Hydrated electron conversion

The conversion of the hydrated electron into the gaseous stale ... 

Our observation of the hydrated electron band at a 5 /xsec. delay cannot be attributed to the thermal reaction H + OH - e aq + H20, because the rate constant of 1.8 X 107Af 1 sec. 1 (21) permits only a negligible conversion of H atoms below pH 10. Therefore, the cases indicated as (+) and (+ + ) are taken as definite proof of photoionization. The cases indicated as (f) are less certain, although a photographic density difference of proper lifetime was measured densitometrically because the weak absorptions made the delineation from other transients, such as short-lived triplets, less certain. The absence of the hydrated electron... 

With the aid of electron tunneling it appears possible to regulate the selectivity of redox conversions. For practically important reactions this has not been realized so far, but that this approach may prove to be useful is demonstrated, e.g. by the data presented in Table 6. In this table, a comparison is made between the rate constants for reactions of three different acceptors with hydrated electrons in liquid water at 298 K and the characteristic times, t, for reactions of the same acceptors with trapped electrons in solid water-alkaline glasses at 77 K. The values of x have been calculated using the values of ve and ae from Ref. [21]. It can be seen in the liquid, when due to diffusion the reagents can approach to within short distances of each other (direct collisions), that the rate constants for all three... 

The conversion of hydrogen atoms to hydrated electrons is an exception to the rule of rapid reactivity in this case, the reaction is relatively slow, first order in [OH-], and has a second-order rate constant of 3 x 107 M-1 s-1.45 In less alkaline solutions, the direct reaction of the hydrogen atom with water is quite slow and yields H2 + OH. As a result of these low rate constants, the interconversion between hydrogen atoms and hydrated electrons can be catalyzed by weak bases such as F-and NH3. 

Introducing the electron pulses into dilute aqueous solutions under anaerobic conditions causes, as stated above, primary changes in the solvent (12). In such experiments, water molecules undergo conversion mainly into OH radicals and hydrated electrons [e (aq)] and, to a lesser extent, H atoms, H2 and H2O2 molecules. 

Thus when hydrogen atoms, generated by a discharge in hydrogen gas, are passed into an alkaline, aqueous solution, reactions characteristic of the hydrated electron are observed [15]. The conversion of hydrogen atoms... 

These studies showed that water decomposes, yielding radicals (69) and molecular entities (5). Later studies have shown that the two molecular products, H2 and H202, are formed with different yields (30), Gh2 < Gh202 The radical products were assumed to be H and OH radicals (69). Later, it was shown that the reducing radical may exist in two forms (12), one being a H atom (20) and the other a hydrated electron (19, 24). It was also shown that an eaq reacts with H+ to yield a H atom (23, 43), while the opposite reaction—the conversion of H atoms by OH" into eaq—has been demonstrated as well (50). 

The same spectral absorption at 270 n.m. was observed by reaction of Au(CN)2" with H atoms generated in an altogether different way in neutral medium. Irradiations were carried out in triply distilled water saturated with N20 but under a high pressure of hydrogen. Under these conditions, the hydrated electrons are converted to OH radicals by Reaction 1, and then the OH radicals are converted to H by the H2 dissolved under high pressure as explained before. In neutral water the conversion of H atoms to e"aq is negligible. 

The present data may be explained if N20" (or N2OH) radicals act as oxidizing radicals of low reactivity. Thus, our results give no evidence for a conversion of hydrated electrons into OH radicals in irradiated solutions of AA containing nitrous oxide. Furthermore, in the present system, N20 (or N2OH) does not seem to behave as stoichiometrically equivalent to OH. 

The yield of hydrogen atoms in neutral or alkaline solutions is very low, G = 0.057 pmol In acidic solutions due to the conversion of hydrated electrons to H atoms in reaction (O 23.28) the yield can be as high as G = 0.34 pmol Therefore, the H atom reactions are... 

The OH reactions are usually studied in N2O saturated solutions. In such solutions the conversion of hydrated electrons to hydroxyl radicals is a two-step process with the formation of intermediate ions (reactions (O 23.6) and (O 23.7)). At higher solute concentrations, above 10 mol dm, the intermediate ions may also react with the solute to not negligible extent and some fraction is not converted to OH radical in reaction (O 23.7). When the rate coefficient of the OH reaction is measured with the usual thiocyanate competitive technique (see also reactions (O 23.20) and (O 23.21)), this reaction may cause an artifact, causing overestimation of the rate coefficient (Schuler and Albarran 2002)... 

Figure 11.10 Schematic representation of photoionization and electron transfer processes in solutions of surfactant micelles containing a solubilized photoactive probe P. The electron acceptor is M" located in the Stern layer of the micelle and the electron is transferred through the Stern layer from the triplet (P ). Hydrated electrons produced by the photoionization process (a) cannot re-enter the micelle and recombine with parent cations. The most likely fate of in micellar solutions is conversion into H2 via the bimolecular reaction ...

With tyrosinase, on the contrary, a two-electron oxidation occurs, as no EPR signal was detected in the catechol oxidation at pH 5.3 Melanins are polymerization products of tyrosine, whereby tyrosinase catalyses the first steps the formation of dopa (3,4-dihydroxyphenylalanine) and of dopaquinone, leading to an indolequi-none polymer The peroxidase mechanism for the conversion of tyrosine into dopa in melanogenesis was not substantiated In natural and synthetic melanins free radicals of a semiquinone type were detected by EPR 4-10 x 10 spins g of a hydrated suspension (the material was modified on drying and the number of free spins increased). The fairly symmetrical EPR signal had a g-value of 2.004 and a line-width of 4-10 G The melanins seem to be natural radical scavengers. 

---