Cyclic reduction potential

One aspect that reflects the electronic configuration of fullerenes relates to the electrochemically induced reduction and oxidation processes in solution. In good agreement with the tlireefold degenerate LUMO, the redox chemistry of [60]fullerene, investigated primarily with cyclic voltammetry and Osteryoung square wave voltammetry, unravels six reversible, one-electron reduction steps with potentials that are equally separated from each other. The separation between any two successive reduction steps is -450 50 mV. The low reduction potential (only -0.44 V versus SCE) of the process, that corresponds to the generation of the rt-radical anion 131,109,110,111 and 1121, deserves special attention. 

Fig. 8 Reactions of various carbocations with Kuhn s anion [2 ] as compared with their reduction potentials (peak potentials measured vs. Ag/Ag in acetonitrile by cyclic voltammetry cf. Tables 1 and 8 and Okamoto et al., 1983). SALT, salt formation COV, covalent bond formation ET, single-electron transfer. 

Figure 16.8 Pt/TiO c-catalyzed oxygen reduction potential, where 0.01 mA cm is reached during the negative scan in a cyclic voltammetry experiment (scan rate 20 mV s ) in oxygen-saturated 0.5 M HCIO4 at 25 °C. (See color insert.)...

FIGURE 3.6 Determination of the reduction potential of rubredoxin by electrochemistry and by EPR monitored bulk titration. The (+) data are from cyclic voltammograms taken at different temperatures the ( ) point is from low temperature EPR monitored titrations at ambient temperature or at 80°C. (Data from Hagedoorn et al. 1998.)... 

Figure 2.87 Schematic of the cyclic voltammogram expected from a reversible electrochemical redox system 0 + e + R having a standard reduction potential °. E is the potential of the working electrode, and I the current.

It is known that the reduction potentials of quinones are related to the aromatic stabilization of the parent conjugated systems. In an attempt to relate the annulenediones 171,177, and 178 to the tetradehydro[18]annuIene system Breslow and coworkers63 have studied their electrochemical reduction by cyclic voltammetry. These diones can easily be reduced to the corresponding dianions, e.g. 171 - 179. These... 

At present, new developments challenge previous ideas concerning the role of nitric oxide in oxidative processes. The capacity of nitric oxide to oxidize substrates by a one-electron transfer mechanism was supported by the suggestion that its reduction potential is positive and relatively high. However, recent determinations based on the combination of quantum mechanical calculations, cyclic voltammetry, and chemical experiments suggest that °(NO/ NO-) = —0.8 0.2 V [56]. This new value of the NO reduction potential apparently denies the possibility for NO to react as a one-electron oxidant with biomolecules. However, it should be noted that such reactions are described in several studies. Thus, Sharpe and Cooper [57] showed that nitric oxide oxidized ferrocytochrome c to ferricytochrome c to form nitroxyl anion. These authors also proposed that the nitroxyl anion formed subsequently reacted with dioxygen, yielding peroxynitrite. If it is true, then Reactions (24) and (25) may represent a new pathway of peroxynitrite formation in mitochondria without the participation of superoxide. 

Hexacyano[3]radialene (50) is a very powerful electron acceptor according to both experiment23,24 35 and MNDO calculations of LUMO energy and adiabatic electron affinity25. The easy reduction to the stable species 50" and 502- by KBr and Nal, respectively, has already been mentioned. Similarly, the hexaester 51 is reduced to 512-by Lil24. Most [3]radialenes with two or three quinoid substituents are reduced in two subsequent, well-separated, reversible one-electron steps. As an exception, an apparent two-electron reduction occurs for 4620. The reduction potentials of some [3]radialenes of this type, as determined by cyclic voltammetry, are collected in Table 1. Due to the occurrence of the first reduction step at relatively high potential, all these radialenes... 

TABLE 1. Reduction potentials of [3]radialenes ( 1/2, V) with quinoid substituents and of some related compounds, as determined by cyclic voltammetry (in CH2CI2 vs SCE)... 

Figure 4. Cyclic voltammetry of adjacent electrodes of a poly(I)-coated microelectrode array driven individually and together at 200 mV/s in the region of the first reduction potential of V2+ in CH3CN/0.1 M [n-Bu4N]PF6.

The one-electron reduction potentials, (E°) for the phenoxyl-phenolate and phenoxyl-phenol couples in water (pH 2-13.5) have been measured by kinetic [pulse radiolysis (41)] and electrochemical methods (cyclic voltammetry). Table I summarizes some important results (41-50). The effect of substituents in the para position relative to the OH group has been studied in some detail. Methyl, methoxy, and hydroxy substituents decrease the redox potentials making the phe-noxyls more easily accessible while acetyls and carboxyls increase these values (42). Merenyi and co-workers (49) found a linear Hammett plot of log K = E°l0.059 versus Op values of substituents (the inductive Hammett parameter) in the 4 position, where E° in volts is the one-electron reduction potential of 4-substituted phenoxyls. They also reported the bond dissociation energies, D(O-H) (and electron affinities), of these phenols that span the range 75.5 kcal mol 1 for 4-amino-... 

In spite of their high pAR+ values, the methyl cations have less negative reduction potentials as compared to those of the cyclic cations e.g., -1.12 V for the tropylium ion 8+ and -2.20 V for cyclopropenium ion 9+ (8). Presence of tert-butyl groups in cations 2b-d+ increased the reversibility of both reduction and oxidation waves. The most negative reduction potential in the dimethylamino derivative 20+ reflects its high electrochemical stability. 

Although separate determination of the kinetic and thermodynamic parameters of electron transfer to transient radicals is certainly important from a fundamental point of view, the cyclic voltammetric determination of the reduction potentials and dimerization parameters may be useful to devise preparative-scale strategies. In preparative-scale electrolysis (Section 2.3) these parameters are the same as in cyclic voltammetry after replacement in equations (2.39) and (2.40) of Fv/IZT by D/52. For example, a diffusion layer thickness S = 5 x 10-2 cm is equivalent to v = 0.01 V/s. The parameters thus adapted, with no necessity of separating the kinetic and thermodynamic parameters of electron transfer, may thus be used to defined optimized preparative-scale strategies according to the principles defined and illustrated in Section 2.4. 

A variety of physical methods has been used to ascertain whether or not surface ruthenation alters the structure of a protein. UV-vis, CD, EPR, and resonance Raman spectroscopies have demonstrated that myoglobin [14, 18], cytochrome c [5, 16, 19, 21], and azurin [13] are not perturbed structurally by the attachment of a ruthenium complex to a surface histidine. The reduction potential of the metal redox center of a protein and its temperature dependence are indicators of protein structure as well. Cyclic voltammetry [5, 13], differential pulse polarography [14,21], and spectroelectrochemistry [12,14,22] are commonly used for the determination of the ruthenium and protein redox center potentials in modified proteins. 

Reduction potentials of the S. obliqms His59 Ru(NH3)5-modified protein have been determined by cyclic voltammetry using as electrode the oxidized surface obtained by polishing the edge plane of pyrolytic graphite [137]. The modified protein responds well at the electrode, whereas the native protein requires multi-eharged cations, e.g. Mg or [Cr(NH3)g] as mediators to give satisfactory reversibility. Separate reduction potentials at 1=0.10 M(NaCl) for native S. obliquus plastocyanin (389 mV) and [Ru(NH3)5 (imidazole)]... 

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