Aquated electron

Cathodic currents are stimulated by the illumination of metal electrodes [55-59]. These currents are often strongly enhanced by the presence in solution of known electron scavengers such as N20 and H30+. Research carried out by Barker [59— 61], Pleskov [62-64], Delahay [65], and their co-workers indicates rather strongly that the impinging photons eject electrons from the electrode surface. They appear to travel some distance ( 50 A) before becoming solvated [55,59,62,65]. If a scavenger is present, the solvated (usually aquated) electrons may react with it irreversibly. For example,... 

Iridium)in ) —Group VII Donors. Pulse radiolysis of [IrCl6]3- in aqueous solution has been shown to give [IrCl6]2 " via oxidation with the hydroxyl radicals produced.2 36 On the other hand, when propan-2-ol was added to the solution, pulse radiolysis led to reduction by aquated electrons, hydrogen radicals, and R to yield [IrCl6]4 ... 

R + X-. Hydrogen extraction can be studied by generating highly reactive hydroxyl radicals as N20 + [e ]aq + H20 -> N2 + OH- + OH and hydrogen atoms from reaction of aquated electrons with protons in solution. 

Nitrite absorbs radiation in the UV region with a high absorption maximum at 210 nm, a shoulder maximum at 280-290 nm and a second maximum near 355 nm ([24,35,36], see also Fig. 1). The absorption band at 355 nm continues well into the visible and accounts for the environmental role of nitrite photochemistry. UV irradiation of nitrite yields OH + NO as the main pathway, while a minor pathway of uncertain importance might also yield NO2 and aquated electrons [22,28,35,37-42] ... 

Even though aquated electrons e(aq) are powerful reducing agents (see Table 10.1 and Section 10.1.1.3), they can also produce OH as follows ... 

Aquated electron aq Photolysis of dissolved organic matter... 

Electron addition or extraction from a complex to cause reduction or oxidation may also be achieved electrochemically (at an electrode in an electrochemical cell) or radiolytically (using an aquated electron or hydroxyl radical). 

K.D. Whitburn, M.Z. Hoffman, and LA. Taub, Simultaneous reaction of hydroxyl radical and aquated electrons with ferrimyoglobin in irradiated solutions, J. Food Sci., 48 (1983) 1888. 

Reduction of [IrCls] by a variety of agents has been cited. The reduction of [IrXs] " (Cl, Br) by 2-thiouracil and 2-thiopyrimidine follows second-order rate laws, first-order with respect to both the concentration of the oxidant and the reductant. A pH-rate profile as well as activation parameters have been presented, and a free radical mechanism proposed." Reduction in aqueous alcoholic solution by aquated electrons, hydrogen and alkyl radicals was investigated by pulse radiolysis, and found to result in the production of [IrClg] " A kinetic examination of reaction (123) in aqueous solution shows that this outer-sphere electron transfer reaction proceeds by parallel paths first and second order in substrate (SCN ). The kinetics of the oxidation of hydrazine by [Ir(Cl)4(H20)2], [Ir(Cl)5(H20)], [IrClef , as well as [IrBrsf , in aqueous acidic perchlorate solution have been investigated." ... 

After T720 was tentatively characterized as an aquated electron with nitrous oxide and electron transfer studies, the quantum yield for the production of the aquated electron In dilute natural waters containing dissolved organic matter and In humic substance extract was measured by the comparlslon method. In this method the literature value for the extinction coefficient of the solvated electron at 720 nm was used (21). The excited state absorption of T720 was measured. These were used In algebraic ratio to an actlnomlter for which a quantum yield had already been thoroughly determined. 

The quantum yield for the production of aquated electrons by 355 nm laser excition of Aldrich HSX (with ground state absorbance of 1.0 at 355 nm) was determined to be 0.014. The quantum yield for the production of aquated electrons by 266 nm excitation of... 

This is the first known observation of such a herblclde-humic substance charge transfer Interaction with flash photolysis. The results shown In this section and the nitrous oxide quenching of T720 seem to be good evidence for the generation of HS charge-transfer complexes Involving an aquated electron upon Irradiation of HS solutions. 

Reaction with O2 produces OJ. A series of rigidly structured Ni(II) macrocyclic and acyclic complexes has been shown to react rapidly with hydroxyl radicals and aquated electrons.Addition of e generates a Ni(I) transient except when the ligand incorporates a nitro group. Reaction with OH leads to a longer-lived Ni(III) species. OH radicals have also been shown to react with Pu(III) with a specific rate of oxidation of 1.83 0.78 x 10 Ms at pH Comparison has been... 

The site where the separational change occurs is called the reaction centre. The first step on the negative side is the formation of an aquated electron (H2O-), cf Fig. 2.7 ... 

The aquated electron undergoes several reactions (see Chapter 5.3.4). It can be transported via water molecules and special cellular substances electron transfer) to other molecules (respectively reactive centers) to achieve reductions (2.48a), but it can also decay into atomic hydrogen and the hydroxyl ion (2.48b). 

Another interesting species is given by the carbon dioxide anion radical COj, produced from aquated electrons (Hart and Anbor 1970), which is an efficient reducing agent in two ways, electron transfer and radical addition /C5 340 = 4 10 L mol" S- (CAPRAM) ... 

As mentioned in section II.B.2 mercaptoetbanol behaves diSaenUy in that only about half the aquated electrons give rise to HjS. Bronsted acids can react with e q and convert them to H, but the pK of the thiol group in mercaptoethanol is not lower than for other thiols, and the explanation must lie elsewhere. 

The aquated electron reacts rapidly with disulphides, rate constants of 1 -3 X10 , 2 X IQi , 9 X10 and 6-4 x 10 1 mol 7 s 7 for cystine, cystamine, homocystine and glutathione disulphide, respectively, at pH 7 being... 

Homocysteine lactonizes readily in acidic solutions, and a study of the aqueous radiation chemistry of this thiolactone was undertaken to see how bonding of the sulphur to a carbonyl carbon modifies its reactivity to the aquated electron . (The normal H atom abstraction from sulphur by OH cannot occur.) The dissodative electron capture reaction which gives H S in the case of free thiols can be formulated ... 

Resonance stabilization of the thiocarboxylate group might have been expected to favour this reaction. The aquated electron reacted fast with the thiolactone, k = 3-6 x 10 lmol s , but it was found that reductive deamination occurred, this being the typical reaction for amino add derivatives . 4,4 -Dithiodibutyric add was found to be a product, and the following steps involving ring opening were postulated ... 

Aquated electron, reaction with disulphides 492 reaction with enzymes 493 reaction with thiols 485, 486 Arizidines, reaction with thiolate anions 728... 

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