Cyclic voltammetry displays

The two cyclic voltammograms shown in Fig. 13 of [Scm(LBu2)] (b) and Scln(LMe-)] (a) show an important feature. Whereas the cyclic voltammetry (CV) of the former compound displays three reversible one-electron transfer waves, the latter shows only two irreversible oxidation peaks. Thus methyl groups in the ortho- and para-positions of the phenolates are not sufficient to effectively quench side reactions of the generated phenoxyls. In contrast, two tertiary butyl groups in the ortho- and para-positions stabilize the successively formed phenoxyls, Eq. (5)... 

The current responses may be displayed as a function of time, as in Figure 1.1c, or as a function of potential, as in Figure 1.1c. The latter presentation is generally preferred and is what is meant in short by the phrase cyclic voltammetry. The fact that the response is symmetrical about the potential axis provides a clear indication of the reversibility of the system, in both the chemical sense (the electron transfer product is chemically stable) and the electrochemical sense (the electron transfer is fast). If the electron transfer product were unstable, the anodic current would be less than the cathodic current, eventually disappearing for high instabilities. For a slow electron transfer and a chemically stable product, the current-potential pattern is no longer symmetrical about the vertical axis, the anodic peak potential being more positive than the cathodic peak potential. 

Several electrochemical techniques may yield the reduction or oxidation potentials displayed in figure 16.1 [332-334], In this chapter, we examine and illustrate the application of two of those techniques cyclic voltammetry and photomodulation voltammetry. Both (particularly the former) have provided significant contributions to the thermochemical database. But before we do that, let us recall some basic ideas that link electrochemistry with thermodynamics. More in-depth views of this relationship are presented in some general physical-chemistry and thermodynamics textbooks [180,316]. A detailed discussion of theory and applications of electrochemistry may be found in more specialized works [332-334],... 

Standard potential of the second electron transfer more cathodic than that of the first electron transfer (AE0 negative). One can consider the case where the formal electrode potential of the second couple is more cathodic, by at least 180 mV, with respect to the first couple (which has, for example, E01 = 0.00 V). If kf is low (compared to the intervention times of cyclic voltammetry i.e. if k[< n F- v/R T), the response will be due to the first electron transfer process, without complications caused by the following chemical reaction. As increases, the second process will have increasing effect up to the limiting case in which kt >n-F-v/R-T. In this limiting case the voltammogram will display two forward peaks, but only the second electron transfer will exhibit a return peak. 

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... 

Pyrrhotite is one of many sulphides which display collectorless flotation resulting from the formation of sulphur on the mineral surface (Hamilton and Woods, 1981 Heyes and Trahar, 1984 Hodgson and Agar, 1984). The anodic scan section of cyclic voltammetry for pyrrhotite electrode in pH = 2.2, 4.7, 7.0, 8.8, 11, 12.1, 12.7 buffer solutions respectively, is presented in Fig. 2.23. The cyclic voltammograms curve at pH = 8.8 is also presented in Fig. 2.23. It can be seen from Fig. 2.23 that anodic current peak emerges at about -0.1—0 V when pH < 11. As pH increases, the peak moves to the left. This peak may correspond to the following reactions ... 

Yes - several analytes can indeed be followed with cyclic voltammetry, with one pair of peaks (anodicplus cathodic) per couple. The couples can represent separate analyses, or can represent one analyte capable of displaying multiple redox states. Figure 6.14 shows a CV of vanadium pentoxide, with the CV comprising two separate redox couples. The anodic peaks are labelled as pa and the cathodic peaks as pc . Additional subscripts, (1) and (2), show how the two sets of forward and reverse peaks relate, one to another. 

The peak potentials from the cyclic voltammetry of 2,1-cyclopentathiazine 46 were registered at 100 mV s in a 5x 10 M solution in DCM <2005JOC9314>. This material displayed a reversible reduction wave at —0.95 V, which is attributed to the stability of the delocalized cyclopentadienyl radical anion, as depicted in resonance structures 66 and 67 (Scheme 8). 

V(IV) complexes that are coordinated by six sulfur donor atoms are also known. For example, [AsPh4]2[V(mnt)3] (mnt = maleonitriledithiolate) displays three redox features on cyclic voltammetry, which correspond to the reversible V(V/IV), V(IV/III), and quasireversible V(III/II) couples at 0.17, —0.87, and —2.12 V versus Cp2Fe/CH2Cl2 [55]. The surface normalized incident Fourier transform infrared spectroscopy (SNIFTIRS) spectroelectro-chemical technique was used to determine that the extent of n bonding of the mnt ligand increases as the metal s oxidation state is lowered through examination of the v(CN) frequencies in the various oxidation states. This technique was particularly effective in the determination of the spectral features ofthe short-lived V(II) species. 

The electrochemical behavior of Np ions in basic aqueous solutions has been studied by several different groups. In a recent study, cyclic voltammetry experiments were performed in alkali ([OH ] = 0.9 — 6.5 M) and mixed hydroxo-carbonate solutions to determine the redox potentials of Np(V, VI, VII) complexes [97]. As shown in Fig. 2, in 3.1 M LiOH at a Pt electrode Np(VI) displays electrode processes associated with the Np(VI)/Np(V) and Np(VII)/Np(VI) couples, in addition to a single cathodic peak corresponding to the reduction of Np(V) to Np(IV). This latter process at Ep —400 mV (versus Hg/HgO/1 M NaOH) is chemically irreversible in this medium. Analysis of the voltammetric data revealed an electrochemically reversibleNp(VI)/Np(V)... 

The examples of polyacetylenes whose main chain is directly bonded to heteroaromatic rings (e.g., silole, carbazole, imidazole, tetrathiafulvalene, ferrocene) are increasing in number. Such polymers are usually obtained by one of catalysts (W, Mo, and Rh). The formed polymers are expected to display interesting (opto)electronic properties such as electrochromism, cyclic voltammetry, electroluminescence, and so on. 

In the linear sweep technique, a recording of the current during the potential sweep (say, from 0.0 V on the normal hydrogen scale to 1.2 V positive to it in a 1 M H2 S04 solution) completes one act of the basic experiment. However, and hence the title of this part of the chapter, the electronics can be programmed so that when the electrode potential reaches 1.20 V, it begins a return sweep, going from 1.2 to 0.00 V, NHS. Completion of the two sweeps and back to the starting point is one act in what is called cyclic voltammetry.16 The current is displayed on a cathode ray oscilloscope screen on an X Y recorder, and it is normal to cany out not one but several and often many cycles. Much information is sometimes contained in the difference between the second and other sweeps in comparison with the first (Fig. 8.10). 

In cyclic voltammetry experiments, the silaimidazolidine 83 shows an oxidation wave at 0.67 V (vs Ag/AgCl) and a reduction wave at —2.67 V. The C-C saturated analogue 84 displays an oxidation wave at 0.95 V and two reduction waves at —1.75 V and —2.35 V. In both cases the voltammetric waves are all irreversible and the oxidation peaks do... 

Electrochemical studies confirmed the presence of redox-active nanoparticles. Differential pulse and cyclic voltammetry studies were conducted. Cyclic voltammetry showed that the complex displays an electrochemically reversible ferrocene/ ferrocenium couple (Figure 9.6). The oxidation potential for the hybrid CPMV-Fc conjugate and free ferrocenecarboxylic acid in solution was determined E1/2 of CPMV-Fc was 0.23 V, and Elj2 of free ferrocenecarboxylic acid was 0.32 V versus the Ag/AgCl electrode, respectively. This shift is expected for the conversion of the carboxyl group of ferrocenecarboxylic acid to an amide on coupling to the virus capsid, since the amide is less electron-withdrawing. 

The family of Ru(II)-Cp ATRA catalysts was recently extended to novel ruthenium amidinate complexes 12 and 13 [40]. Complex 12 displays two successive one-electron oxidation waves in cyclic voltammetry, assigned to Ru(II)/Ru(III) and Ru(III)/Ru(IV) oxidation steps, respectively. This opens the door to chemical transformations of organic molecules on 12 either byway of one-electron redox processes [i.e.,Ru(II) to Ru(III) or Ru(III) to Ru(IV)],or via two-electron processes [i.e., Ru(II) to Ru(IV)]. 

When the reactions between various aryl isocyanides ArNC and (Bu4N)2Re2Cl8 are carrried out in methanol at room temperature, then rhenium(III)-containing complexes of the types [Re(C-NAr)6]+[ReCl4(CNAr)2] and ReCl3(CNAr)3 can be isolated.21 These compounds display well-defined electrochemistry (cyclic voltammetry) in 0.2 M (Bu4 N)PF6-CH2Cl2, the mononuclear Re111... 

---