Reduction electrochemical data

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). 

One of the most useful applications of standard potentials is in the calculation of equilibrium constants from electrochemical data. The techniques that we develop here can be applied to any kind of reaction, including neutralization and precipitation reactions as well as redox reactions, provided that they can be expressed as the difference of two reduction half-reactions. 

The above predictions are partially confirmed by the available electrochemical data. Specifically, the half wave reduction potentials for cis and trans 1,2-disubsti-tuted ethylenes have been measured and it is found that it is easier to reduce the trans isomer, i.e. E, > E°fa243 248). 

Electrochemical data for the solid state are rare, but one example comprises reduction potential measurements of oligo(p-phenylene-vinylenes) with a convergence limit of —2.18 V vs. Fc/Fc+ for long chains. In contrast to the solution measurements, all solid-state voltammograms exhibit a strong hysteresis [240]. 

Thus, electrochemical data involving both thermodynamic and kinetic parameters of hydrocarbons are available for only olefinic and aromatic jr-systems. The reduction of aromatics, in particular, had already attracted much interest in the late fifties and early sixties. The correlation between the reduction potentials and molecular-orbital (MO) energies of a series of aromatic hydrocarbons was one of the first successful applications of the Hiickel molecular orbital (HMO) theory, and allowed the development of a coherent picture of cathodic reduction [1], The early research on this subject has been reviewed several times [2-4],... 

The experimental kinetic data obtained with the butyl halides in DMF are shown in Fig. 13 in the form of a plot of the activation free energy, AG, against the standard potential of the aromatic anion radicals, Ep/Q. The electrochemical data are displayed in the same diagrams in the form of values of the free energies of activation at the cyclic voltammetry peak potential, E, for a 0.1 V s scan rate. Additional data have been recently obtained by pulse radiolysis for n-butyl iodide in the same solvent (Grim-shaw et al., 1988) that complete nicely the data obtained by indirect electrochemistry. In the latter case, indeed, the upper limit of obtainable rate constants was 10 m s", beyond which the overlap between the mediator wave and the direct reduction wave of n-BuI is too strong for a meaningful measurement to be carried out. This is about the lower limit of measurable... 

Electrochemical data have been collected for a selection of the antimony OEP and TPP complexes including [Sb(Por)Me2] and [Sb(Por)(R)(OH)] (R = Me, Et). The complexes show one-electron oxidations and reductions at the porphyrin rings. Spectroelectrochemistry indicated that small amounts of antimony(III) products may be formed through a chemical reaction following the first reduction. " ... 

In aqueous solution, thorium exists as Th(IV), and no definitive data have been presented for the presence of lower-valent thorium ions in this medium. The standard potential for the Th(IV)/Th(0) couple has not been determined from experimental electrochemical data. The values presented thus far for the standard reduction potential have been calculated from thermodynamic data or estimated from spectroscopic measurements. The standard potential for the four-electron reduction of Th(IV) ions has been estimated as —1.9 V in two separate references 12. The reduction of Th(OH)4 to Th metal was estimated at —2.48 V in the same two publications. Nugent et al. calculated the standard potential for the oxidation ofTh(III) to Th(IV) as +3.7 V versus SHE, while Miles provides a value of +2.4 V [13]. The standard potential measurements from studies in molten-salt media have been the subject of some controversy. The interested reader is encouraged to look at the summary from Martinot [10] and the original references for additional information [14]. 

Radiopolarography measurements for the cathodic reduction of Bk(III) to Bk(0) at a dropping mercury electrode in 0.1 M LiCl at pH 2 give an amalgamation halfwave potential value of 1.63 V versus SHE and an estimated of 2.18 V [169]. Analysis of the electrochemical data leads the authors to conclude that the Bk(III)/Bk(0) electrode process is irreversible. 

The voltammetric reduction of a series of dialkyl and arylalkyl disulfides has recently been studied in detail, in DMF/0.1 M TBAP at the glassy carbon electrode The ET kinetics was analyzed after addition of 1 equivalent of acetic acid to avoid father-son reactions, such as self-protonation or nucleophilic attack on the starting disulfide by the most reactive RS anion. Father-son reactions have the consequence of lowering the electron consumption from the expected two-electron stoichiometry. Addition of a suitable acid results in the protonation of active nucleophiles or bases. The peak potentials for the irreversible voltammetric reduction of disulfides are strongly dependent on the nature of the groups bonded to the sulfur atoms. Table 11 summarizes some relevant electrochemical data. These results indicate that the initial ET controls the electrode kinetics. In addition, the decrease of the normalized peak current and the corresponding increase of the peak width when v increases, point to a potential dependence of a, as discussed thoroughly in Section 2. 

Finally, the electrochemical data for the cobalt system enable evaluation of the effect on redox properties of the thiometalate ligands (TM) varying from M02S2- to MS2-. It is evident that an-increasing oxygen content of the ligand makes the reduction more difficult. 

Fig. 12.8. Test of reproducibility of electrochemical oxidation/reduction and of completeness of thermoinjections. Dependence of the oxidative charge on the reduction charge is shown in (a, b). The oxidation was performed in pure water immediately after reduction (all data in (b) and in (c)), after 72 h storage at room temperature ( ) and after 96h storage at -20°C ( ). Kinetics of the limiting oxidation current of wire gold electrode with reduced mercury before (1) and after (2) thermoinjection is shown in (c) [45].

For the outer-sphere Co(NH3)63+ reduction, the SERS and current-potential data are closely compatible in that the SERS intensities drop sharply at potentials towards the top of the voltammetric wave where the overall interfacial reactant concentration must decrease to zero. Some discrepancies between the SERS and electrochemical data were seen for the inner-sphere Cr(NH3)sBr2 and Cr(NH3)sNCS2+ reductions, in that the SERS intensities decrease sharply to zero at potentials closer to the foot of the voltammetric wave. This indicates that the inner-sphere reactant bound to SERS-active sites is reduced at significantly lower overpotentials than is the preponderant adsorbate. (15)This suggests that SERS-active surface sites might display unusual electrocatalytic activity in some cases. 

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