DFT Studies of Solvation Effects on the Nanosize Bare, Thiolated, and Redox Active Ligated Au55 Cluster

The structural and electronic properties of the bare Au-55 cluster, and of the model thiol passivated Au-55(SCH3)(42), the redox active ligated Au55S(CH2)(2)CO2(CH2)(10)bpy center dot 2Cl and Au-55(SCH3)(41)(S(CH2)(2)CO2(CH2)(10)bpy)center dot 2Cl (bPY = N-methyl-4,4'-bipyridinium) complexes are studied at the DFT level in the gas phase and with an explicit water layer. For all complexes, neutral, positive, and negative charge states are investigated. The thiol ligation distorts the outer layer of the approximate icosahedral geometry of the bare cluster and induces a charge transfer from the gold core to the ligand shell. The anchoring of a single redox active ligand on the bare Au55 leads to the formation of a cavity around the S-Au bond. We show that this cavity formation is prevented by the thiol ligands in Au-55(SCH3)(41)(S(CH2)(2)CO2(CH2)(10)bpy)center dot 2Cl. The vertical addition of one electron to the [Au55S(CH2)(2)CO2(CH2)(10)bpy center dot 2Cl](0) cluster is followed by a charge transfer from the Au55 core to the bpy(2+) ligand, which is accompanied by a mechanical motion of the redox active bpy arm driven by electrostatic interactions. The presence of a solvent shell does not alter the structure but significantly decreases the computed charging energies of the clusters, making them comparable with experimental values. The computed redox potential differences are in good agreement with the experimental values.