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

Gold clusters, which display a variety of unusual geometric structures due to their strong relativistic effects, have attracted much attention. Among them, Au has a high-symmetry tubular structure (D) with a large HOMO-LUMO energy gap, but its electronic stability still remains unclear. In this paper, the electronic nature of the Au cluster is investigated using the density functional theory method. Depending on the super valence bond model, the tubular Au 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 (σ, 2π) between the two cages. Based on this new cage-cage superatomic structural model, a series of similar tubular clusters are predicted from the Au skeleton. The two capped Au atoms are replaced by Cu, Ag and In atoms, respectively, to form tubular D AuCu and AuAg (26e) and AuIn (30e) clusters, where the super triple bonds also exist. Moreover, tubular D AuIn (26e) is obtained by replacing hexatomic Au rings in the bulk of AuIn with pentagonal Au rings. Chemical bonding analysis reveals that there is a super quintuple bond (σ, 2π, 2δ) between two open (AuIn) cages, in accordance with the 26e LiMg 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.