The [(AL2O3)2]- anion cluster: electron localization-delocalization isomerism.

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 zero-dimensional gas phase aluminum oxide clusters. 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. In particular, the [(Al2O3)3 5] + clusters were the first example of polynuclear main-group metal oxide cluster that are able to thermally activate CH4. [7] Over the past decades gasphase aluminum oxide clusters have been extensively studied both experimentally and computationally, 4, 7, 11–14] but definitive structural assignments were made for only a handful of them: the planar [Al3O3] and [Al5O4] cluster anions, and the [(Al2O3)1 4ACHTUNGTRENNUNG(AlO)] + cluster cations. For stoichiometric clusters only the atomic structures of [(Al2O3)4] + /0 have been unambiguously resolved. Herein we report on the structures of the [(Al2O3)2] /0 clusters combining photoelectron spectroscopy (PES) and quantumchemical calculations employing a genetic algorithm as a global optimization technique. The [(Al2O3)2] cluster anion shows 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 is of considerable value for future computational studies of aluminum oxide and related systems. Figure 1 shows the PES spectra of [(Al2O3)2] at 355 nm (3.496 eV) and 193 nm (6.424 eV) photon energies. The 355 nm spectrum reveals two bands (labeled as X and X’; Figure 1a). Band X exhibits a vibrational progression with an average spacing of 570 cm 1 (Table 1). The 0–0 transition at 1.65 eV defines the ground-state adiabatic detachment energy (ADE),