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| - Exposing Solvent's Roles in Electron Transfer Reactions: Tunneling Pathway and Solvation
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| - We describe studies of electron transfer in donor−spacer−acceptor molecules for which the highly curvedspacer topology imparts a vacant cleft along the “line-of-sight” between the electron donor and electronacceptor moieties. The electron transfer kinetics in nondipolar and weakly polar solvents allow experimentaldetermination of the reaction free energy as a function of solvent structure and temperature. These data wereused to parametrize a molecular solvation model developed by Matyushov. The model provides reasonableestimates of reaction free energy in solvents that are too polar for its direct measurement and provides reasonablevalues of the solvent reorganization energy in all solvents. Successful modeling of the solvation enablesquantitative study of the factors that control electron tunneling through molecules located in the solute'scleft, i.e., the electronic coupling. Electron tunneling in these systems is mediated by the unoccupied orbitalsof the solvent (“electron-mediated superexchange”). The solvent molecule's presence within the cleft is criticalfor effective electronic coupling, and its motion modulates the electronic coupling magnitude. These studiesdemonstrate and quantify the importance of electron tunneling “pathways” through noncovalent contacts forthis model system and indicate that such pathways can contribute significantly to electron-transfer processesin biological and chemical systems.
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