Anodic shift

Anodic Inhibitors. Passivating or anodic inhibitors produce a large positive shift in the corrosion potential of a metal. There are two classes of anodic inhibitors which are used for metals and alloys where the anodic shift in potential promotes passivation, ie, anodic protection. The fkst class includes oxidking anions that can passivate a metal in the absence of oxygen. Chromate is a classical example of an oxidking anodic inhibitor for the passivation of steels. 

The (photo)electrochemical behavior of p-InSe single-crystal vdW surface was studied in 0.5 M H2SO4 and 1.0 M NaOH solutions, in relation to the effect of surface steps on the crystal [183]. The pH-potential diagram was constructed, in order to examine the thermodynamic stability of the InSe crystals (Fig. 5.12). The mechanism of photoelectrochemical hydrogen evolution in 0.5 M H2SO4 and the effect of Pt modification were discussed. A several hundred mV anodic shift of the photocurrent onset potential was observed by depositing Pt on the semiconductor electrode. 

In each series the oxidations show monotonic cathodic shifts, while the reductions show monotonic anodic shifts as the heteroatom increases in atomic number. Thus it is easier both to oxidize and reduce the heavier diheteroferrocenes. 

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. 

Ruj1,111 11]0/ 1 11 11]- (E j2 T Interestingly, with increase of the tt-electron accepting capability in 2,2/-bipyridyl ligands, redox potentials are progressively anodic-shifted in the order 31 —> 32 —> 33 —> 34 [8]. The stabilization against... 

Ru3 /Ru3 , and Ru3 /Ru3 processes. The corresponding redox potentials in 48 show 0.12-0.30 V anodic shift compared with those in 47, probably due to the better 7t-accepting ability for pyrimidime-containing abcp than that for pyridyl-containing abpy. 

Fig. 8 Plots of cyclic voltammograms of abcp-substituted triruthenium species 48 and the parent triruthenium complex [Ru30(0Ac)6(py)3]+ in chloromethane solution of (Bu4N)(PFg), showing anodic shifts of redox potentials in 48 relative to those in [Ru30(0Ac)6(py)3] +...

The reduction potentials were determined by polarography, and by CV on a hanging Hg cathode, and were irreversible, though the presence of a reverse wave was noted. Evidently, here the group 14 elements are directly affected (Table 14). The cathodic shift is understandable when L in Ph3MCo(CO)3L is changed from CO (an acceptor ligand) to the donors phosphine or phosphite. Anodic shift of the first reduction potential in the... 

The synthesis of C60-based dyads in which the Ccm core is covalently attached to a strong electron acceptor moiety, has been carried out by 1,3-dipolar cycloaddition of in situ generated nitrile oxides with C(,o- As expected, the obtained adducts show reduction waves of the fullerene core that are anodically shifted in comparison with the parent Cr>o. This indicates that they are remarkably stronger acceptors than Ceo-The electron acceptor organic addend also undergoes an anodic shift due to the electronic interaction with the C(,o moiety (545). 

Determination of electrochemical oxidation potentials and electrochemical reduction of 13 p-phosphorylated acyclic nitrones shows that phosphorylated compounds have a clear anodic shift of potentials of both, oxidation (Ep 1.40 to 2.00 V versus SCE in CH3CN) and reduction (Ep—0.94 to —2.06 V). This is caused by a strong electron-acceptor influence of the diethoxyphosphoryl group (430). In contrast, a reversible one-electron oxidation of azulene nitrones (233) (Scheme 2.80) occurs 0.6 V below the Ep potential of PBN, that is at the value one observes the oxidation of AH -imidazole-1,3-dioxides (219) (428, 429). In other words, the corresponding RC (234) is 14 kcal more stable than the RC of PBN. Although the EPR spectrum of RC (234) was not recorded, RC (236) from dinitrone (235) turned out to be rather stable and gave an EPR spectrum (170). 

In 1990 we reported the synthesis of new redox-responsive crown ether molecules that contain a conjugated link between the crown ether unit and a ferrocene redox-active centre (Beer et al., 1990a). Examples of some of the species synthesized are shown in Fig. 5. The electrochemical behaviour of these species was investigated and also the electrochemical behaviour of their analogues with a saturated link between the ferrocene unit and the crown ether. The changes in the CVs of [2a] upon addition of magnesium cations are shown in Fig. 6. The metal cation-induced anodic shifts of [2a], [2b] and also their saturated analogue [3] and vinyl derivatives [4a], [4b] are shown in Table 1. 

These results show that significant anodic shifts in the ferrocene oxidation wave result if cations are added to the conjugated receptor systems where the 7r-electron system links the heteroatoms of the ionophore to the redox centre. 

Table 1 The electrochemical anodic shifts of the ferrocene oxidation wave of [2a], [2b], [3], [4a] and [4b] upon addition of 4 equiv of cation.

The bis-benzo-15-crown-5 ferrocene compound [7] containing two vinylic linkages was formed in a mixture of three isomeric components, the cis-cis, cis-trans and trans-trans isomers, which proved inseparable. However, the precedent of insignificant differences found between the magnitudes of the metal cation-induced anodic shifts in the ferrocenyl redox potentials of the respective separated cis and trans isomers [2a] and [2b] led us to use the same isomeric mixture of [7] throughout the subsequent FABMS and electrochemical group 1 and 2 metal cation complexation experiments,... 

CVs of [7] were recorded after addition of calculated equivalents of Na+, K+ and Mg2+ and equimolar mixtures of Na+/K+ and Na+/K+/Mg2+. The results obtained are presented in Table 2. One-wave metal cation-induced anodic shifts of the ferrocenyl redox couple are observed (mediated by a through-bond coupling pathway), and interestingly the magnitudes of these are... 

Table 5 Anodic shifts in the redox couples of [17] and [18] upon addition of Li+ or...

The addition of stoichiometric amounts of Ni2+, Cu2+ and Zn2+ to solutions of [28]—[32] in acetonitrile led to large anodic shifts of the respective ferrocene/ferrocenium redox couple of up to 190 mV in the case of [29] and Cu2+ (Table 8). Analogous experiments in water at pH values 10.5-12 revealed that [28]—[32] electrochemically recognize these transition metal cations in the aqueous environment (Table 9). 

Data were obtained in aqueous solution containing 0.2 mol dm"3 KC1 as supporting electrolyte. Solutions were 3 X 10 3 mol dm"3 in compound and potentials were determined with reference to SCE at 21 1°C at 50 mV s"1 scan rate. The solution pH was adjusted with 0.5 mol dm"3 KOH and 0.1 mol dm"3 HC1. 6For [28], [29], [30] and [31] the CVs were reversible one electron oxidations at pH < 6. At pH = 11, an EC mechanism was observed for [28], [29] and [31]. Minor oxidation waves of the amino groups appeared after that of the Fc+/Fc couple at slow scan rate. The CV of [32] was a one-electron reversible oxidation wave, less dependent on the solution pH, and showed no oxidation of the amino groups in the pH range explored. Anodic shifts of anodic current peak potential of the Fc+/Fc couple produced by the presence of metal cations (1 or 2 equiv added as their perchlorate salts). 

Table 10 Anodic shifts (mV) in the formal reduction potentials of [50]—[53] upon addition of alkali metal cations.

Table 13 Reduction potentials of [57] and the anodic shifts in the presence of 1.0 or 2.0 equivalents of different cationic species. 1...

Obtained by both cyclic (100 mV s-1) and square-wave (10 Hz, Osteryoung-type) voltammetry in acetonitrile solution containing 0.1 mol dm-3 BuJNBF4 as supporting electrolyte. Solutions were 1 x 10-3 mol dm-3 in compound with reference to an Ag/Ag+ electrode (330 10 mV vs SCE) at 21 1°G b Anodic shift of the reduction waves of [57] in the presence of 1.0 equiv of the respective cationic species added as their perchlorate or hexafluorophosphate salts. "Anodic shift in the presence of 2.0 equiv of the respective cations. The second reduction wave of [57] became obscure or disappeared in the presence of more than 1 equiv of the respective cations. 

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