Self-exchange electron-transfer reaction kinetic parameters

The Marcus theory provides an appropriate formalism for calculating the rate constant of an outer-sphere redox reaction from a set of non-kinetic parameters[347 350]. The simplest possible process is a self-exchange reaction, where AG = 0. In an outer-sphere electron self-exchange reaction the electron is transferred within the precursor complex (Eq. 11.5). 

Using rate constants derived from reaction of H2A and HA" with [Co(ox)3] and [Fe(phen)3], comparisons of the Marcus-derived one-electron potentials for H2A /H2A and HA /HA" with molecular orbital calculations for the homo energy confirm the greater reactivity of HA" over H2A. It is pointed out that the Marcus-derived potential for HA /HA", 0.85-1.0 V, is greater than the best available measurement for this parameter, 0.68 V. The self-exchange rate for ascorbate radical is lO -lO" M s" and indicates a considerable barrier to electron transfer. The ascorbate radical A also has a high intrinsic barrier to electron transfer, and detection of second-order kinetics in the decomposition of A" suggests a dimerization step with subsequent acid catalysis. 

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