Gold-Thiolate Clusters: A Relativistic Density Functional Study of the Model Species Au₁₃(SR)_n, R = H, CH₃, n = 4, 6, 8

The binding of sülfanyl and alkylsülfanyl model ligands to gold clusters was studied for the case of Aüi3(SR)_n with R = H, CH₃ and n = 4, 6, 8. Accurate all-electron electronic structure calculations and geometry optimizations of these gold-thiolate clusters have been performed with a scalar relativistic Kohn-Sham procedure as implemented in the density functional program PARAGAUSS. In all structures obtained, bridge coordination was preferred for both types of ligands; no higher coordinated sites where occupied. While in many cases ligand decoration did not change the overall structure of the Au₁₃ core, also more open structures with Au-Au distances elongated beyond the bulk value have been obtained. The effects due to increasing ligand decoration were small: a small decrease of the binding energy per ligand does not exclude higher ligand coverages. The differences between the model ligands SH and SCH₃ were consistent in all cases considered: SCH₃ exhibits weaker binding and a slightly smaller charge separation between cluster core and ligand shell, which amounts up to about 1.5 e for 8 ligands. Overall, the Au₁₃ core of the clusters was found to be quite flexible. This can be rationalized by the fact that the calculated binding energy per ligand is comparable or even exceeds the binding energy per atom in Au₁₃.