Experiments on mass-selected boron clusters date back more than 20 years.[1] However, only recently have experimental and theoretical methods advanced enough to allow for structural assignments over a wide range of cluster sizes. To date, most is known about boron cluster anions as a result of the pioneering work of Wang and co-workers, who used a combination of photoelectron spectroscopy and quantum chemistry to determine structures for clusters with up to 20 atoms. [2,3] Throughout this size range, B x À ions appear to have planar, sheet-like structures comprising webs of triangles and occasionally squares. These two-dimensional structures are often bowed and sometimes partly corrugated. This distortion is likely due to strain arising from shorter bond distances at the periphery, where atoms form stronger bonds because they have fewer bond partners. In a recent study it has been suggested that whereas the B 20 À ion forms the planar isomer in experiment, there is an energetically very close-lying regularcylindrical isomer comprising two stacked ten-atom rings. [4] The recent report of the preparation and electron-microscopic characterization of 3-nm-diameter single-walled boron nanotubes [5] has led to speculations that boron clusters may assume cylindrical structures beyond a critical size. We have explored this question further by structurally probing boron cluster cations using a combination of collision cross section measurements and density functional theory (DFT) calculations.The theoretical determination of low-energy boron cluster structures faces various problems, as is apparent from studies like those reviewed in reference [2] (e.g. B 13+ [6][7][8][9] and B 20 [4,10] ). Electronic structures often show multiple-reference character, which makes reliable calculations expensive or virtually impossible for systems larger than B 20 . Even more problematic are the unusual and unexpected features of geometries, which diminish the hope of locating structures of interest by experienced guesses alone. Thus, a computational procedure is needed that is unbiased, efficient, and tolerant of multiplereference cases. As a compromise of these requirements, we have chosen the following strategy. A first set of structures for the neutral clusters was obtained with a genetic algorithm. [11,23] This procedure required on the order of 100 generations resulting in 1000 to 2000 geometry optimizations for each B n cluster. As this approach necessitates a low-cost procedure, we chose the DFT with the BP86 functional, which has been shown to yield reliable structure constants.[12] The relatively small def2-SVP [13] orbital and auxiliary bases were considered sufficient for this purpose.The genetic algorithm converged rapidly for small test cases like B 6 or B 12 , that is, 20 to 40 generations sufficed for convergence. Trial runs for larger clusters (B 16 , B 20 , B 24 ) failed to find some of the low-energy structures even after 80 generations. To speed up convergence, we seeded the initial population with optimized structures...

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.

We employ a combination of ion mobility measurements and an unbiased systematic structure search with density functional theory methods to study structure and energetics of gas phase tin cluster cations, Sn(n)(+), in the range of n = 3-15. For Sn(13)(+) we also carry out trapped ion electron diffraction measurements to ascertain the results obtained by the other procedures. The structures for the smaller systems are most easily described by idealized point group symmetries, although they are all Jahn-Teller distorted: D(3h) (trigonal bipyramid), D(4h) (octahedron), D(5h) (pentagonal bipyramid) for n = 5, 6, and 7. For the larger systems we find capped D(5h) for Sn(8)(+) and Sn(9)(+), D(3h) (tricapped trigonal prism) and D(4d) (bicapped squared antiprism) plus adatoms for n = 10, 11, 14, and 15. A centered icosahedron with a peripheral atom removed is the dominant motif in Sn(12)(+). For Sn(13)(+) the calculations predict a family of virtually isoenergetic isomers, an icosahedron and slightly distorted icosahedra, which are about 0.25 eV below two C(1) structures. The experiments indicate the presence of two structures, one from the I(h) family and a prolate C(1) isomer based on fused deltahedral moieties.

Experimente an massenselektierten Borclustern werden seit mehr als 20 Jahren durchgeführt; [1] erst kürzlich wurde es aber dank der Weiterentwicklung der experimentellen und theoretischen Methoden möglich, Strukturen von Clustern eines weiten Größenbereichs zu bestimmen. Die meisten Eigenschaften von Bor-Anionen wurden von Wang et al. [2,3] Für die endgültige Behandlung wurde die letzte Population des genetischen Algorithmus erneut geometrieoptimiert, allerdings mit der größeren def2-TZVPP [13] -Basis, dem besseren TPSS-Funktional [14] und der gewünschten Ladung. Alle Rechnungen erfolgten mit dem TURBOMOLE-Programmpaket, und die Nicht-Hybrid-Dichtefunktionalrechnung wurde mit dem RI-DFT-Modul durchgeführt. [15,24] Rechnungen liefern im Allgemeinen Strukturparameter zuverlässiger als Energien, für die wir einen Fehler von einigen Zehnteln eV erwarten. Um die vorgeschlagenen Kandidatenstrukturen zu bestätigen oder auszuschließen, vergleichen wir ihre Stoßquerschnitte mit den auf Ionenbeweglichkeitsmessungen basierenden experimentellen Werten. [16,17] Der experimentelle Aufbau zur Bestimmung des Stoßquer-schnitts wird in Lit.[18] detailliert beschrieben. Die Borcluster-Kationen werden mit einer Laserverdampfungsquelle erzeugt, anschließend massenselektiert, in eine mit 7 mbar Helium gefüllte Driftzelle injiziert und mithilfe eines elektrischen Feldes durch die Zelle geleitet. Ionen, die die Zelle

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