Theoretical Investigation of Optimized Structures of Thiolated Gold Cluster Au25(SCH3)18+

Geometric and electronic structures of a gold−methanethiolate Au25(SCH3)18+ are investigated by using density functional theory. Three types of optimized structures are derived from two different Au25 core clusters protected by 18 methanethiolates. The most probable optimized structure consists of a Au7 core cluster and Au−S complex-like ring clusters, Au12(SCH3)12 and Au3(SCH3)3. The Au7 core cluster is enclosed by the Au12(SCH3)12 ring cluster and then the Au7−Au12(SCH3)12 core−ring subsystem is capped with the two Au3(SCH3)3 ring clusters from both sides of the top and the bottom. This structural feature is in contrast to a general notion of gold−thiolate clusters that a core gold cluster is superficially protected by thiolate molecules. The optimized structure provides a large HOMO-LUMO gap, and its X-ray diffraction and absorption spectra successfully reproduce the experimental results.