J. Am. Chem. Soc. 130 (2008) 15143–15149

The vibrational spectroscopy of the electronically closed-shell (Al₂O₃)_n(AlO)⁺ cations with n = 1−4 is studied in the 530−1200 cm⁻¹ range by infrared predissociation spectroscopy of the corresponding ion−He atom complexes in combination with quantum chemical calculations. In all cases we find, assisted by a genetic algorithm and the TURBOMOLE program package, global minimum structures that differ considerably from those derived from known modifications of bulk alumina. The n = 1 and n = 4 clusters exhibit an exceptionally stable conical structure of C_3v symmetry, whereas for n = 2 and n = 3, multiple isomers of lower symmetry and similar energy may contribute to the recorded spectra. A blue shift of the highest energy absorption band is observed with increasing cluster size and attributed to a shortening of Al−O bonds in the larger clusters. This intense band is assigned to vibrational modes localized on the rim of the conical structures for n = 1 and n = 4 and may aid in identifying similar, highly symmetric structures in larger ions.