Reductants hydrated electron

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). 

The reaction H + OH— eh is undoubtedly responsible for the increase of G(eh) at high pH. Similarly, the reaction eh + H+—H must be responsible for the reduction of the hydrated electron yield in acid solution. The increase of total reducing yield and water decomposition yield at pH = 1.3 is not clearly understood, but it may also be due to secondary reactions. 

The hydrated electron reacts with H202 with a diffusion-controlled rate (see Table 6.6), giving OH and OH-. An intermediate product of this reaction, H202, may be responsible for prolonged conductivity in pulse-irradiated water. The rate of this reaction is consistent with rates of similar one-electron reduction reactions of H202. 

Ionizing radiations (a, ft and y) react unselectively with all molecules and hence in the case of solutions they react mainly with the solvent. The changes induced in the solute due to radiolysis are consequences of the reactions of the solute with the intermediates formed by the radiolysis of the solvent. Radiolysis of water leads to formation of stable molecules H2 and H2O2, which mostly do not take part in further reactions, and to very reactive radicals the hydrated electron eaq, hydrogen atom H" and the hydroxyl radical OH" (equation 2). The first two radicals are reductants while the third one is an oxidant. However there are some reactions in which H atom reacts similarly to OH radical rather than to eaq, as e.g. abstraction of an hydrogen atom from alcohols, addition to a benzene ring or to an olefinic double bond, etc. 

Pulse radiolysis has also been used to study the reduction of various Mb derivatives by hydrated electrons 151-154). With this technique, it was possible to study reduction of ligand-bound forms of metMb at cryogenic temperature and thereby identify reduced, ligand-bound forms of the protein 152) and to reduce oxyMb to produce ferryl (Fe(IV)=0)... 

Mb. Subsequent application of this technique to reduction of various derivatives of reduced and oxidized myoglobin led to the observation that the rate of reduction by hydrated electrons depends primarily on the net charge of the protein and the dissociation constant for formation of ligand bound derivatives of metMb. 

Photolysis of ion pairs of cobalt(III) complexes with iodide ions leads to oxidation of iodide and reduction of the complex.55,63-86 Under the normal experimental conditions, however, most of the light is absorbed by free iodide and the reduction of the complex is effected by hydrated electrons produced as in reaction (36).86... 

Other than water, protein is the major constituent of meat averaging nearly 21% in heef or chicken meat, with fat varying fiom 4.6 to 11.0% in beef and fiom 2.7 to 12.6% in chickoi. The principal radiolytic reactions of aqueous solutions of aliphatic amino acids are reductive deamination and decarboxylation. Alanine yields NH3, pyruvic add, acetaldehyde, propionic acid, CO2, H2, and ethylamine (6). Sulfur-containing amino adds are espedally sensitive to ionizing radiation. Cysteine can be oxidized to cystine by the hydroxyl radical or it can react with the hydrated electron and produce... 

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 ... 

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)]. 

Baxendale JH, Dixon RS (1963) Some unusual reductions by the hydrated electron. Proc Chem Soc 148-149... 

It is noteworthy that although DMP+ catalyzes reduction of fluorobenzene it does not affect benzene. The respective electron affinities in the gas phase are —0.89 and —1.15 eV27) and reaction of fluorobenzene with hydrated electrons is only 6 times faster than that of benzene 82). 

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. 

The formation of the transient species Ag° by reduction of Ag+ from hydrated electrons using a double flash photolysis technique has been observed.199 This species may be photodissociated at 315 nm, probably via charge-transfer to solvent, to produce Ag+ and solvated electrons. 

The Hydrated Electron. In aqueous media electrons become hydrated in 10-12 s to become the ultimate base, nucleophile, and reductant 37... 

The conjugate acid of the hydrated electron is the hydrogen atom (H ), which is the effective reductant under acidic conditions ... 

In the case of metal deposition at the ionic liquid plasma interface two possible reduction processes can conceivably take place. First, the metal cations of the dissolved metal salt can be reduced. Secondly, the cations of the ionic liquid can be reduced to neutral radicals, which can further react as described by Witkamp and as summarized above. As a first guess of which process is preferred, the rate constants of the reaction for example of silver ions (k > 3.2 x 1010 Lmol-1 s-1) and imidazolium ions (k < 4.3 xlO9 Lmoh1 s 1) with hydrated electrons, taken from the data collection of Buxton et al., can be considered [52]. Thus, as long as sufficient silver ions are still in solution the reduction of the imidazolium cations of the ionic liquid represent the minor reaction pathway and the ionic liquid should not decompose significantly. 

Deposition of platinum metal In the case of platinum no solid product was found. The ionic liquid darkened more and faster the smaller the distance between the surface of the ionic liquid [EMIM][TfO] containing tetrabutylammonium hexachloro-platinate ([n-Bu4N]2[PtCl6]) and the Ar/H2-plasma (3 1, overall pressure 100 Pa) was chosen. So far no other ionic liquid has been tested. The rate constant for the reduction of the tetrabutylammonium ion with a hydrated electron is only 1.4 x 106 LmoH1 s 1, hence the main rival pathway for reduction of platinum(IV) is the reduction of the imidazolium ion of the ionic liquid. As in the case of copper, a suitable platinum salt - maybe made by electro-oxidation of metallic platinum in a suitable ionic liquid - has to be found. 

Fig. 14. A schematic mode) of the dissociation of earthworm hemoglobin induced by the reduction by hydrated electron. The molecule consists of 10 subunits with the total molecular weight of 4000000 [84]...

Cytochrome c, a small heme protein (mol wt 12,400) is an important member of the mitochondrial respiratory chain. In this chain it assists in the transport of electrons from organic substrates to oxygen. In the course of this electron transport the iron atom of the cytochrome is alternately oxidized and reduced. Oxidation-reduction reactions are thus intimately related to the function of cytochrome c, and its electron transfer reactions have therefore been extensively studied. The reagents used to probe its redox activity range from hydrated electrons (I, 2, 3) and hydrogen atoms (4) to the complicated oxidase (5, 6, 7, 8) and reductase (9, 10, 11) systems. This chapter is concerned with the reactions of cytochrome c with transition metal complexes and metalloproteins and with the electron transfer mechanisms implicated by these studies. 

Radiolysis has also been employed to generate the hydroxyl radical. However, because very energetic particles are used (x-rays, 7-rays, electron beams, etc.) aqueous solutions are used instead of hydrogen peroxide. Water molecules can be cleaved homolytically and heterolytically to produce three radical species hydroxyl radical, hydrated electron and hydrogen atom.30 A great many rate constants of hydroxyl radical with reductants, especially alcohols, have been measured using radiolysis combined with EPR or electronic spectroscopy.31... 

The rate constants for the reduction of aquo cations by hydrated electrons and for the oxidation of aquo cations by hydroxyl radicals at 25°C are given in Tables 7.25b and 7.25c, respectively. 

Second-order rate constants (10-8 A2 (dm3 mol-1 s l)) for reduction of aquo cations by hydrated electrons at... 

Rapid reduction of BrO- by the hydrated electron has been reported to yield O- and Br- (67). The pertinent half-cell is... 

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. 

Reduction of CO by the hydrated electron might be expected to yield CO or HCO. However, by the use of pulse radiolysis Raef and Swallow demonstrated that the first detectable species is HC(OH)2 (250). Presumably CO- undergoes very rapid hydrolysis. It is difficult to estimate accurately the energetics of the CO/CO couple, but the CO/HCO couple is more easily discussed. The JANAF tables indicate a value of AfG° = 28.3 + 8 kj/mol for HCO in the gas phase. By neglecting the hydration free energy a value of —1.54 V is calculated for E° for the CO/HCO couple. The chemistry of HC(OH)2 is beyond the scope of this review. 

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