Reduction potentials hydrated electron

Although OH reacts at near-diffusion-controlled rates with inorganic anions [59], there seems to bean upper limit of ca. 3 x 10 dm mol sec in the case of simple hydrated metal ions, irrespective of the reduction potential of M"". Also, there is no correlation between the measured values of 43 and the rates of exchange of water molecules in the first hydration shell of, which rules out direct substitution of OH for H2O as a general mechanism. Other mechanisms that have been proposed are (i) abstraction of H from a coordinated H2O [75,76], and (ii) OH entering the first hydration shell to increase the coordination number by one, followed by inner-sphere electron transfer [77,78]. Data reported [78] for M" = Cr, for which the half-life for water exchange is of the order of days, are consistent with mechanism (ii) ... 

The Hydrated Electron and Absolute Values of Reduction Potentials... 

The value of ArG of —267.2 kJ mol-1 for the hydrated electron/ hydrated proton reaction converts into a reduction potential of 267.2/ 96.485= +2.77 V on the conventional scale and indicates that the hydrated electron is approximately equal to sodium in its reducing power ... 

The first element, hydrogen, has an Allred Rochow electronegativity coefficient of 2.1, and an electronic configuration Is1. The atom may lose the single electron to become a proton, which exists in aqueous solutions as the hydroxonium ion, H30+(aq), in which the proton is covalently bonded to the oxygen atom of a water molecule. The ion is hydrated, as is discussed extensively in Chapter 2. The reduction of the hydrated proton by an electron forms the reference standard half-reaction for the scale of reduction potentials ... 

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. 

The hydrated electron is the most powerful reductant (E7 = -2.9 V) IP has a somewhat higher reduction potential (E7 = -2.4 V for a compilation of reduction potentials, see Wardman 1989). Often, both H and eaq are capable of reducing transition metal ions to their lower oxidation states [e.g., reactions (4) and (5)]. 

This mechanism is reasonable as a) reduction of benzene occurs at a cathode potential of -2,5 V vs. S.C.E., roughly corresponding to the standard potential of the hydrated electron 293 while the potential for the direct electron transfer to benzene is more negative ( -3,0 V) and b) in situ electrolysis in the ESR cavity produces at -100 °C the characteristic singlet of the solvated electron 293a>, which changes to the septett of the benzene radical anion, when benzene is added to the solution. 

It is claimed that Br03 is generated in the relatively slow oxidation of Br03 by OH (11), but no thermochemical data are available for this radical. Reduction of Br04 by the hydrated electron yields Br03 and O (237). A potential of 0.06 V can be calculated for the Br047(Br03", 0 ) couple. 

The hydrated electron may be described as an electron surrounded by a small number of oriented water molecules. It reacts rapidly with many substrates M having more positive reduction potentials by one-electron transfer processes according to the general Eq. 6-15 where n represents the positive charge on the substrate. 

The alkali metal cations Li, Na, K, Rb and Cs cannot be reduced by hydrated electrons because they have a more negative reduction potential. 

Throughout the literature, many authors argue for the high reduction potential of hydroxyl radicals being responsible for the oxidative reactions observed in AOPs. However, simple electron transfer reactions such as those of Eq. 6-21 seem to be unlikely because of the large solvent reorganization energy involved in the formation of the hydrated hydroxide ion (Buxton et al., 1988). Instead, in the case of halide ions X or pseudo-halide ions, the formation of intermediate adducts with hydroxyl radicals is observed (Eq. 6-24). 

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... 

Electrochemical reduction of [Ru(NH3)e] and related halide complexes, e.g. [Ru(NH3)5C1], produces Ru products hydrolysis of the chloride ligands then occurs. Reduction potentials of [Ru(NH3)sL] and [Ru(NH3)4L2] (L = py-X or RCN) have been measured by cyclic voltammetry. Nitriles have more positive E[ values than py and unsaturated R groups give the largest values electron-withdrawing X substituents increase Ef. Co-ordination of the N-bound 4-pyridine car-boxaldehyde slows the acid-catalysed hydration. 

The Hydrated Electron. The hydrated electron is the quintessential reducing agent, with a reduction potential of -2.77 V vs. normal hydrogen electrode (NHE) (9). Because of its high molar absorptivity, reactions of ejq... 

As the viologen moiety of the anion exchange membrane is reduced by photoirradiation, electrons are released from the membrane. Though the first reduction potential (MV2+ + e —> MV+ = —0.45V) cannot reduce water molecules,121 the released electron might behave like an hydrated electron in the... 

Following the discovery of the hydrated electron in radiation chemistry, the reexamination of some fields of aqueous chemistry gave rise to a new concept of primary reduction processes. This paper surveys aspects of these investigations in which it appears that e aq, as opposed to its conjugate acid (H atom), is invariably the precursor to H2 when water is reduced. Evidence is reviewed for the production of e aq (a) photochemically, (b) by chemical reduction of water, (c) electrolytically, (d) by photo-induced electron emission from metals, (e) from stable solvated electrons, and (f) from H atoms. The basis of standard electrode potentials and various aspects of hydrated electron chemistry are discussed briefly. 

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