Structural Evolution of Doped Gold Clusters: MAu

We report a joint experimental and theoretical study on the structures of a series of gold clusters doped with a group-14 atom: MAux (M ) Si, Ge, Sn; x ) 5-8). Well-resolved photoelectron spectra were obtained and compared to calculations at several levels of theory to identify the low-lying structures of MAu5-8. We found that the structure of SiAu5 is dominated by the tetrahedrally coordinated Si motif, which can be viewed as built from the tetrahedral SiAu4 by an extra Au atom bonded to a terminal gold atom. However, SiAu6 and SiAu7 have quasi-planar structures, similar to those of GeAu6/SnAu6 and GeAu7/SnAu7, respectively. SiAu8 again has a tetrahedrally coordinated Si structure, which displays a structural motif of a dangling Au-Si unit sitting on a gold cluster surface, resembling that of the larger Si-doped gold cluster SiAu16. For M ) Ge, Sn, our results show that the major isomers of GeAu5-8 have structures similar to those of the corresponding SnAu5-8 clusters, and they can be viewed as grown from the previously suggested square-pyramidal GeAu4 and SnAu4, respectively. Population of minor isomers was observed for SnAu5, GeAu6, SnAu6, and GeAu8. The 3D to quasi-2D to 3D structural evolution for SiAu5 to SiAu8 and the structural convergence for MAux (M ) Si, Ge, Sn) at x ) 6, 7 manifest competitions between the tendency of forming molecule-like structures around the group-14 dopant (optimizing M-Au interactions) and the strong tendency of forming planar structures for small gold anion clusters (optimizing Au-Au interactions).