Advances in Heterocycling Chemistry. Vol. 32 by Donald Bethell

By Donald Bethell

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1993). The substituted fullerene bipyridyl-C61 (R2C61) shown in Fig. , 1995). Each pair of isomers either has the CH3C(Bipy) unit on the junction between two adjacent six-membered rings ([6,6] isomers) or between a five-membered ring and a six-membered MODERN VOLTAMMETRY 43 ring ([5,6] isomers). Furthermore, each pair of isomers may have the bridgehead atoms bonded (closed form) or not bonded (open form). 05 M (C4H9)4NPF6 showed seven main reduction processes. The similarity of the voltammetry to free C60 suggested that the electron-transfer processes were primarily fullerene based.

Comparison of (37) and (35) under equivalent conditions reveals that the peak current for an irreversible process is lower than the equivalent value for a reversible one. This feature emerges because the kinetics of the electron transfer are relatively slow in the irreversible case, so that during the course of the potential scan, diffusion has more time to relax the concentration gradient of A at the electrode surface. Consequently, when [A]xϭ0 ϭ 0, the flux of A to the electrode surface is lower than for the equivalent reversible case, resulting in the occurrence of a decrease in the value of the peak current.

The relationship between all these parameters and the current is given by the Butler–Volmer equation (20), I ϭ nFAk0([A]xϭ0 exp[(1 Ϫ ␣)(E Ϫ E0f ) nF/RT] Ϫ [B]xϭ0 exp[Ϫ␣(E Ϫ E0f ) nF/RT]) (20) where [A]xϭ0 and [B]xϭ0 represent the electrode surface concentrations of A and B. e. the appropriate version of (18)]. In the case of fast electron transfer kinetics, the theoretical expression becomes equivalent to that obtained by combining the Nernst equation with the mass transport equation. An alternative theory to the Butler–Volmer theory for electron transfer is provided by the Marcus–Hush theory (Marcus, 1968; Hush, 1968) which assumes a potential-dependent ␣.

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