Irreversible electron transfer, cyclic

In addition, all complexes display a reversible, one-electron reduction at a very negative potential Em —1.70 to -1.90 V vs Fc+/Fc, which is metal centered and nearly invariant with respect to the substitution pattern of the coordinated pheno-lates. It demonstrates the enormous stabilization of the high-spin ferric state by three phenolato ligands. The electrochemistry also nicely shows that unprotected ortho- or para positions of these phenolates lead to irreversible electron-transfer waves on the time scale of a cyclic voltammogram and that methyl substituents are inefficient protecting groups. 

In this case, the cyclic voltammetric response is essentially similar to the preceding case, with the difference that, given the irreversibility of the electron transfer, the return peak is missing. Thus, if kf is low, the response is that of a simple irreversible electron transfer. As k increases, the greater the potential scan rate the higher the peak current (compared to simple irreversible electron transfer). This continues up to a maximum value at which the response assumes a S-like shape. 

For irreversible systems the peak potential of a reduction process is shifted toward more negative potentials by about 0.030 V for a decade increase in the scan rate [Eq. (3.43)]. By analogy, a peak of an anodic process is shifted toward more positive potentials. The most characteristic feature of a cyclic voltammogram of a totally irreversible system is the absence of a reverse peak. However, it does not necessarily imply an irreversible electron transfer but could be due to a fast following chemical reaction. 

Similar equations can be derived for the peak current for voltammetric responses for irreversible electron transfer ( irrev mechanism - Eirrev diagnostics in cyclic voltammetry) or for other mechanistic cases. 

Fig. la-c. Schematic representation of the cyclic voltammetric response exhibited by the system Ox + e - Red, having a formal electrode potential E° = 0.00 V, which ideally proceeds through a) a reversible electron transfer b) a quasireversible electron transfer, c) an irreversible electron transfer... 

In practice, anMytical approaches based, e.g. on Eq. II. 1.20 are of limited use and advanced numerical simulation methods [16] and fitting of experimental to simulation data for a range of experimental data sets is the most reliable procedure to confirm the Butler-Volmer kinetics and to accurately obtain kinetic parameters such as ks and a from cyclic voltammetric data. Some criteria for the case of irreversible electron transfer (symbol Ejnev) are listed below. 

Fig. 7.20 The cyclic voltammogram of an ideally adsorbed surface species, showing irreversible electron transfer kinetics.

Cyclic voltammetry provides a simple method for investigating the reversibility of an electrode reaction (table Bl.28.1). The reversibility of a reaction closely depends upon the rate of electron transfer being sufficiently high to maintain the surface concentrations close to those demanded by the electrode potential through the Nemst equation. Therefore, when the scan rate is increased, a reversible reaction may be transfomied to an irreversible one if the rate of electron transfer is slow. For a reversible reaction at a planar electrode, the peak current density, fp, is given by... 

FIGURE 2-6 Cyclic voltammograms for a reversible electron transfer followed by an irreversible step for various ratios of chemical rate constant to scan rate, k/a, where a = nFv/RT. (Reproduced with permission from reference 1.)... 

The two cyclic voltammograms shown in Fig. 13 of [Scm(LBu2)] (b) and Scln(LMe-)] (a) show an important feature. Whereas the cyclic voltammetry (CV) of the former compound displays three reversible one-electron transfer waves, the latter shows only two irreversible oxidation peaks. Thus methyl groups in the ortho- and para-positions of the phenolates are not sufficient to effectively quench side reactions of the generated phenoxyls. In contrast, two tertiary butyl groups in the ortho- and para-positions stabilize the successively formed phenoxyls, Eq. (5)... 

FIGURE 1.17. Cyclic voltammetry of slow electron transfer involving immobilized reactants and obeying a Butler Volmer law. Normalized current-potential curves as a function of the kinetic parameter (the number on each curve is the value of log A ) for a. — 0.5. Insert irreversible dimensionless response (applies whatever the value of a). 

FIGURE 2.5. EC reaction scheme in cyclic voltammetry. Mixed kinetic control by an electron transfer obeying the Butler-Volmer law (with a = 0.5) and an irreversible follow-up reaction, a Variation of the peak potential with the scan rate, b Variation of the peak width with scan rate. Dots represent examples of experimental data points obtained over a six-order-of-magnitude variation of the scan rate. 

TABLE 2.1. Characteristics of the Irreversible Cyclic Voltammetric Responses (Pure Kinetic Conditions) for the Main Mechanisms That Involve the Coupling of a Fast Electron Transfer and a Homogeneous Rate-Determining Follow-up Reaction... 

In cyclic voltammetry, the current-potential curves are completely irreversible whatever the scan rate, since the electron transfer/bond-breaking reaction is itself totally irreversible. In most cases, dissociative electron transfers are followed by immediate reduction of R, as discussed in Section 2.6, giving rise to a two-electron stoichiometry. The rate-determining step remains the first dissociative electron transfer, which allows one to derive its kinetic characteristics from the cyclic voltammetric response, ignoring the second transfer step aside from the doubling of the current. 

Diagnostic criteria to identify an irreversible dimerization reaction following a reversible electron transfer. In the presence of a chemical reaction following an electron transfer, the dependence of the cyclic voltammetric parameters from the concentration of the redox active species are sufficient by themselves to reveal preliminarily a second-order complication (a ten-fold change in concentration from = 2 10-4 mol dm-3 to 2 10-3 mol dm-3 represents a typical path). 

Diagnostic criteria to identify an irreversible disproportionation reaction following a reversible electron transfer. Once again the dependence of the parameters of the cyclic voltammetric response from the concentration of the species Ox preliminarily reveals the second-order complication. 

Figure 34 Typical cyclic voltammograms for a two-electron transfer process in which one of the steps is irreversible. Case R-I (a) A E° = —180 mV (b) A E° = 0 mV. Case I-R (c) AE01 = -180 mV. Case I-I (d) AE01 = -180 mV...

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