Delocalized electronic states in small clusters. Comparison of Nan, Cun, Agn, and Aun clusters

Structure and energetics of small gold nanoclusters and their positive ions

We performed density functional theory calculations at the PW91PW91/LANL2DZ, PW91PW91/Stuttgart 1997, PW91PW91/CRENBL, B3LYP/LANL2DZ, and SVWN5/LANL2DZ levels of theory to attain the minimum-energy structures of neutral and cationic gold clusters of up to nine gold atoms. We locate the 2D-to-3D (two-dimensional to three-dimensional) transition in cationic clusters as occurring between Au(8) (+) and Au(9) (+). We also demonstrate that we can correlate the 2D-to-3D transition in cationic clusters with a linear extrapolation of the energy differences of the lowest-lying 2D and 3D structures of cluster sizes below the transition. We then use the same approach to predict that the 2D-to-3D transition occurs in neutral clusters at Au(11); this is confirmed by locating 3D Au(11) structures that are lower in energy than the best 2D structures reported to date. We examine the effects of choice of basis sets and exchange-correlation functionals on the relative stabilities and other properties of the calculated structures. Finally we find that there is good agreement between calculated and experimental data for clusters with up to six constituent atoms. For clusters with more than seven atoms, there are significant differences observed between the calculated and experimental properties using SVWN5/LANL2DZ, but there is still good agreement for the other levels of theory used.

Anomalous Behavior of Atomic Hydrogen Interacting with Gold Clusters

The change in the electronic structure of Au(n)- clusters induced by the exchange of an Au atom by hydrogen is studied using photoelectron spectroscopy. Au anion clusters react with one hydrogen atom but not with molecular hydrogen. The spectra of Au(n)- and Au(n-1)H- clusters show almost identical features for n > 2 suggesting that hydrogen behaves as a protonated species by contributing one electron to the valence pool of the Au(n)- cluster. This behavior is in sharp contrast to that of the commonly understood electronic structure of hydrogen in metals; namely, it attracts an electron from the conduction band of the metal and remains in an "anionic" form or forms covalent bonding. We discuss the influence of the unique electronic structure of H on the unusual catalytic behavior of Au clusters.

REACTIONS OFTRANSITIONMETALCLUSTERS WITHSMALLMOLECULES

Atoms and small molecules react with transition metal clusters in ways that are analogous to the physisorption and chemisorption reactions observed on the corresponding extended metal surface. However, often underlying these similarities are size-dependent variations in the reaction mechanisms and rates, the interpretation of which requires a detailed understanding of the structures of both the bare metal cluster substrates and the cluster-molecule complexes. Although polyatomic transition metal clusters cannot be characterized by the traditional methods of molecular spectroscopy, the combination of other physical and chemical probes can provide qualitative and semiquantitative structural information. These techniques, when combined with equilibrium geometries calculated using ab initio or semiempirical methods, provide a detailed picture of the structural origin of metal cluster reactivity and its variation with size.