Methyl viologen cation radical

An interesting aspect of this mechanism is the implicit unreactivity of Z at the electrode, although it reacts with electrogenerated O. This situation has been frequently encountered in the reaction of certain types of biological materials such as heme proteins. An example is the reduction of cytochrome c by electrogenerated methyl viologen cation radical, MV [2] ... 

Lozano MC, Perez RT, Tomas V, et al. 1989. Coulometric determination of organic compounds with methyl viologen cation-radical. Microchem J 39 59-64. 

Scheme 13 Oxygenation of methyl viologen cation radical via radical-radical coupling with O2...

Figure 9. Spectra of cytochrome c (17.5 /tM)-cytochrome c oxidase U2.5 fiM heme a) mixture during reductive indirect coulometric titration using methyl viologen. Titrant increments of 1.04x10" equivalents spectra recorded after each addition of reductive charge. Final two spectra at 605 nm were taken after excess methyl viologen cation radical was present. Adapted from Reference (60) with permission.

Mandler and coworkers have studied the case of twin microdisks (tip and substrate) in which the substrate is unbiased and acts to catalyze the reaction of the product formed at the tip (methyl viologen cation radical) with H+(aq) [83]. This arrangement allows the determination of the substrate potential and exchange current density and was used to analyze the kinetics of proton reduction at the Pt substrate. 

Organic Molecules It can be seen from our earlier discussion that the presence of a transition metal ion is not always required for an electrochromic effect. Indeed, many organic molecules can yield colored products as a result of reversible reduction or oxidation. 4,4 -Bipyridinium salts are the best known example of such compounds. These compounds can be prepared, stored, and purchased in colorless dicationic form (bipm +). One electron reduction of the dication leads to the intensely colored radical cation (bipm+ ). Such radical cations exist in equilibrium with their dimers (bipm ). In the case of methyl viologen, the radical cation is blue and the dimer is red. By varying the substient group in the molecule, different colors can be obtained. 

Numerous papers dealing with the photoreduction of viologens have appeared. Rate constants for electron transfer between aromatic hydrocarbon triplets and bipyridinium and pyridinium salts have been measured. The results suggest that the process is both facile and efficient. All the rate constants necessary for the determination of the quantum yield of formation of the viologen cation-radical in the methyl viologen-Eosin-EDTA system are also now available. " The high activity of dihydroxytin(rv) uroporphyrin in the photoreduction of methyl viologen has been ascribed to the failure of a photochemi-cally inactive tt—tt complex to be formed between substrate and sensitizer. 

The SERS of methyl viologen cations, and MV, and a wide variety of cationic metal complexes have also been observed (Table VIII). With the MV radical cation, both a SERS and a RR process can be observed from the surface Raman spectrum. " The MV and MV cations seem to interact with the Ag surface via adsorbed halides. On adding an electron, the SERS carbon-carbon stretching frequency shifts from 1292 cm for MV to 1352 cm" for MV for the carbons bridging the pyridine rings in the molecule. This upward frequency shift is indicative of increased electron density in the bridging carbon-carbon bond on electron transfer. 

Indeed, the (200-fs) laser excitation of the EDA complexes of various benz-pinacols with methyl viologen (MV2+) confirms the formation of all the transient species in equation (59). A careful kinetic analysis of the decay rates of pinacol cation radical and reduced methyl viologen leads to the conclusion that the ultrafast C—C bond cleavage (kc c = 1010 to 1011 s- ) of the various pinacol cation radicals competes effectively with the back electron transfer in the reactive ion pair. 

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. 

A cation radical chain cycloaddition-polymerization catalysed by tris(4-bromophenyl)aminium hexachloroantimonate has been reported to afford polymers with an average molecular weight up to 150000. Both cyclobutanation and Diels-Alder polymers were obtained. " The reactivity of the phospine radical cation towards nucleophiles was studied. Tributylphosphine reacted with l,l-dimethyl-4,4-bipyridinium (methyl viologen, MV) in the presence of an alcohol or thiol (RXH X = O, S), which resulted in the gradual formation of the one-electron reduced form... 

For instance, inclusion of l,T-dimethyl-4,4 -dipyridyl cation-radical (the cation-radical of methyl viologen) into a cavity of sulfocalixarenes prevents the disadvantageous n dimerization. This is important for more effective use of viologens as components of electrochromic displays and electric condnctors (Gno et al. 2007, references therein). 

Nonetheless, sensitization by dyes held within the cores of microemulsions can be easily accomplished [69]. Such sensitization is an important component of photogalvanic effects, the magnitude of which are significantly enhanced in the non-homogeneous environment of a microemulsion [70], The hydrophilic core of an water-in-oil microemulsion can concentrate cation radicals formed via interfacial electron transfer and hence increase the yield of subsequent dimerization the dimethylnaphthalene cation radical exhibits a dimerization equilibrium constant of nearly 500 in a microemulsion [71]. For similar reasons, hexylviologen acts as a much more efficient relay than methyl viologen in a CTAB/hexanol microemulsion [72]. 

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. 

Direct spectroscopic evidence for charge transfer was also obtained using methyl viologen as an acceptor. Formation of the reduced species (methyl vio-logen radical cation) was observed upon irradiation of 132 and 154 [101,106] (Scheme 26). 

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