrated electrons. 

The irradiation of starch in aqueous gels, sols, and solutions leads to somewhat different results because of the operation of different mechanisms. Although all reactions in the solid state involve free-radical processes, those in aqueous solutions may involve hydrated electrons, which with oxygen give O2 ions. Kochetkov et al. 90 have assumed that deoxy sugars result from the involvement of Oj ions alone, whereas the formation of deoxy ketoses is assisted by hydroxyl radicals. Stockhausen et a/.191 have proposed the following sequences of reactions. Ionization of water leads to solvated electrons and further radicals. 

The reactivity of hydroxide ion (and that of other oxyanions) is interpreted in terms of two unifying principles (a) the redox potential of the YO /YO- (Y = H, R, HO, RO, and O) couple (in a specific reaction) is controlled by the solvation energy of the YO anion and the bond energy of the R-OY product (RX - - YO R-OY - - X ), and (b) the nucleophilic displacement and addition reactions of YO occur via an inner-sphere single-electron shift. The electron is the ultimate base and one-electron reductant which, upon introduction into a solvent, is transiently solvated before it is leveled (reacts) to give the conjugate base (anion reductant) of the solvent. Thus, in water the hydrated electron... 

For solvents in which the lifetime of the solvated electron is short, it cannot be observed in this way. For instance in water, the hydrated electron may be formed by dissolving alkali metals. But the metal dissolution timescale is much longer (a hundred of milliseconds) than the lifetime of the electron (a few microseconds) and, as soon as solvated electrons are produced, a very fast reaction occurs between two solvated electrons producing molecular hydrogen, leading to the explosive combustion in air that accounts for the hazardous contact of alkali metal and water. 

This narrative echoes the themes addressed in our recent review on the properties of uncommon solvent anions. We do not pretend to be comprehensive or inclusive, as the literature on electron solvation is vast and rapidly expanding. This increase is cnrrently driven by ultrafast laser spectroscopy studies of electron injection and relaxation dynamics (see Chap. 2), and by gas phase studies of anion clusters by photoelectron and IR spectroscopy. Despite the great importance of the solvated/ hydrated electron for radiation chemistry (as this species is a common reducing agent in radiolysis of liquids and solids), pulse radiolysis studies of solvated electrons are becoming less frequent perhaps due to the insufficient time resolution of the method (picoseconds) as compared to state-of-the-art laser studies (time resolution to 5 fs ). The welcome exceptions are the recent spectroscopic and kinetic studies of hydrated electrons in supercriticaF and supercooled water. As the theoretical models for high-temperature hydrated electrons and the reaction mechanisms for these species are still rmder debate, we will exclude such extreme conditions from this review. 

---