Aluminum Avoids the Central Position in AlB9- and AlB10-: Photoelectron Spectroscopy and ab Initio Study

The structures and the electronic properties of two Al-doped boron clusters, AlB9- and AlB10-, were investigated via joint photoelectron spectroscopy and high-level ab initio study. The photoelectron spectra of both anions are relatively broad and have no vibrational structure. The geometrical structures were established by unbiased global minimum searches using the Coalescence Kick method and comparison between the experimental and calculated vertical electron detachment energies. The results show that both clusters have quasi-planar structures and that the Al atom is located at the periphery. Chemical bonding analysis revealed that the global minimum structures of both anions can be described as doubly (sigma- and pi-) aromatic systems. The nona-coordinated wheel-type structure of AlB9- was found to be a relatively high-lying isomer, while a similar structure for the neutral AlB9 cluster was previously shown to be either a global minimum or a low-lying isomer.