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Rudolph A. Marcus
Arthur Amos Noyes Professor of Chemistry

Professor Marcus' group formulates and investigates theories of chemical reactions, including electron transfer processes in solution, in proteins, and at interfaces, and of unimolecular reactions, and intramolecular dynamics.

In the field of electron transfer reactions, recent theoretical studies here include solvent dielectric dispersion effects on electron transfer reaction rates and on the relaxation of polar solvent molecules around photoexcited charge transfer solutes. Another set of studies are of the effect of donor/acceptor electronic coupling on the reaction rates, as in the dependence of the electronic coupling matrix elements for electron transfer in rigid bridged systems. The method was extended to electron transfers in proteins, initially using an artificial intelligence (AI) technique to select the more important amino acids involved in the donor/acceptor electron transfer. The mechanism is principally one of electronic superexchange. Most recently, a fast "sparse matrix" technique was introduced to include all of the amino acids and to test the AI results. Each of these studies and those described below were stimulated by experiments, and one focus in this work, and that described below, is on the comparison of theory and experiment.

The electron coupling studies were extended by introducing a recursion method to treat successive bonds or groups between donor and acceptor. Applications include the treatment of electron transfer along long chain alkanethiols attached to metal electrodes. It is planned to adapt it to treat long range electron transfer in proteins.

Related topics being studied are rates of electron transfer at semiconductor electrode/liquid interfaces and at interfaces between liquids and different metal electrodes. One aspect of the latter is the relative effectiveness of s(Au, Ag) vs sand d(Pt, Pd) electrons for these electron transfer rates. Further studies are being made on electric field effects on electron transfers in photosynthesis.

In another area, unimolecular dissociation reactions and bimolecular association reactions, such as ozone formation from O + O₂, the dynamics of the processes are being explored, with the goal of explaining some unusual experimental results.

Another problem stimulated by current experimental results concerns intramolecular vibrational energy randomization in unimolecular reactions. When the amount of vibrational energy is low, the density of directly coupled zeroth-order vibrational states in a molecule is relatively low, and we treated the behavior using a superexchange "tier structure" (off-resonant) mechanism, using an AI search. The ideas were tested by comparing with spectral data on overtones. At higher energies, important in unimolecular reactions, a different ("resonant") mechanism is expected, because of the high density of directly coupled states. It is planned to treat recent data on the time scale of randomization using such a description.

The research is supported by NSF and ONR.