Irreversible electron transfer, cyclic voltammograms

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. 

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. 

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

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 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...

In the case of an irreversible charge-transfer process the rate of electron transfer is insufficient to maintain the charge-transfer process at equilibrium. The shape of the cyclic voltammogram is modified and peak positions shift as a function of scan rate (unlike the reversible case). A more detailed discussion can be found elsewhere.93... 

Peak potential — is a term used in -> voltammetry for the potential of the working (indicator) electrode at which the peak current is attained. In the cyclic voltammogram of a reversible redox couple, anodic and cathodic peak p. are separated by 2.2RT/nF, which is considered as a diagnostic feature and gives a possibility to determine the formal p. as a mid-peak p. For irreversible couples, quantitative relations exist, interconnecting peak p. and the rate constant of the -> electron transfer. 

The direct process includes the reduction of A to B (i) followed by the irreversible reaction of B to a product C (iv). If now a compound P, a so-called mediator, that is easier to reduce than A is added to the solution, the direct reduction of A is replaced by the reaction sequence (ii)-(iii). During the homogeneous electron transfer process (iii) the oxidized form P of the mediator is regenerated. In other words, the reduction of A to B is catalyzed by the redox couple P/Q. Cyclic voltammograms typical for the catalyzed process are shown in Fig. 18. The effect of increasing values of kiji is shown in Fig. 18(a) and the effect of an increasing concentration ratio C /Cp is shown in Fig. 18(b). 

The electrochemistry of [Th(Por)(OH)2]3 (Por=OEP,TPP) is of particular interest as they contain three redox active metalloporphyrin units (Kadish et al. 1988). The cyclic voltammogram of the OEP complex recorded in THE at -72 C shows three reversible one-electron reduction couples at -1.49, -1.70, and -1.87 V vs. SCE. As the temperature rises to room temperature, the third reduction becomes irreversible, and it has been shown that it involves a one-electron transfer followed by a fast chemical reaction (probably dissociation) and an additional one or more electron reduction (an electrochemical ECE-type mechanism). The UV-Vis spectrum of the electroreduced species [Th(OEP)(OH)2]3 shows absorption bands at 411, 456, and 799 nm, and its ESR spectrum displays a signal at g=2.003, both of which are characteristic of a porphyrin ti radical anion. Further one-electron reduction doubles the molar absorptivities of the absorption bands at 456 and 799 nm, indicating that the second reduction is also based on porphyrin. The TPP analog [Th(TPP)(OH)2]3 also exhibits three reversible one-electron reductions at -1.13, -1.27, and -1.36V at -55 C, which are shifted by 360-510mV relative to the respective processes for [Th(OEP)(OH)2]3 at —72 C. Three additional irreversible reductions at —1.76, -2.00, and —2.10 V are also observed for this complex when the potential is scanned to -2.20 V which may be due to the formation of dianions localized on each of the three porphyrin units. Spectroelectrochemical data also indicate that the initial three reductions occur at porphyrin based orbitals. 

Fig. 12. Simulated cyclic voltammograms for CrgyE mechanisms. Normalized current functions j/i vs. n(E —Ef) equilibrium constant K=10 . (A) CV in the diffusion zone, 2 = ksRT/(nFv) = 10- (1) reversible electrode process, /I = kh(D

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. 

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