A combined experimental and theoretical study of small gold cluster anions is performed. The experimental effort consists of ion mobility measurements that lead to the assignment of the collision cross sections for the different cluster sizes at room temperature. The theoretical study is based on ab initio molecular dynamics calculations with the goal to find energetically favorable candidate structures. By comparison of the theoretical results with the measured collision cross sections as well as vertical detachment energies ͑VDEs͒ from the literature, we assign structures for the small Au n Ϫ ions (nϽ13) and locate the transition from planar to three-dimensional structures. While a unique assignment based on the observed VDEs alone is generally not possible, the collision cross sections provide a direct and rather sensitive measure of the cluster structure. In contrast to what was expected from other metal clusters and previous theoretical studies, the structural transition occurs at an unusually large cluster size of twelve atoms.

We have performed ion mobility measurements on gold cluster cations Aun+ generated by pulsed laser vaporization. For clusters with n<14, experimental cross sections are compared with theoretical results from density functional calculations. This comparison allows structural assignment. We find that room temperature gold cluster cations have planar structures for n=3–7. Starting at n=8 they form three dimensional structures with (slightly distorted) fragments of the bulk phase structure being observed for n=8–10.

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...

No abstract

No abstract

Binding energies and entropies have been measured for the attachment of up to six H2 ligands to ground-state Cu+ ions (1S, Ar3d10), to electronically excited Cu+* ions (1,3D, Ar4s13d9), and to hydrated H2O·Cu+ ions. The ground-state Cu+ ion added four H2 ligands in the first solvation shell with bond dissociation energies (BDEs) of 15.4, 16.7, 8.8, and 5.1 kcal/mol. The fifth and sixth ligands begin a new solvation sphere and were very weakly bound. The BDEs for addition of H2 to electronically excited Cu+* were also small (4.2, 2.5, and 1.4 kcal/mol for the first three ligands). The difference between ground- and excited-state association energies is almost entirely due to the repulsive nature of the 4s electron. Hydration of the Cu+ ion significantly increased the BDE of the first H2 ligand (to 19.6 kcal/mol) but greatly reduced that of the second (to 3.8 kcal/mol). Theoretical calculations with large basis sets at the DFT-B3LYP and MP2 levels were done on all species both to determine geometries and vibration frequencies and to examine the origin of the bonding and its variation with Cu+ coordination. The calculations show that covalent interactions are important in these Cu+ clusters and that the observed changes in BDE as different ligands are added are due to electronic rather than steric effects. These sources of bonding are discussed, and comparisons are made to the Co+(H2) n and Ni+(H2) n systems. It is also shown that the M+−ligand interactions are similar for H2 and CO ligands. Special attention is paid to the origin of the highly symmetric D 3 h planar structure found in Ni+(H2)3, Cu+(H2)3, and Cu+(CO)3.

V n (C 6 H 6 ) m + cluster ions were produced by laser vaporization/ionization of a vanadium rod into a mixture of helium and benzene. The clusters with m ) n + 1 show strongly enhanced intensities. Ion mobility measurements and collision-induced dissociation experiments were used to determine the structure of the different species. Density functional theory calculations using the B-LYP parametrization were performed to provide candidate structures and energetics of these clusters. Comparison of experimental and theoretical model cross sections indicated these clusters have sandwich structures with an alternation of vanadium atoms and benzene molecules. We also found that mass-selected V n + clusters (n < 5) react with benzene, sequentially adding benzene neutrals to form terminal ions of formula V n (C 6 H 6 ) n+1 + . Mobility measurements on these ions strongly suggest they also have the sandwich structure, indicating major structural reorganization occurs when benzene interacts with the vanadium cluster.

Ligand-free metal clusters can be prepared over a wide size range, but only in comparatively small amounts. Determining their size-dependent properties has therefore required the development of experimental methods that allow characterization of sample sizes comprising only a few thousand mass-selected particles under well-defined collisionfree conditions. In this review, we describe the application of these methods to the geometric structural determination of Au + n and Au − n with n = 3-20. Geometries were assigned by comparing experimental data, primarily from ion-mobility spectrometry and trapped ion electron diffraction, to structural models from quantum chemical calculations.