Reversible electron transfer, criteria

Here, we have demonstrated the differences between turnover and nontumover CVs for G. sulfurreducens biofilms. Turnover and nonturnover CVs can be used to correlate catalytic current under turnover conditions to redox peaks under nonturnover conditions. Scan rate analysis can provide a qualitative understanding of the biofilm electron-transfer mechanisms occurring within a biofilm. However, caution should be used when applying the Randles-Sevcik criterion to biofilms, as it was derived for a single-step, reversible electron-transfer step for diffusing mediators. 

Thus, as in the case of alkenes in the preceding chapter, we start with the radical type of activation that is much older. Transition-metal compounds play a key role in radical activation, because they provide very strong oxidants that can oxidize hydrocarbons either by (reversible) electron transfer or H-atom transfer (more rarely by hydride transfer). Biological oxidation of hydrocarbons involves reactive metal-0X0 species in methane mono-oxygenases and many related synthetic models, and a number of simple metal-oxo complexes also work. The clear criterion of distinction between an organometallic C-H activation and a radical activation is the above selectivity in activated C-H bonds. 

Electrochemical reversibility presupposes that Nemstian concentrations will exist at the electrode surface because die forward and reverse electron transfers are fast and occur simultaneously, near to equilibrium. The following criterion has been suggested by Matsuda and Ayabe [5] for confirmation of electrochemical reversibility ... 

The voltammograms at the microhole-supported ITIES were analyzed using the Tomes criterion [34], which predicts ii3/4 — iii/4l = 56.4/n mV (where n is the number of electrons transferred and E- i and 1/4 refer to the three-quarter and one-quarter potentials, respectively) for a reversible ET reaction. An attempt was made to use the deviations from the reversible behavior to estimate kinetic parameters using the method previously developed for UMEs [21,27]. However, the shape of measured voltammograms was imperfect, and the slope of the semilogarithmic plot observed was much lower than expected from the theory. It was concluded that voltammetry at micro-ITIES is not suitable for ET kinetic measurements because of insufficient accuracy and repeatability [16]. Those experiments may have been affected by reactions involving the supporting electrolytes, ion transfers, and interfacial precipitation. It is also possible that the data was at variance with the Butler-Volmer model because the overall reaction rate was only weakly potential-dependent [35] and/or limited by the precursor complex formation at the interface [33b]. 

This criterion is slightly different from that assessed in Section 1.4.3 on the reversibility or irreversibility of an electron transfer ... 

The diagnostic criterion of the reversibility of electron transfer is usually provided by the following dimensionless parameter, ip, asssuming the diffusion coefficients of the oxidized and reduced species are equal ... 

An example of an electronic energy transfer collision that is more efficient than the high nominal translational exoergicity wonld suggest on the basis of tiie adiabaticity criterion is the process of charge neutralization, e.g., Na+ -I- I Na -I- I or the reverse process of collisional ionization ... 

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