Reactive species hydrated electron

Discovery of the hydrated electron and pulse-radiolytic measurement of specific rates (giving generally different values for different reactions) necessitated consideration of multiradical diffusion models, for which the pioneering efforts were made by Kuppermann (1967) and by Schwarz (1969). In Kuppermann s model, there are seven reactive species. The four primary radicals are eh, H, H30+, and OH. Two secondary species, OH- and H202, are products of primary reactions while these themselves undergo various secondary reactions. The seventh species, the O atom was included for material balance as suggested by Allen (1964). However, since its initial yield is taken to be only 4% of the ionization yield, its involvement is not evident in the calculation. 

The Reactivity of Different Chemical Species toward Hydrated Electrons and the Products of these Reactions... 

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. 

The hydrated electron is obviously a nucleophile and its reactions are affected by substituents correspondingly. The hydroxyl radical is expected to behave as an electrophile and this behaviour was, indeed, demonstrated with aromatic compounds. The low reactivity of O toward aromatic and olefinic ir-systems suggests that this species behaves as a nucleophile because of its charge. The behaviour of hydrogen atoms is not easily predictable the effect of substitution in benzene demonstrated a slight electrophilicity. 

Well studied primary reactive species in radiation- or photo-initiated reactions of auxiliary oxidants in an aqueous phase are hydrated electrons (eaq), hydrogen atoms (H ) and hydroxyl radicals ( OH), the last being by far the most important ones in photo-initiated AOPs. The formation and reactivity of ejq and of H were described by Hart and Anbar (1970) and by Buxton et al. (1988). Hydrated electrons can be produced by VUV photolysis of water, by photolysis of aqueous solutions of [FelCNlq]" or of V with formation of [Fe(CN)5] and il2, respectively (cf. Buxton et al, 1988). 

A specific free radical can be produced from a precursor molecule either in an initiation step or a propagation step in which a reagent radical reacts with the precursor. Initiation requires either removal or addition of an electron or homolysis. Chemically this can be done in a number of ways, by using one-electron oxidants or reductants or by inducing homolysis in some way examples of these types of reactions include autoxidation [84-86], photochemical oxidation and reduction [87-90], and oxidation and reduction by metal ions and their complexes [91-93], In propagation reactions, the reagent radical might be the hydroxyl radical, the hydrated electron, or any other suitably reactive species that will interact with the precursor molecule in the desired manner. We will consider initiation reactions first. 

Baxendale, Fielden, and Keene (7) identified a transient lower oxidation state of silver obtained by reaction of Ag+ solutions with hydrated electrons or atomic H. In the present work, similarly, transient—i.e., chemically reactive species of the lower oxidation state of gold were obtained by reducing Au1. Kinetic properties of these species, which might be of zero valency have, moreover, been examined. 

In dilute solutions the chemical changes of a specific solute primarily reflect its reactivity towards e q, H- and -OH. The hydrated electron e q" is a strongly reducing species (Eq = —2.9 V) whereas the hydrogen atom is a less powerful reductant (Eq = —2.3 V). The H atom can be considered as a weak acid with a pKa of 9.6... 

A final concern in our quest for an understanding of oxygen activation and reactive intermediates is the tendency for many to formulate oxy anions (XO ) in aqueous solutions at pH 7.0. Table IV summarizes the aqueotis acidities for various XO-H species, and confirms that free superoxide ion (02 ) and bicarbonate ion [H0C(0)0 ] are the only dominant oxy anions at pH 7. Even hydrated electrons (eaq ) are as their conjuate acid (H-). 

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