No abstract

Size-selected, ligand-free gold clusters with diameters less than 2 nm can be routinely generated in the gas phase. The pronounced size dependence of their physical and chemical properties is one of their most important features. Surfacedeposited gold clusters are particularly interesting for applications in nanotechnology and heterogeneous catalysis. [1][2][3][4][5][6][7] One prerequisite for such applications is a detailed knowledge of the cluster structures.The extensive literature on theoretical studies of gold clusters has been surveyed in recent reviews. [8,9] In principle, quantum-chemical methods allow many properties of small gold clusters to be predicted with high accuracy. However, for larger clusters, the large number of possible isomers impedes the search for a global energy minimum. Often, only a small set of the possible structures can be considered, without any guarantee that the global minimum is included in the set. Consequently, progress in the intermediate size regime can only be made through the joint use of experiment and theory.In this fashion, the structures of small gold cluster anions and cations with up to 13 atoms have been inferred through a comparison of theoretical and experimental collision cross sections from ion-mobility measurements. [10,11] A remarkable finding in this study was that the 2D!3D structural transition for Au n À occurs at the surprisingly large cluster sizes of n = 11 and 12. This result was later confirmed through a comparison of photoelectron spectroscopy (PES) data with calculated density of states (DOS) curves.[12] For Au n À with n = 16-18 and 21-24, experimental and theoretical evidence for hollow cage-like structures has been reported. [13][14][15] Au 20 À possesses a tetrahedral structure, [16] which corresponds to a fragment of the face-centered cubic (fcc) structure of bulk gold; the cluster consists only of surface atoms and does not contain any inner atoms.It has been suggested, on the basis of quantum-chemical calculations, that medium-sized gold clusters, such as Au n with n = 32-35, [17,18] 42, [19] , and 50, [20] also have cage-like structures. In contrast, in a recent study, Au 32 À was assigned an amorphous, but dense, structure on the basis of a comparison of data from PES and the calculated DOS.[21] Low-symmetry "disordered" structures have also been proposed by Garzón et al. for Au 28 and Au 55 .[22] These results were supported by a combined PES and theoretical study by Häkkinen et al., which excluded high-symmetry structures for Au 55 À (whereas Ag 55 À and Cu 55 À have icosahedral structures).[23]The PE spectra of several Au n À clusters, notably for n = 14, 20, 34, and 58, show prominent band gaps, [24] reflecting the large gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the corresponding neutral clusters. The associated valence-electron counts correlate with the sequence of jellium-shell closings obtained from a simple free-electron model invoking (volume-filling) spherical ...

The structures of mass-selected gold cluster anions Au(14)(-)-Au(19)(-) have been reinvestigated using an improved low temperature trapped ion electron diffraction experiment. Structures were assigned by comparing experimental with simulated scattering functions using model structures obtained by density functional calculations. Flat three-dimensional structures are found for Au(14)(-) and Au(15)(-), hollow cages for Au(16)(-)-Au(18)(-) and a tetrahedral structure is found for Au(19)(-). For several clusters in this series, our assignments differ distinctly from previous assignments.

The structures and energetics of small tin cluster Sn(n)(-) anions up to n=15 were determined by a combination of density-functional theory and three different experimental methods: Ion mobility spectrometry, trapped ion electron diffraction, and collision induced dissociation. We find compact, quasispherical structures up to n=12. Sn(12)(-) is a slightly distorted hollow icosahedron while Sn(13)(-) to Sn(15)(-) have prolate structures, consisting of merged, hollow, in part incomplete, deltahedral subunits: Sn(13)(-) consists of a face-sharing pentagonal bipyramid and tricapped trigonal bipyramid, Sn(14)(-) comprises a face-sharing dicapped trigonal prism and capped square-antiprism, and Sn(15)(-) consists of two face-sharing tricapped trigonal prisms.

Structural, electronic, and magnetic properties of the golden cage doped with a transition-metal atom, MAu 16 − ͑M = Fe,Co,Ni͒, are investigated using trapped ion electron diffraction, photoelectron spectroscopy, and density-functional theory. The best agreement to experiment is obtained for endohedral M @Au 16 − structures but with considerable distortions to the parent Au 16 − cage. Fe@ Au 16 − and Co@ Au 16 − are found to have similar structures with C 2 symmetry while a C 1 structure is obtained for Ni@ Au 16 −. The 4s electrons are observed to transfer to the Au 16 cage, whereas atomiclike magnetism due to the unpaired 3d electrons is retained for all the doped clusters.