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Solvent effects on the kinetics of simple electrochemical reactions

1985; Elsevier BV; Volume: 191; Issue: 2 Linguagem: Inglês

10.1016/s0022-0728(85)80028-0

2590-2954

Jane Kim Farmer, Thomas Gennett, Michael J. Weaver,

Photochemistry and Electron Transfer Studies

The dependence of the kinetics of Co(en)33+/2+ exchange (en=ethylenediamine) at mercury electrodes upon the nature of the solvent is discussed further by examining the preexponential factors, Acorr, as well as double-layer corrected standard rate constants, kcorrs, and comparing them with the corresponding quantities, Acalc and kcalcs, calculated from electron-transfer theory. The solvents examined are water, formamide, acetonitrile, propylene carbonate (PC), N-methylformamide, N,N-dimethylformamide (DMF), and dimethylsulfoxide (DMSO). Whereas in most nonaqueous solvents kcorrskcalcs and Acorr≈ Acalc, in aqueous media kcorrs≈102kcalcs yet Acorr≈10−5Acalc. The unexpectedly large solvent dependences of Acorr as well as kcorrs appear to be due primarily to differences in the degree of interfacial reactant solvation, rather than to variations in the elementary electron-transfer barrier resulting from specific reactant-solvent interactions as was speculated previously. Estimates of Acorr obtained for Co(NH3)63+ reduction showed a similarly large solvent dependence to that for Co(en)33+/2+. Values of kcorrs and Acorr for Ru(NH3)63+/2+ exchange are also reported in water, PC, DMF, and DMSO. The solvent dependence of kcorrs for this couple is attributable in part to the influence of solvent reorganization dynamics upon the frequency factor.