No abstract

As a guide to the thermodynamics of intramolecular electron-transfer processes, the redox thermodynamics of three dinuclear transition-metal systems have been investigated in mixtures of acetonitrile and dimethyl sulfoxide (DMSO) as solvent. The specific systems are ( 2,2'-bipyridine)2C1Ru-L-Ru(NH3)4(pyridine)s+/4'/3t, where L is pyrazine, 4,4'-bipyridine, or bis(pyridy1)ethane. A special feature is that the tetraammineruthenium redox site in each interacts specifically with hydrogen-bond-accepting (electron-pair-donating) solvents (as demonstrated by various optical and electrochemical measurements (Curtis et al. Inorg. Chem. 1986, 25, 4233; 1987, 26, 2660)) whereas the (polypyridy1)ruthenium site does not. Thus, the formal potential (Ef) for the ammine fragment is shifted to progressively less positive values as the solvent is enriched in DMSO. Measurements of Ef for the Rulrr/ll-polypyridyl fragment demonstrate that the solvational effects are readily transmitted electronically from the ammine fragment when pyrazine is the bridge. Variable-temperature Ef measurements reveal a sharp positive "spike" in plots of the half-reaction entropy for each of the tetraammine couples versus mixed-solvent composition. A statistical calculation shows that the entropy spike is a direct consequence of the unsymmetrical preferential solvation of (2,2'-bipyridine)2Ru11-L-Ru111(NH3)4-(pyridine)4+ versus (2,2'-bipyridine)2Ru11-L-Ru11(NH3)4(pyridine)'+, as found elsewhere for monomeric redox couples and as earlier predicted from optical intervalence studies. Further examination shows that the entropy effects also are transmitted electronically from the tetraammine site to the polypyridyl site and are detectable in both the pyrazine-and 4,4'-bipyridine-bridged cases. An evaluation of the overall intervalence thermodynamics reveals that ASo "leads" AGO on a solvent molar composition coordinate. This suggests that unusual mixed-solvent-induced variations in activation parameters might also be observed in related kinetic experiments.

No abstract

No abstract

A number of ingredients are known or thought to be important in determining rates of electrochemical reactions and, therefore, mechanisms of electrocatalysis. These ingredients may include bond reorganization, solvent reorganization, electronic coupling, ion pairing, proton demand, "work terms", and free-energy driving-force effects. Although evidence exists, in general, for a detectable role for each one of these, for specific cases there is inevitably uncertainty about the relative importance of each effect. Fortunately, optical electron-transfer measurements provide an unambiguous way of partitioning rates into their component effects. The theme of this article is the use of optical processes to learn about molecular level kinetic events and, by inference, molecular level electrocatalysis. We begin by exploring the relationships between electrode processes and analogous optical reactions. Following that, some applications to problems of electrochemical interest-largely drawn from our own recent work-are illustrated. Although the focus is mainly on uncomplicated, single-electron-transfer reactions, preliminary studies of more complicated processes like proton-coupled electron transfer are also mentioned.