Electronic and atomic structure of the AlnHn+2 clusters

The electronic and atomic structure of the family of hydrogenated Al clusters AlnHn+2 with n=4-11 has been studied using the density functional theory with the generalized gradient approximation (GGA) for exchange and correlation. All these clusters have substantial gaps between the highest occupied and the lowest unoccupied molecular orbitals (HOMO-LUMO) and, consequently, they are chemically very stable. The largest gap of 2.81 eV occurs for Al6H8. Five clusters of the family, Al4H6, Al5H7, Al6H8, Al7H9, and Al10H12, fulfill the Wade-Mingos rule. That is, in AlnHn+2, the Al matrix forms a polyhedron of n vertices and n H atoms form strong H-Al terminal bonds; one pair of electrons is involved in each of those bonds. The remaining n+1 electron pairs form a delocalized cloud over the surface of the Al cage. The clusters fulfilling the Wade-Mingos rule have wider HOMO-LUMO gaps and are chemically more stable. The trends in the gap have some reflections in the form of the photoabsorption spectra, calculated in the framework of time-dependent density functional theory using the GGA single-particle energies and orbitals and a local density approximation exchange-correlation kernel.