Weakly supported voltammetry

E. J. F. Dickinson and R. G. Gompton. The zero-field approximation for weakly supported voltammetry A critical evaluation, Chem. Phys. Lett. 497, 178 183 (2010). 

Since the Debye length is of the order of tens of nanometres even at weak support, this is certainly the case for a microelectrode with re > 1 fim. Therefore, weakly supported voltammetry can be simulated using the Poisson equation to describe the electric field, with the boundary condition dtp/dr = 0 at the electrode surface. This is useful since we can simulate the problem without recourse to electroneutrality, but also without any detailed knowledge of the double layer structure. 

Fig. 7.1. Cyclic voltammetry (left) and time variation of the potential difference between the working electrode and the electron transfer place (right) for an electron transfer under fully supported (solid line) and weakly supported (dashed line) conditions.

J. G. Limon-Petersen, I. Streeter, N. V. Rees, and R. G. Compton. Voltammetry in weakly supported media The stripping of thaUium from a hemispherical amalgam drop. Theory and experiment, J. Phys. Chem. C 112, 17175 17182 (2008). 

Voltammetry in Weakly Supported Media Migration and Other Effects... 

Voltammetry for the oxidation of ferrocene in increasingly weakly supported solutions of acetonitrile is shown at Fig. 10.2. The scan rate is 200 mV s and the electrode radius is 0.3 mm. Answer the following about the voltammetry. 

FIGURE 2.15 Schematic illustration of the different ET probability for an electroactive molecule on a large electrode and a spherical nanoelectrode. (Reprinted with permission from Limon-Petersen, J.G., Streeter, L, Rees, N.V., and Compton, R.G., Quantitative voltammetry in weakly supported media Effects of the applied overpotential and supporting electrolyte concentration on the one electron oxidation of ferrocene in acetonitrile, J. Phys. Chem. C, 2009, 113, 333-337. Copyright 2009 American Chemical Society.)... 

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]. 

For further contributions on the dia-stereoselectivity in electropinacolizations, see Ref. [286-295]. Reduction in DMF at a Fig cathode can lead to improved yield and selectivity upon addition of catalytic amounts of tetraalkylammonium salts to the electrolyte. On the basis of preparative scale electrolyses and cyclic voltammetry for that behavior, a mechanism is proposed that involves an initial reduction of the tetraalkylammonium cation with the participation of the electrode material to form a catalyst that favors le reduction routes [296, 297]. Stoichiometric amounts of ytterbium(II), generated by reduction of Yb(III), support the stereospecific coupling of 1,3-dibenzoylpropane to cis-cyclopentane-l,2-diol. However, Yb(III) remains bounded to the pinacol and cannot be released to act as a catalyst. This leads to a loss of stereoselectivity in the course of the reaction [298]. Also, with the addition of a Ce( IV)-complex the stereochemical course of the reduction can be altered [299]. In a weakly acidic solution, the meso/rac ratio in the EHD (electrohy-drodimerization) of acetophenone could be influenced by ultrasonication [300]. Besides phenyl ketone compounds, examples with other aromatic groups have also been published [294, 295, 301, 302]. 

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