Au₃₂: A 24‐Carat Golden Fullerene

Abstract

For more than a decade, gold nanostructures have attracted the attention of experimentalists and theoreticians alike. Recent years have witnessed increased interest in goldcontaining structures, as several fields of application have found the metal to be not only aesthetical, but also of practical use.[1] In contrast to carbon where the familiar buckminsterfullerene [2] came first, later to be followed by carbon nanotubes, gold research started in reverse; gold nanotubes are already a synthetic reality.[3] Au 32 has to date been considered a moderately uninteresting molecule, just one among the many gold clusters. The most stable structure has been suggested to be space-filling, [4,5] like the majority of all metal clusters studied to date. Using relativistic quantum chemical calculations, we show the existence of another stable isomer: the icosahedral "golden" fullerene Au 32 , the first all-gold fullerenic species. It is spherical and hollow (with a diameter of % 0.9 nm) and structurally very similar to C 60 . Au 32 has a record value of magnetic shielding at its center, and appears to be aromatic.Considering its place in the periodic table, gold is an unusually relativistic element.[6] Among other things, this is expressed in its bonding properties. The element manifests aurophilicity, which further enhances the strong gold-gold interactions.[7] Relativistic effects make many interesting pure gold species such as clusters and nanotubes possible. In addition, heterogenic species are also studied with great interest. The bimetallic icosahedron WAu 12 , first predicted by Pyykkö and Runeberg [8] and later synthesized by Li et al., [9] is representative of these. Recently, images of multiwalled gold nanowires were published.[3] No reports of pure "golden" fullerenes exist, however. The closest match is WAu 12 , where the Au 12 shell engulfs a tungsten atom. The icosahedral form of Au 12 is itself, however, unstable. [10] Few studies of Au 32 exist. Work with empirical potentials suggest that the global energy minimum for the molecule is a low-symmetry, lumplike structure of either C 2[4] or D 2 symmetry.[5] The scalar relativistic density functional theory (DFT) calculations presented here show that Au 32 has another minimum: the icosahedral fullerenic form. To determine the stability of the Au 32 fullerene, we first optimized its structure. Two different functionals, the popular generalized gradient approximation (GGA) functional BP86 [11] and the nonempirical hybrid GGA functional PBE0, [12] were used throughout this work. Au 32 is composed of triangles in icosahedral symmetry, making a near perfect rhombic triacontahedron. Each atom binds to either five or six neighboring gold atoms. Thus, the symmetry is the same as for the truncated icosahedron C 60 , with the vertices and planes interchanged. Figure 1 shows the structure of Au 32 , compared with C 60 .Au 32 is a closed-shell molecule with an appreciable energy gap between the frontier orbitals, factors important for stability. The gap between the highest oc...

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