Electrochemical activation parameters for surface-attached reactions Evaluation of unimolecular frequency factors for representative ligand-bridged reactions

Abstract

The temperature dependence of unimolecular rate constant are reported for the one-electron reduction of various Co(III) and Cr(III) reactants attached to mercury or silver surfaces via either thiocyanate or various thiophenecarboxylate ligands. Combining the “ideal” activation enthalpies obtained at a constant electrode potential using a non-isothermal cell arrangement with the corresponding activation entropies estimated from reaction entropy data enables values of the unimolecular frequency factor, A et , for the elementary electron-transfer step to be evaluated. Values of A et close to 10 13 s −1 were obtained in this manner for the thiocyanate-bridged reactions at mercury, indicating that the electronic transmission coefficient, κ el , approaches unity, i.e. the reactions are adiabatic. Although a comparable value of A et was obtained for Co(III) reduction via a thiophenecarboxylate bridge featuring uninterrupted conjugation, substantially smaller A et values were observed with bridges containing one or more saturated carbon atoms, suggesting that κ el ≪1 under the latter conditions.