We use density functional theory (DFT) to investigate the bonding of propene to small gas-phase gold clusters and to a Au(111) surface. The desorption energy trends and the geometry of the binding sites are consistent with the following set of rules. (1) The bond of propene to gold is formed by donation of electron density from the highest occupied molecular orbital (HOMO) of propene to one of the low-lying empty orbitals [denoted by LUMO1, LUMO2, em leader (LUMO-lowest unoccupied molecular orbital)] of the gold cluster. (2) Propene binds to a site on the Au cluster where one of the low-lying LUMOs protrudes in the vacuum. Different isomers (same cluster, but different binding sites for propene) correspond to sites where different low-lying LUMOs protrude in space. (3) The desorption energy of the lowest energy isomer correlates with the energy of the lowest empty orbital of the cluster; the lower the energy of that LUMO, the higher the desorption energy. (4) If the lowest-lying LUMO protrudes into space at two nonequivalent sites at the edge of a cluster, propene binds more strongly to the site with the lowest coordination. These rules are consistent with the calculated bond energies and geometries for [Au(n)(C(3)H(6))](q), for n=1-5 and n=8 and q=-1, 0, +1. Based on them we have made a number of predictions that have been confirmed by DFT calculations. The bond of propene to gold is strengthened as the net charge of the cluster varies from -1, to zero, to +1. Compared to a gas-phase cluster, a cluster on a support binds propene more strongly if the support takes electron density from the cluster (e.g., a Au cluster on a gold surface) and more weakly if the support donates electron density to the cluster (e.g., a Au cluster on an oxygen vacancy on an oxide surface).

We use density functional theory to investigate the binding of propene to small mixed Au-Ag clusters, in the gas phase. We have found that the rules proposed by us for propene binding to Au and Ag clusters, also work for binding to mixed Au-Ag clusters. The rules state that propene binds to those sites on the edge of the cluster where the equal density plots of the LUMO of the naked cluster protrude into the vacuum. Furthermore, the desorption energy of propene correlate with the LUMO energy: the lower the LUMO energy, the stronger the propene bond. We have also found an additional rule that is specific to mixed clusters. We call active the atoms on which the LUMO of the naked cluster protrude in the vacuum, and inactive those for which such protrusions do not exist. To define the rules we use the following notation: A is an active site to which propene is bound B is another active site, and C is an inactive site. If the atom in C (Ag or Au) is replaced with another atom (Au or Ag) propene desorption energy changes very little. If we replace the atom B with a more electronegative atom (i.e., we replace Ag by Au) the propene bond to A becomes stronger. If we replace the atom B with a less electronegative atom (i.e., we replace Au by Ag) the propene bond to A becomes weaker. © 2004 American Institute of Physics.

Several levels of theory, including both Gaussian-based and plane wave density functional theory (DFT), second-order perturbation theory (MP2), and coupled cluster methods (CCSD(T)), are employed to study Au6 and Au8 clusters. All methods predict that the lowest energy isomer of Au6 is planar. For Au8, both DFT methods predict that the two lowest isomers are planar. In contrast, both MP2 and CCSD(T) predict the lowest Au8 isomers to be nonplanar.