Redox groups

The studies by Biermann et al. [28] indicate that the carbon blacks used as the conductive matrix in Leclanche cells remain chemically inert, that is, they do not undergo oxidation during storage or discharge of the cell. However, Caudle et al. [29] found evidence that the ion-exchange properties of carbon black, which exist because of the presence of surface redox groups, are responsible for electrochemical interactions with Mn02. The extent of MnO, reduction to MnOOH depends on the carbon black (i.e., furnace black > acetylene black). 

Providing an ion exchanger with a sufficient number of redox groups so that conduction can occur by a relay-type redox-change mechanism. Examples are hydroquinone-derived redox polymers and polyvinyl polymers with a tetrathia-fulvalene, ferrocene, or carbazole group, which have been found useful for research and analytical applications. 

The common feature of compounds [5]-[15] is that the electrophoric units are linked by saturated spacers, thus establishing only weak electronic (through-bond or through-space) interaction of the Tt-systems. In contrast, the binaphthyl [16], the biperylenyl [17] and the bianthryl [18] as well as the structurally related homologues [19], [20] and [21] allow for a direct 7r,7r-interaction of the subunits it will be shown, however, that for both steric and electronic reasons the inter-ring conjugation can be weak and thus lead to electronically independent redox groups in a similar fashion as in [5]-[15]. 

One example of a — Hg- linked TTF dimer has been reported [106]. Linear coordination is expected for the mercury atom. Extended Hiickel calculations indicate that the mercury orbitals are essentially not involved in the HOMOs, and thus the two TTF redox groups can be regarded as being completely isolated from each other. The voltammograms confirm these expectations in that both TTF... 

It has been reported that the electrical properties of single molecules incorporating redox groups (e.g. viologens [114, 119, 120, 123, 124], oligophenylene ethynylenes [122, 123], porphyrins [111, 126], oligo-anilines and thiophenes [116, 127], metal transition complexes [118,128-132], carotenes [133], ferrocenes [134,135],perylene tetracarboxylic bisimide [93, 136, 137] and redox-active proteins [138-143]), can be switched electrochemically. Such experiments, typically performed by STM on redox-active molecules tethered via Au-S bonds between a gold substrate and a tip under potential control, allow the possibility to examine directly the correlation between redox state and the conductance of individual molecules. 

The reorganization free energy /.R represents the electronic-vibrational coupling, ( and y are fractions of the overpotential r] and of the bias voltage bias at the site of the redox center, e is the elementary charge, kB the Boltzmann constant, and coeff a characteristic nuclear vibration frequency, k and p represent, respectively, the microscopic transmission coefficient and the density of electronic levels in the metal leads, which are assumed to be identical for both the reduction and the oxidation of the intermediate redox group. Tmax and r max are the current and the overvoltage at the maximum. 

Electrochemical oxidation of X produces a polymer film with polythiophene as the backbone and viologen centers as pendant redox groups. The electrochemical properties of the polymer are the combination of polythiophene and viologen. Using viologen subunits as the internal standard (one per repeat unit of the polymer), the "doping level" of the oxidized polythiophene backbone at its maximum conductivity can be measured and is about 25%. The charge transport via the pendant V2+/+ of poly(l) has been studied by... 

The redox groups can be introduced by coordination, electrostatic binding or covalent attachment to the polymer backbone. 

Electrolytic oxidation or reduction may occur upon contact with the electrospray solution, triggering the formation of ions that normally do not exist in the bulk. Derivatization can be used to induce ionization for those compounds that do not carry redox groups [27,28],... 

As the names of the component proteins imply, iron-containing redox groups are essential to nitrogenase function. These clusters are of the Fe S type, but they exhibit many unique features that are not present in simpler protein and model systems. A brief summary of the properdes of the nitrogenase proteins, with emphasis on the metal clusters, follows. 

Cytochrome oxidase (cytochrome aa3) represents the most important cytochrome of the a class. This is the terminal oxidase used in animals, plants, yeasts, algae and some bacteria. It contains two copper centres, giving four redox groups in total. This oxidase is discussed with other cytochromes that have a terminal oxidase function in Sections 62.1.12.4 and 62.1.12.5. These are cytochromes o, d and cd,. The oxidases fed719 and ax are not included in that discussion. The situation regarding cytochrome ax has been confused, partly due to uncertainty in the definition of this cytochrome. In some respects, the properties of cytochrome ax resemble those of mitochondrial and bacterial aa3. It functions as a terminal oxidase in some bacteria,720 but its role in E. coli is unknown. A soluble fraction from disrupted E. coli cells grown anaerobically on glycerol and fumarate contains a hemoprotein similar to cytochrome ax, which has catalase and peroxidase activity.721... 

In relation to the application of the Marcus-Hush model to immobilized redox groups in LSV and CV, the behavior predicted by MH matches that of the BV... 

Catalytically functional acidic-basic and redox groups must possess advantageous geometry (disposition) structurally shaped similar to the active zone of the appropriate enzyme [14],... 

A three-zone DC model for treating electrochemical ET at a self-assembled monolayer (SAM) film-modified metal electrode surface [49] is displayed in Figure 3.27, where zones I, II, and III, defined by parallel infinite planes, correspond, respectively, to an aqueous electrolyte, a hydrocarbon film, and the metal, and the ET-active redox group is represented by a point charge shift (Aq) in a spherical solute cavity [22]. The Poisson equation has been solved for this system, and the results analyzed in terms of image charge contributions to As [22] (see below). 

Another example of ET in an inhomogeneous medium is the three-zone interfacial assembly depicted in Figure 3.27. To model As for ET between a film-modified metal electrode and a ferrocene/ferrocenium redox couple in contact with aqueous solvent [49], the Poisson equation was solved with the following parameters s = 1.8 and 0I = 78.0 (water) = s0ll = 2.25 (alkane film) = eom = oo (metal), a = d = 3.8 A (effective radius of redox group), and Aq = e [23]. 

---