Inorganic and Organometallic Charge Transfer

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Clifford P. Kubiac of the University of California, San Diego is supported by the Inorganic, Bioinorganic, and Organometallic Chemistry program for research on charge transfer in inorganic and organometallic systems. These studies involve experimental characterization of dynamical averaging of infra-red spectra by electron exchange in a variety of triruthenium cluster-compounds. Differences in electronic interactions among the ruthenium centers in such trinuclear molecular ions account for rates of electron transfer that range both above and below the vibrational time scale. Systematic study of the effects of variation in adjustable parameters (ligands, solvent, etc.) are used to test theories of electron transfer (e.g., Marcus-Hush) at the threshold between weakly-localized and fully-delocalized behavior. Other, related, studies will involve polysilanes and polychromium compounds.Electron-transfer reactions are of central importance in many diverse areas of chemistry and related scientific areas, including biological charge transfers of many types. Since the electron is very light with respect to atomic nuclei, electron-transfer reactions can be discussed theoretically in a level of detail that is impossible for most other kinds of chemical change. Powerful theories (e.g. Marcus -Hush) can deal successfully with charge-transfer reactions in which the electron-donor and electron-acceptor centers are only weakly coupled --- or when these two centers are strongly mixed. The intermediate case, in which interaction between donor and acceptor centers are neither very strong or very weak, has proved more difficult to understand. The PI and his group have shown that the fine details of the vibrational (IR) spectra of trinuclear ruthenium complexes are sensitive to seemingly small changes in adjustable parameters that influence the rate of electron transfer in those complexes. This project uses these changes in IR spectra to probe intra-molecular coupling in these species, and to test and extend theories of electron-transfer for systems with intermediate donor-acceptor interaction. This is a fundamentally important, but previously poorly-understood, region of parameter-space.