ET Reactions at Electrodes and the Inverted Region

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.  

It was suggested that the absence of an inverted region for the ET reactions at spacer-covered metal electrodes is due to the availabihty of a continuum of electronic states in metal electrodes below the Fermi level. For the same reason, the inverted region is also not expected to be seen for the homogeneous intermolecular ET reactions because a continuum of electronic states are also available below and above the respective ground states of acceptor and donor ions in solutions involved in homogeneous ET reactions. 

It is important to note that as early as 1931, the density of electronic states in metals, the distribution of electronic states of ions in solution, and the effect of adsorption of species on metal electrode surfaces on activation barriers were adequately taken into account in the seminal Gurney-Butler nonquadratic quantum mechanical treatments, which provide excellent agreement with the observed current-overpotential dependence. 

Many conceptual inconsistencies of the continuum theory mentioned above and the discrepancy with experiments (see also the appendix of Chapter 4 in Ref. 55) warrant new theories based on molecular models. We focus on molecular models in the following sections. 

MOLECULAR MODELS OE ELECTRON TRANSFER REACTIONS AT ELECTRODES 

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