First half wave potential

Table 8 First half-wave potentials for ring oxidation of various tetraphenylporphyrin and octaethylporphyrin derivatives a...

The first half-wave potentials in Table 8 have a bearing on shelf life for example, other things being equal, SnIV complexes are expected to be much more stable towards aerial oxidation in solution in the dark than are Mg11 complexes. 

Controlled electrolytic reduction of humic acid and hydroxyquinones in dimethylformamide in our laboratory (27) have shown small initial decreases in spin content followed by an increase up to the first half-wave potential. Since this was carried out in the absence of base, it would correspond to the portion of the curve in Figure 7 after the first maximum. The probable series of competing reactions is illustrated in Figure 8. Austen and co-workers (1) have observed similar behavior in coal extracts that were electrolytically reduced. 

A -Alkylniiroazoles at first half-wave potentials form stable RA and their ESR spectra parameters as given in Table 3.36 [850-853] (Scheme 3.18). 

By increasing the medium pH the first half-wave potential (E 1/2) value of all investigated compounds is displaced in the negative region (Table 3.41). Besides, for TV-substituted nitroazoles a considerably greater displacement is observed, as in alkaline media these compounds form anions. This allows the determination in alkaline media the presence of both TV-substituted and TV-unsubstituted forms of nitroazoles. The authors also utilized the E1/2/pH dependence for identification of the earlier-stated nitroazole forms (Table 3.41) [903, 904, 909, 910],... 

A- Alky In i tropy razo I cs are also reduced in two stages the first stage corresponds to reversible one-electron transfer (Scheme 3.35). In comparison with nitropyrazoles not substituted on nitrogen atom, the first half-wave potentials of A-alkylnitro-pyrazoles are essentially moved in cathodic region. Using the ESR method the signals of primary radical anions are recorded. 

The mechanism of nitroimidazole EC reduction has been fairly well considered in the literature [921-926], The way of iV-substituted nitroimidazoles EC reduction at the first half-wave potentials is presented in Scheme 3.37 ... 

The results of two parameter correlations AE m and AE"m (A 1/2=E /2X-H /2H) with constants of the substituents CTjCTr, a ak", FR and CTjCTr+ (Table 3.49) show that the substituent influence on the first half-wave potential follows both induction and resonance mechanisms, the ratio of contributions of these effects being approximately equal and, practically, independent of a choice of the substituent parameters. The correlation results between AE"1/2 and substituent parameters indicate that the substituent influence is mainly achieved by the resonance mechanism (approximately 80%) (Table 3.49) [991],... 

Cyclic voltammograms of C60F46 in a CH2CI2/O.I M TBAPF6 (tetrabutyl-ammonium hexafluorophosphate) solution are shown in Fig. 20 [33], The reduction of C60F46 to its anion radical occurs at the first half wave potential "1/2 = 0.51... 

A free energy of reduction AGred bears the relation, AGred = -nFE0, to the standard reduction potential <) > which is approximately identical to the first E /2- A combination of these relationships and Born s solvation model yields the following equation between the difference in electron affinity A a and the difference in the first half wave potential A i/2 for two species [33]. 

Fig. 7. Free energy of formation, AF, in relation to first half-wave potential of acceptor in donor-acceptor complexes with hexamethylbenzene as donor. Thermodynamic data from ref. 15 polarographic data from ref. 17.

The two principal categories of compounds studied have been diaryl or aryl alkyl sulphones (not dialkyl sulphones, which are polarographically not reducible) and S-dioxides of certain heterocyclic compounds, such as thiophene (also benzo- and dibenzothiophenes) and phenothiazines. The first named have half-wave potentials in the region of — 2.0 V, the thiophene dioxides near — 1.0 V. Some examples of each category may be given. 

Copper(II) ions in the presence of chloride ions are reduced at the dropping mercury electrode (dme) in two steps, Cu(II) -> Cu(I) and Cu(I) -> Cu(0) producing a double wave at -1-0.04 and 0.22 V versus sce half-wave potentials. In the presence of peroxydisulphate , when the chloride concentration is large enough, two waves are also observed the first limiting current corresponds to the reduction of the Cu(II) to Cu(I) plus reduction of a fraction of peroxydisulphate and the total diffusion current at a more negative potential is equal to the sum of the diffusion currents of reduction of Cu(II) to Cu(0) and of the peroxydisulphate. There is evidence that peroxydisulphate is not reduced at the potential of the first wave because of the adsorption of the copper(I) chloride complex at... 

A prime example of this is the crucial ease of such oxidation in the magnesium(II) complex of the chlorin system present in chlorophyll during photosynthesis. Table 8 shows the half-wave potentials for the first ring oxidation of some metalloporphyrins, the examples chosen being based on 5,10,15,20-tetraphenyl porphyrin, TPP (2) and on 1,2,7,8,12,13,17,18-octaethylporphyrin, OEP (16). 

TABLE 13. First and second half-wave potentials, Ey2 and Ey2 in V (vs SCE), for reduction of J75-CpFe(CO)2MPh3 in THF, TBAP 0.1 M178... 

Half-wave potentials for each of the electron transfer steps shown in Scheme V are listed in Table II. The first oxidations of (0EP)Ge(C6H5)Cl and (0EP)Ge(C H5)0H are irreversible and occur at Ep = 1.00 V for X = 0H. The second oxidations of these complexes are reversible and both occur at Ei/2 = 1.40 V. These second oxidation processes occur at identical to the Ei/2 values for oxidation of (0EP)Ge(C6H5)C10i, and this was presented as strong evidence for an oxidation of Cl on (OEP)Ge(CsHs)Cl and OH on (0EP)Ge(C6Hs)0H(35). 

Thermodynamic reduction potentials of numerous aromatics were first measured by Hoijtink and van Schooten in 96% aqueous dioxane, using polarography [15, 16]. These fundamental works were decisive tests of the HMO theory, showing that the polarographic half-wave potentials vary linearly with the HMO energies of the lowest unoccupied molecular orbitals (LUMO) of the hydrocarbons [1]. Hoijtink etal. had already noticed that most aromatics can be further reduced to their respective dianions [17]. They proposed a... 

Neutral (1 M LiCl) and alkaline (1 M NaOH) solutions of TcO in the concentration range of 10 to 10 M have been investigated polarographically with direct and alternating current . Four irreversible waves are observed (Fig. 12). The first two waves with the half-wave potentials of —0.85 V and —1.15 V vs. SCE are clearly recognizable in alkaline solution. They correspond to the electron transition n = 2 and n = 3 and can be used for the analytical determination of technetium. The third and fourth wave are not diffusion controlled and influenced... 

Figure 8.1. Variation of half-wave potential with pH for the first oxidation wave of phenidone 33 in aqueous buffers. Data fttim ref [147]...

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