Electronic shells of a tubular Au-26 cluster: a cage-cage superatomic molecule based on spherical aromaticity

摘要

Gold clusters, which display a variety of unusual geometric structures due to their strong relativistic effects, have attracted much attention. Among them, Au-26 has a high-symmetry tubular structure (D-6d) with a large HOMO-LUMO energy gap, but its electronic stability still remains unclear. In this paper, the electronic nature of the Au-26 cluster is investigated using the density functional theory method. Depending on the super valence bond model, the tubular Au-26 cluster with 26 valence electrons could be viewed as a superatomic molecule composed of two open cages based on spherical aromaticity, and its molecule-like electronic shell closure is achieved via a super triple bond (sigma, 2 pi) between the two cages. Based on this new cage-cage superatomic structural model, a series of similar tubular clusters are predicted from the Au-26 skeleton. The two capped Au atoms are replaced by Cu, Ag and In atoms, respectively, to form tubular D-6d Au24Cu2 and Au24Ag2 (26e) and Au24In2 (30e) clusters, where the super triple bonds also exist. Moreover, tubular D-5d Au20In2 (26e) is obtained by replacing hexatomic Au-6 rings in the bulk of Au24In2 with pentagonal Au-5 rings. Chemical bonding analysis reveals that there is a super quintuple bond (sigma, 2 pi, 2 delta) between two open (Au10In) cages, in accordance with the 26e Li20Mg3 superatomic molecule composed of two icosahedral superatoms. Our study proposes the new cage-cage structural model of superatomic molecules based on spherical aromaticity, which extends the range of the super valence bonding pattern and gives inferences for further study of superatomic clusters.