Time-resolved photodissociation and threshold collision-induced dissociation of anionic gold clusters

Probing the binding and activation of small molecules by gas-phase transition metal clusters via IR spectroscopy

Isolated transition metal clusters have been established as useful models for extended metal surfaces or deposited metal particles, to improve the understanding of their surface chemistry and of catalytic reactions. For this objective, an important milestone has been the development of experimental methods for the size-specific structural characterization of clusters and cluster complexes in the gas phase. This review focusses on the characterization of molecular ligands, their binding and activation by small transition metal clusters, using cluster-size specific infrared action spectroscopy. A comprehensive overview and a critical discussion of the experimental data available to date is provided, reaching from the initial results obtained using line-tuneable CO₂ lasers to present-day studies applying infrared free electron lasers as well as other intense and broadly tuneable IR laser sources.

Structures of Nitrogen Oxides Attached to Anionic Gold Clusters Au4- Revealed by Infrared Multiple Photon Dissociation Spectroscopy

The adsorption forms of NO and NO₂ on anionic Au₄^- clusters were investigated by a combination of IR multiple photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) calculations. For all three species investigated (Au₄NO^-, Au₄N₂O₂^-, and Au₄NO₂^-), the spectra were found to be consistent with a Y-shaped Au₄^- cluster with triangular Au₃ and one Au atom sticking out, on which NO and NO₂ molecules adsorb molecularly. These species are considered as intermediates of the Au₄^--mediated disproportionation reaction of NO, Au₄(NO)₃^- → Au₄(NO₂)(N₂O)^-. We discuss the reaction path on the basis of the found geometries and energies and conclude that the disproportionation reaction of NO can occur catalytically on the Au₄^- cluster.

Dual-polarity SALDI FT-ICR MS imaging and Kendrick mass defect data filtering for lipid analysis

Lipids are biomolecules of crucial importance involved in critical biological functions. Yet, lipid content determination using mass spectrometry is still challenging due to their rich structural diversity. Preferential ionisation of the different lipid species in the positive or negative polarity is common, especially when using soft ionisation mass spectrometry techniques. Here, we demonstrate the potency of a dual-polarity approach using surface-assisted laser desorption/ionisation coupled to Fourier transform-ion cyclotron resonance (SALDI FT-ICR) mass spectrometry imaging (MSI) combined with Kendrick mass defect data filtering to (i) identify the lipids detected in both polarities from the same tissue section and (ii) show the complementarity of the dual-polarity data, both regarding the lipid coverage and the spatial distributions of the various lipids. For this purpose, we imaged the same mouse brain section in the positive and negative ionisation modes, on alternate pixels, in a SALDI FT-ICR MS imaging approach using gold nanoparticles (AuNPs) as dual-polarity nanosubstrates. Our study demonstrates, for the first time, the feasibility of (i) a dual-polarity SALDI-MSI approach on the same tissue section, (ii) using AuNPs as nanosubstrates combined with a FT-ICR mass analyser and (iii) the Kendrick mass defect data filtering applied to SALDI-MSI data. In particular, we show the complementarity in the lipids detected both in a given ionisation mode and in the two different ionisation modes. Graphical abstract.

The structural, electronic and catalytic properties of Aun (n = 1–4) nanoclusters on monolayer MoS2

The structural stability, electronic and catalytic properties of Au_n (n = 1–4) nanoclusters supported on monolayer MoS₂ have been investigated based on first principle DFT calculation with van der Waals (vdW) corrections.

Determining the size-dependent structure of ligand-free gold-cluster ions

Ligand-free metal clusters can be prepared over a wide size range, but only in comparatively small amounts. Determining their size-dependent properties has therefore required the development of experimental methods that allow characterization of sample sizes comprising only a few thousand mass-selected particles under well-defined collision-free conditions. In this review, we describe the application of these methods to the geometric structural determination of Au(n)(+) and Au(n)(-) with n = 3-20. Geometries were assigned by comparing experimental data, primarily from ion-mobility spectrometry and trapped ion electron diffraction, to structural models from quantum chemical calculations.

Reactions of mixed silver-gold cluster cations AgmAun+ (m+n=4,5,6) with CO: Radiative association kinetics and density functional theory computations

Near thermal energy reactive collisions of small mixed metal cluster cations Ag(m)Au(n) (+) (m+n=4, 5, and 6) with carbon monoxide have been studied in the room temperature Penning trap of a Fourier transform ion-cyclotron-resonance mass spectrometer as a function of cluster size and composition. The tetrameric species AgAu(3) (+) and Ag(2)Au(2) (+) are found to react dissociatively by way of Au or Ag atom loss, respectively, to form the cluster carbonyl AgAu(2)CO(+). In contrast, measurements on a selection of pentamers and hexamers show that CO is added with absolute rate constants that decrease with increasing silver content. Experimentally determined absolute rate constants for CO adsorption were analyzed using the radiative association kinetics model to obtain cluster cation-CO binding energies ranging from 0.77 to 1.09 eV. High-level ab initio density functional theory (DFT) computations identifying the lowest-energy cluster isomers and the respective CO adsorption energies are in good agreement with the experimental findings clearly showing that CO binds in a "head-on" fashion to a gold atom in the mixed clusters. DFT exploration of reaction pathways in the case of Ag(2)Au(2) (+) suggests that exoergicities are high enough to access the minimum energy products for all reactive clusters probed.

Threshold collision-induced dissociation of diatomic molecules: A case study of the energetics and dynamics of O2− collisions with Ar and Xe

The energetics and dynamics of collision-induced dissociation of O2- with Ar and Xe targets are studied experimentally using guided ion-beam tandem mass spectrometry. The cross sections and the collision dynamics are modeled theoretically by classical trajectory calculations. Experimental apparent threshold energies are 2.1 and 1.1 eV in excess of the thermochemical O2- bond dissociation energy for argon and xenon, respectively. Classical trajectory calculations confirm the observed threshold behavior and the dependence of cross sections on the relative kinetic energy. Representative trajectories reveal that the bond dissociation takes place on a short time scale of about 50 fs in strong direct collisions. Collision-induced dissociation is found to be remarkably restricted to the perpendicular approach of ArXe to the molecular axis of O2-, while collinear collisions do not result in dissociation. The higher collisional energy-transfer efficiency of xenon compared with argon is attributed to both mass and polarizability effects.

Structure and energetics of small gold nanoclusters and their positive ions

We performed density functional theory calculations at the PW91PW91/LANL2DZ, PW91PW91/Stuttgart 1997, PW91PW91/CRENBL, B3LYP/LANL2DZ, and SVWN5/LANL2DZ levels of theory to attain the minimum-energy structures of neutral and cationic gold clusters of up to nine gold atoms. We locate the 2D-to-3D (two-dimensional to three-dimensional) transition in cationic clusters as occurring between Au(8) (+) and Au(9) (+). We also demonstrate that we can correlate the 2D-to-3D transition in cationic clusters with a linear extrapolation of the energy differences of the lowest-lying 2D and 3D structures of cluster sizes below the transition. We then use the same approach to predict that the 2D-to-3D transition occurs in neutral clusters at Au(11); this is confirmed by locating 3D Au(11) structures that are lower in energy than the best 2D structures reported to date. We examine the effects of choice of basis sets and exchange-correlation functionals on the relative stabilities and other properties of the calculated structures. Finally we find that there is good agreement between calculated and experimental data for clusters with up to six constituent atoms. For clusters with more than seven atoms, there are significant differences observed between the calculated and experimental properties using SVWN5/LANL2DZ, but there is still good agreement for the other levels of theory used.

Structure and energetics of two- and three-dimensional neutral, cationic, and anionic gold clusters Au5⩽n⩽9Z (Z=0,±1)

Low-energy structures are found on the potential energy surfaces of the neutral, cationic, and anionic gold clusters Au(5< or = n < or =9)Z (Z=0,+/-1) and on the neutral potential energy surface of Au(9). These structures provide insights on the two to three dimensional (2D-->3D) transition in small neutral and charged gold clusters. It is demonstrated that the size threshold for the 2D-3D coexistence is lower for cationic than neutral gold clusters: the 2D-3D coexistence develops for Au(5) (+) and Au(7) (+) on the cationic potential energy surfaces while only for Au(9) on the neutral. Two metastable long-lived dianions of gold clusters are also reported.

Structures of mixed gold-silver cluster cations (AgmAun+,m+n<6): Ion mobility measurements and density-functional calculations

The collision cross sections of Ag(m)Au(n)+ (m+n)<6 cluster ions were determined. For bimetallic clusters, we observe a significant intracluster charge transfer leaving most of the ions positive charge on the silver atoms. The mixed trimeric ions Ag2Au+ and AgAu2+ are triangular like the pure gold and silver trimers. Most of the tetrameric clusters are rhombus shaped, with the exception of Ag3Au+, which has a Y structure with the gold atom in the center. Among the pentamers we find distorted X structures for all systems. For Ag2Au3+ we find an additional isomer which is a trigonal bipyramid. These findings are in line with predictions based on density-functional theory calculations, i.e., all these structures either represent the global minima or are within less than 0.1 eV of the predicted global minimum.

Photodissociation of small group-11 metal cluster ions: fragmentation pathways and photoabsorption cross sections

Noble metal cluster ions Cu(n)(+), Ag(n)(+) and Au(n)(+) (n = 3-21) have been stored in a Penning trap and photodissociated by low intensity laser pulses of 10 ns at photon energies of 3.49 eV and 4.66 eV. The fragmentation pathways, neutral monomer and dimer evaporation, have been monitored as a function of cluster size, excitation energy and element. It is found that the behavior of the branching ratio between monomer and dimer evaporation as a function of excitation energy depends on the metal under investigation. In particular, the slope of the energy dependence is positive for silver but negative for gold and copper cluster ions. Furthermore, photoabsorption cross sections are determined from observed total fragment yields in single-photon dissociation.

Reactions and thermochemistry of small transition metal cluster ions

This review discusses the reactivities and thermodynamics of small-size-specific transition metal clusters and focuses on thermodynamic information, which has not been comprehensively discussed before. Because of this focus, guided-ion-beam mass spectrometry was used to acquire much of the data. The details of this technique and the associated data analysis methods are provided. Results on the stabilities of bare transition metal clusters are provided for neutral, cationic, and anionic species. Implications for the electronic and geometrical structures are discussed, as well as the extrapolation of these values to bulk phase behavior. Detailed results for reactions of transition metal clusters with D2 and the oxygen donors O2 and CO2 are reviewed. Available bond energies between size-specific clusters and one D atom and one and two O atoms are compiled, and their implications are evaluated and favorably compared with bulk phase analogs. Several additional thermodynamic studies of various cluster systems are also discussed.