Ionic atmosphere effects on the energetics of thermal and optical electron-exchange reactions: Application to ferrocenium-ferrocene self exchange
✍ Scribed by A. M. Kuznetsov; D. K. Phelps; M. J. Weaver
- Publisher
- John Wiley and Sons
- Year
- 1990
- Tongue
- English
- Weight
- 858 KB
- Volume
- 22
- Category
- Article
- ISSN
- 0538-8066
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✦ Synopsis
Abstract
A treatment of ionic‐atmosphere effects upon symmetrical electron‐transfer reactions resulting from added electrolyte is outlined. Relationships are derived on the basis of the extended Debye‐Huckel model for the increase in the activation free energy, Δ__G__~ia~*, associated with reorganization of the ionic atmosphere for homogeneous‐phase reactions involving a pair of spherical reactants with varying internuclear distance R. Similar relationships apply to the energetics of symmetrical optical electron transfer, since the increase in the optical transition energy, Δ__E__, should equal the corresponding ionic atmosphere reorganization energy, E; under the anticipated linear response conditions, E = 4Δ__G__~ia~*. The predicted Δ__G__~ia~* (and hence Δ__E__) values increase sharply with increasing R, as a consequence of the diminished “sharing” of the ionic cloud surrounding the donor and acceptor sites under these conditions. Outer‐sphere electrochemical reactions, featuring a single “near‐isolated” reactant, are predicted to feature substantially larger Δ__G__~ia~* values than for homogeneous processes proceeding with the reaction partners in contact. The influence of more specific “ionic atmosphere” effects upon Δ__G__~ia~*, especially involving reactant‐electrolyte ion pairing, is also discussed. Unlike that of the nonspecific ionic atmosphere, the nuclear reorganization process associated with counterion transport between donor and acceptor sites coupled with electron transfer is nonlinear in nature, so that E ≠ 4Δ__G__~ia~*. Some recent experimental data for electrolyte effects upon the rate constants for ferrocenium‐ferrocene self exchange and related systems are examined in the light of these considerations.