Electron stoichiometries

The peak-shaped response of differential-pulse measurements results in unproved resolution between two species with similar redox potentials, hi various situations, peaks separated by 50 mV may be measured. Such quantitation depends not only upon the corresponding peak potentials but also on the widths of the peak. The width of the peak (at half-height) is related to the electron stoichiometry ... 

Their most distinctive feature compared to the EC case is the passage from a one-electron stoichiometry to a two-electron stoichiometry as X... 

When, as it is assumed here, the B —> C reaction is the rate-determining step, the dimensionless rate parameter, 2, is the same as in the ECE case. As 2 increases, the wave loses its reversibility while the electron stoichiometry passes from 1 to 2, as in the ECE case. Unlike the latter, there is no trace crossing upon scan reversible. This is related to the fact that now only the reduction of A contributes to the current. C has indeed disappeared by means of its reaction with B before being able to reach back to the electrode surface. The characteristic equations, their dimensionless expression, and their resolution are detailed in Section 6.2.1. The dimensionless peak current, tjj, thus varies with the kinetic parameter, 2, from 0.446, the value characterizing the reversible uptake of one electron, to 2 x 0.496 = 0.992, the value characterizing the irreversible exchange of two electrons (Figure 2.11a). 

More complicated mechanisms of the same category are encountered in SrnI reactions (Section 2.5.6) where the electrocatalytic reaction, which corresponds to a zero-electron stoichiometry, is opposed to two-electron consuming side reactions (termination step in the chain process). 

If no acid (or electrophile) is present, the initial molecule may serve this purpose, thus triggering a father-son reaction. Myriads of such reactions have been described. A simple example is given in Scheme 2.22, where the reduction of a phosphonium cation generates the corresponding ylid in a 50% yield. It should be emphasized that the overall electron stoichiometry is unity despite the fact that the reduction consumes two electrons. This type of reaction should thus be envisaged before concluding that one-electron stoichiometries are necessarily indicative of radical coupling reactions. 

In the absence of nucleophile, the aryl halide undergoes a two-electron reductive cleavage according to an ECE-DISP mechanism (Scheme 2.21). The two-electron stoichiometry occurs because the aryl radical produced on the one-electron reductive cleavage is easier to reduce than the substrate. The competition between the ECE and DISP pathways is governed by the parameter... 

In cyclic voltammetry, the current-potential curves are completely irreversible whatever the scan rate, since the electron transfer/bond-breaking reaction is itself totally irreversible. In most cases, dissociative electron transfers are followed by immediate reduction of R, as discussed in Section 2.6, giving rise to a two-electron stoichiometry. The rate-determining step remains the first dissociative electron transfer, which allows one to derive its kinetic characteristics from the cyclic voltammetric response, ignoring the second transfer step aside from the doubling of the current. 

Figure 3.29b). Unlike the reaction with the iron(O) porphyrin, the electron stoichiometry is of two electrons per molecule. The alkyl iron(III) porphyrin, now formed is indeed easier to reduced than the starting iron(II) porphyrin, thus giving rise to an ECE-DISP mechanism. The rate constant may again be derived from the loss of reversibility or from the positive shift of the wave when it has become totally irreversible, and also, this time, from the passage from a two- to a one-electron stoichiometry upon raising the scan rate (see Section 2.2.2). 

Reaction mechanisms are also defined for electroorganic reactions, induced by or including an electron transfer at an electrode. Knowledge of such electrode reaction mechanisms includes, preferably but not exclusively, the potential at which the reaction proceeds, the proof of intermediates, the electron stoichiometry, the kinetics of the various reaction steps, and the transport properties of the species involved. Recently, the terms... 

The voltammetric reduction of a series of dialkyl and arylalkyl disulfides has recently been studied in detail, in DMF/0.1 M TBAP at the glassy carbon electrode The ET kinetics was analyzed after addition of 1 equivalent of acetic acid to avoid father-son reactions, such as self-protonation or nucleophilic attack on the starting disulfide by the most reactive RS anion. Father-son reactions have the consequence of lowering the electron consumption from the expected two-electron stoichiometry. Addition of a suitable acid results in the protonation of active nucleophiles or bases. The peak potentials for the irreversible voltammetric reduction of disulfides are strongly dependent on the nature of the groups bonded to the sulfur atoms. Table 11 summarizes some relevant electrochemical data. These results indicate that the initial ET controls the electrode kinetics. In addition, the decrease of the normalized peak current and the corresponding increase of the peak width when v increases, point to a potential dependence of a, as discussed thoroughly in Section 2. 

Often the first step in the electrochemical characterization of a compound is to ascertain its oxidation-reduction reversibility. In our opinion, cyclic voltammetry is the most convenient and reliable technique for this and related qualitative characterizations of a new system, although newer forms of pulse polarography may prove more suitable for quantitative determination of the electrochemical parameters. The discussion in Chapter 3 outlines the specific procedures and relationships. The next step in the characterization usually is the determination of the electron stoichiometry of the oxidation-reduction steps of the compound. Controlled-potential coulometry (discussed in Chapter 3) provides a rigorously quantitative means for such evaluations. 

Voltammetric methods also provide a convenient approach for establishing the thermodynamic reversibility of an electrode reaction and for the evaluation of the electron stoichiometry for the electrode reaction. As outlined in earlier... 

Figure 9.1 illustrates the electrochemical reduction of 02 at platinum electrodes in aqueous media (1.0 M NaC104). The top curve represents the cyclic voltammogram (0.1 V s-1) for 02 at 1 atm ( 1 mM), and the lower curve is the voltammogram with a rotated-disk electrode (900 rpm, 0.5 V min-1). Both processes are totally irreversible with two-electron stoichiometries and half-wave potentials (EU2) that are independent of pH. The mean of the Em values for the forward and reverse scans of the rotated-disk voltammograms for 02 is 0.0 V versus NHE. If the experiment is repeated in media at pH 12, the mean Em value also occurs at 0.0 V. 

Rate-controlling step Overall stoichiometry Electron stoichiometry (in the limit as v — 0)°... 

Most of these metal-oxygen intermediates are expected to be reactive toward organic substrates and electrode surfaces. Hence, the presence of metal cations enhances the electron stoichiometry for the reduction of 02, but frequently passivates the electrode surface. Thus, the formation of (H20)4Znn(02) on the surface of a platinum electrode probably initiates a metathesis reaction ... 

The products and the observed electron stoichiometries for the electrochemical reduction of HOOH are consistent with a mechanism in which the primary step is a one-electron transfer... 

The multicopper oxidases (laccase, ascorbate oxidase, and ceruloplasmin) catalyze a four-electron reduction of dioxygen to water (285-287). Consistent with the four-electron stoichiometry, the enzymes contain four copper ions. One of the copper ions is type I, causing an intensely blue color of the proteins, thus the enzymes of this family are referred to as blue oxidases. They also contain a monomeric copper site that exhibits normal spectroscopic features, whereas the other two copper... 

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