Electron transfer reduction potential values

This review is concerned with the formation of cation radicals and anion radicals from sulfoxides and sulfones. First the clear-cut evidence for this formation is summarized (ESR spectroscopy, pulse radiolysis in particular) followed by a discussion of the mechanisms of reactions with chemical oxidants and reductants in which such intermediates are proposed. In this section, the reactions of a-sulfonyl and oc-sulfinyl carbanions in which the electron transfer process has been proposed are also dealt with. The last section describes photochemical reactions involving anion and cation radicals of sulfoxides and sulfones. The electrochemistry of this class of compounds is covered in the chapter written by Simonet1 and is not discussed here some electrochemical data will however be used during the discussion of mechanisms (some reduction potential values are given in Table 1). 

It is seen that q = 0 for a clean coupling reaction and that q = 1 for a clean electron transfer reaction. The value of k,n is close to that for a diffusion-controlled process, whereas the value of kjv increases when E /a- moves in the negative direction. Accordingly, a plot of q versus Ea/a- will give rise to an S-shaped curve from which the potential E, corresponding to q = 0.5 may be determined. Once the value of Ej, is known, the standard potential for the reduction of R of the Marcus theory [128]. 

TCNQ and TCAQ acceptors exhibit a significantly lower rate of photoinduced electron transfer, despite the fact that they are better acceptors than TCTQ, 5,14-TCPQ and Ceo- A qualitative interpretation for such behaviour was predicted from the Marcus theory for electron transfer and referred to as the inverted region [139]. On the other hand, the efficient photoinduced charge transfer of MEH-PPV in combination with acceptors TCTQ, diMeO-TCTQ, 5,14-TCPQ and C6o, compared with the lack of activity for 1,4-benzoqui-nones with a similar reduction potential value, indicates that the reduction potential is not the only parameter that affects the electron-transfer rate. Nevertheless, phase segregation and the solubility of the acceptors within the conjugated polymer might make difficult the interpretation of the observed results [135]. 

Double potential steps are usefiil to investigate the kinetics of homogeneous chemical reactions following electron transfer. In this case, after the first step—raising to a potential where the reduction of O to occurs under diffrision control—the potential is stepped back after a period i, to a value where tlie reduction of O is mass-transport controlled. The two transients can then be compared and tlie kinetic infomiation obtained by lookmg at the ratio of... 

More recent research provides reversible oxidation-reduction potential data (17). These allow the derivation of better stmcture-activity relationships in both photographic sensitization and other systems where electron-transfer sensitizers are important (see Dyes, sensitizing). Data for an extensive series of cyanine dyes are pubflshed, as obtained by second harmonic a-c voltammetry (17). A recent "quantitative stmcture-activity relationship" (QSAR) (34) shows that Brooker deviations for the heterocycHc nuclei (discussed above) can provide estimates of the oxidation potentials within 0.05 V. An oxidation potential plus a dye s absorption energy provide reduction potential estimates. Different regression equations were used for dyes with one-, three-, five-methine carbons in the chromophore. Also noted in Ref. 34 are previous correlations relating Brooker deviations for many heterocycHc nuclei to the piC (for protonation/decolorization) for carbocyanine dyes the piC is thus inversely related to oxidation potential values. 

In contrast, aromatic sulphoxides do not need extreme experimental conditions to give a well-defined step in polarography and voltammetry. Thus methyl phenyl sulphoxide (80) exhibits69 a well-defined wave in strongly acidic media at very moderate potential values. The reduction scheme assumes the transient formation of a protonated form prior to the electron transfer ... 

Electrochemical reductions and oxidations proceed in a more defined and controllable fashion because the potential can be maintained at the value suitable for a one-electron transfer and the course of the electrolysis can be followed polarographically and by measurement of the esr or electronic spectra. In some cases, conversion is low, which may be disadvantageous. Electrolytic generation of radical ions is a general method, and it has therefore become widely used in various applications. In Figures 3 and 4, we present electrochemical cells adapted for esr studies and for measurements of electronic spectra. Recently, electrochemical techniques have been developed that permit generation of unstable radicals at low temperatures (18-21). 

FIG. 10 Potential dependence of the electron-transfer rate constant k i) normalized to the value at the potential of zero charge TCNQ reduction by hexacyanoferrate at the water-DCE... 

Electrochemical redox studies of electroactive species solubilized in the water core of reverse microemulsions of water, toluene, cosurfactant, and AOT [28,29] have illustrated a percolation phenomenon in faradaic electron transfer. This phenomenon was observed when the cosurfactant used was acrylamide or other primary amide [28,30]. The oxidation or reduction chemistry appeared to switch on when cosurfactant chemical potential was raised above a certain threshold value. This switching phenomenon was later confirmed to coincide with percolation in electrical conductivity [31], as suggested by earlier work from the group of Francoise Candau [32]. The explanations for this amide-cosurfactant-induced percolation center around increases in interfacial flexibility [32] and increased disorder in surfactant chain packing [33]. These increases in flexibility and disorder appear to lead to increased interdroplet attraction, coalescence, and cluster formation. 

E° and E2° values of +76 and +21 mV, respectively, have been measured for Hox from M. trichosporium OB3b by similar methods (63). These values are more closely spaced and imply that Hmv from this organism is thermodynamically less stable with respect to disproportionation. Addition of protein B lowered the potentials to -52 mV and -115 mV, respectively. The regulation of electron transfer to the hydroxylase with protein B and reductase observed with the M. capsulatus (Bath) MMO was not seen with this system. Instead, it was reported that the potentials of Hox and of Hox with added protein B are shifted slightly to more positive values in the presence of reductase (Table II), and the reduction was not substrate-dependent. 

Table 6.2 Apparent formal redox potentials of systems present in the electron-transfer chain (pH = 7). It should be noted that the potential values were obtained in the homogeneous phase. Due to stabilization in a membrane, the oxidation-reduction properties vary so that the data listed below are of orientation character...

The values of E° for Eqs. (15)-(20) indicate that if multielectron reductions of C02 take place, for example, by using suitable catalysts, the potentials required are much less negative than that for single-electron transfer, C02/C0J, and are also less negative... 

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