The [(Al2O3)2]− Anion Cluster: Electron Localization–Delocalization Isomerism

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. Recently, structural diversity has also been demonstrated for zerodimensional gas phase aluminum oxide clusters. 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. In particular, the [(Al2O3)3-5]+ clusters were the first example of polynuclear maingroup metal oxide cluster that are able to thermally activate CH4. Over the past decades gas phase aluminum oxide clusters have been extensively studied both experimentally and computationally, but definitive structural assignments were made for only a handful of them: the planar [Al3O3]- and [Al5O4]- cluster anions, and the [(Al2O3)1-4(AlO)]+ cluster cations. For stoichiometric clusters only the atomic structures of [(Al2O3)4]+/0 have been nambiguously resolved. Here we report on the structures of the [(Al2O3)2]-/0 clusters combining photoelectron spectroscopy (PES) and quantum chemical calculations employing a genetic algorithm 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 resultsmore » 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.« less