Indirect electron transfer

Redox reactions can proceed by direct transfer of electrons between chemical species. Examples include the rusting of iron and the metabolic breakdown of carbohydrates. Redox processes also can take place by indirect electron transfer from one chemical species to another via an electrical circuit. When a chemical reaction is coupled with electron flow through a circuit, the process is electrochemical. Flashlight batteries and aluminum smelters involve electrochemical processes. 

Spontaneous redox reactions can aiso occur by indirect eiectron transfer. In an indirect electron transfer, species invoived in the redox chemistry are not aiiowed to come into direct contact with one another. Instead, the oxidation occurs at one end of a wire and transfers eiectrons to the wire. Reduction occurs at the other end of the wire and removes eiectrons from the wire. The wire conducts eiectrons between the oxidation site and the reduction site. 

In the example above, the electron transfer was direct, that is, the electrons were exchanged directly from the zinc metal to the cupric ions. But such a direct electron transfer doesn t allow for any useful work to be done by the electrons. Therefore, in order to use these electrons, indirect electron transfer must be done. The two half-reactions are physically separated and connected by a wire. The electrons that are lost in the oxidation half-reaction are allowed to flow through the wire to get to the reduction half-reaction. While those electrons... 

As the thickness of a superficial film increases, the transfer of redox electrons through electron levels in the film (indirect electron transfer) becomes predominant over the direct electron transfer between the electrode metal and the redox particles the electron transfer takes place between the electron level in the film and the electron level in the redox particles. 

The free-radicals are generated by discharge of proton, peroxides and easily reducible compounds at the cathode according to Eq. (1—4). The radial-anion of monomer is obtained by both direct and indirect electron transfer process [Eq. (5—6)]. The indirect process means that the active oxidizing species is formed from the electrolytes by electrode reaction, followed by interaction with the monomer. An unstable monomer like a,a -2-trichloro-p-xylene is formed and polymerizes instantaneously [Eq. (7)]. The regeneration of ferrous ion from the pool of used up ferric ion in a redox system is electrolytically successful [Eq. (8)] and an... 

Based on the polarographic examination, the initiation has been verified to be caused by indirect electron transfer through the reduction... 

The indirect electron transfer sensitization reported earlier when biphenyl was present in dicyanoanthracene-sensitized epoxide openings also finds analogy in the sensitized oxygenation of a mixture of trans-stilbene and tetraphenylethylene, eq. 76 (233) ... 

The assumption that some dyes can spectrally sensitize latent image formation in silver halides by direct electron transfer from the excited dye to the conduction band and other dyes by indirect electron transfer from the dye radical formed by photoinjection of a hole into the valence band is in good accord with experiment. The locations of the highest filled and lowest vacant energy levels of the dye relative to the valence and conduction bands of the silver halide determine which mode of sensitization will occur, or whether both can occur. 

Fig. 8.5. Fluxes of holes (g) and electrons (J ) into the interface. The possible reaction routes for holes are direct or indirect electron transfer to redox species and recombination.

The near-IR transient absorption spectrum produced in the indirect electron-transfer experiment is shown in Fig. 2. The shape of the spectrum is very similar to that in the absence of BP, but about ten times as strong. The risetime suggests diffusion-controlled quenching of BP by Cgo-... 

Hence there is no gas-phase experiment yet which fully encompasses all aspects of an electron-transfer reaction in solution. In solution, the solvent acts first as a polarization medium, which affects the energetics of direct transfers from the donor to the acceptor. It can also act as a transport medium for indirect electron transfers. The first aspect has been addressed in various cluster experiments [276]. The second aspect was addressed more recently by considering the femtosecond dynamics of iodide-(water) anion clusters, as reviewed below [277]. Finally, clusters present the advantage of isolating one reaction pair free from secondary collisions, except those, which are desired, with the solvent molecules (or atoms). The latter consideration motivated the cluster isolated chemical reaction (CICR) technique reviewed in Section 2.8.3. 

There remains some doubt about the first step of the overall reaction in MeCN the final products are usually the A -alkylacetamides, as shown in Eq. (35). In neutral solution, at extreme anodic potentials, it is difficult to decide between direct [Eqs. (1), (2), and/or (3)] and indirect electron transfer [Eqs. (16), (17), and/or (18)]. For oxidation in MeCN-BF4 solutions the variation in potentials is best explained in terms of the direct mechanism. An indirect oxidation mechanism involving hydrogen abstraction by electrogenerated nitrate radical has recently been proposed for the electrolysis of linear alkanes in tert-BUOH/H2O mixtures containing HNO3 and saturated with O2 [25]. 

Frew, J.E. and Hill, H.A.O. (1988) Review direct and indirect electron transfer between electrodes and redox proteins. European Journal of Biochemistry, 172, 261-269. 

One of the main advantages of the optically transparent thin-layer spectroelectrochemical technique (OTTLSET) is that the oxidized and reduced forms of the analyte adsorbed on the electrode and in the bulk solution can be quickly adjusted to an equilibrium state when the appropriate potential is applied to the thin-layer cell, thereby providing a simple method for measuring the kinetics of a redox system. The formal potential E° and the electron transfer number n can be obtained from the Nernst equation by monitoring the absorbance changes in situ as a function of potential. Other thermodynamic parameters, such as AH, AS, and AG, can also be obtained. Most redox proteins do not undergo direct redox reactions on a bare metal electrode surface. However, they can undergo indirect electron transfer processes in the presence of a mediator or a promoter the determination of their thermodynamic parameters can then... 

Indirect electron transfer based on the electron shuttle mediated process. 

Cosnier and coworkers coupled the polymer entrapment strategy with the use of CNTs as porous conductive support [28]. Methyl viologen as a redox mediator covalently bound to the polymer ensured a high indirect electron transfer from D. jructosovorans hydrogenase to the CNTs. Again, a 10-fold increase in... 

Fig.l Electron transfer mechanisms (D T direct electron transfia-, MET mediated electron transfer, lET indirect electron transfer)... 

MET - natural extracellular flavins [17, 18, 26, 27] and artificial compounds [28]. Furthermore, as a variant of MES, it is possible to indirectly transfer electrons from electrodes to microorganisms via the production of H2 or other carriers. The lET (indirect electron transfer) can be regarded as two independent steps the electrochemical generation of a suitable electron carrier and the consumption of this natural or artificial carrier by the microorganism. Figure 1 summarizes the different electron transfer principles. 

The electrochemical oxidation of an organic pollutant can occur by direct electron transfer to the anode, indirect electron transfer mediated by hydroxyl radicals ( OH) or other species electrogenerated from inorganic salts. In the direct electrochemical oxidation, which yields very poor decontamination, the organic... 

But if you separate those two half-reactions so that when the zinc is oxidized, the electrons it releases are forced to travel through a wire to get to the Cu, you get something useful a galvanic or voltaic cell, a redox reaction that produces electricity. In this section, 1 show you how that Zn/Cu reaction may be separated out so that you have an indirect electron transfer and can produce some useable electricity. I cdso show you how electrolytic cells do the reverse, using electricity to cause a redox reaction. Finally, you see how rechargeable batteries both generate electricity and cause chemical reactions. 

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