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.

We present an experimental and theoretical study of the structure of small, neutral gold clusters-Au 3 , Au 4 and Au 7 -'tagged' by krypton atoms. Infrared (IR) spectra of Au N · Kr M complexes formed at 100 K are obtained via far-IR multiple photon dissociation in a molecular beam. The theoretical study is based on a statistical (canonical) sampling of the Au N · Kr M complexes through ab initio molecular dynamics using density-functional theory in the generalized gradient approximation, explicitly corrected for long-range van-der-Waals (vdW) interactions. The choice of the functional is validated against higherlevel first-principle methods. Thereby finite-temperature theoretical vibrational spectra are obtained that are compared with the experimental spectra. This enables us to identify which structures are present in the experimental molecular 5 Only the structures of neutral Au 7 , Au 19 and Au 20 have been recently derived from farinfrared (IR) multiple photon dissociation (FIR-MPD) spectra of their complexes with krypton atoms and comparison to theoretical predictions [19]. However, this study posed the question of the influence of the Kr messenger on the spectra and the type of the interaction between Kr and the neutral Au clusters. It has been recognized before that even 'inert' rare gas (RG) atoms may influence the IR spectra of metal clusters and their explicit consideration can improve the agreement between experimental and predicted spectra [20,21]. Whereas this was somehow unexpected for neutral clusters, where RG atoms are considered to only physisorb, binding of RG atoms to cationic clusters, especially of late transition metals, is considerably stronger. For instance, it has been previously shown for cationic cobalt clusters [22] how the presence of Ar ligands significantly modifies the vibrational spectrum of the cluster. Similar effects of RG binding, leading even to changes in the energetic ordering of isomers, have been seen, e.g. for cationic vanadium or cerium oxide clusters [23,24].So far, in most cases the experimental FIR spectra of metal clusters are interpreted only by comparison to calculated harmonic spectra (at T = 0 K). However, experiments are performed at finite temperature and even at lowest temperatures anharmonic effects can have a noticeable influence on the vibrational spectra. Nevertheless, a theoretical investigation of Au 7 and Au 7 · Kr motivated by our previous study [19] applying a vibrational configurationinteraction approach did not identify significant anharmonicities for this particular cluster [25] and instead supported the initial conclusion that the Kr binding does not significantly change the vibrational frequencies, 'but has an effect on the IR intensities, which become very similar to those in the experimental spectrum' [19].In this paper, we thoroughly investigate if this indeed holds, by studying how RG atoms bind to small, neutral gold clusters, and how this binding influences the vibrational spectra at T = 0 K as well as at finite temperatures. We report F...

The geometry of cationic silicon clusters doped with vanadium, Si n V þ (n ¼ 12-16), is investigated by using infrared multiple photon dissociation of the corresponding rare gas complexes in combination with ab initio calculations. It is shown that the clusters are endohedral cages, and evidence is provided that Si 16 V þ is a fluxional system with a symmetric Frank-Kasper geometry. DOI: 10.1103/PhysRevLett.107.173401 PACS numbers: 36.40.Mr, 33.20.Ea, 61.46.Bc The ongoing trend towards further miniaturization in microelectronics triggers a quest for nanostructured building blocks. Given the importance of silicon in the semiconductor industry, it is straightforward to consider small silicon particles for nanostructuring. The search for silicon building blocks was initiated in the 1990s and revealed strong size dependencies and cluster geometries that do not correspond to bulk fragments [1]. Unfortunately, elemental silicon clusters have dangling bonds, which renders them chemically reactive and therefore not suitable as nanoscale building blocks [2]. In contrast to carbon, silicon prefers the formation of bonds through sp 3 hybridization, resulting in three-dimensional structures [3]. Ion mobility studies demonstrated that silicon structures follow a prolate growth sequence and no fullerenelike caged particles are formed [4,5]. However, incorporating a metal atom or hydrogenation can saturate the dangling bonds and induce the formation of caged silicon clusters [3,6,7].Theoretical investigations of doped silicon clusters have considered dopants from almost every group of the periodic table [6]. Most interestingly, it is predicted that transition-metal atoms possibly stabilize the clusters and induce the formation of symmetric endohedral cages with the dopant atom at the center of the cage, which is of relevance for novel silicon based nanostructured devices [8][9][10][11][12]. For example, fullerenelike cages and Frank-Kasper (FK) polyhedrons, which are tetrahedrally close-packed structures containing interpenetrating polyhedra with coordination numbers 12, 14, 15, or 16 [13], are predicted for Ti and V doped silicon clusters with at least 12 Si atoms [10,14,15]. However, if there are insufficient Si atoms to fully enclose the dopant atom, basketlike structures are formed [11]. Although mass spectrometry and photodissociation experiments [8,[16][17][18][19] show an enhanced stability of specific transition-metal doped silicon clusters, no single experiment has yet provided detailed information on their structure. Up to now, mainly indirect evidence is found for the formation of symmetric species by photoelectron and x-ray spectroscopy studies [20][21][22] and chemical probe methods [20,23].In this Letter, the structure of size selected endohedrally doped silicon clusters is obtained by combining experimental infrared multiple photon dissociation (IR-MPD) spectroscopy with quantum chemical calculations. It will be shown that the V dopant atom in the cationic Si n V þ (n ¼ 12-16) clusters locates at the center ...

Vibrational spectra of neutral silicon clusters Si(n), in the size range of n = 6-10 and for n = 15, have been measured in the gas phase by two fundamentally different IR spectroscopic methods. Silicon clusters composed of 8, 9, and 15 atoms have been studied by IR multiple photon dissociation spectroscopy of a cluster-xenon complex, while clusters containing 6, 7, 9, and 10 atoms have been studied by a tunable IR-UV two-color ionization scheme. Comparison of both methods is possible for the Si(9) cluster. By using density functional theory, an identification of the experimentally observed neutral cluster structures is possible, and the effect of charge on the structure of neutrals and cations, which have been previously studied via IR multiple photon dissociation, can be investigated. Whereas the structures of small clusters are based on bipyramidal motifs, a trigonal prism as central unit is found in larger clusters. Bond weakening due to the loss of an electron leads to a major structural change between neutral and cationic Si(8).

Uranium atoms activate methane upon ultraviolet excitation to form the methyl uranium hydride CH3-UH, which undergoes alpha-H transfer to produce uranium methylidene dihydride, CH2=UH2. This rearrangement most likely occurs on an excited-quintet potential-energy surface and is followed by relaxation in the argon matrix. These simple U+CH4 reaction products are identified through isotopic substitution (13CH4, CD4, CH2D2) and density functional theory frequency and structure calculations for the strong U-H stretching modes. Relativistic multiconfiguration (CASSCF/CASPT2) calculations substantiate the agostic distorted C1 ground-state structure for the triplet CH2=UH2 molecule. We find that uranium atoms are less reactive in methane activation than thorium atoms. Our calculations show that the CH2=UH2 complex is distorted more than CH2=ThH2. A favorable interaction between the low energy open-shell U(5f) sigma orbital and the agostic hydrogen contributes to the distortion in the uranium methylidene complexes.

Tunable far-infrared-vacuum-ultraviolet two color ionization is used to obtain vibrational spectra of neutral silicon clusters in the gas phase. Upon excitation with tunable infrared light prior to irradiation with UV photons we observe strong enhancements in the mass spectrometric signal of specific cluster sizes. This allowed the recording of the infrared absorption spectra of Si6, Si7, and Si10. Structural assignments were made by comparison with calculated linear absorption spectra from quantum chemical theory.