Homogeneous and electrochemical

As depicted in Scheme 1, reductive and oxidative cleavages may follow either a concerted or a stepwise mechanism. How the dynamics of concerted electron transfer/bond breaking reactions (heretofore called dissociative electron transfers) may be modeled, and particularly what the contribution is of bond breaking to the activation barrier, is the first question we will discuss (Section 2). In this area, the most numerous studies have concerned thermal heterogeneous (electrochemical) and homogeneous reactions. 

Some Comparisons Between the Energetics of Electrochemical and Homogeneous Electron-Transfer Reactions... 

Relationship Between Electrochemical and Homogeneous Reaction Energetics... 

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. 

Comparison between Experimental Reorganization Parameters for some Electrochemical and Homogeneous Exchange Reactions... 

Additional insight can be obtained by comparing the electrochemical and homogeneous activation parameters. Table II... 

Comparison of the corresponding electrochemical and homogeneous reorganization parameters reveal that 2A( e > AG ... 

It therefore seems reasonable that the deviations of the activation free energies for highly exoergic electrochemical and homogeneous reactions, illustrated in Figures 2 and 5, may arise partly from the same source, i.e., from values of for the oxidation half reactions that are unexpectedly small. That is not to say that other factors are not responsible, at least in part, for these discrepancies. Nonadiabaticity, work terms, specific solvation, and other environmental effects may all play important roles depending on the reactants. For example, there is evidence to suggest that the true rate constant for outer-3+/2+... 

Fig. 3 Electrochemical and homogeneous standard free energies of activation for self-exchange in the reduction of aromatic hydrocarbons in iV.A -dimethylformamide as a function of their equivalent hard sphere radius, a. 1, Benzonitrile 2, 4-cyanopyridine 3, o-toluonitrile 4, w-toluonitrile 5, p-toluonitrile 6, phthalonitrile 7, terephthalonitrile 8, nitrobenzene 9, w-dinitrobenzene 10, p-dinitrobenzene 11, w-nitrobenzonitrile 12, dibenzofuran 13, dibenzothiophene 14, p-naphthoquinone 15, anthracene 16, perylene 17, naphthalene 18, tra 5-stilbene. Solid lines denote theoretical predictions. (Adapted from Kojima and Bard, 1975.)...

COMPARISONS BETWEEN REACTIVITIES OF CORRESPONDING ELECTROCHEMICAL AND HOMOGENEOUS REDOX PROCESSES... 

III) Relative Rate Comparisons between Corresponding Electrochemical and Homogeneous Reactions. Providing each reaction is outer sphere, and work terms can be neglected or cancel, the relative rates for electrochemical reduction (or oxida-... 

As outlined in 12.3.7.2, there are close parallels in the kinetic parameters for corresponding electrochemical and homogeneous reactions, at least for outer-sphere pathways. Aside from the fundamental virtues of testing theoretically based relationships between electrochemical and homogeneous reactivities, such correlations can have predictive value, especially for estimating electrochemical reactivities from the much larger body of homogeneous rate data. 

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