PNNL

Abstract

Three-dimensional bulk alumina and its two-dimensional thin films show great structural diversity, posing considerable challenges to their experimental structural characterization and computational modeling.[1,2] Recently, structural diversity has also been demonstrated for zerodimensional gas phase aluminum oxide clusters.[3,4] Mass-selected clusters not only make systematic studies of the structural and electronic properties as a function of size possible, but lately have also emerged as powerful molecular models of complex surfaces and solid catalysts.[5-8] In particular, the [(Al2O3)3-5]+ clusters were the first example of polynuclear maingroup metal oxide cluster that are able to thermally activate CH4.[7] Over the past decades gas phase aluminum oxide clusters have been extensively studied both experimentally[3,4,7-10] and computationally,[3,4,7,11-14] but definitive structural assignments were made for only a handful of them: the planar [Al3O3]- and [Al5O4]- cluster anions, [9c,13f] and the [(Al2O3)1-4(AlO)]+ cluster cations.[4] For stoichiometric clusters only the atomic structures of [(Al2O3)4]+/0 have been nambiguously resolved.[3] Here we report on the structures of the [(Al2O3)2]-/0 clusters combining photoelectron spectroscopy (PES) and quantum chemical calculations employing a genetic algorithm[3] as a global optimization technique. The [(Al2O3)2]- cluster anion show energetically close lying but structurally distinct cage and sheet-like isomers which differ by delocalization/localization of the extra electron. The experimental results are crucial for benchmarking the different computational methods applied with respect to a proper description of electron localization and the relative energies for the isomers which will be of considerable value for future computational studies of aluminum oxide and related systems.