Homogeneous electron exchange rates, determined by a modified e.s.r. method in both the slow and fast exchange limits, are compared with published heterogeneous rates at mercury for aromatic hydrocarbon reduction in dimethylformamide ; the ratio is approximately constant. This constant can be derived from theories of Marcus and Hush on a model in which image forces are negligible in the electrode process and homogeneous exchange occurs only on contact. Potential-step methods are used to study the source of electrons transferred to radicals formed by protonation of the hydrocarbon radical anions (e.c.e. mechanism) : electron transfer occurs from other radical anions for anthracene and from the electrode for naphthalene. This result is interpreted in terms of relative rates of cross-exchange reactions as predicted by Marcus theory from the " isotopic " exchange rates and redox potentials of the systems. General considerations imply that cross-exchange reactions will predominate in many aromatic systems. Proton addition from phenol to the radical anions appears to involve direct contact of acid and base rather than the free (solvated) proton.
DETERMINATION OF HOMOGENEOUS EXCHANGE RATES BY ELECTRON S P I N RESONANCESince Ward and Wei~sman,~ who measured the homogeneous electron exchange rate between naphthalene and its radical anion, many similar studies have been made.5-12 The work mostly employs the methods of Ward and Weissman in which the exchange rates are determined from measurements of the exchange contributions to the hyperfine line widths in the e.s.r. spectra. The homogeneous electron exchange rate k,, in 1. mol-' s-1 is given by k,, = ,/3n(2.83 x 106)AHR/[R].
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