André Fielicke

Structures of neutral Au7, Au19, and Au20 clusters in the gas phase.

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 Au7 and a pyramidal structure for neutral Au20 can be unambiguously assigned. The reduction of the symmetry when a corner atom is cut from the tetrahedral Au20 cluster is directly reflected in the vibrational spectrum of Au19.

Size and charge effects on the binding of CO to late transition metal clusters

We report on the size and charge dependence of the C-O stretching frequency, nu(CO), in complexes of CO with gas phase anionic, neutral, and cationic cobalt clusters (Co(n)CO(-0+)), anionic, neutral, and cationic rhodium clusters (Rh(n)CO(-0+)), and cationic nickel clusters (Ni(n)CO(+)) for n up to 37. We develop models, based on the established vibrational spectroscopy of organometallic carbonyl compounds, to understand how cluster size and charge relate to nu(CO) in these complexes. The dominating factor is the available electron density for backdonation from the metal to the CO pi* orbital. Electrostatic effects play a significant but minor role. For the charged clusters, the size trends are related to the dilution of the charge density at the binding site on the cluster as n increases. At large n, nu(CO) approaches asymptotes that are not the same as found for nu(CO) on the single crystal metal surfaces, reflecting differences between binding sites on medium sized clusters and the more highly coordinated metal surface sites.

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.

Structures of silicon cluster cations in the gas phase.

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.

The adsorption of CO on group 10 (Ni, Pd, Pt) transition-metal clusters

The adsorption of a single CO molecule on clusters of the Group 10 transition metals is characterized by infrared multiple photon dissociation (IR-MPD) spectroscopy. The cationic, neutral, and anionic carbonyl complexes contain between 3 and up to 25 metal atoms. The C-O stretching frequency nu(CO) shows that while both nickel and platinum clusters adsorb CO only in atop positions, palladium clusters exhibit a variety of binding sites. These findings can be rationalized by considering the increasing role relativistic effects play in the electronic structure of the cluster complexes going down the group. Conclusions for the cluster-support interactions for size-selected supported particles are drawn from the charge dependence of nu(CO) for the gas-phase species.

Vibrational spectroscopy of neutral silicon clusters via far-IR-VUV two color ionization

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 Si(6), Si(7), and Si(10). Structural assignments were made by comparison with calculated linear absorption spectra from quantum chemical theory.

Structure determination of neutral MgO clusters—hexagonal nanotubes and cages

Structural information for neutral magnesium oxide clusters has been obtained by a comparison of their experimental vibrational spectra with predictions from theory. (MgO)(n) clusters with n = 3-16 have been studied in the gas phase with a tunable IR-UV two-color ionization scheme and size-selective infrared spectra have been measured. These IR spectra are compared to the calculated spectra of the global minimum structures predicted by a hybrid ab initio genetic algorithm. The comparison shows clear evidence that clusters of the composition (MgO)(3k) (k = 1-5) form hexagonal tubes, which confirm previous theoretical predictions. For the intermediate sizes (n≠ 3k) cage-like structures containing hexagonal (MgO)(3) rings are identified. Except for the cubic (MgO)(4) no evidence for bulk like structures is found.

Structural diversity and flexibility of mgo gas-phase clusters

Magnesium oxide (MgO) is a prototype material of (simple) metal oxides. The NaCl-type structure of bulk MgO is the only phase observed in experiments up to a pressure of 227 GPa. 2] This indicates that MgO has an inherent structural stability, which can be expected to persist when passing from the bulk solid to molecular clusters. Indeed, mass spectra of (MgO)n + and (MgO)nMg + cluster ions along with calculations using rigid ion and polarizable ion shell model potentials indicate compact cubic structures similar to fragments of the MgO crystal lattice, with the most abundant clusters based on a (MgO)3 subunit. [4] The spectra and cluster compositions observed in IR resonance-enhanced multiphoton ionization experiments on large neutral (MgO)n clusters (n 15) have also given indications for cubic structures. Up to now, computational studies have almost exclusively investigated neutral MgO clusters, without direct comparison to experiment, 6–8, 10–17] despite the fact that most experiments were performed on cationic clusters. The main conclusion from these studies has been that the most stable structures for a given value of n are cubelike, except for (MgO)3n clusters, for which rings and stacks of rings are preferred. The geometric structures of the cationic MgO clusters have been assumed to be the same as for neutral ones (vertical ionization approximation), except for small hypermagnesium ions. So far, no systematic theoretical studies of the stoichiometric cationic clusters have been reported. Herein we demonstrate that, in contrast to the bulk material, neutral and cationic gas-phase clusters of MgO display unusual structural diversity and flexibility. Not only are the structures of the clusters in most cases noncubic, but the neutral and charged ones also differ. The atomic structures of cationic stoichiometric (MgO)n + (n = 2–7) clusters were determined by combining quantum chemical calculations with infrared multiple photon dissociation (IRMPD) experiments. In particular, global structure optimizations using density functional theory (DFT) have been performed on all the cluster sizes. Although several of the geometric structures reported here (but not all of them) have been found before with neutral 7, 10–17] and anionic clusters by different computational techniques, our calculations reveal unequivocally the global minima of all these configurations. Cationic clusters and their weakly bound complexes with Ar and O2 have been investigated experimentally in a molecular beam. Changes in this cluster distribution induced by the interaction with tunable infrared radiation were used to obtain the cluster-size-specific IR-MPD spectra. Figure 1 shows the global minimum structures of neutral (MgO)n and cationic (MgO)n + clusters with n = 2–7; for other low-energy isomers see Figures 1S and 2S in the Supporting Information. Figures 2 and 3 show a comparison between the experimental IR-MPD spectra and the calculated linear IR

Activation of molecular oxygen by anionic gold clusters

A golden opportunity: Molecular oxygen is found to be converted into a superoxo (O2−) species upon complexation to gold-cluster anions containing an even number of Au atoms. Vibrational spectra (see scheme) reveal small variations in the extent of O[BOND]O bond activation dependent upon the electron affinity of the parent cluster.

Gas-phase structures of neutral silicon clusters

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).