Daniel J. Harding

Probing the structures of neutral boron clusters using infrared/vacuum ultraviolet two color ionization: B11, B16, and B17.

The structures of neutral boron clusters, B(11), B(16), and B(17), have been investigated using vibrational spectroscopy and ab initio calculations. Infrared absorption spectra in the wavelength range of 650 to 1550 cm(-1) are obtained for the three neutral boron clusters from the enhancement of their near-threshold ionization efficiency at a fixed UV wavelength of 157 nm (7.87 eV) after resonant absorption of the tunable infrared photons. All three clusters, B(11), B(16), and B(17), are found to possess planar or quasi-planar structures, similar to their corresponding anionic counterparts (B(n) (-)), whose global minima were found previously to be planar, using photoelectron spectroscopy and theoretical calculations. Only minor structural changes are observed between the neutral and the anionic species for these three boron clusters.

Probing the structures of gas-phase rhodium cluster cations by far-infrared spectroscopy

The geometric structures of small cationic rhodium clusters Rh(n)(+) (n = 6-12) are investigated by comparison of experimental far-infrared multiple photon dissociation spectra with spectra calculated using density functional theory. The clusters are found to favor structures based on octahedral and tetrahedral motifs for most of the sizes considered, in contrast to previous theoretical predictions that rhodium clusters should favor cubic motifs. Our findings highlight the need for further development of theoretical and computational methods to treat these high-spin transition metal clusters.

Infrared induced reactivity on the surface of isolated size-selected clusters: dissociation of N2O on rhodium clusters.

Multiple photon infrared excitation of size-selected Rh(6)N(2)O(+) clusters drives surface chemistry resulting in partially oxidized clusters.

Infrared-Induced Reactivity of N2O on Small Gas-Phase Rhodium Clusters

Far- and mid-infrared multiple photon dissociation spectroscopy has been employed to study both the structure and surface reactivity of isolated cationic rhodium clusters with surface-adsorbed nitrous oxide, Rh(n)N(2)O(+) (n = 4-8). Comparison of experimental spectra recorded using the argon atom tagging method with those calculated using density functional theory (DFT) reveals that the nitrous oxide is molecularly bound on the rhodium cluster via the terminal N-atom. Binding is thought to occur exclusively on atop sites with the rhodium clusters adopting close-packed structures. In related, but conceptually different experiments, infrared pumping of the vibrational modes corresponding with the normal modes of the adsorbed N(2)O has been observed to result in the decomposition of the N(2)O moiety and the production of oxide clusters. This cluster surface chemistry is observed for all cluster sizes studied except for n = 5. Plausible N(2)O decomposition mechanisms are given based on DFT calculations using exchange-correlation functionals. Similar experiments pumping the Rh-O stretch in Rh(n)ON(2)O(+) complexes, on which the same chemistry is observed, confirm the thermal nature of this reaction.

Activated methane on small cationic platinum clusters.

The catalytic activation of C H bonds in small hydrocarbons, particularly methane, is a reaction that is of significant technological interest, as it allows valuable, functionalized products to be made from plentiful, cheap feedstocks. However, even on well-characterized platinum surfaces, the determination of the details of methane activation, in particular the earliest steps, remains difficult. Challenges include the weak physisorption of molecular methane on platinum surfaces, its ready dissociation, and the difficulty associated with determining hydrogen atom positions in many surface experiments, as hydrogen atoms are weak scatterers of X-rays or electrons and have no electronic core levels. Despite these challenges, Yoshinobu et al. have used infrared reflection absorption spectroscopy to show that CH4 adsorbed on Pt(111) has at most C3v symmetry. [4] strçm et al. have determined the adsorption geometry of methane on Pt(977) using X-ray absorption spectroscopy, reporting methane to bind by a single hydrogen atom, though they were unable to determine whether it was bound atop or in hollow sites. Partially dehydrogenated reaction intermediates/products, including methyl, methylene, and methylidyne, have been extensively studied (see for example Ref. [6]). The reactions of methane with platinum atoms and clusters have been studied in some detail. In the case of small ionic clusters reacting with CH4 under single collision conditions, Ptn[C,2H] + complexes were found to be the favored products. There have been a number of computational studies of the interactions of platinum clusters and surfaces 14, 15] with methane, primarily using density functional theory (DFT). Such calculations are challenging, owing to the large system size, number of electrons and possible paths, and the fact that several electronic states and crossings between them may need to be treated. Experimental spectroscopic characterization of these species, particularly the reaction intermediates, can therefore provide important information about their structures and benchmark data for theory. Recently, we have demonstrated the possibility of forming reactive intermediate species under thermalized conditions in a flow reactor. This approach was unsuccessful for the Ptn +

Communications: The structure of Rh(8) (+) in the gas phase.

The geometric structure of the Rh(8) (+) cation is investigated using a combination of far-infrared multiple photon dissociation spectroscopy and density functional theory (DFT) calculations. The energetic ordering of the different structural motifs is found to depend sensitively on the choice of pure or hybrid exchange functionals. Comparison of experimental and calculated spectra suggests the cluster to have a close-packed, bicapped octahedral structure, in contrast to recent predictions of a cubic structure for the neutral cluster. Our findings demonstrate the importance of including some exact exchange contributions in the DFT calculations, via hybrid functionals, when applied to rhodium clusters, and cast doubt on the application of pure functionals for late transition metal clusters in general.

Infrared Spectroscopy and Binding Geometries of Oxygen Atoms Bound to Cationic Tantalum Clusters

The binding of isolated oxygen atoms to small tantalum clusters is investigated using vibrational spectroscopy. Infrared spectra of Ta(n)O(0,1,2)(+) (n = 6-11) are reported in the range of 90-1100 cm(-1), comprising both the internal cluster modes and the adatoms vibrations. The vibrational spectra are compared to the results of DFT calculations for n = 6-8, which show that the oxygen atoms bind preferentially as 2-fold bridging adatoms. Addition of one or two O atoms induces only minor distortions of the underlying metal cluster core.

Platinum group metal clusters: from gas-phase structures and reactivities towards model catalysts.

Transition-metal clusters have long been proposed as model systems to study heterogeneous catalysts. In this Concept article we show how advanced spectroscopic techniques can be used to determine the structures of gas-phase transition-metal clusters and their complexes with small molecules. Combined with computational studies, this can help to develop an understanding of the reactivity of these catalytic models.

Structures of Platinum Oxide Clusters in the Gas Phase

The structures of small gas-phase Pt(n)O(2m)(+) (n = 1-6, m = 1, 2) cluster cations have been investigated in a combined infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) study. On the basis of the infrared spectra obtained, it is concluded that in most clusters oxygen is bound dissociatively, preferring 2-fold bridge binding motifs, sometimes combined with singly coordinated terminal binding. Comparison of the oxide cluster structures with those of bare cationic platinum clusters reported previously reveals major structural changes induced in the platinum core upon oxygen binding. For some cluster sizes the presence of the Ar messenger atom(s) is found to induce a significant change in the observed cluster structure.

Oxides of small Rhodium clusters: Theoretical investigation of experimental reactivities

Density functional theory is used to investigate the structures of cationic rhodium cluster oxides, Rh(6)O(m) (+) (m=1,4). On the monoxide and dioxide, the oxygen atoms occupy bridge sites, while on trioxide and tetroxide clusters, high-coordination sites are favored. A range of spin multiplicities are investigated for each cluster, with high spin multiplicities found to be less favored for the oxides compared with the naked metal clusters. The dissociation of nitric oxide on low-energy isomers of Rh(6)O(4) (+) is investigated and found to be unfavorable compared to molecular adsorption due to a combination of thermodynamic and kinetic factors. These calculations are consistent with, and help to account for, the experimentally observed reactivity of rhodium and rhodium oxide clusters with nitric oxide [M. S. Ford et al., Phys. Chem. Chem. Phys. 7, 975 (2005)].