Structure, Energetics, Electronic, and Hydration Properties of Neutral and Anionic Al 3 O 6 , Al 3 O 7 , and Al 3 O 8 Clusters S. Gowtham, Kah Chun Lau, Mrinalini Deshpande,* and Ravindra Pandey Department of Physics, Michigan Technological UniVersity, Houghton, Michigan 49931 Anita K. Gianotto and Gary S. Groenewold Idaho National Engineering and EnVironmental Laboratory, Idaho Falls, Idaho 83415 ReceiVed: December 30, 2003; In Final Form: March 26, 2004 We report the results of a theoretical study of neutral and anionic Al 3 O n (n ) 6-8) and an experimental investigation of Al 3 O 6 H 2 - clusters, focusing on their structural and electronic properties. Our results, based on density functional calculations, reveal that sequential oxidation of Al 3 O 5 induces significant structural changes in the cluster configurations in which an O 2 molecule tends to replace an O atom. The neutral Al 3 O n (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 Al 3 O n (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 Al 3 O 6 H 2 - was examined experimentally using an ion trap-secondary ion mass spectrometer under vacuum conditions, and the gas-phase clusters were shown to add three H 2 O molecules. Since H 2 O 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. I. Introduction Aluminum oxide, Al 2 O 3 , traditionally referred to as alumina, is a very important ceramic material that has many technological applications. Its great usefulness as a ceramic material rests primarily on its extreme hardness (15 Gpa), high melting point (2327 K), and low electrical conductivity (10 -12 S/m at 20 °C). It has a wide range of applications, from electronics, optics, biomedical, and mechanical engineering to catalyst support. In the bulk Al 2 O 3 , bonding interactions are chiefly ionic. 1 The valance electrons of aluminum (i.e. 3s 2 3p 1 ) are transferred to oxygen, thus producing closed shell Al 3+ and O 2- ions whose electrostatic interactions are primarily responsible for alumina’s cohesive energy. Clusters of aluminum oxide have, conse- quently, been studied both theoretically and experimentally to better understand the relationship between structure and bonding between aluminum and oxygen. In Al m O n clusters, 2,3 for a given m, transformation from metallic to ionic bonding was observed with increasing n. As an example of hypermetalated species, Al 3 O was considered for theoretical calculations. 4,5 Quantum mechanical calculations on neutral and anionic Al 3 O n (n e 5) have shown how structure reflects ionic bonding and localization of electrons on aluminum atoms. 6-11 There have been many experimental studies 2,3 of small aluminum-oxygen clusters, including a systematic pho- toelectron spectroscopy study of Al 3 O n - (n ) 0-5). Wu et al. 3 reported that the sequential oxidation of Al 3 has led to a higher value of the electron affinity. For Al 3 O 5 - , the photoelectron spectrum suggests a complete transfer of valence electrons from Al to O in the cluster. Additionally, calculations based on the electron propagator method 7 for Al 3 O 5 and Al 3 O 5 - find that Dyson orbitals associated with the lowest electron detachment energies are dominated by p functions on terminal oxygens in the cluster. Recent mass spectrometry studies conducted by Groenewold and co-workers showed that alumina clusters were formed by projectile bombardment of alumina surfaces using an ion trap- secondary ion mass spectrometer (IT-SIMS). 12,13 The approach could not provide any direct insight into the electronic structure of the clusters formed, but it did show that the reactivity with respect to H 2 O and H 2 S could be readily studied. The results of the reactivity studies might be correlated with structure if the same systems were studied. In the present study, we now investigate the effect of sequential oxidation of Al 3 O 5 on structural and electronic properties of the oxygen-rich alumina clusters. We focus on neutral and anionic Al 3 O n ,(n ) 6-8) clusters and report here their equilibrium properties, including configurational param- eters, binding and fragmentation energies, vibrational frequen- cies, highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap, and electron affinity (EA). It is noted that none of the previous experimental and theoretical studies considered Al 3 O n for n g 5. Therefore, the work presented here provides new results and insight into the structure and bonding of Al 3 O n (n ) 6-8), enabling us to assess both size and charge dependence of the structural properties at the same level of theory. We also present experimental results describing the extent of hydration observed for Al 3 O 6 H 2 - species * Permanent Address: Department of Physics, H. P. T. Arts and R. Y. K. Science College, Nasik, India. 5081 J. Phys. Chem. A 2004, 108, 5081-5090 10.1021/jp038040n CCC: $27.50 © 2004 American Chemical Society Published on Web 05/14/2004