Transition from exohedral to endohedral structures of AuGe_n^- (n = 2-12) clusters: Photoelectron spectroscopy and ab initio calculations

Gold-doped germanium clusters, AuGe_n^- (n = 2-12), were investigated by using anion photoelectron spectroscopy in combination with ab initio calculations. Their geometric structures were determined by comparison of the theoretical calculations with the experimental results. The results show that the most stable isomers of AuGe_n^- with n = 2-10 are all exohedral structures with the Au atom capping the vertex, edge or face of Ge_n clusters, while AuGe₁₁^- is found to be the critical size of the endohedral structure. Interestingly, AuGe₁₂^- has an I_h symmetric icosahedral structure with the Au atom located at the center. The molecular orbital analysis of the AuGe₁₂^- cluster suggests that the interactions between the 5d orbitals of the Au atom and the 4s4p hybridized orbitals of the Ge atoms may stabilize the I_h symmetric icosahedral cage and promote the Au atom to be encapsulated in the cage of Ge₁₂. The NICS(0) and NICS(1) values are calculated to be -143.7 ppm and -36.3 ppm, respectively, indicating that the icosahedral AuGe₁₂^- cluster is significantly aromatic.