Outer-sphere electrochemical reaction

Bridge mediation mechanisms in heterogeneous outer sphere electrochemical reactions has also been theoretically treated using the pull—push and push-pull mechanistic concepts [84]. Schmidt [85] has considered theoretically homogeneous inner sphere bridge electron transfer reactions without atom or ion transfer. Bridge mediation in electron transfer reactions may also involve simultaneous atom or ion transfer. Heyrovsky [86] invoked mediation of electron transfer by formation of bridges to explain the enhancement of the rate of electroreduction of indium (III) ions in the presence of specifically adsorbed halide ions on mercury. 

Strictly, then, we can write for outer-sphere electrochemical reactions (cf. ref. 28)... 

These theoretical considerations also gave a basis for the consideration of the optimal distance of discharge, which is a result of competition between the activation energy AG and the overlap of electronic wave functions of the initial and final states. The reaction site for outer-sphere electrochemical reactions is presumed to be separated from the electrode surface by a layer of solvent molecules (see, for instance, [129]). In consequence, the influence of imaging interactions on AGJ predicted by the Marcus equation is small, which explains why such interactions are neglected in many calculations. However, considerations of metal field penetration show that the reaction sites close to the electrode are not favored [128], though contributions to ks from more distant reaction sites will be diminished by a smaller transmission coefficient. If the reaction is strongly nonadiabatic, then the closest approach to the electrode is favorable. 

We consider an outer-sphere electrochemical reaction between a metal and a simple redox system, once again using the reaction depicted in Eq. 35 ... 

In addition, the analysis of the kinetics of outer-sphere electrochemical reactions in terms of the preequilibrium model has the important advantage of allowing a common... 

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. 

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

The best way to search for the existence of an inverted region (if any) would be to use a single electrochemical electron transfer reaction in one solvent medium at a particular electrode and determine the effect of high overpotential on the reaction rate or the current density. Many experiments were carried out at organic spacer-covered ( 2.0 nm thick) electrodes to search for the inverted region for the outer-sphere ET reactions however, no inverted region was observed." ... 

Therefore, if the Marcus theory describes properly the effect of solvents of k, a linear correlation between In and ( op -fis ) should be observed in the experimental results. Before turning to the experimental studies, the (Sop - s ) parameter for various solvents used in electrochemical work is presented in Table 1. Inspection of these data reveals that the largest difference of the (Cop -Ss ) parameter for the listed solvents amounts to 0.263. Thus, on the basis of the Marcus theory for the outer-sphere electrode reactions, the largest change of the reaction rate for different solvents should amount to exp (const 0.263). In this estimation any double-layer effect on the rate constant was neglected. 

In most electrochemical studies, one employs solutions where the concentration of the electroactive species, i, is 1 mM. With these concentrations, the diffusion flux of electroactive species to the electrode, /, is of the order of otjC, where OTj is the mass transfer coefficient (cm/s) and C is the bulk concentration (mol/cm ). With m, 10 cm/s, this produces fluxes of the order of 10 mol/s/ cm or 6 X10 " molecules/s/cm, producing currents of 10 " A/cm. Under these conditions, even with very small electrodes, one measures the behavior of large ensembles of molecules. However, if the concentration of electroactive species is dropped to 1 pM, these fluxes drop to/j=10 mol/s/ cm or 6 X10 molecules/s/cm with a current density of 10 A/cm. Thus, with an ultramicroelectrode (UME) with about a 10 pm size or area of about 10 cm, the number of molecules arriving by diffusion to the electrode is about 1/s. In our previous work, we showed that by using very small ( pm) nanoelectrocatalytic C or Au electrodes with relatively small background currents, that nanometer-size electrocatalytic NPs, for example, of Pt, amplify the current of an appropriate inner-sphere (IS) reaction (e.g., hydrazine oxidation or proton reduction) to the pA level, and the frequency, size, and shape of collision events could be investigated. More recent work with different approaches has shown that interactions of the NPs with the electrode can be detected, even for outer-sphere (OS) reactions, as described in later sections of this chapter. 

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. 

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. 

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. 

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. 

In the case of stepwise electron-transfer bond-breaking processes, the kinetics of the electron transfer can be analysed according to the Marcus-Hush theory of outer sphere electron transfer. This is a first reason why we will start by recalling the bases and main outcomes of this theory. It will also serve as a starting point for attempting to analyse inner sphere processes. Alkyl and aryl halides will serve as the main experimental examples because they are common reactants in substitution reactions and because, at the same time, a large body of rate data, both electrochemical and chemical, are available. A few additional experimental examples will also be discussed. 

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