Reversible reaction cyclic voltammetry

FIGURE 2.30. Redox catalysis induction of Srn1 reactions. Cyclic voltammetry in liquid ammonia + 0.1 M KC1 at —40°C of (a) redox catalyis of the reductive cleavage of 2-chlorobenzonitrile, RX, by 4-cyanopyridine, P. The dotted reversible cyclic voltammogram corresponds to P in the absence of RX. The solid line shows the catalytic increase of the current, (b) Transformation of the voltammogram upon addition of the nucleophile PhS. Adapted from Figure 1 in reference 23, with permisison from the American Chemical Society. 

The remarkable hexanuclear complex [ NiCp ] (81), prepared by the sodium naphthalenide reduction of nickelocene, undergoes an extensive series of reversible one-electron transfer reactions cyclic voltammetry shows waves relating the six species [ NiCp 6] (Z = -2 to 3). Chemical oxidation of 81, with Ag, gave the monocation whose structure shows only a small tetragonal distortion from the octahedral array of nickel atoms in the neutral precursor (198). 

The two major classes of voltammetric technique 4 Evaluation of reaction mechanisms 6 General concepts of voltammetry 6 Electrodes roles and experimental considerations 8 The overall electrochemical cell experimental considerations 12 Presentation of voltammetric data 14 Faradaic and non-Faradaic currents 15 Electrode processes 17 Electron transfer 22 Homogeneous chemical kinetics 22 Electrochemical and chemical reversibility 25 Cyclic voltammetry 27 A basic description 27 Simple electron-transfer processes 29 Mechanistic examples 35... 

Kong et al. [90] applied the electrochemical approach to the study of a two-phase azo coupling facilitated by reverse PTC. Cyclic voltammetry and chronoamperometry were employed to evaluate quantitatively the rate constants for the reaction. The process was interpreted in terms of an EC mechanism, i.e., diffusion-controlled electrochemical charge transfer followed by a homogeneous chemical reaction. The authors highlighted the usefulness of this approach based on the factors that enable the estimation of the contributions of the chemical reaction, mass transfer, partitioning, and the adsorption of reactants at the interface to the overall two-phase reaction. 

Cyclic voltammetry provides a simple method for investigating the reversibility of an electrode reaction (table Bl.28.1). The reversibility of a reaction closely depends upon the rate of electron transfer being sufficiently high to maintain the surface concentrations close to those demanded by the electrode potential through the Nemst equation. Therefore, when the scan rate is increased, a reversible reaction may be transfomied to an irreversible one if the rate of electron transfer is slow. For a reversible reaction at a planar electrode, the peak current density, fp, is given by... 

In addition, Bryce et al. have studied the binding of palladium to other S/N-ferrocenyloxazoline ligands by cyclic voltammetry and proved that it was reversible.These redox-active liganding systems were successfully used in the test reaction, providing the product in both high yield and enantioselectivity of up to 93% ee, as shown in Scheme 1.70. 

A.464 A purple-black, mixed-valence Ir11 Ir1 binuclear compound, [(Ir(cod)(/u-L) 2]BF4 (L = pz, 4-Mepz), is synthesized from the reaction of [Ir(cod)(//-L)]2 with NOBF4. The binuclear cationic radical exhibits an EPR spectrum showing hyperflne coupling to two equivalent Ir. Cyclic voltammetry studies have shown a reversible, one-electron oxidation.4... 

Cyclic voltammetry, square-wave voltammetry, and controlled potential electrolysis were used to study the electrochemical oxidation behavior of niclosamide at a glassy carbon electrode. The number of electrons transferred, the wave characteristics, the diffusion coefficient and reversibility of the reactions were investigated. Following optimization of voltammetric parameters, pH, and reproducibility, a linear calibration curve over the range 1 x 10 6 to 1 x 10 4 mol/dm3 niclosamide was achieved. The detection limit was found to be 8 x 10 7 mol/dm3. This voltammetric method was applied for the determination of niclosamide in tablets [33]. 

The first CNT-modified electrode was reported by Britto et al. in 1996 to study the oxidation of dopamine [16]. The CNT-composite electrode was constructed with bro-moform as the binder. The cyclic voltammetry showed a high degree of reversibility in the redox reaction of dopamine (see Fig. 15.3). Valentini and Rubianes have reported another type of CNT paste electrode by mixing CNTs with mineral oil. This kind of electrode shows excellent electrocatalytic activity toward many materials such as dopamine, ascorbic acid, uric acid, 3,4-dihydroxyphenylacetic acid [39], hydrogen peroxide, and NADH [7], Wang and Musameh have fabricated the CNT/Teflon composite electrodes with attractive electrochemical performance, based on the dispersion of CNTs within a Teflon binder. It has been demonstrated that the electrocatalytic properties of CNTs are not impaired by their association with the Teflon binder [15]. 

The electrogeneration of [(TPP)Co] from (TPP)Co, and the reaction of this species with CHjI can be followed by cyclic voltammetry as shown in Figures lc and Id. In the absence of any added reagent, there are two reversible reduction waves which occur at Ei/2 = 0.85 jind -1.86 V (see Figure lc). These are due to the formation of [(TPP)CoJ and [(TPP)Co]2-, where the second reduction has occurred at the porphyrin ir ring system. The first reduction of (TPP)Co is not reversible in the presence of CH3I, and occurs at Ep = -0.86 V (see Figure Id). A new reversible reduction also appears at Ej/2 = -1.39 V. This process is due to (TPP)Co(CHj) which is formed as shown by Equation 8. The formation of (TPP)Co(CHj) as the final product of the electrosynthesis was confirmed by spectroelectrochemical experiments which were carried out under the same experimental conditions(26). 

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)... 

FIGURE 2.3. EC reaction scheme in cyclic voltammetry. Variation of the peak potential and of reversibility upon crossing the kinetic zone diagram (Figure 2.1) for K= 103. 

FIGURE 2.4. EC reaction scheme in cyclic voltammetry. Derivation of the rate constant from the anodic-to-cathodic peak current ratio in zone KO. In this example the scan is reversed 200 mV (at 25° C) after the peak. 

As shown in Section 2.2.7, chemical reactions may be triggered by electrons or holes from an electrode as illustrated by SrnI substitutions (Section 2.5.6). Instead of involving the electrode directly, the reaction may be induced indirectly by means of redox catalysis, as illustrated in Scheme 2.15 for an SrnI reaction. An example is given in Figure 2.30, in which cyclic voltammetry allows one to follow the succession of events involved in this redox catalysis of an electrocatalytic process. In the absence of substrate (RX) and of nucleophile (Nu-), the redox catalysis, P, gives rise to a reversible response. A typical catalytic transformation of this wave is observed upon addition of RX, as discussed in Sections 2.2.6 and 2.3.1. The direct reduction wave of RX appears at more negative potentials, followed by the reversible wave of RH, which is the reduction product of RX (see Scheme 2.21). Upon addition of the nucleophile, the radical R is transformed into the anion radical of the substituted product, RNu -. RNu -... 

One specific variant of the technique is known as direct current cyclic voltammetry (DCCV), in which the voltage sweep is over a limited range and a short time and is immediately reversed. The cycle is repeated many times and the pattern of current change is monitored. The technique uses relatively simple electrodes and is used to study redox reactions and there are a range of sophisticated variants of the technique. 

The most popular voltammetric technique is probably cyclic voltammetry (CV), partly because of its early development in theory and the availability of the corresponding commercial equipment. In this technique, the electrode potential is first scanned linearly with time from a starting potential, where no reaction occurs, passing E°, towards another potential, and then reversed back to the starting potential. In this case, the time variable can be conveniently represented by the scan rate, v. 

In the case when the preceding chemical reaction occurs at a rate of the same order as the intervention time scale of cyclic voltammetry, the repercussions of the chemical complication on the potential of the electrode process are virtually negligible, whereas there is a significant effect on the current. In particular, it is characteristic of this mechanism that the forward current decreases with the scan rate much more than the reverse current. This implies that the current ratio ipr/ipf is always greater than 1, increasing as scan rates are increased. 

As a general criterion one must bear in mind that, in cyclic voltammetry, the presence of a following chemical reaction has little influence on the forward peak, whereas it has a considerable effect on the reverse peak. 

If kfchemical reaction is very slow compared to the intervention times of cyclic voltammetry) the response is very similar to that of a simple reversible electron transfer and occurs at the formal potential, E°, of the couple Ox/Red. 

As mentioned in the introduction to controlled potential electrolysis (Section 2.3), there are various indirect methods to calculate the number of electrons transferred in a redox process. One method which can be rapidly carried out, but can only be used for electrochemically reversible processes (or for processes not complicated by chemical reactions), compares the cyclic voltammetric response exhibited by a species with its chronoamperometric response obtained under the same experimental conditions.23 This is based on the fact that in cyclic voltammetry the peak current is given by the Randles-Sevcik equation ... 

It is worthwhile emphasizing that there are important differences between the 2 + /3 + oxidation of the [Mn(terpy)2]2+ complex, which is reversible on the time scale of cyclic voltammetry, and that of the [Mn(phen)j]2+ and [Mn(bipy)3]2+ complexes, which is complicated by following chemical reactions. A study of these chemical complications has shown that oxidation of the [Mn(phen)3]2 + and [Mn(bipy)3]2+ complexes leads to the formation of the oxo-dimers [Mn202(phen)4]" + and [Mn202(bipy)4]"+ (n = 3, 4), respectively. Figure 24 shows the molecular structures of the dimers with n = 3.40,41... 

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