Transfer coefficient, electrochemical

It follows that the value of the electrochemical transfer coefficient may allow the distinction between stepwise and concerted electron-transfer-bond-breaking reactions when a chemical bond of normal strength is involved (Andrieux and Saveant, 1986b Andrieux et al., 1990b). If the reduction wave possesses the characteristics of a process controlled by slow electron transfer rather than controlled by a follow-up reaction, and if a is significantly larger than 0.5, then one can conclude that the reaction proceeds in a stepwise manner. The same is true when the wave exhibits the characteristics of a process controlled by a follow-up reaction, electron transfer remaining at equilibrium. 

Here, k° is the standard heterogeneous electron transfer rate constant and a is the electrochemical transfer coefficient [33], which corresponds in electrochemistry to the Bronsted coefficient in organic chemistry. It is seen from Equations 6.10 and 6.11 that kTsei and k°x are both equal to k° at E = E°. 

Electron transfer properties of polyhalogenated biphenyls were investigated by cyclic voltammetry. The primary reduction peak of 4,4 -dichlorobiphenyl, involving replacement of halide with hydrogen in an irreversible ECE- type reaction, are under kinetic control of the initial ET step. Electrochemical transfer coefficients, standard potentials and standard heterogeneous rate constants were also estimated from the voltammetric data230. 

Charge transfer coefficient— (also called transfer coefficient or electrochemical transfer coefficient or symmetry coefficient (factor)) [i-vi]. 

In nonaqueous aprotic solvents, such as dimethoxyethane [25] or acetonitrile [26,27], the reduction product from tertiary nitroalkanes is the radical anion. Cyclic voltammetric data of 2-nitro-2-methylpropane showed that the electrochemical rate constant was rather low and depended on the size of the supporting electrolyte cation the electrochemical transfer coefficient a was found to be potential dependent [28]. The nitro-t-butyl radical anion is rather unstable (half-life of 0.66s) and decomposes into nitrite ion and t-butyl radical. Continued electrolysis results in the formatrion of di-t-alkyl nitroxide radical [25,27]. 

Ynn+i = Yn+in are the fractions of the voltage drops in the n n+1 and n+1 n transitions inside the chain. The quantity a coincides approximately with the electrochemical transfer coefficient taken as a = /2 in the following. Equation (6-12) can then be given the shorter form... 

W(s rj) is the rate constant and j(s rj) the (infinitesimal) current density from a given electronic level s, f(s) the Fermi function, p(s) ihs electronic level density. Fox and Fred the population of the oxidized and reduced state, respectively, of the redox (bio)molecule close to the electrode surface, either in a monolayer film, or of mobile reactants, and e the electronic charge, a is the observable electrochemical transfer coefficient... 

Figure 5.1 shows that Eqs (5.5)-(5.8) reproduce the observed AV well in several cases (abbreviations Figure 5.2). Agreement is less good for the Co(diamsar) / couple, although the diprotonated form conforms well to expectations despite a rather wide error bar and the very high charges (5+/4+) [14] solutions containing the 3+/2+ couple were buffered with morpholine/CFsSOsH, and it is suspected that some interaction with the buffer was occurring (perhaps significantly, the electrochemical transfer coefficient y for this couple was only 0.27 rather than the 0.4-0.5 normally found for the complexes considered in this chapter [16]).

Then A versus log(/ f results in a single kinetic working curve, as shown in Fig. 8, and provides a convenient way of determining Xf h- A plot of log Xf h against overpotential (17) yields the formal heterogeneous electron transfer rate constant (X h) from the intercept and the value of the electrochemical transfer coefficient (a) from the slope. - ... 

Figure 18. (A) Cyclic voltammetry of purified cytochrome c at doped indium oxide optically transparent electrodes. Solution contained 73 /uiM cytochrome c, 0.21 M Tris, 0.24 M cacodylic acid, pH 7.0, 0.20 M ionic strength. Electrode area = 0.71 cm. Potential scan rates in mV/s are (a) 100 (b) 50 (c) 20 (d) 10 (e) 5.0 (f) 2.0. (B) Derivative cyclic voltabsorptometry of purified cytochrome c at a tin-doped indium oxide optically transparent electrode. Same conditions as described above. Circles are calculated derivative cyclic voltabsorptometric responses for 73 /iM cytochrome c, formal potential = 0.260 V, n = 1.0, diffusion coefficient of oxidized and reduced cytochrome c = 1.2 x 10 cm /s, difference molar absorptivity at 416 nm = 57,000 cm" formal heterogeneous electron transfer rate constant = 1.0 x 10 cm/s, and electrochemical transfer coefficient = 0.5. Adapted from Reference (126) with permission.

---