Gas-Phase Production of Hydroxylated Silicon Oxide Cluster Cations: Structure, Infrared Spectroscopy, and Astronomical Relevance.

The interaction of free cationic silicon oxide clusters, Si O ( = 2-5, ≥ ), with dilute water vapor, was investigated in a flow tube reactor. Product mass distributions indicate cluster size-dependent dissociative water adsorption. To probe the structure and vibrational spectra of the resulting Si O H ( = 2-4) clusters, we employed infrared multiple photon dissociation spectroscopy and density functional theory calculations.

Cluster size dependent coordination of formate to free manganese oxide clusters.

The interaction of free manganese oxide clusters, MnO ( = 1-9, = 0-12), with formic acid was studied infrared multiple-photon dissociation (IR-MPD) spectroscopy together with calculations using density functional theory (DFT). Clusters containing only one Mn atom, such as MnO and MnO, bind formic acid as an intact molecule in both the - and -configuration. In contrast, all clusters containing two or more manganese atoms deprotonate the acid's hydroxyl group.

Spin-Gated Selectivity of the Water Oxidation Reaction Mediated by Free Pentameric CaMnO Clusters.

We report on the first preparation of isolated ligand-free CaMnO gas-phase clusters, as well as other pentameric CaMnO ( = 0-4) clusters with varying Ca contents, which serve as molecular models of the natural CaMnO inorganic cluster in photosystem II. Ion trap reactivity studies with DO and HO reveal a pronounced cluster composition-dependent ability to mediate the oxidation of water to hydrogen peroxide. First-principles density functional theory simulations elucidate the mechanism of water oxidation, proceeding via formation of a terminal oxyl radical followed by oxyl/hydroxy (O/OH) coupling.

Nonmetal-to-Metal Transition of Magnesia Supported Au Clusters Affects the Ultrafast Dissociation Dynamics of Adsorbed CHBr Molecules.

The detection of intermediate species and the correlation of their ultrafast dynamics with the morphology and electronic structure of a surface is crucial to fully understand and control heterogeneous photoinduced and photocatalytic reactions. In this work, the ultrafast photodissociation dynamics of CHBr molecules adsorbed on variable-size Au clusters on MgO/Mo(100) is investigated by monitoring the CH transient evolution using a pump-probe technique in conjunction with surface mass spectrometry. Furthermore, extreme-UV photoemission spectroscopy in combination with theoretical calculations is employed to study the electronic structure of the Au clusters on MgO/Mo(100).

Size, Stoichiometry, Dimensionality, and Ca Doping of Manganese Oxide-Based Water Oxidation Clusters: An Oxyl/Hydroxy Mechanism for Oxygen-Oxygen Coupling.

Gas-phase ion-trap reactivity experiments and density functional simulations reveal that water oxidation to HO mediated by (calcium) manganese oxide clusters proceeds via formation of a terminal oxyl radical followed by oxyl/hydroxy O-O coupling. This mechanism is predicted to be energetically feasible for MnO ( = 2-4) and the binary CaMnO, in agreement with the experimental observations. In contrast, the reaction does not proceed for the tetramanganese oxides MnO ( = 4-6) under these experimental conditions.

Cluster Size Dependent Interaction of Free Manganese Oxide Clusters with Acetic Acid and Methyl Acetate.

We have employed infrared multiple-photon dissociation (IR-MPD) spectroscopy together with density functional theory (DFT) calculations to study the interaction of series of subnanometer sized manganese oxide clusters, MnO ( = 1-6, = 0-9) with acetic acid (HOAc) and methyl acetate (MeOAc). Reaction with HOAc leads to strongly cluster size and composition dependent IR-MPD spectra, indicating molecular adsorption on MnO clusters and thermodynamically favorable but kinetically hampered HOAc dissociation (deprotonation) on MnO and MnO. Other cluster sizes exhibit the preferred formation of a dissociative bidentate chelating structure.

Energetic Stabilization of Carboxylic Acid Conformers by Manganese Atoms and Clusters.

Free cationic manganese atoms and clusters Mn ( = 1-3) have been reacted with small carboxylic acids (formic, acetic, and propionic acids) and methyl acetate in a flow tube reactor held at room temperature. The geometry of the thus formed complexes has subsequently been studied via infrared multiple-photon dissociation (IR-MPD) spectroscopy and density-functional theory (DFT) calculations. The IR-MPD spectra of the acid complexes show two signals in the C═O stretch region indicating the coexistence of two conformers.

Infrared Spectroscopy of Gas-Phase MnO(CO) Complexes.

The interaction of manganese oxide clusters MnO ( = 2-5, ≥ ) with CO is studied via infrared multiple-photon dissociation spectroscopy (IR-MPD) in the spectral region of 630-1860 cm. Along with vibrational modes of the manganese oxide cluster core, two bands are observed around 1200-1450 cm and they are assigned to the characteristic Fermi resonance of CO arising from anharmonic coupling between the symmetric stretch vibration and the overtone of the bending mode. The spectral position of the lower frequency band depends on the cluster size and the number of adsorbed CO molecules, whereas the higher frequency band is largely unaffected.

Infrared photodissociation spectroscopy of di-manganese oxide cluster cations.

Infrared multiple-photon dissociation (IR-MPD) spectroscopy and density functional theory (DFT) calculations have been employed to elucidate the geometric structure of a series of di-manganese oxide clusters MnO (x = 4-7). The theoretical exploration predicts that all investigated clusters contain a rhombus-like MnO core with up to four, terminally bound, oxygen atoms. The short Mn-O bond length of the terminal oxygen atoms of ≤1.

Co-adsorption of O and CH on a Free Gold Dimer Probed via Infrared Photodissociation Spectroscopy.

Infrared multiple photon dissociation (IR-MPD) spectroscopy in conjunction with density functional theory (DFT) calculations has been employed to study the activation of molecular oxygen and ethylene co-adsorbed on a free gold dimer cation Au. Both studied complexes, AuO(CH) and AuO(CH), show distinct features of both intact O and ethylene co-adsorbed on the cluster. However, the ethylene C=C double bond is activated, increasing in length by up to 0.

A Gas-Phase Ca Mn O Cluster Model for the Oxygen-Evolving Complex of Photosystem II.

One of the fundamental processes in nature, the oxidation of water, is catalyzed by a small CaMn O ⋅MnO cluster located in photosystem II (PS II). Now, the first successful preparation of a series of isolated ligand-free tetrameric Ca Mn O (n=0-4) cluster ions is reported, which are employed as structural models for the catalytically active site of PS II. Gas-phase reactivity experiments with D O and H O in an ion trap reveal the facile deprotonation of multiple water molecules via hydroxylation of the cluster oxo bridges for all investigated clusters.

The interaction of ethylene with free gold cluster cations: infrared photodissociation spectroscopy combined with electronic and vibrational structure calculations.

The interaction of ethylene with free gold clusters of different sizes and charge states has been previously shown theoretically to involve two different adsorption modes of the CH molecule, namely: the di-σ- and π-bonded ethylene adsorption configurations. Here, we present the first experimental investigation of the structure of a series of gas-phase gold-ethylene complexes, [Formula: see text]. By employing infrared multiple-photon dissociation spectroscopy in conjunction with first-principles calculations it is revealed that up to three CH molecules preferably bind to gold cations in a π-bonded configuration.

Thermal stability of iron-sulfur clusters.

The thermal decomposition of free cationic iron-sulfur clusters FeS (x = 0-7, y = 0-9) is investigated by collisional post-heating in the temperature range between 300 and 1000 K. With increasing temperature the preferential formation of stoichiometric FeS (y = x) or near stoichiometric FeS (y = x ± 1) clusters is observed. In particular, FeS represents the most abundant product up to 600 K, FeS and FeS are preferably formed between 600 K and 800 K, and FeS clearly dominates the cluster distribution above 800 K.

Selective C-H Bond Cleavage in Methane by Small Gold Clusters.

Methane represents the major constituent of natural gas. It is primarily used only as a source of energy by means of combustion, but could also serve as an abundant hydrocarbon feedstock for high quality chemicals. One of the major challenges in catalysis research nowadays is therefore the development of materials that selectively cleave one of the four C-H bonds of methane and thus make it amenable for further chemical conversion into valuable compounds.

Cluster size and composition dependent water deprotonation by free manganese oxide clusters.

In the quest for cheap and earth abundant but highly effective and energy efficient water splitting catalysts, manganese oxide represents one of the materials of choice. In the framework of a new hierarchical modeling strategy we employ free non-ligated manganese oxide clusters MnxOx+y(+) (x = 2-5, y = -1, 0, 1, 2) as simplified molecular models to probe the interaction of water with nano-scale manganese oxide materials. Infrared multiple-photon dissociation (IR-MPD) spectroscopy in conjunction with first-principles spin density functional theory calculations is applied to study several series of MnxOx+y(H2O)n(+) complexes and reveal that the reaction of water with MnxOx+y(+) leads to the deprotonation of the water molecules via hydroxylation of the cluster oxo-bridges.

The Interaction of Water with Free Mn4 O4 (+) Clusters: Deprotonation and Adsorption-Induced Structural Transformations.

As the biological activation and oxidation of water takes place at an inorganic cluster of the stoichiometry CaMn4 O5 , manganese oxide is one of the materials of choice in the quest for versatile, earth-abundant water splitting catalysts. To probe basic concepts and aid the design of artificial water-splitting molecular catalysts, a hierarchical modeling strategy was employed that explores clusters of increasing complexity, starting from the tetramanganese oxide cluster Mn4 O4 (+) as a molecular model system for catalyzed water activation. First-principles calculations in conjunction with IR spectroscopy provide fundamental insight into the interaction of water with Mn4 O4 (+) , one water molecule at a time.

Water activation by small free ruthenium oxide clusters.

The reactions of ruthenium clusters, Rux(+) (x = 2-5), and ruthenium oxide clusters, RuxOy(+) (x = 2-5, y = 1-2), with water molecules have been investigated by gas phase ion trap mass spectrometry and first principle density functional calculations. The joint experimental and theoretical study reveals that the reactions of the ruthenium oxide clusters with water are considerably more efficient. This is assigned theoretically to the stronger binding of the water molecules to RuxOy(+) and, more importantly, to water activation leading to an efficient hydrogen transfer reaction from the water molecules to the oxygen atoms of the ruthenium oxide clusters.

Size-dependent self-limiting oxidation of free palladium clusters.

Temperature-dependent gas phase ion trap experiments performed under multicollision conditions reveal a strongly size-dependent reactivity of Pd(x)(+) (x = 2-7) in the reaction with molecular oxygen. Yet, a particular stability and resistance to further oxidation is generally observed for reaction products with two oxygen molecules, Pd(x)O4(+). Complementary first-principles density functional theory simulations elucidate the details of the size-dependent bonding of oxygen to the small palladium clusters and are able to assign the pronounced occurrence of Pd(x)O4(+) complexes to a dissociatively chemisorbed bridging oxygen atomic structure which impedes the chemisorption of further oxygen molecules.

The origin of the selectivity and activity of ruthenium-cluster catalysts for fuel-cell feed-gas purification: a gas-phase approach.

Gas-phase ruthenium clusters Ru(n)(+) (n=2-6) are employed as model systems to discover the origin of the outstanding performance of supported sub-nanometer ruthenium particles in the catalytic CO methanation reaction with relevance to the hydrogen feed-gas purification for advanced fuel-cell applications. Using ion-trap mass spectrometry in conjunction with first-principles density functional theory calculations three fundamental properties of these clusters are identified which determine the selectivity and catalytic activity: high reactivity toward CO in contrast to inertness in the reaction with CO2; promotion of cooperatively enhanced H2 coadsorption and dissociation on pre-formed ruthenium carbonyl clusters, that is, no CO poisoning occurs; and the presence of Ru-atom sites with a low number of metal-metal bonds, which are particularly active for H2 coadsorption and activation. Furthermore, comprehensive theoretical investigations provide mechanistic insight into the CO methanation reaction and discover a reaction route involving the formation of a formyl-type intermediate.

Gas-phase synthesis and structure of Wade-type ruthenium carbonyl and hydrido carbonyl clusters.

The gas-phase reaction of size-selected Ru(n)(+) (n = 4-6) clusters with CO in an ion trap yields only one specific ruthenium carbonyl complex for each cluster size, Ru4(CO)14(+), Ru5(CO)16(+), and Ru6(CO)18(+). First-principles density functional theory calculations reveal structures for these hitherto unknown carbonyl compounds that are in perfect agreement with the geometries predicted by Wade's electron counting rules. Furthermore, reactions with D2 show that for Ru4(+) and Ru6(+), CO molecules can be partially replaced by D2 to form hydrido carbonyl complexes while preserving the total ligand count corresponding to the Wade cluster sizes.

Dimensionality dependent water splitting mechanisms on free manganese oxide clusters.

The interaction of ligand-free manganese oxide nanoclusters with water is investigated, aiming at uncovering phenomena which could aid the design of artificial water-splitting molecular catalysts. Gas phase measurements in an ion trap in conjunction with first-principles calculations provide new mechanistic insight into the water splitting process mediated by bi- and tetra-nuclear singly charged manganese oxide clusters, Mn2O2(+) and Mn4O4(+). In particular, a water-induced dimensionality change of Mn4O4(+) is predicted, entailing transformation from a two-dimensional ring-like ground state structure of the bare cluster to a cuboidal octa-hydroxy-complex for the hydrated one.

Pd6O4+: an oxidation resistant yet highly catalytically active nano-oxide cluster.

The palladium oxide cluster Pd(6)O(4)(+) is formed as the sole product upon reaction of a bare palladium cluster Pd(6)(+) with molecular oxygen in an octopole ion trap under multicollision conditions. This oxide cluster is found to be resistant to further oxidation over a large temperature range, and further O(2) molecules merely physisorb on it at cryogenic temperatures. The particular stability of Pd(6)O(4)(+) is confirmed by the observation that the reaction of Pd(7)(+) with O(2) leads to fragmentation resulting in the formation of Pd(6)O(4)(+).