The catalytic properties of gold nanoparticles are determined by their electronic and geometric structures. We revealed the geometries of several small neutral gold clusters in the gas phase by using vibrational spectroscopy between 47 and 220 wavenumbers. A two-dimensional structure for neutral Au 7 and a pyramidal structure for neutral Au 20 can be unambiguously assigned. The reduction of the symmetry when a corner atom is cut from the tetrahedral Au 20 cluster is directly reflected in the vibrational spectrum of Au 19 .

We present gas-phase infrared spectra for small silicon cluster cations possessing between 6 and 21 atoms. Infrared multiple photon dissociation (IR-MPD) of these clusters complexed with a xenon atom is employed to obtain their vibrational spectra. These vibrational spectra give for the first time experimental data capable of distinguishing the exact internal structures of the silicon cluster cations. By comparing the experimental spectra with theoretical predictions based on density functional theory (DFT), unambiguous structural assignments for most of the Si(n)(+) clusters in this size range have been made. In particular, for Si(8)(+) an edge-capped pentagonal bypriamid structure, hitherto not considered, was assigned. These structural assignments provide direct experimental evidence for a cluster growth motif starting with a pentagonal bipyramid building block and changing to a trigonal prism for larger clusters.

Laser-ablated Cr, Mo, and W atoms react with di-, tri-, and tetrahalomethanes to form XCtMX 3 (M ) Mo, W; X ) H, F, Cl) methylidyne molecules as major products. Dihalomethanes also give a minor yield of CH 2 dMX 2 methylidenes. The electronic state and bonding changes in the CH 2 dCrCl 2 , CH 2 d CrFCl, and CH 2 -CrF 2 methylidene series, but the Mo and W counterparts are calculated to be triplet state CH 2 dMX 2 molecules. Identifications of these new carbon-metal multiple bond species are made through isotopic substitution (D, 13 C) and isotopic frequency calculations using density functional theory. The HCtMX 3 molecules exhibit C-H stretching frequencies in the 3030-3090 cm -1 region and CtM stretching frequencies in the 1007-980 cm -1 range, which vary slightly with the carbon hybridization as determined by the substituents employed here. The XCtMX 3 molecules show very high C-X stretching frequencies in the 1540-1520 cm -1 region for X ) F and 1300-1230 cm -1 for X ) Cl due to strong bonds and the antisymmetric nature of the X-C-M vibrational mode.