Structure, Energetics and Electronic, and Hydration Properties of Neutral and Anionic Al3O6, Al3O7 and Al3O8 Clusters

We report the results of a theoretical study of neutral and anionic Al3On (n = 6-8) and an experimental investigation of Al3O6H2- clusters, focusing on their structural and electronic properties. Our results, based on density functional calculations, reveal that sequential oxidation of Al3O5 induces significant structural changes in the cluster configurations in which an O2 molecule tends to replace an O atom. The neutral Al3On (n = 6-8) clusters are found to be in doublet electronic states, with a planar to three-dimensional close-packed structure being most stable. The triplet state is found to be the optimum electronic state for the ground state of anionic Al3On (n = 6-8). The clusters showed an energetic preference for a twisted-pair rhombic structure, although for n = 6 and 8, a planar hexagonal structure was only 0.16 eV higher in energy. It is also shown that the strength of the oxygen-oxygen bond dominates the preferred fragmentation path for both neutral and anionic clusters. The hydration behavior of an n = 6 cluster Al3O6H2- was examined experimentally using an ion trap-secondary ion mass spectrometer under vacuum conditions, and the gas-phase clusters were shown to add three H2O molecules. Since H2O addition is consistent with the presence of under-coordinated metals in oxide clusters, the experimental result for n = 6 was consistent with the planar hexagonal structure, which contained three under-coordinated Al sites