Gold Clusters in the Gas Phase

Gold clusters exhibit strong size and charge state dependent variations in their properties. This is demonstrated by significant changes in their geometric structures and also in their chemical properties. Here we focus on clusters containing up to about 20 gold atoms and briefly review their structural evolution emphasising the role of isomerism and structural fluxionality. The discussion of chemical properties is limited to the interaction of gold clusters with molecular oxygen and carbon monoxide, separately, and their interaction in CO/O2 co-adsorbates on gold clusters eventually leading to CO oxidation. Whilst highlighting results obtained using different experimental approaches, special attention is given to the insights obtained using infrared multiple photon dissociation (IR-MPD) spectroscop

CO adsorption on neutral iridium clusters

The adsorption of carbon monoxide on neutral iridium clusters in the size range of n = 3 to 21 atoms is investigated with infrared multiple photon dissociation spectroscopy. For each cluster size only a single v(CO) band is present with frequencies in the range between 1962 cm-1 (n = 8) and 1985 cm-1 (n = 18) which can be attributed to an atop binding geometry. This behaviour is compared to the CO binding geometries on clusters of other group 9 and 10 transition metals as well as to that on extended surfaces. The preference of Ir for atop binding is rationalized by relativistic effects on the electronic structure of the later 5d metals

Nature of Ar bonding to small Co_n^+ clusters and its effect on the structure determination by far-infrared absorption spectroscopy

Far-infrared vibrational spectroscopy by multiple photon dissociation has proven to be a very useful technique for the structural fingerprinting of small metal clusters. Contrary to previous studies on cationic V, Nb and Ta clusters, measured vibrational spectra of small cationic cobalt clusters show a strong dependence on the number of adsorbed Ar probe atoms, which increases with decreasing cluster size. Focusing on the series Co_4^+ to Co_8^+ we therefore use density-functional theory to analyze the nature of the Ar-Co_n^+ bond and its role for the vibrational spectra. In a first step, energetically low-lying isomer structures are identified through first-principles basin-hopping sampling runs and their vibrational spectra computed for a varying number of adsorbed Ar atoms. A comparison of these fingerprints with the experimental data enables in some cases a unique assignment of the cluster structure. Independent of the specific low-lying isomer, we obtain a pronounced increase of the Ar binding energy for the smallest cluster sizes, which correlates nicely with the observed increased influence of the Ar probe atoms on the IR spectra. Further analysis of the electronic structure motivates a simple electrostatic picture that not only explains this binding energy trend, but also why the influence of the rare-gas atom is much stronger than in the previously studied systems.Comment: 12 pages including 10 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm

Reaktivitätsstudien an Metalloxidclustern in der Gasphase

Aussagen zur Reaktivität von Metalloxidclustern in der Gasphase wurden aus Messungen von totalen integralen Wechselwirkungsquerschnitten an Gastargets abgeleitet und diese mit Untersuchungen des Reaktionsverlaufes unter thermalisierten Bedingungen in einem Fließreaktor korreliert. Diese Untersuchungen konnten mit einer speziell entwickelten Molekularstrahlapparatur ausgeführt werden, die detailliert beschrieben wird. Mittels dieser beider Methoden wurden die Reaktionen von Bismutoxid-Clusterkationen mit Alkenen untersucht. Als Hauptreaktionskanal wird hier eine Assoziation des Alkens festgestellt. Einzelne Clusterkationen lagern zusätzlich zum Alken molekularen Sauerstoff an. Aus der massenspektrometrisch beobachteten Reaktionsfolge wird ein Mechanismus für die Aktivierung des molekularen Sauerstoffs und die Übertragung auf den Kohlenwasserstoff abgeleitet. Diese Resultate werden durch jüngste quantenchemische Rechnungen bestätigt. (M. Bienati, Dissertation, Humboldt-Universität zu Berlin, 2001)Insights into the reactivity of metal oxide clusters in the gas phase have been gained from total integral cross sections measured with gas targets and investigations of the reaction sequences at thermalized conditions in a fast flow reactor. These experiments have been carried out with a newly designed molecular beam apparatus, which is described in detail. Applying these two techniques, the reactions of bismuth oxide cluster cations with alkenes have been probed. The main reaction channel is the association of alkenes, but particular clusters bind molecular oxygen additionally. A mechanism for this activation of molecular oxygen and its transfer towards the hydrocarbon has been derived from the mass spectrometrically measured reaction sequence, which is supported by recent theoretical calculations (M. Bienati, Doctoral Thesis, Humboldt-Universität zu Berlin, 2001

Imaging Photoelectron Circular Dichroism in the Detachment of Mass‐Selected Chiral Anions

Photoelectron Circular Dichroism (PECD) is a forward-backward asymmetry in the photoemission from a non-racemic sample induced by circularly polarized light. PECD spectroscopy has potential analytical advantages for chiral discrimination over other chiroptical methods due to its increased sensitivity to the chiral potential of the molecule. The use of anions for PECD spectroscopy allows for mass-selectivity and provides a path to simple experimental schemes that employ table-top light sources. Evidence of PECD for anions is limited, and insight into the forces that govern PECD electron dynamics in photodetachment is absent. Here, we demonstrate a PECD effect in the photodetachment of mass-selected deprotonated 1-indanol anions. By utilizing velocity map imaging photoelectron spectroscopy with a tunable light source, we determine the energy-resolved PECD over a wide range of photon energies. The observed PECD reaches up to 11%, similar to what has been measured for neutral species

Comparison of Conventional and Nonconventional Hydrogen Bond Donors in Au^- Complexes

Although gold has become a well-known nonconventional hydrogen bond acceptor, interactions with nonconventional hydrogen bond donors have been largely overlooked. In order to provide a better understanding of these interactions, two conventional hydrogen bonding molecules (3-hydroxytetrahydrofuran and alaninol) and two nonconventional hydrogen bonding molecules (fenchone and menthone) were selected to form gas-phase complexes with Au-. The Au-[M] complexes were investigated using anion photoelectron spectroscopy and density functional theory. Au-[fenchone], Au-[menthone], Au-[3-hydroxyTHF], and Au-[alaninol] were found to have vertical detachment energies of 2.71 ± 0.05, 2.76 ± 0.05, 3.01 ± 0.03, and 3.02 ± 0.03 eV, respectively, which agree well with theory. The photoelectron spectra of the complexes resemble the spectrum of Au- but are blueshifted due to the electron transfer from Au- to M. With density functional theory, natural bond orbital analysis, and atoms-in-molecules analysis, we were able to extend our comparison of conventional and nonconventional hydrogen bonding to include geometric and electronic similarities. In Au-[3-hydroxyTHF] and Au-[alaninol], the hydrogen bonding comprised of Au-···HO as a strong, primary hydrogen bond, with secondary stabilization by weaker Au-···HN or Au-···HC hydrogen bonds. Interestingly, the Au-···HC bonds in Au-[fenchone] and Au-[menthone] can be characterized as hydrogen bonds, despite their classification as nonconventional hydrogen bond donors

Atomic Cluster Au₁₀⁺ Is a Strong Broadband Midinfrared Chromophore

We report an intense broadband midinfrared absorption band in the Au10+ cluster in a region in which only molecular vibrations would normally be expected. Observed in the infrared multiple photon dissociation spectra of Au10Ar+, Au10(N2O)+, and Au10(OCS)+, the smooth feature stretches 700–3400  cm-1 (λ=14–2.9  μm). Calculations confirm unusually low-energy allowed electronic excitations consistent with the observed spectra. In Au10(OCS)+, IR absorption throughout the band drives OCS decomposition resulting in CO loss, providing an alternative method of bond activation or breaking

Argon tagging of doubly transition metal doped aluminum clusters: The importance of electronic shielding

The interaction of argon with doubly transition metal doped aluminum clusters, AlnTM2+ (n = 1–18, TM = V, Nb, Co, Rh), is studied experimentally in the gas phase via mass spectrometry. Density functional theory calculations on selected sizes are used to understand the argon affinity of the clusters, which differ depending on the transition metal dopant. The analysis is focused on two pairs of consecutive sizes: Al6,7V2+ and Al4,5Rh2+, the largest of each pair showing a low affinity toward Ar. Another remarkable observation is a pronounced drop in reactivity at n = 14, independent of the dopant element. Analysis of the cluster orbitals shows that this feature is not a consequence of cage formation but is electronic in nature. The mass spectra demonstrate a high similarity between the size-dependent reactivity of the clusters with Ar and H2. Orbital interactions provide an intuitive link between the two and further establish the importance of precursor states in the reactions of the clusters with hydrogen

Aufklärung der Struktur von Metallclustern durch Schwingungsspektroskopie

Application of a new method of infrared spectroscopy delivers insights into the atomic structure of clusters composed of only a few metal atoms

Structural determination of niobium-doped silicon clusters by far-infrared spectroscopy and theory

In this work, the structures of cationic SinNb+ (n = 4-12) clusters are determined using the combination of infrared multiple photon dissociation (IR-MPD) and density functional theory (DFT) calculations. The experimental IR-MPD spectra of the argon complexes of SinNb+ are assigned by comparison to the calculated IR spectra of low-energy structures of SinNb+ that are identified using the stochastic 'random kick' algorithm in conjunction with the BP86 GGA functional. It is found that the Nb dopant tends to bind in an apex position of the Si-n framework for n = 4-9 and in surface positions with high coordination numbers for n = 10-12. For the larger doped clusters, it is suggested that multiple isomers coexist and contribute to the experimental spectra. The structural evolution of SinNb+ clusters is similar to V-doped silicon clusters (J. Am. Chem. Soc., 2010, 132, 15589-15602), except for the largest size investigated (n = 12), since V takes an endohedral position in Si12V+. The interaction with a Nb atom, with its partially unfilled 4d orbitals leads to a significant stability enhancement of the Si-n framework as reflected, e.g. by high binding energies and large HOMO-LUMO gaps.EC/FP7/226716/EU/European Light Sources Activities - Synchrotrons and Free Electron Lasers/ELISADFG, FOR 1282, Controlling the electronic structure of semiconductor nanoparticles by doping and hybrid formatio