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The role of solvent dynamics in electron transfer

1989; Elsevier BV; Volume: 260; Issue: 1 Linguagem: Inglês

10.1016/0022-0728(89)87095-0

2590-2954

Roger M. Nielson, Michael J. Weaver,

Electrochemical sensors and biosensors

Rate constants for electrochemical exchange, kex, have been measured for the cobalt clathrochelate couples Co(dmg)3(BF)2+/0 and Co(dmg)3(BC4H9)2+/0 (dmg = double deprotonated dimethylglyoxime) at gold and mercury electrodes in six solvents to ascertain whether the solvent relaxation, dynamics provide a significant contribution to the solvent-dependent reaction rates. Unlike most systems examined previously with regard to solvent dynamical effects, a substantial component of the activation barrier for the cobalt clathrochelate couples is associated with inner-shell (reactant vibrational) distortions in addition to outer-shell (solvent) reorganization. The solvents examined: acetonitrile, acetone, methylene chloride, benzonitrile, nitrobenzene and propylene carbonate, were chosen so as to provide large (up to ca. 30-fold) variations in the frequency of barrier passage from overdamped solvent relaxation, vos. The relative barrier-crossing frequencies, vn extracted from the kex values by correcting for the solvent-dependent activation barriers, show a marked dependence upon vos, indicating that overdamped solvent relaxation contributes significantly to the barrier-crossing frequency for these systems. Qualitative comparisons are made with recent theoretical descriptions of the combined influence of reactant vibrational and solvent relaxation tovn.