ABINITIO PREDICTION OF THE STRUCTURES AND STABILITIES OF THE HYPERALUMINUM MOLECULES - AL3O AND SQUARE-PLANAR AL4O

The concept of hypermetalation, characterized by molecules with unprecedented stoichiometries, is extended to the aluminum-oxygen combinations, Al3O and Al4O. Their equilibrium geometries and fundamental frequencies, as well as those of the isolated reference species AlO, Al2, Al2+, Al2O, Al3O+, Al3O, Al3+, Al4(2+), and Al4O, were calculated at HF/6-31G* and at various correlated levels, e.g., MP2(full)/6-31G*. Extensive searches of possible structures and electronic states were carried out. The global minima are: linear Al2O (D-infinity-h, 1-SIGMA-g+), planar Al3O+ (D3h, 1A1'), planar Al3O (the C2-nu Y(2A1) and T(2B2) forms have nearly the same energy), tetrahedral Al4(2+) (T(d), 1A1), and planar Al4O (D4h, 1A1g). All these species are very stable (with the exception of Al4(2+)) with regard to all possible decomposition pathways. Representative dissociation energies (in kcal/mol at PMP4SDTQ/6-311+G*//MP2(full)/6-31G*+ZPE) are: Al3O+, into Al2O + Al+ (34.1); Al3O into Al2O + Al (19.9); A14O, into Al3O + Al (45.5) or into Al2O + Al2 (37.1). Although the aluminum-oxygen attraction is largely ionic, aluminum-aluminum bonding contributes significantly to the stability of the hyperaluminum Al3O and Al4O molecules.