Au-21 cage structures and their magic number tri-cations

摘要

We examine the stability and properties of three Au-21 cage structures, one with D-3 symmetry and denoted as Au-21 (D-3), which is novel, and the other two with C-2v symmetry. One, denoted as Au-21 (C-2v-1), has been previously reported but the other, denoted as Au-21 (C-2v-2), is novel. As reference Au-21 structures, we also examine a sheet isomer and a compact isomer, Au-21 (C-s-Tetra), formed by adsorbing an Au atom on Au-20 (T-d). For all structures, we consider charge ranging from -1 to +4. For the Au-21 cage structures, a primary property of interest is their spherical aromaticity, as measured by their nucleus independent chemical shift. Our focus is on charge +3 since each gold atom is assumed to contribute one (6s) valence electron, and 18 is a magic number for shell closing. We find that, although Au-21 (D-3)(+3) has the largest aromaticity, it is not the most stable Au21+3 cage species. Surprisingly, Au-21 (C-2v-2), which is not even stable as a neutral cage species, is the most stable tri-cation cage species. We also examine the stability of (neutral) Au21Xn cage structures relative to Au21Xn structures derived from Au-21 (C-s-Tetra) (with X = F, Cl, Br, I and n = 1, 3, 5). We find that, although Au21F3 derived from Au-21 (D-3) has the largest aromaticity, it is not the most stable Au21F3 cage structure. Nonetheless, all cage structures are stabilized relative to Au-21 (C-s-Tetra) and, remarkably, for the trichlorinated, tribrominated, and triiodineated clusters, at least one cage structure is more stable than its Au-21 (C-s-Tetra) counterpart.