One-electron potential

A relatively low potential, one-electron oxidation is observed (Equation (72)), followed above pH 2.2 by a two-electron oxidation, two-proton step (Equation (73)) and a one-electron oxidation (Equation (74)). In more acidic solutions a direct three-electron oxidation occurs leading also to the [Ruv O Ruv]4+ species. In various studies the Rulv O Rulv, RuIV-0 Ruv, and Ruv O Ruv species have been considered as the catalytically active form. Although these species have been characterized by resonance Raman and EPR spectroscopies,475,476,480 no definitive conclusion about the mechanism involved in the catalysis can be drawn and the question remains largely open. 

In the case of two flavoenzyme oxidase systems (glucose oxidase (18) and thiamine oxidase s where both oxidation-reduction potential and semiquinone quantitation values are available, semiquinone formation is viewed to be kinetically rather than thermodynamically stabilized. The respective one-electron redox couples (PFl/PFl- and PFI7PFIH2) are similar in value (from essential equality to a 50 mV differential) which would predict only very low levels of semiquinone (32% when both couples are identical) at equilibrium. However, near quantitative yields (90%) of semiquinone are observed either by photochemical reduction or by titration with dithionite which demonstrates a kinetic barrier for the reduction of the semiquinone to the hydroquinone form. The addition of a low potential one-electron oxidoreductant such as methyl viologen generally acts to circumvent this kinetic barrier and facilitate the rapid reduction of the semiquinone to the hydroquinone form. 

The first step consists of the formation of the dioxygen adduct which can have either a superoxo structure (1) if the metal is a potential one-electron donor, or a peroxo structure (2) if the metal is a potential two-electron donor. These superoxo or peroxo complexes can be considered as the formal, but not chemical, analogs of the superoxide 02 and peroxide 022- anions. The superoxo complex (1) can further react with a second reduced metal atom to give the /x-peroxo species (3), which can cleave itself into the oxo species (4), which may be hydrolyzed to give the hydroxo species (6) or react with a second metal atom to give the p.-oxo species (5). The alkylperoxo (7) and hydroperoxo (8) species can result from the alkylation or protonation of the peroxo species (2), or from anion exchange from metal salts by alkyl hydroperoxides or hydrogen peroxide. 

Bearing in mind that one of the products of electroreduction of 4-aminopyrimidine, which may be regarded as a formal analogue of adenine, undergoes photodissociation 102), it might be expected that similar behaviour would be exhibited by the product of the potential one-electron reduction step of the neutral molecule of adenine. It is therefore of interest that the product of the one-electron reduction of adenine in DMF (non-protic solvent) exhibits a UV spectrum with a maximum at 300 nm 164), like the product of reduction of 4-aminopyrimidine 104). 

Charge Transport. - As guanine has the lowest ionisation potential, one electron oxidation of DNA leads to a guanine radical cation which then... 

The lack of a counterion implies intramolecular charge compensation and therefore mixed-valence character for [(Fe )2Fe OL3 ] 16a. This was unambiguously confirmed by a Mossbauer spectrum which exhibits two quadruple doublets with a peak area ratio of 1 2. Cyclic voltammetric investigation of the redox-active iron centers of neutral 16a shows a reversible three-potential one-electron transfer process. The half-wave potentials of -635 and -1230 mV correspond to the redox processes [(Fe 02Fe OL3 ] [Fe (Fe )20L3 ] [(Fe lsOL ] , whereas the oxidation of... 

Measurements of the oxidation potentials yielded results which reflected the close proximity of the two phthalocyanine planes. The first two oxidation potentials (one-electron oxidations from each phthalocyanine ring) were 100 mV apart, suggesting the delocahzation of the cation radical over the two phthalocyanines jt-electronic framework. This behavior shows some similarity to that of the porphyrin dimers, and is expected to favor energy- and electron-transfer reactions. Another characteristic feature of this phthalocyanine dimer is that its fluorescence quantum yields are almost the same as those of the corresponding monomers (0.45, 0.26, and 0.76 for Zn(OBu), Zn(f-Bu), and Mg(f-Bu) phthalocyanines, respectively). Such a highly fluorescent phthalocyanine dimer has never been reported before. 

The chemistry of complex 11 with AgPFg was evaluated because Ag(I) is a common additive in Pd-catalyzed oxidative C-H coupling reactions [57-64] and a potential one-electron oxidant, similar to Fc". Treatment of dimethyl Pd(II) complex 11 with AgPFg resulted in the immediate formation of an intermediate (16), as observed by NMR spectroscopy, which subsequently generated 13, ethane, and Ag mirror (Fig. 9). The authors proposed that Ag(I) acts as an inner-sphere one-electron oxidant. Initial coordination of Ag(I) to Pd to generate 16, followed by electron transfer, would furnish proposed Pd(III) intermediate 12. Disproportionation of Pd(III) intermediate 12 to Pd(II) complex 13 and Pd(IV) intermediate 15... 

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