Long-range electron transfer: what is rate-determining-the geometric distance or the pathway?

Properties of monolayers of the long-chain aliphatic alcohols and acids and their mixtures has been studied and compared with their three-dimensional computer models. For cetyl alcohol and its mixtures with stearic acid, molecules of monolayers are in an all-trans stretched conformation oriented normally to the electrode surface; in the other cases, the monolayer structure is “crumpled” and less regular. The kinetics of the redox reaction of hemin adsorbed on these monolayers has been studied; the exchange currents were corrected for the change of reorganization energy with the layer thickness. The reaction rate differs by several orders of magnitude and even by sign from the predictions of a simple rate-distance dependence. In accordance with the superexchange mechanism, the electron transfer probability is determined by the length of the transfer pathway. For the all-trans stretched molecules, the pathway length is determined only by the length of the hydrocarbon chain. The interlacement of these chains in a film of an irregular structure results in the interchain coupling through van der Waals contacts between molecules facilitating the electron transfer.