Abstract
Adiabatic, outer-sphere electron transfer models are applied to the calculation of electron self-exchange rates in transition metal complexes and organic molecules. Large discrepancies between calculated and experimental values are found in the spin-allowed transfers of Fe(CN) 4−/3− 6 and tetracyanoethylene −/0, and in the spin-forbidden transfers of Co(H 2O) 6 2+/3+, Co(NH 3) 6 2+/3+ and Co(phen) 3 2+/3+. The “diabatic path” of Evans and Polanyi (Trans. Faraday Soc., 31 (1935) 875) is utilized to define an alternative reaction profile which emphasizes the role of the changes occurring in the internal coordinate. The intersecting-state model (ISM) is employed to estimate the free-energy barriers for the self-exchange reactions along this path, and good agreement is found with the observed rate constants, provided the σ *-d electron transfers have appreciable non-adiabatic factors. Evidence for such non-adiabaticity is discussed. The electron self-exchange rate of cytochrome c calculated with this model is also in close agreement with the experimental value.
Published Version
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