Oxidation potential shift

Generally, the oxidation potentials shift to less positive potentials, while the reduction potentials shift to less negative potentials, with the shift in oxidation potentials being more remarkable. The comparison of voltammetric behaviors of 2 and 3 with that of 53 [25] showed that one-electron redox processes I and II of each dimer 2 and 3 should correspond to the oxidation of two Pcs rings of each dimer, and the processes III and IV to their reduction. 

The peak potential is a function of scan rate, unlike the case for a reversible process when the peak potentials are independent of scan rate. As the scan rate increases, the voltammetric peak becomes wider. Thus, the peak oxidation potential shifts to more positive potentials as the scan rate increases. 

The formal replacement of a CH group in a ferrocene by an sp2 phosphorus atom is associated with an anodic oxidation potential shift of about 185 mV. This value is essentially independent of the substitution pattern of the metallocene [33]. This electronic destabilization of the ferricinium cation upon incorporation of the phosphorus atom probably reflects the high n electronegativity of 2-coordinate sp2 hybridized phosphorus which has been observed in MCD stud-... 

The deprotonated form, 1-FcAq, shows reversible two-step 1 e reduction at = -1.26 and -1.71 V versus ferrocenium/ferrocene (Fc /Fc) derived from the anthraquinone moiety, and reversible 1 e" oxidation at = 0.22 V due to the fer-rocenyl moiety in BU4NCIO4-CH2CI2 (Table 3.4). The first reduction potential shifts dramatically in the positive direction to E = -0.06 V, and the oxidation potential shifts moderately in the positive direction to E = 0.33 V in the protonation product, [1-FvAqH], whereas the second reduction potential is little changed. These results correspond to the structural changes in both ferrocenyl and anthraquinone moieties by protonation. 

Diaz et al. [599] studied the electrooxidation of various aromatic monomers and oligomers, including pyrrole and its oligomers, to identify relationships between the number of repeat units in the substrate molecule and the oxidation potential as well as the UV-vi absorption maximum of the oxidation product (soluble as well as deposited on the electrode). As can be expected based on theoretical calculations alreacfy mentioned, the oxidation potential shifts to less positive values with an increasing number of pyrrole units in the educt, whereas the absorption maximiun shifts considerably to longer wavelengths. 

Other Coordination Complexes. Because carbonate and bicarbonate are commonly found under environmental conditions in water, and because carbonate complexes Pu readily in most oxidation states, Pu carbonato complexes have been studied extensively. The reduction potentials vs the standard hydrogen electrode of Pu(VI)/(V) shifts from 0.916 to 0.33 V and the Pu(IV)/(III) potential shifts from 1.48 to -0.50 V in 1 Tf carbonate. These shifts indicate strong carbonate complexation. Electrochemistry, reaction kinetics, and spectroscopy of plutonium carbonates in solution have been reviewed (113). The solubiUty of Pu(IV) in aqueous carbonate solutions has been measured, and the stabiUty constants of hydroxycarbonato complexes have been calculated (Fig. 6b) (90). 

Various other observations of Krapcho and Bothner-By are accommodated by the radical-anion reduction mechanism. Thus, the position of the initial equilibrium [Eq. (3g)] would be expected to be determined by the reduction potential of the metal and the oxidation potential of the aromatic compound. In spite of small differences in their reduction potentials, lithium, sodium, potassium and calcium afford sufficiently high concentrations of the radical-anion so that all four metals can effect Birch reductions. The few compounds for which comparative data are available are reduced in nearly identical yields by the four metals. However, lithium ion can coordinate strongly with the radical-anion, unlike sodium and potassium ions, and consequently equilibrium (3g) for lithium is shifted considerably... 

A knowledge of the concentrations of all reactants and products is necessary for a description of the equilibrium state. However, calculation of the concentrations can be a complex task because many compounds may be Imked by chemical reactions. Changes in a variable such as pH or oxidation potential or light intensity can cause large shifts in the concentrations of these linked species. Aggregate variables may provide a means of simplifying the description of these complex systems. Here we look at two cases that involve acid-base reactions. 

This iron-ate complex 19 is also able to catalyze the reduction of 4-nitroanisole to 4-methoxyaniline or Ullmann-type biaryl couplings of bis(2-bromophenyl) methylamines 31 at room temperature. In contrast, the corresponding bis(2-chlor-ophenyl)methylamines proved to be unreactive under these conditions. A shift to the dianion-type electron transfer(ET)-reagent [Me4Fe]Li2 afforded the biaryl as well with the dichloro substrates at room temperature, while the dibromo substrates proved to be reactive even at —78°C under these reaction conditions. This effect is attributed to the more negative oxidation potential of dianion-type [Me4Fe]Li2. 

The consequences of polychlorination of porphyrins on redox properties of complexes has been investigated.1404 The highly chlorinated porphyrin 3-octachloro-/ /c.vo-tetrakis(3,5-dichloro-2,6-dimethoxyphenyl)porphyrin exhibits a substantial anodic shift for reduction of over 0.5 V and a smaller shift for oxidation versus the unchlorinated precursor. Contrastingly, small potential shifts for the octabromo-substituted 5,10,15,20-tetraphenylporphyrinate arise from the dominance of macrocycle ruffling over electronic effects. In the polychloro complex, distortion does not compensate fully for electron-withdrawing effects of the Cl substituents. 

In order to understand the observed shift in oxidation potentials and the stabilization mechanism two possible explanations were forwarded by Kotz and Stucki [83], Either a direct electronic interaction of the two oxide components via formation of a common 4-band, involving possible charge transfer, gives rise to an electrode with new homogeneous properties or an indirect interaction between Ru and Ir sites and the electrolyte phase via surface dipoles creates improved surface properties. These two models will certainly be difficult to distinguish. As is demonstrated in Fig. 25, XPS valence band spectroscopy could give some evidence for the formation of a common 4-band in the mixed oxides prepared by reactive sputtering [83],... 

The position of the 4-derived t2g band in the mixed oxides shifts from 0.8 eV for Ru02 to 1.5 eV for Ir02 proportional to the composition of the oxide. As a consequence of common 4-band formation the delocalized electrons are shared between Ir and Ru sites. In chemical terms, Ir sites are oxidized and Ru sites are reduced and electrochemical oxidation potentials are shifted. Oxidation of Ru sites to the VIII valence state is now prohibited. Thus corrosion as well as 02 evolution on Ru sites is reduced which explains the Tafel slope and overpotential behaviour. Most probably Ru sites function as Ir activators [83]. 

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