Al62- − Fusion of Two Aromatic Al3- Units. A Combined Photoelectron Spectroscopy and ab Initio Study of M+[Al62-] (M = Li, Na, K, Cu, and Au)

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Abstract
Photoelectron spectroscopy is combined with ab initio calculations to elucidate the structure and chemical bonding of a series of MAl6- (M = Li, Na, K, Cu, and Au) bimetallic clusters. Well-resolved photoelectron spectra were obtained for MAl6- (M = Li, Na, Cu, and Au) at several photon energies. The ab initio calculations showed that all of the MAl6- clusters can be viewed as an M+ cation interacting with an Al62- dianion. Al62- was found to possess an Oh ground-state structure, and all of the MAl6- clusters possess a C3v ground-state structure derived from the Oh Al62-. Careful comparison between the photoelectron spectral features and the ab initio one-electron detachment energies allows us to establish firmly the C3v ground-state structures for the MAl6- clusters. A detailed molecular orbital (MO) analysis is conducted for Al62- and compared with Al3-. It was shown that Al62- can be considered as the fusion of two Al3- units. We further found that the preferred occupation of those MOs derived from the sums of the empty 2e‘ MOs of Al3-, rather than those derived from the differences between the occupied 2a1‘ and 2a2‘ ‘ MOs of Al3-, provides the key bonding interactions for the fusion of the two Al3- into Al62-. Because there are only four bonding MOs (one π and three σ MOs), an analysis of resonance structures was performed for the Oh Al62-. It is shown that every face of the Al62- octahedron still possesses both π- and σ-aromaticity, analogous to Al3-, and that in fact Al62- can be viewed to possess three-dimensional π- and σ-aromaticity with a large resonance stabilization.