Conformation Cyclic voltammograms

Electrochemistry. The cyclic voltammogram of Compound 87a was measured and is compared to octa. S -benzyl porphyrazine, Ni[pz(A4)] A = difS -benzyl), 60a (from Section IV.B.l) in Table XXIV. Compound 87a has two reversible ring reductions, which are more positive than those measured for H2 pc and more negative than those measured for Ni[pz(.V-benzyl)8], 60a, suggesting that the conformational influences of the peripheral seven-membered ring make this pz harder to reduce than the pz with unconstrained peripheral thioethers. Because these compounds are of limited solubility and cannot be oxidized or reduced readily, they appear to be unsuitable for use as building blocks for molecular conductors. 

The chelation effect also brings about a stabilization of the — 1 state of the peptide model complexes as indicated by the thermal stability and redox behavior. Only [Fe(Z-cys-Pro-Leu-cys-OMe)2] exhibits a relatively reversible redox couple in the cyclic voltammogram measurement, but the others do not (20). The bulkiness of side chains of the X and Y residues in Cys-X-Y-Cys probably restricts the adoption of the inherent by preferable conformation (ift = 0°), resulting in a more restricted orientation of Fe-S-C. In fact, the X-ray analysis of native rubredoxin shows that two of the Fe-S torsion angles are restricted and the other two are normal, i.e., conformationally more stable. 

The difference in oxidation potentials (A ) detected for the two waves found for the poly(ferrocenylsilanes) 15 (R = R = Me, Et, -Bu, -Hex), which provides an indication of the degree of interaction between the iron sites, varies from 0.21 V (for 15 (R = R = Me)) to 0.29 V (for 15 (R = R = -Bu or -Hex)) (63). This indicates that the extent of the interaction between the ferrocenyl units in poly(ferrocenylsilanes) depends significantly on the nature of the substituents at silicon, which may be a result of electronic or conformational effects (63). Unsymmetrically substituted poly(ferrocenylsilanes) show similar electrochemical behavior (59). In addition, polymer 15 (R = Me, R = Fc) shows a complex cyclic voltammogram which indicates that interactions exist between the iron centers in the polymer backbone and the ferrocenyl side groups (59). 

While we characterized this conformational transition using nuclear magnetic resonance investigations, the cyclic voltammograms of MQC exhibits two clear reversible redox reactions (Figure 23). In aprotic media, quinones... 

The cyclic voltammograms of MQC exhibits two clear reversible redox reactions (Fig. 34.14). In aprotic media, quinones exhibit two reduction peaks separated by 0.7 V, which corresponds to the formation of a radical anion species and a dianion species of quinones, respectively. This is in agreement with the reduction characteristics of MQC. Two well-separated reduced states of MQC are formed in the aprotic solvent of acetonitrile upon reduction. Therefore, the electronic states of MQC and MHQC can be easily transformed into each other by simple electrochemical control of the redox reaction, which results in large conformational flapping motions due to a preference for the stable conformation caused by the change in the electronic state of the quinone moiety. 

Electrochemical measurements of the Cu(II/I) potentials with the nS4 ligands (n = 12-16) indicate that the Cu(II) and Cu(I) species each exist in two different conformational states [170]. Conformational rearrangement may either precede or succeed electron transfer. Rorabacher and coworkers interpreted their results in light of a square mechanistic scheme that neatly reconciles the sweep rate dependence of the cyclic voltammograms with the requisite change in coordination geometry at Cu. Kinetic studies on the electron transfer [149, 170, 176-177] support this scheme application of the Marcus cross relationship to reduction of Cu(II) and oxidation of Cu(I) yields widely discrepant values, presumably because of the different conformational states involved. 

Fig. 10. Cyclic voltammogram of horse cytochrome c at pH 9.30. Solution contained 0.35 mM oxidized cytochrome c in a medium consisting of 0.10 M sodium perchlorate, 0.02 M sodium borate and 0.01 M 4,4 -bipyridyl as promoter. An Au electrode, area 0.0079 cm was used. Scan rate 5mVs , temperature 25 °C. Two chemically non-reversible redox processes are observed. Wave 2c is associated with reduction of the State IV conformer which prevails at this pH. Note the virtual absence of wave Ic, which would be observed for reduction of the State III conformer. The corresponding return wave 2a is not observed because the Fe(II) product reverts immediately to the State IIj, conformer resembling State HI of the Feflll) form. Instead, re-oxidation (wave la) is observed at a potential appropriate for the native State III system. From Ref. 62, redrawn with kind permission of the authors...

Fig. 12 (A) Cyclic voltammograms (scan rate 100 mV s ) for Co-MOF-71 particles on a bppg electrode immersed in a 0.1 M NaOH over 10 scans (first scan shown as dashed line). (B) As before but with Co(OH)2 particles. (C) Photographs of crystal colours during transformation. (D) Microscopy images of a crystal (ca. 20 pm) during conformal transformation from Co-MOF-71 (red) to CoOOH (brown) (taken from ref. 86).

---