Quasi-reversible electron transfer, cyclic

Cyclic Voltammetry. However, experimental use of this technique has been restricted almost exclusively to the analysis of the limiting currents of the signals obtained. One reason for this could be that when a quasi-reversible electronic transfer is analyzed in RPV, two very close waves are obtained, which are difficult to resolve from an experimental viewpoint. This problem can be eliminated by using the triple pulse technique Reverse Differential Pulse Voltammetry (RDPV), proposed in references [80, 84, 85] and based in the application of the waveform presented in Scheme 4.5. 

With faster scan cyclic voltammetry, a new two-electron anodic peak was detected, at more negative potentials, for the first stage of the oxidation process, with an accompanying cathodic peak on the reverse scan (11). The ratio of the forward to the reverse peak currents increased towards unity as the scan rate was raised to —200 V s 1 (Fig. 15). This behavior was attributed to the initial two-electron process being accompanied by a fairly rapid follow-up chemical reaction and was successfully analyzed in terms of an EqCi process (quasi-reversible electron transfer followed by a first-order irreversible chemical process), with a rate constant for the chemical step, k, = 250 s 1. 

Obviously, therefore there must be an intermediate case in which the kinetics of both the forward and reverse electron-transfer processes have to be taken account of. Such systems are described as being quasi-reversible and as would be expected, the scan rate can have a considerable effect on the nature of the cyclic voltammetry. At sufficiently slow scan rates, quasi-reversible processes appear to be fully reversible. However, as the scan rate is increased, the kinetics of the electron transfer are not fast enough to maintain (Nernstian) equilibrium. In the scan-rate region when the process is quasi-reversible, the following observations are made. 

The electrode reduction mechanism of benzenodicarbonitrile isomers was examined by polarography, cyclic voltammetry and controlled potential electrolysis in DMF solutions at a Hg cathode. 1,2- and 1,4-dicyanobenzenes were reduced in two successive steps under polarographic conditions, where the first step corresponds to a quasi-reversible one-electron transfer. Cyclic voltammetric experiments provided more information on the electrode reduction mechanism and allowed one to suggest the mechanistic scheme for 1,2-and 1,4-dicyanobenzenes shown in Scheme 16. 

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...

Electron transfer reactions are classified as reversible, quasi-reversible or irreversible depending on the ability of the reaction to respond to changes in E, which, of course, is related to the magnitude of k°. The distinction is important, in particular, for the (correct) application of linear sweep and cyclic voltammetry, and for that reason further discussion of this classification will be postponed until after the introduction of these techniques in Section 6.7.2. 

Tohnan and Lee constructed a type 1-like site linked to a type 2-like site and investigated the potential for intramolecular electron transfer between the two. Their coordination scheme is illustrated in Figme 6(a) the structure of their complex in shown in Figure 6(b). Cyclic voltammetry showed the two Cu(II) atoms to undergo quasi-reversible, independent reduction with 1/2 values consistent with the two copper atoms coordination -0.911 V for the CuSR site, -0.112 V for the CuPyr one. These studies did not address the question of intramolecular electron transfer between the two sites, however. 

A typical example in heteroaromatic chemistry is the preparative electrolysis of some 2,5-diaryl-1,4-dithiins 13, which give two well resolved quasi-reversible one-electron transfer waves to the radical cation and the di-cation, respectively, in cyclic voltammetry. The electrolysis gives low yields of the 2,2 dimers (see Scheme 12) via the radical cation coupling mechanism [46]. 

The anodic oxidation of a series of 2,5-diaryl-1,4-dithiins (LVIII), as in Eq. (96), has been studied in detail [189]. Cyclic voltammetry (CV) experiments showed that all compounds undergo two quasi-reversible one-electron transfers to form the radical cations and the dications. Linear correlations between E° and a were observed. Preparative electrolyses gave the corresponding 2,2 -dimers (LIX) as the major products in yields up to 20%. The low yields are due to workup difficulties and formation of polymeric materials of unknown composition. 

Mechanistic studies can employ CPE if the coupled chemical reactions are slow. Conventional bulk electrolyses require typically 10-30 min for completion, longer than the typical longest time for voltammetric techniques (ca. 20 s maximum for cyclic voltammetry, CV, ca. 8 s for polarography, etc.). This is important to recall when comparing CPE with voltammetry data. An electrode reaction that is chemically reversible in a slow CV experiment may be irreversible in bulk electrolysis if the electrode product has a half-life of, e.g., a minute or two. Conversely, an electron transfer that is quasi- or irreversible in a relatively fast voltammetric experiment may be electrochemically reversible in the long timescale of bulk electrolysis. 

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... 

As discussed before in the case of nucleic acids the authors have also considered the incidence of the interfacial conformation of the hemoproteins on the appearance of the SERRS signals from the chromophores. Although under their Raman conditions no protein vibration can be observed, the possibility of heme loss or protein denatura-tion are envisaged to explain a direct interaction of the heme chromophores with the electrode surface in the case of the adsorl Mb. extensive denaturation of Cytc at the electrode appears unlikely to the authors on the basis of the close correspondence of the surface and solution spectra. Furthermore, the sluggish electron transfer kinetics measured by cyclic voltammetry in the case of Cytc is also an argument in favour of some structural hindrance for the accessibility to the heme chromophore in the adsorbed state of Cytc. This electrochemical aspect of the behaviour of Cytc has very recently incited Cotton et al. and Tanigushi et al. to modify the silver and gold electrode surface in order to accelerate the electron transfer. The authors show that in the presence of 4,4-bipyridine bis (4-pyridyl)disulfide and purine an enhancement of the quasi-reversible redox process is possible. The SERRS spectroscopy has also permitted the characterization of the surface of the modified silver electrode. It has teen thus shown, that in presence of both pyridine derivates the direct adsorption of the heme chromophore is not detected while in presence of purine a coadsorption of Cytc and purine occurs In the case of the Ag-bipyridyl modified electrode the cyclicvoltammetric and SERRS data indicate that the bipyridyl forms an Ag(I) complex on Ag electrodes with the appropriate redox potential to mediate electron transfer between the electrode and cytochrome c. 

This equation is often used to determine the formal potential of a given redox system with the help of cyclic voltammetry. However, the assumption that mid-peak potential is equal to formal potential holds only for a reversible electrode reaction. The diagnostic criteria and characteristics of cyclic voltammetric responses for solution systems undergoing reversible, quasi-reversible, or irreversible heterogeneous electron-transfer process are discussed, for example in Ref [9c]. An electro-chemically reversible process implies that the anodic to cathodic peak current ratio, lpa/- pc equal to 1 and fipc — pa is 2.218RT/nF, which at 298 K is equal to 57/n mV and is independent of the scan rate. For a diffusion-controlled reduction process, Ip should be proportional to the square root of the scan rate v, according to the Randles-Sevcik equation [10] ... 

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