Photoelectron spectroscopy revealed that a 20-atom gold cluster has an extremely large energy gap, which is even greater than that of C60, and an electron affinity comparable with that of C60. This observation suggests that the Au20 cluster should be highly stable and chemically inert. Using relativistic density functional calculations, we found that Au20 possesses a tetrahedral structure, which is a fragment of the face-centered cubic lattice of bulk gold with a small structural relaxation. Au20 is thus a unique molecule with atomic packing similar to that of bulk gold but with very different properties.

An interesting feature of elemental boron and boron compounds is the occurrence of highly symmetric icosahedral clusters. The rich chemistry of boron is also dominated by three-dimensional cage structures. Despite its proximity to carbon in the periodic table, elemental boron clusters have been scarcely studied experimentally and their structures and chemical bonding have not been fully elucidated. Here we report experimental and theoretical evidence that small boron clusters prefer planar structures and exhibit aromaticity and antiaromaticity according to the Hückel rules, akin to planar hydrocarbons. Aromatic boron clusters possess more circular shapes whereas antiaromatic boron clusters are elongated, analogous to structural distortions of antiaromatic hydrocarbons. The planar boron clusters are thus the only series of molecules other than the hydrocarbons to exhibit size-dependent aromatic and antiaromatic behaviour and represent a new dimension of boron chemistry. The stable aromatic boron clusters may exhibit similar chemistries to that of benzene, such as forming sandwich-type metal compounds.

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Aromaticity is a concept invented to account for the unusual stability of an important class of organic molecules: the aromatic compounds. Here we report experimental and theoretical evidence of aromaticity in all-metal systems. A series of bimetallic clusters with chemical composition MAl4- (M = Li, Na, or Cu), was created and studied with photoelectron spectroscopy and ab initio calculations. All the MAl4- species possess a pyramidal structure containing an M+ cation interacting with a square Al4(2-) unit. Ab initio studies indicate that Al4(2-) exhibits characteristics of aromaticity with two delocalized pi electrons (thus following the 4n + 2 electron counting rule) and a square planar structure and maintains its structural and electronic features in all the MAl4- complexes. These findings expand the aromaticity concept into the arena of all-metal species.

We report a joint experimental and theoretical study of the electronic and atomic structures of small gold clusters with up to 14 atoms. Well-resolved photoelectron spectra were obtained for Au N -(N ) 1-14) at several photon energies. Even-odd alternations were observed, where the even-sized clusters (except Au 10 -) exhibit an energy gap between the lowest binding energy peak and the rest of the spectrum, indicating that all the neutral even-sized clusters have closed shells. The Au 10spectrum reveals the existence of isomers, with the ground-state cluster exhibiting an extremely high electron binding energy. Evidence of multiple isomers was also observed in the spectra of N ) 4, 8, 12, and 13. The structures of the gold cluster anions in the range N ) 4-14 were investigated using first-principles simulations. A striking feature of the anionic clusters in this range is the occurrence of planar ground-state structures, which were predicted in earlier theoretical studies et al. Phys. ReV. Lett. 2002, 89, 033401) and observed in ion-mobility experiments et al. J. Chem. Phys. 2002, 117, 6982) and the existence of close-lying isomers. The calculated electron detachment energies and density of states were compared with the measured data, which confirmed the ground-state structures of the anions. It is found that the main isomers observed experimentally indeed consist of planar clusters up to Au 12 -, whereas for Au 13and Au 14the theoretical results from threedimensional isomers agree better with the experiment, providing further support for the 2D to 3D structural transition at Au 12 -, as concluded from previous ion mobility experiments. We also find that small neutral clusters exhibit a tendency to form two-dimensional structures up to a size of 13 atoms.

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Experimental and computational simulations revealed that boron clusters, which favor planar (2D) structures up to 18 atoms, prefer 3D structures beginning at 20 atoms. Using global optimization methods, we found that the B20 neutral cluster has a double-ring tubular structure with a diameter of 5.2 Å. For the B 20 ؊ anion, the tubular structure is shown to be isoenergetic to 2D structures, which were observed and confirmed by photoelectron spectroscopy. The 2D-to-3D structural transition observed at B20, reminiscent of the ring-to-fullerene transition at C20 in carbon clusters, suggests it may be considered as the embryo of the thinnest single-walled boron nanotubes.photoelectron spectroscopy ͉ density functional calculation ͉ global minimum search S mall atomic clusters often exhibit structures and properties remarkably different from those of their bulk counterparts. For example, the most stable form of carbon is graphite, consisting of layers of two-dimensional (2D) graphene sheets. Yet small carbon clusters form chains, rings, and fullerenes (1-5). Boron, carbon's lighter neighbor, is also a strongly covalent material consisting of B 12 icosahedral cages (6-8). But small boron clusters were predicted to be planar (9-11), in stark contrast to the bulk three-dimensional (3D) cages. Planar boron clusters have been recently produced in the gas phase and experimentally confirmed up to B 15 (12)(13)(14). However, it is still unclear at what critical size the 2D-to-3D structural transition occurs. We show from concerted photoelectron spectroscopy (PES) and global geometry optimization theoretical studies (15-17) that the transition occurs at the size of 20 atoms. The B 20 neutral cluster is found to overwhelmingly favor a double-ring tubular-type structure over any 2D isomers, whereas in the anion the tubular and several 2D structures are close in energy. The 2D-to-3D transition at B 20 is reminiscent of the ring-to-cage transition at C 20 , which forms the smallest fullerene (5). The tubular B 20 is the smallest stable 3D boron cluster and can be viewed as the embryo of the thinnest boron nanotube, with a diameter of 5.2 Å. Methods PES.The experiments were carried out by using a magnetic-bottle time-of-flight PES apparatus equipped with a laser vaporization supersonic cluster source (15, 17). B n Ϫ cluster anions were produced by laser vaporization of a disk target made of enriched 10 B isotope (99.75%) in the presence of a helium carrier gas and were analyzed with a time-of-flight mass spectrometer. The B 20 Ϫ clusters were mass-selected and decelerated before irradiation by a photodetachment laser beam. Photoelectrons were collected at nearly 100% efficiency by the magnetic bottle and analyzed in a 3.5-m-long electron flight tube. The photoelectron spectra were calibrated by the known spectrum of Rh Ϫ , and the energy resolution of the apparatus was ⌬E k ͞E k ϳ 2.5%, i.e., 25 meV for 1-eV electrons. Effort was devoted to control the cluster temperatures (Fig. 4, which is published as supporting information on th...

The fullerenes are the first ''free-standing'' elemental hollow cages identified by spectroscopy experiments and synthesized in the bulk. Here, we report experimental and theoretical evidence of hollow cages consisting of pure metal atoms, Au n ؊ (n ‫؍‬ 16 -18); to our knowledge, free-standing metal hollow cages have not been previously detected in the laboratory. These hollow golden cages (''bucky gold'') have an average diameter >5.5 Å, which can easily accommodate one guest atom inside.anion photoelectron spectroscopy ͉ density functional calculation ͉ hollow gold cages ͉ lowest-energy clusters