Quasi-reversible electron transfer, cyclic voltammograms

Fig. 8 Typical cyclic voltammograms of pure electron transfer reactions (a) effect of quasi-reversibility ks decreases from solid to dashed line) (b) effect of relative values of...

Enemark et al,98 reported solution redox potentials and heterogeneous electron transfer rate constants for [W(E)(Tp )(SS)] (E = O, NO SS = dithiolate). Cyclic voltammograms reveal a one-electron quasi-reversible oxidation process for [WO(Tp )(tdt)], which has the most negative electrochemical potentials among a large series of Mo and W investigated.98... 

The ec scheme, which is a very common mechanism in organic electrochemistry, is described by Equations (6.17) and (6.18). The cyclic voltammogram observed depends on the relative rates of the two steps. The simplest situation is where the electron transfer is totally irreversible the presence of the chemical reaction has no effect on the voltammogram obtained and no kinetic data related to the chemical reaction can be derived. This situation leads to the properties in Table 6.2. Similar properties can also arise when the rate of the electron transfer step is relatively fast if the rate constant for the chemical reaction is very large. The full range of other possibilities where the chemical reaction can be reversible or irreversible and the electron transfer either reversible or quasi-reversible has been considered in detail by Nadjo Saveant [7], and the various kinetic zones have been identified. In this chapter the only case to be discussed in detail is that where the electron transfer is reversible and the chemical reaction is irreversible. 

Fig. 2 shows the cyclic voltammogram of an uncoated gold electrode in the presence of hydroquinone (a) and its redox reaction (b). The separation of the two Faradaic peaks of 0.33 V is larger than theoretically predicted from the theoretical two electron transfer. This can be attributed to the specific cell properties and indicates a quasi-reversible system. 

Fig. 2.10 a Typical cyclic voltammogram depicting the peak position Ep and peak height Ip. b Cyclic voltammograms for reversible (a), quasi-reversible b) and irreversible (c) electron transfer... 

Figure 1 shows a cyclic voltammogram at the RVC electrode of a 0.133 mM cofactor solution prepared by dilution of the oxidized form of FeMoco into acidic 0.1 M TBAPFe/NMF and recorded at the relatively fast sweep rate of 0.4 V s. Two quasi-reversible electrode reactions were observed at -0.30 and -0.98 V vs. NHE. These are assigned to sequential one-electron transfers involving conversion of FeMoco from its oxidized (ox) to semi-reduced (s-r) and semi-reduced to fully reduced (red) forms ... 

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