Viologen radicals

With ft2 and ft3 known, k could thus be determined. In a related example, a number of reactions of alkyl radicals (CH, C2H, etc.) have been studied by the application of similar principles. An intensely colored probe that reacts with R was added with (or without) the substrate of interest. The probe is a species like the methyl viologen radical ion, MV,+. If S represents the substrate, the scheme is... 

The recombination between Ru(bipy)j and viologen radicals nearby the inner surface of the vesicle is found to be strongly inhibited as compared to homogeneous solution, presumably due to a fast extraction of hydrophobic viologen radicals into the depth of the membrane. 

Two mechanisms of transmembrane electron transfer were elucidated (i) via the translocation of viologen radical across membrane and (ii) via... 

Figure 17. Formation of viologen radicals on visible light irradiation in the presence of EDTA as a sacrificial electron donor.

Worked Example 8.11 We encountered the dimerization of methyl viologen radical cation MV+ in Equation (8.6) and Worked Example 8.4. Calculate the value of the second-order rate constant k2 if the initial concentration of MV+ was 0.001 mol dm-3 and the concentration dropped to 4 x 10-4 mol dm-3 after 0.02 s. (The temperature was 298 K.)... 

Similarly, the pyruvate dehydrogenase complex (PDC) can be activated directly by electrogenerated methyl viologen radical cations (MV +) as mediator. Thus, the naturally PDC-catalyzed oxidative decarboxylation of pyruvic acid in the... 

However, in certain cases, the rate of electron uptake by a particular species just happens to be slow. For example, electron transfer between the methyl viologen radical cation (MV ) and hydrogen peroxide has a rate constant of 2.0 (mol dm ) s , while the reaction between MV and just about any other chemical oxidant known is so fast as to be dijfusion-controlled. The reason for this is simply not known at the present time. 

Litter et al. (1991) found that the dissolution of maghemite was also considerably speeded up, once a dissolved Fe "-oxalate or Fe" -EDTA complex was reduced to an Fe" complex by UV irradiation 1 = 254 nm). This system also showed an induction period which could be eliminated by addition of Fe " (see Fig. 12.28). In a study concerned with dissolution of corrosion oxide, electrons from viologen radicals produced by y-radiation ( Co) were used to dissolve hematite and goethite (Mulvaney et al., 1988) it was observed that the Fe " appearing in solution could only account for a fraction of the electrons consumed. The remainder was involved in conversion of the Fe " oxide into magnetite. 

Table 1.13 Optical properties of viologen radical cations...

Figure 54 Comparison of the yields of the viologen radical as a function of photolysis time for ZnTPPC in Li-Al LDH with 0.01 M EDTA (A) 0.01 M PSV and (+) 0.01 M HV2+. (From Ref. lOld. Copyright 1996 The American Chemical Society.)...

Table 2. Rate constants of transmembrane electron transfer (k,) by viologen radical cations, their recombination with Ru(bpy)j+ in homogeneous solution (k ) and inside the vesicle cavity (kr and kb) and efficiencies of spatial charge separation (t ) for viologens with various substituents [201]...

Fig. 5. Viologen-mediated electron transport across the vesicle membrane (a) — scheme of the process (b) — variations of viologen radical cation concentration in the presence (/) and the absence (2) of 0.75 mol/1 K3Fe(CN)6 in the inner cavity. Sodium dithionite concentration is 0.01 mol/1. The molar ratio viologen lipid =1 20...

Tabushi et al. [161] studied the dependence of the stationary rate of dark electron transfer across a lipid membrane by dialkyl viologen radical cations CnV+ on the number n of carbon atoms in the alkyl substituents (n = 1, 3, 4, 8 and 16) in the... 

Hurst and co-workers [172, 180-185] extensively studied the mechanisms of dark electron transfer by alkylmethylviologen radical cations across the membranes of DHP vesicles. The experiments carried out and the results obtained are quite similar to those described above. In particular, it has been demonstrated that during transmembrane electron transfer in steady-state conditions viologen radical cations migrate from one side of the membrane to the other with the rate constant kt = 2 x 10-2 s-1. 

Artificial electron carriers are recognizable by the active sites of different redox enzymes and specifically biocatalysts containing Fe of Mo sulfur clusters as active sites. Bipyridinium radical cations, i.e. methyl viologen radical, MV+, exhibit proper electrical and size properties to penetrate into protein structures and to mediate reduction processes at the enzymes active sites. 

The mechanism was proposed to involve reduction of the neutral nickel bis(dithiolene) complex by methyl viologen radical cation, followed by dimerization to form the active catalyst Ni(SS)212 2 - as shown in Eqs. 2-6 ... 

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