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.

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.

Carbide Dihydrides: Carbonaceous Species Identified in Ta -Mediated Methane Dehydrogenation.

The products of methane dehydrogenation by gas-phase Ta clusters are structurally characterized using infrared multiple photon dissociation (IRMPD) spectroscopy in conjunction with quantum chemical calculations. The obtained spectra of [4Ta,C,2H] reveal a dominance of vibrational bands of a H Ta C carbide dihydride structure over those indicative for a HTa CH carbyne hydride one, as is unambiguously verified by studies employing various methane isotopologues. Because methane dehydrogenation by metal cations M typically leads to the formation of either MCH carbene or HMCH carbyne hydride structures, the observation of a H MC carbide dihydride structure implies that it is imperative to consider this often-neglected class of carbonaceous intermediates in the reaction of metals with hydrocarbons.

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.

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.

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.

Confirmation of a de novo structure prediction for an atomically precise monolayer-coated silver nanoparticle.

Fathoming the principles underpinning the structures of monolayer-coated molecular metal nanoparticles remains an enduring challenge. Notwithstanding recent x-ray determinations, coveted veritable de novo structural predictions are scarce. Building on recent syntheses and de novo structure predictions of Au Ag (TBBT), where is a countercation, = 0 or 1, and TBBT is 4--butylbenzenethiol, we report an x-ray-determined structure that authenticates an a priori prediction and, in conjunction with first-principles theoretical analysis, lends force to the underlying forecasting methodology.

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.

M3Ag17(SPh)12 Nanoparticles and Their Structure Prediction.

Although silver nanoparticles are of great fundamental and practical interest, only one structure has been determined thus far: M4Ag44(SPh)30, where M is a monocation, and SPh is an aromatic thiolate ligand. This is in part due to the fact that no other molecular silver nanoparticles have been synthesized with aromatic thiolate ligands. Here we report the synthesis of M3Ag17(4-tert-butylbenzene-thiol)12, which has good stability and an unusual optical spectrum.

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.

Hydrogen-bonded structure and mechanical chiral response of a silver nanoparticle superlattice.

Self-assembled nanoparticle superlattices-materials made of inorganic cores capped by organic ligands, of varied structures, and held together by diverse binding motifs-exhibit size-dependent properties as well as tunable collective behaviour arising from couplings between their nanoscale constituents. Here, we report the single-crystal X-ray structure of a superlattice made in the high-yield synthesis of Na(4)Ag(44)(p-MBA)(30) nanoparticles, and find with large-scale quantum-mechanical simulations that its atomically precise structure and cohesion derive from hydrogen bonds between bundledp-MBA ligands. We also find that the superlattice's mechanical response to hydrostatic compression is characterized by a molecular-solid-like bulk modulus B(0) = 16.

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.

Ultrastable silver nanoparticles.

Noble-metal nanoparticles have had a substantial impact across a diverse range of fields, including catalysis, sensing, photochemistry, optoelectronics, energy conversion and medicine. Although silver has very desirable physical properties, good relative abundance and low cost, gold nanoparticles have been widely favoured owing to their proved stability and ease of use. Unlike gold, silver is notorious for its susceptibility to oxidation (tarnishing), which has limited the development of important silver-based nanomaterials.

Oxidative thymine mutation in DNA: water-wire-mediated proton-coupled electron transfer.

One-electron oxidation of A/T-rich DNA leads to mutations at thymine. Experimental investigation of DNA containing methyl-deuterated thymine reveals a large isotope effect establishing that cleavage of this carbon-hydrogen bond is involved in the rate-determining step of the reaction. First-principles quantum calculations reveal that the radical cation (electron hole) generated by DNA oxidation, initially located on adenines, localizes on thymine as the proton is lost from the methyl group, demonstrating the role of proton-coupled electron transfer (PCET) in thymine oxidation.

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)(+).

Total structure and electronic properties of the gold nanocrystal Au36(SR)24.

A golden opportunity: the total structure of a Au(36)(SR)(24) nanocluster reveals an unexpected face-centered-cubic tetrahedral Au(28) kernel (magenta). The protecting layer exhibits an intriguing combination of binding modes, consisting of four regular arch-like staples and the unprecedented appearance of twelve bridging thiolates (yellow). This unique protecting network and superatom electronic shell structure confer extreme stability and robustness.

Dielectron attachment and hydrogen evolution reaction in water clusters.

Binding of excess electrons to nanosize water droplets, with a focus on the hitherto largely unexplored properties of doubly-charged clusters, were investigated experimentally using mass spectrometry and theoretically with large-scale first-principles simulations based on spin-density-functional theory, with all the valence electrons (that is, 8e per water molecule) and excess electrons treated quantum mechanically. Singly-charged clusters (H(2)O)(n)(-1) were detected for n = 6-250, and our calculated vertical detachment energies agree with previously measured values in the entire range 15 ≤ n ≤ 105, giving a consistent interpretation in terms of internal, surface and diffuse states of the excess electron. Doubly-charged clusters were measured in the range of 83 ≤ n ≤ 123, with (H(2)O)(n)(-2) clusters found for 83 ≤ n < 105, and mass-shifted peaks corresponding to (H(2)O)(n-2)(OH(-))(2) detected for n ≥ 105.

Size-dependent binding energies of methane to small gold clusters.

The reactions of small gold cluster cations Au(x)(+) (x=2-6) with CH(4) were studied by joint gas-phase kinetics and first-principles density functional theory calculations. The experimentally obtained temperature-dependent low pressure rate constants were analyzed by employing the Lindemann energy transfer model for association reactions in conjunction with statistical RRKM theory. In this way cluster-size-dependent binding energies of methane to the gold cluster cations were determined from the experimental data for two different transition-state models.