Electrochemical outer-sphere

Schmickler W. 1976. The effect of quantum vibrations on electrochemical outer sphere redox reactions. Electrochim Acta 21 161-168. 

The two models discussed were developed for electrochemical outer-sphere reactions. Most electrochemical reactions, especially those of technological usefulness, however, involve the breaking or formation of chemical bonds during the electron transfer. 

Khan [174] studied the electrooxidation of ferrocene at a Pt electrode in polar solvents ranging from methanol to heptan-l-ol. Experimental data concorded well with the calculated results when solvent influence on the pre-exponential coefficient was considered. In calculations v = rb was used. Khan [174] points out that expressed by Eq. (36) exhibits a temperature dependence different from that predicted by the classical expression = k T/h. Another conclusion which may follow from the same paper is that the transmission coefficient for the electrochemical outer-sphere electron-transfer reactions in polar alcoholic solvents may not be equal to unity. 

The field of modified electrodes spans a wide area of novel and promising research. The work dted in this article covers fundamental experimental aspects of electrochemistry such as the rate of electron transfer reactions and charge propagation within threedimensional arrays of redox centers and the distances over which electrons can be transferred in outer sphere redox reactions. Questions of polymer chemistry such as the study of permeability of membranes and the diffusion of ions and neutrals in solvent swollen polymers are accessible by new experimental techniques. There is hope of new solutions of macroscopic as well as microscopic electrochemical phenomena the selective and kinetically facile production of substances at square meters of modified electrodes and the detection of trace levels of substances in wastes or in biological material. Technical applications of electronic devices based on molecular chemistry, even those that mimic biological systems of impulse transmission appear feasible and the construction of organic polymer batteries and color displays is close to industrial use. 

Electrochemical reactions only involving a change of charge of simple or complex ions but not any change in inner geometry are commonly called outer-sphere electron transfer reactions. For some time, the reduction and oxidation of simple and... 

The elementary electrochemical reactions differ by the degree of their complexity. The simplest class of reactions is represented by the outer-sphere electron transfer reactions. An example of this type is the electron transfer reactions of complex ions. The electron transfer here does not result in a change of the composition of the reactants. Even a change in the intramolecular structure (inner-sphere reorganization) may be neglected in many cases. The only result of the electron transfer is then the change in the outer-sphere solvation of the reactants. The microscopic mechanism of this type of reaction is very close to that for the outer-sphere electron transfer in the bulk solution. Therefore, the latter is worth considering first. 

FIGURE 34.4 Dependence of electrochemical rate constant on the electrode potential for outer-sphere electron transfer. An exponential increase in the normal region changes for the plateau in the activationless region. 

Experimental tests of the theoretical predictions have involved the electrochemical reduction of alkyl and benzyl halides as well as their reduction by homogeneous electron donors.22,29-31 In the first case, AG° = E - rx r.+x=f where E is the electrode potential and rx r.+x=f is the standard potential of the RX/R + XT couple. In the homogeneous case, AG° = E q — rx r-+xt> where E Q is the standard potential of the outer-sphere electron donor or acceptor couple P/Q, and + stands for a reduction and — for an oxidation. 

Chemical and electrochemical reactions in condensed phases are generally quite complex processes only outer-sphere electron-transfer reactions are sufficiently simple that we have reached a fair understanding of them in terms of microscopic concepts. In this chapter we give a simple derivation of a semiclassical theory of outer-sphere electron-transfer reactions, which was first systematically developed by Marcus [1] and Hush [2] in a series of papers. A more advanced treatment will be presented in Chapter 19. 

Coordinatively unsaturated complexes and those giving easily such species by ligand dissociation favor pathways related to that described in Eqs. (10) and (13). Coordinatively saturated complexes reduce halocarbons via outer-sphere ET [193, 194]. In cases of electrochemical dehalogenations, the species formed by one-electron reduction of the mediators on the cathode often react in this way [156, 157, 198], For example (Eq. (14)) [157, 166] ... 

In outer-sphere SET reductions (e.g., in electrochemical dehalogenations), hydrogen abstraction by R leads to the product RH (i.e., no step related to (ii) is required to occur). Process (ii) follows generally the activation of the substrate in the proposed hydrodehalogenation cycles, but we know also of opposite examples [77, 82, 106, 112],... 

Chemical reactivity of unfunctionalized organosilicon compounds, the tetraalkylsilanes, are generally very low. There has been virtually no method for the selective transformation of unfunctionalized tetraalkylsilanes into other compounds under mild conditions. The electrochemical reactivity of tetraalkylsilanes is also very low. Kochi et al. have reported the oxidation potentials of tetraalkyl group-14-metal compounds determined by cyclic voltammetry [2]. The oxidation potential (Ep) increases in the order of Pb < Sn < Ge < Si as shown in Table 1. The order of the oxidation potential is the same as that of the ionization potentials and the steric effect of the alkyl group is very small. Therefore, the electron transfer is suggested as proceeding by an outer-sphere process. However, it seems to be difficult to oxidize tetraalkylsilanes electro-chemically in a practical sense because the oxidation potentials are outside the electrochemical windows of the usual supporting electrolyte/solvent systems (>2.5 V). 

Electrochemical reactions can be broken down into two groups outer-sphere electron-transfer reactions and inner-sphere electron transfer reactions. Outer-sphere reactions are reactions that only involve electron transfer. There is no adsorption and no breaking or forming of chemical bonds. Because of their simplicity, numerous studies have been performed, many entirely theoretical.18-25 By definition, though, electrode reactions are not outer-sphere reactions. However, if charge transfer is rate limiting for an electrode reaction, it typically takes a form similar to that of an outer-sphere reaction, which is described later in this section. 

Another strategy consists in the application of convolution in the same manner as depicted in Section 1.4.3 for outer-sphere electron transfers. The activation-driving force law is then obtained directly from the variation of the rate constant, k(E), with the electrode potential. An example of the successful application of this strategy is provided by the electrochemical reduction of alkyl peroxides7 ... 

The stage is set in the first chapter, with the depiction of a typical electrochemical experiment and application to the determination of the thermodynamic and kinetic characteristics of outer-sphere electron transfer reaction, with no further chemical steps in the reaction mechanism. In this chapter as well as in the others, we describe both the experimental data and the methods by which they can be gathered. 

In the following sections, we shall explore the applicability of such relationships to experimental data for some simple outer-sphere reactions involving transition-metal complexes. In keeping with the distinction between intrinsic and thermodynamic barriers [eq 7], exchange reactions will be considered first, followed by a comparison of driving force effects for related electrochemical and homogeneous reactions. 

It should be emphasized that the majority of electrochemically induced redox processes in inorganic chemistry proceed (or are assumed to proceed) through outer-sphere mechanisms. 

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