Peak potentials, electrochemical

A study of the electrochemical oxidation and reduction of certain isoindoles (and isobenzofurans) has been made, using cyclic voltammetry. The reduction wave was found to be twice the height of the oxidation wave, and conventional polarography confirmed that reduction involved a two-electron transfer. Peak potential measurements and electrochemiluminescence intensities (see Section IV, E) are consistent vidth cation radicals as intermediates. The relatively long lifetime of these intermediates is attributed to steric shielding by the phenyl groups rather than electron delocalization (Table VIII). 

Emission spectra have been recorded for four aryl-substituted isoindoles rmder conditions of electrochemical stimulation. Electrochemiluminescence, which was easily visible in daylight, was measured at a concentration of 2-10 mM of emitter in V jV-dimethylformamide with platinum electrodes. Emission spectra due to electrochemi-luminescence and to fluorescence were found to be identical, and quantum yields for fluorescence were obtained by irradiation with a calibrated Hght source. Values are given in Table X. As with peak potentials determined by cyclic voltammetry, the results of luminescence studies are interpreted in terms of radical ion intermediates. ... 

For a totally irreversible electrochemical process, the heterogeneous rate constant ke for electron transfer at the CV peak potential Ep is given by... 

Cyclooctatetraene and some of its derivatives are electrochemically reducible in dry degassed DMF containing BU4NCIO4 as the supporting electrolyte. The first reduction peak potentials which are required to form the corresponding anion radical are shown in Table 824, though a further reaction of the intermediates is not known. 

An alternative electrochemical method has recently been used to obtain the standard potentials of a series of 31 PhO /PhO- redox couples (13). This method uses conventional cyclic voltammetry, and it is based on the CV s obtained on alkaline solutions of the phenols. The observed CV s are completely irreversible and simply show a wave corresponding to the one-electron oxidation of PhO-. The irreversibility is due to the rapid homogeneous decay of the PhO radicals produced, such that no reverse wave can be detected. It is well known that PhO radicals decay with second-order kinetics and rate constants close to the diffusion-controlled limit. If the mechanism of the electrochemical oxidation of PhO- consists of diffusion-limited transfer of the electron from PhO- to the electrode and the second-order decay of the PhO radicals, the following equation describes the scan-rate dependence of the peak potential ... 

The current responses may be displayed as a function of time, as in Figure 1.1c, or as a function of potential, as in Figure 1.1c. The latter presentation is generally preferred and is what is meant in short by the phrase cyclic voltammetry. The fact that the response is symmetrical about the potential axis provides a clear indication of the reversibility of the system, in both the chemical sense (the electron transfer product is chemically stable) and the electrochemical sense (the electron transfer is fast). If the electron transfer product were unstable, the anodic current would be less than the cathodic current, eventually disappearing for high instabilities. For a slow electron transfer and a chemically stable product, the current-potential pattern is no longer symmetrical about the vertical axis, the anodic peak potential being more positive than the cathodic peak potential. 

V -dimethylformamide (DMF) and for the reaction of the same compound with anthracene anion radical in the same solvent.12 The results are shown in Figure 3.3. In the electrochemical case, the values predicted for the cyclic voltammetric peak potential (at 0.2 V/s) and the entropy of activation are plotted as functions of the ratio C/AS. Validation of the theory derives from the observation that the agreement between theoretical and experimental values is reached for the same value of AS C/AS for the peak potential and the entropy of activation. The same is true for the homogeneous reactions. That this common value of AS°F c/ASi]h- is smaller in the latter case than in the former falls in line with the fact that the presence of anthracene renders more difficult the mutual displacement of the R and X moieties within the solvent cage. 

In the electrochemical case, using, for example, cyclic voltammetry, one way of driving the potential toward more negative values is to increase the scan rate. This is true whether the linearization procedure or the convolution approach is followed. In the first case, equation (3.4) shows that the activation free energy at the peak, AG, is a decreasing function of the scan rate as a result of the kinetic competition between electron transfer and diffusion. The larger the scan rate, the faster the diffusion and thus the faster the electron transfer has to be in order to compete. This implies a smaller value AG, which is achieved by a shift of the peak potential toward more negative values. 

These derivatives undergo an irreversible oxidation process. Assuming that such electron removal involves an electrochemically reversible process complicated by fast chemical reactions, a thermodynamic meaning can be assigned to the different peak potential values. 

The electrocatalytic activity of the nanostructured Au and AuPt catalysts for MOR reaction is also investigated. The CV curve of Au/C catalysts for methanol oxidation (0.5 M) in alkaline electrolyte (0.5 M KOH) showed an increase in the anodic current at 0.30 V which indicating the oxidation of methanol by the Au catalyst. In terms of peak potentials, the catalytic activity is comparable with those observed for Au nanoparticles directly assembled on GC electrode after electrochemical activation.We note however that measurement of the carbon-supported gold nanoparticle catalyst did not reveal any significant electrocatalytic activity for MOR in acidic electrolyte. The... 

The resulting isopotential point confirms that H-Tg-H reacts to produce a new electroactive species without side reactions. The optical absorption of the electrochemically generated product is red-shifted and its cathodic peak potentials upon discharging (reduction) lie negative to those of the educt, indicating that the product consists of larger molecules with a more extended redox system. If experiments are carried out at higher sweep rates (v > 100 m V s ), broad waves are... 

The simplest way of generating and observing aryl halide anion radicals is to use an electrochemical technique such as cyclic voltammetry. With conventional microelectrodes (diameter in the millimetre range), the anion radical can be observed by means of its reoxidation wave down to lifetimes of 10" s. Under these conditions, it is possible to convert, upon raising the scan rate, the irreversible wave observed at low scan rates into a one-electron chemically reversible wave as shown schematically in Fig. 9. Although this does not provide any structural information about RX , besides the standard potential at which it is formed, it does constitute an unambiguous proof of its existence. Under these conditions, the standard potential of the RX/RX " couple as well as the kinetics of the decay of RX-" can be derived from the electrochemical data. Peak potential shifts (Fig. 9) can also be used... 

The experimental kinetic data obtained with the butyl halides in DMF are shown in Fig. 13 in the form of a plot of the activation free energy, AG, against the standard potential of the aromatic anion radicals, Ep/Q. The electrochemical data are displayed in the same diagrams in the form of values of the free energies of activation at the cyclic voltammetry peak potential, E, for a 0.1 V s scan rate. Additional data have been recently obtained by pulse radiolysis for n-butyl iodide in the same solvent (Grim-shaw et al., 1988) that complete nicely the data obtained by indirect electrochemistry. In the latter case, indeed, the upper limit of obtainable rate constants was 10 m s", beyond which the overlap between the mediator wave and the direct reduction wave of n-BuI is too strong for a meaningful measurement to be carried out. This is about the lower limit of measurable... 

When the characteristic time for charge diffusion is lower than the experiment timescale, not all the redox sites in the film can be oxidized/reduced. From experiments performed under these conditions, an apparent diffusion coefficient for charge propagation, 13app> can be obtained. In early work choroamperometry and chronocoulometry were used to measure D pp for both electrostatically [131,225] and covalently bound ]132,133] redox couples. Laviron showed that similar information can be obtained from cyclic voltammetry experiments by recording the peak potential and current as a function of the potential scan rate [134, 135]. Electrochemical impedance spectroscopy (EIS) has also been employed to probe charge transport in polymer and polyelectrolyte-modified electrodes [71, 73,131,136-138]. The methods... 

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