Cationic gold clusters with eight valence electrons: possible spherical aromatic systems with Sigma holes

The energetically preferred structures of the gold clusters Au9+, Au113+, and Au124+ with eight skeletal electrons have been studied by density functional theory for comparison with the 8-electron Au102+ cluster shown previously to have a highly favored Td tetracapped octahedral structure. The low-energy structures for the Au9+ and Au113+ clusters are found to be similar relatively spherical polyhedra. Such systems can be considered to exhibit spherical aromaticity in accord with their filled 1S21P6 shells, their diatropic NICS(0) values ranging from -21.4 to -44.3 ppm, and their shielding cone surfaces. However, the preferred spherical polyhedra for Au9+ and Au113+ are not the same as the closo deltahedra found in the BnHn2- borane dianions. Instead they have smaller internal cavities formed by capping faces of smaller deltahedra or by formation of internal Au-Au bonds. The lowest energy Au124+ structures are not similar nearly spherical polyhedral structures. Instead they are derived from planar gold subclusters by adding more gold atoms to form tetrahedral Au4 bubbles. The planar origin of the low-energy Au124+ structures relates to the energetic preference for neutral Au<14 clusters for planar structures or nearly planar structures containing small polyhedral bubbles. The presence of σ-holes has been identified on the surfaces of the complete series of the Aun(n-8)+ (n = 9 to 12) clusters. The strength of their electrostatic interactions is predicted to increase upon increasing cluster size.