Theoretical investigation of optimized structures of thiolated gold cluster [Au25(SCH3)18]+

Geometric and electronic structures of a gold-methanethiolate [Au-25(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 Au-7 core cluster and Au-S complex-like ring clusters, Au-12(SCH3)(12) and Au-3(SCH3)(3). The Au-7 core cluster is enclosed by the Au-12(SCH3)(12) ring cluster and then the Au-7-Au-12(SCH3)(12) core-ring subsystem is capped with the two Au-3(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.