Gas-phase Clusters Research Articles

Bonding, Structure, and Stability of [(Na) n(Phe) m] n+ Clusters: Some Surprising Results from an Experimental and Theoretical Investigation in Gas Phase

Structure and stability of clusters in the ground state were analyzed at the theoretical and experimental levels. Our experimental and theoretical findings showed that the clusters in gas phase tend to form mainly planar rings of four members. The symmetry and the small dipole moment in these specific configurations suggested that their stability could be associated with an alignment of the water molecules, maximizing attractive electrostatic interactions caused by changes in the charge distribution of the clusters.

Hydration and speciation studies of Mn2+ in aqueous solution with simple monovalent anions (ClO4−, NO3−, Cl−, Br−)

Raman spectroscopic measurements have been performed on aqueous solutions of Mn(ClO4)2, MnCl2, MnBr2, and Mn(NO3)2 in the terahertz region (40-600 cm(-1)) and to wavenumbers up to 4200 cm(-1) employing an absorption gap in these light pink coloured solutions. In Mn(ClO4)2 solutions of water and heavy water, the hexahydrate and its deuterate, [Mn(OH2)6](2+) and [Mn(OD2)6](2+), were characterized as showing a very weak, strongly polarized band at 354 cm(-1) and 338 cm(-1), respectively. These modes were assigned to ν1 MnO6 of [Mn(OH2)6](2+) and [Mn(OD2)6](2+). In Mn(NO3)2(aq), the undisturbed mode at 354 cm(-1), representative of manganese hexahydrate, was also detected in dilute solutions up to ~3 mol L(-1) and no sign of nitrato complex formation could be obtained. In fairly dilute MnCl2(aq) up to ~1.5 mol L(-1) the hexaaqua ions were identified but in concentrated solutions, chloro-complexes of Mn(2+) were detected in the form of [Mn(OH2)(6-n)Cl(n)]((+2-n)) with n equal to one or two. In MnBr2(aq) a comparable picture to the one in MnCl2(aq) could be obtained. Density functional theory (DFT) calculations on Mn(2+) water clusters were carried out to optimize the geometry and calculate the frequencies of the [Mn(OH2)6](2+) cluster. For this purpose an unrestricted B3LYP functional was used with a triple-ζ basis set 6-311+G(d,p). In order to include the hydration effects around the [Mn(OH2)6](2+), a continuum model approach was employed where the solvent is described by a structureless dielectric polarizable continuum using the Polarizable Continuum Model of Solvation (PCM). The gas phase cluster of [Mn(OH2)6](2+) led to lower MnO6 frequencies compared to the measured ones. A second, much larger cluster model, with 18 water molecules containing 6 waters in the first shell and an explicit second hydration shell, [Mn(OH2)6(OH2)12](2+), was modelled. Again, the cluster in vacuo and the cluster surrounded by a structureless polarizable continuum were considered. The larger cluster including the polarizable continuum gives the most realistic frequency value of ν1 MnO6 and the other MnO6 skeleton modes. The hydration enthalpy, ΔH(hyd(l)) at 298 K, of [Mn(OH2)6](2+)(aq) was calculated by applying a Born-Haber cycle and correcting for the heat of vaporization, ΔH(vap), of water and the solvation enthalpy, ΔH(solv), released by transferring the gas phase cluster into the solution. The theoretical hydration enthalpy is in fair agreement with the measured hydration enthalpy.

Band structure quantization in nanometer sized ZnO clusters

Nanometer sized ZnO clusters are produced in the gas phase and subsequently deposited on clean Au(111) surfaces under ultra-high vacuum conditions. The zinc blende atomic structure of the approximately spherical ZnO clusters is resolved by high resolution scanning transmission electron microscopy. The large band gap and weak n-type conductivity of individual clusters are determined by scanning tunnelling microscopy and spectroscopy at cryogenic temperatures. The conduction band is found to exhibit clear quantization into discrete energy levels, which can be related to finite-size effects reflecting the zero-dimensional confinement. Our findings illustrate that gas phase cluster production may provide unique possibilities for the controlled fabrication of high purity quantum dots and heterostructures that can be size selected prior to deposition on the desired substrate under controlled ultra-high vacuum conditions.

Noble metal alloy clusters in the gas phase derived from protein templates: unusual recognition of palladium by gold

Matrix assisted laser desorption ionization of a mixture of gold and palladium adducts of the protein lysozyme (Lyz) produces naked alloy clusters of the type Au24Pd(+) in the gas phase. While a lysozyme-Au adduct forms Au18(+), Au25(+), Au38(+) and Au102(+) ions in the gas phase, lysozyme-Pd alone does not form any analogous cluster. Addition of various transition metal ions (Ag(+), Pt(2+), Pd(2+), Cu(2+), Fe(2+), Ni(2+) and Cr(3+)) in the adducts contributes to drastic changes in the mass spectrum, but only palladium forms alloys in the gas phase. Besides alloy formation, palladium enhances the formation of specific single component clusters such as Au38(+). While other metal ions like Cu(2+) help forming Au25(+) selectively, Fe(2+) catalyzes the formation of Au25(+) over all other clusters. Gas phase cluster formation occurs from protein adducts where Au is in the 1+ state while Pd is in the 2+ state. The creation of alloys in the gas phase is not affected whether a physical mixture of Au and Pd adducts or a Au and Pd co-adduct is used as the precursor. The formation of Au cores and AuPd alloy cores of the kind comparable to monolayer protected clusters implies that naked clusters themselves may be nucleated in solution.

Spectroscopy and Reaction Study on Isomer Ions of Gas-Phase Clusters Separated by Mobility Spectrometry

Spectroscopy and Reaction Study on Isomer Ions of Gas-Phase Clusters Separated by Mobility Spectrometry

微視的溶媒和と気相クラスターの化学：孤立分子と溶液との橋渡し

微視的溶媒和と気相クラスターの化学：孤立分子と溶液との橋渡し

Cluster-Continuum Calculations of Hydration Free Energies of Anions and Group 12 Divalent Cations.

Understanding aqueous phase processes involving group 12 metal cations is relevant to both environmental and biological sciences. Here, quantum chemical methods and polarizable continuum models are used to compute the hydration free energies of a series of divalent group 12 metal cations (Zn(2+), Cd(2+), and Hg(2+)) together with Cu(2+) and the anions OH(-), SH(-), Cl(-), and F(-). A cluster-continuum method is employed, in which gas-phase clusters of the ion and explicit solvent molecules are immersed in a dielectric continuum. Two approaches to define the size of the solute-water cluster are compared, in which the number of explicit waters used is either held constant or determined variationally as that of the most favorable hydration free energy. Results obtained with various polarizable continuum models are also presented. Each leg of the relevant thermodynamic cycle is analyzed in detail to determine how different terms contribute to the observed mean signed error (MSE) and the standard deviation of the error (STDEV) between theory and experiment. The use of a constant number of water molecules for each set of ions is found to lead to predicted relative trends that benefit from error cancellation. Overall, the best results are obtained with MP2 and the Solvent Model D polarizable continuum model (SMD), with eight explicit water molecules for anions and 10 for the metal cations, yielding a STDEV of 2.3 kcal mol(-1) and MSE of 0.9 kcal mol(-1) between theoretical and experimental hydration free energies, which range from -72.4 kcal mol(-1) for SH(-) to -505.9 kcal mol(-1) for Cu(2+). Using B3PW91 with DFT-D3 dispersion corrections (B3PW91-D) and SMD yields a STDEV of 3.3 kcal mol(-1) and MSE of 1.6 kcal mol(-1), to which adding MP2 corrections from smaller divalent metal cation water molecule clusters yields very good agreement with the full MP2 results. Using B3PW91-D and SMD, with two explicit water molecules for anions and six for divalent metal cations, also yields reasonable agreement with experimental values, due in part to fortuitous error cancellation associated with the metal cations. Overall, the results indicate that the careful application of quantum chemical cluster-continuum methods provides valuable insight into aqueous ionic processes that depend on both local and long-range electrostatic interactions with the solvent.

Experimental and Theoretical Study of Reactions between Manganese Oxide Cluster Cations and Hydrogen Sulfide

Manganese oxide cluster cations Mnm18On+ were prepared by laser ablation and reacted with hydrogen sulfide (H2S) in a fast flow reactor under thermal collision conditions. A time-of-flight mass spectrometer was used to detect the cluster distributions before and after the interactions with H2S. The experiments suggest that oxygen-for-sulfur (O/S) exchange reaction to release water took place in the reactor for most of the manganese oxide cluster cations: Mnm18On++H2S→Mnm18On−1S++H218O. Density functional theory calculations were performed for reaction mechanisms of Mn2O2++H2S, Mn2O3++H2S, and Mn2O4++H2S. The computational results indicate these O/S exchange reactions are both thermodynamically and kinetically favorable, thus in good agreement with the experimental observations. The O/S exchange reactions identified in this gas-phase cluster study parallel similar behavior of related condensed phase reaction systems.

Ageing of mass-selected Cu/Au and Au/Cu core/shell clusters probed with atomic resolution

Bimetallic clusters can display new characteristics that cannot be obtained by varying either the size of pure metallic systems or the composition of bulk bimetals. We have demonstrated that both copper (Cu)-rich core/gold (Au)-rich shell and Au-rich/Cu-rich shell clusters can be prepared using the radio frequency (RF) magnetron-sputtering gas phase cluster beam synthesis method with subsequent mass selection. Here we present an aberration-corrected scanning transmission electron microscopy (STEM) study of the ageing of the deposited clusters. We find that the Au-rich core/Cu-rich shell structure is unstable against both storage in vacuum and exposure to air, losing the Cu-rich shell. However, the Cu-rich core/Au-rich shell structure is found to be much more stable, even when exposed to air.

Capillary and Coulombic Effects on the Gas Phase Structure of Electrosprayed Concanavalin A Ions and Its Clusters Cn+z(n= 1–6)

Ion mobility spectrometry (IMS) coupled to mass spectrometry (MS) is used to study the gas phase collision cross section Ω(z, n) in CO(2) of multimers C(n) (n = 1-4, 6) of concanavalin A, whose tetramer C(4) has a crystal structure resembling four tetrahedrically arranged globules. C(n)(+z) ions electrosprayed from aqueous solutions of triethylammonium formate (Et(3)AF) are moderately charged (up to z = 6 and 17 for n = 1 and 6) and produce narrow mobility peaks. Charge states down to z = 1 obtained with a charge-reducing radioactive (63)Ni source are studied for the dimer and the tetramer via pure IMS (no MS). The mobilities are independent of pH in the range 6-8, controlled by addition of triethylamine to the Et(3)AF. The measured compactness group Ω(z, n)/n(2/3) is practically independent of n and z, whereas mobility calculations with clusters of touching spheres show that it should vary with n by 20-30% for a variety of scattering models. This contrast suggests that, irrespective of ambiguities on the scattering model, all multimers adopt globular shapes, precluding in particular a tetrahedral tetramer. Acetic acid solutions (87 mM aqueous) yield ions with substantially higher z, mostly with broad mobility distributions. Exceptionally high z tetramers (z = 25-29) and trimers have narrowly defined mobilities with compact but nonspherical shapes. Addition of 2-4 mM Et(3)AF to the 87 mM aqueous acetic acid solution yields narrowly defined mobilities almost identical at all z values to those from the Et(3)AF buffer, although with higher charge states showing also a transition to nonspherical shapes. We conclude that all gas phase clusters charged below a Rayleigh-like charge, z(R), are globular without regard to solution conditions, some undergoing a sharp shape transition at a critical z = z(R). We confirm that gas phase protein cross sections differ from those expected from the crystal structure, with a trend to compact probably driven by their high surface energy (and opposed by Coulombic stresses). The Rayleigh-like shape transitions seen are similar to those arising in linear homopolymers, although not as sharply defined. They yield a surface energy for protein matter almost as high as the surface tension of water. This quantitative conclusion is corroborated by prior data on cytochrome c and apomyoglobin (also showing a critical shape transition) as well as measurements of the maximum charge versus mass in aggregates of dipeptides.

Bismuth-Doped Tin Clusters: Experimental and Theoretical Studies of Neutral Zintl Analogues

The electron count of gas-phase clusters is increased gradually by element substitution in order to mimic the total number of electrons of known stable closo-clusters. A combination of elements from the fourth and fifth group of the periodic table such as Sn and Bi is well-suited for this approach. Hence, these small Sn-Bi clusters are investigated by employing the electric field deflection method. For clusters in the series Sn(M-N)Bi(N) (M = 5-13, N = 1-2), the beam profiles obtained in cryogenic experiments are dominated by beam broadening, indicating the presence of a permanent electric dipole moment that is sensitive to the (rigid) cluster structure. An intensive search for the global minimum structure employing a density functional theory/genetic algorithm method is performed. Dielectric properties for the identified low-energy isomers are computed. The structural and dielectric properties are used in beam profile simulations in order to discuss the experimental data. Comparison of theoretical and experimental results enables identification of the growing pattern of these small bimetallic clusters. For multiply doped clusters, it is concluded that the dopant atoms do not form direct Bi-Bi bonds, but more interestingly, a rearrangement of the cluster skeleton becomes apparent. The structural motifs are different from pure tin clusters but rather are rationalized using the corresponding structures of tin anions or are based on the Wade-Mingos concept. Further evidence for this idea is deduced from nuclear independent chemical shift calculations, which show nearly identical behavior for negatively charged pure and neutral bimetallic clusters. All of these findings are consistent with the idea of neutral Zintl analogues in the gas phase.

Characterization of Nanoporous WO3 Films Grown via Ballistic Deposition

We report on the preparation and characterization of high surface area, supported nanoporous tungsten oxide films prepared under different conditions on polished polycrystalline Ta and Pt(111) substrates via direct sublimation of monodispersed gas phase clusters of cyclic (WO3)3. Scanning electron microscopy and transmission electron microscopy were used to investigate the film morphology on a nanometer scale. The films consist of arrays of separated filaments that are amorphous. The chemical composition and the thermal stability of the films were investigated by means of X-ray photoelectron spectroscopy. The surface area and the distribution of binding sites on the films were measured as functions of growth temperature, deposition angle, and annealing conditions using temperature-programmed desorption of Kr. Films deposited at 20 K and at an incident angle of 65° from the substrate normal display the greatest specific surface area of ∼560 m2/g.

Response behavior of a palladium nanoparticle array based hydrogen sensor in hydrogen–nitrogen mixture

Closely spaced palladium nanoparticle arrays were fabricated in between a pair of interdigital electrodes by means of gas phase cluster beam deposition with controlled coverage. The quantum conductance change induced by hydrogen absorption was used for hydrogen sensing. The hydrogen response behavior of a sensor chip was investigated in N2+H2 mixture gas. The sensor could be calibrated with three linear response hydrogen concentration regimes. A sub-second fast response was shown at 2.2% H2 concentration. The influence of the temperature variation to the quantitative hydrogen concentration measurement with this sensor was discussed.

Identification of twinned gas phase clusters by single-shot scattering with intense soft x-ray pulses

Scattering experiments on xenon nanoclusters with high-intensity soft x-ray laser pulses from the Free-Electron LASer in Hamburg (FLASH) are performed to investigate different cluster morphologies in the gas phase. Three different types of scattering patterns can be identified. The most frequent pattern of concentric rings reflects the event of a single spherical cluster in focus. Fine interference rings similar to Newton rings appear when two clusters are illuminated at μm distance, revealing three-dimensional information about the location of the clusters. Between 10 and 30% of all hits show a previously unknown twin cluster configuration with two clusters in direct contact. Simulations of scattering patterns for twin clusters with different sizes of the two particles, degree of fusion and orientation in space allow us to explain all the observed patterns.

Experimental and Theoretical Study of the Reactions between Vanadium Oxide Cluster Cations and Hydrogen Sulfide

Vanadium oxide cluster cations (Vm16On+ and Vm18On+) are prepared by laser ablation and reacted with hydrogen sulfide (H2S) in a fast flow reactor under thermal collision conditions. A time-of-flight mass spectrometer is used to detect the cluster distributions before and after the interactions with H2S. The experiments suggest that, in addition to H2S adsorption to form association products VmOnH2S+, three types of reactions are evidenced in the reactor: (1) VmOn+ + H2S → VmOnH+ + SH, (2) VmOn+ + H2S → VmOnH2+ + S, and (3) VmOn+ + H2S → VmOn–1S+ + H2O. Density functional theory calculations are performed for reaction of VO2+ with H2S, and the results indicate that the above three types of reaction channels are both thermodynamically and kinetically favorable, which supports the experimental observations. This gas-phase cluster study may provide insights into selective oxidation of H2S to elemental sulfur over vanadium-based oxide catalysts.

Structural and electronic properties of Ag–Pd bimetallic clusters on Al2O3 substrates: A first principles study

We report the structure and electronic properties of AgmPdn bimetallic clusters (m, n=1–2) in gas phase and deposited on the α-Al2O3(0001) surface using plane wave pseudopotential approach. The ground state geometry of the mixed clusters is significantly modified after deposition. For Ag2Pd2 tetramer, the gas phase compact tetrahedron structure reorients to open bent rhombus. The charge distribution analysis shows that small amount of charge is accumulated at the Pd site and depleted at the Ag site. The electronic density of states analysis reveals that after deposition, the bands near the Fermi energy become wider.

Reactions of CH3SH and CH3SSCH3 with Gas-Phase Hydrated Radical Anions (H2O)n•–, CO2•–(H2O)n, and O2•–(H2O)n

The chemistry of (H(2)O)(n)(•-), CO(2)(•-)(H(2)O)(n), and O(2)(•-)(H(2)O)(n) with small sulfur-containing molecules was studied in the gas phase by Fourier transform ion cyclotron resonance mass spectrometry. With hydrated electrons and hydrated carbon dioxide radical anions, two reactions with relevance for biological radiation damage were observed, cleavage of the disulfide bond of CH(3)SSCH(3) and activation of the thiol group of CH(3)SH. No reactions were observed with CH(3)SCH(3). The hydrated superoxide radical anion, usually viewed as major source of oxidative stress, did not react with any of the compounds. Nanocalorimetry and quantum chemical calculations give a consistent picture of the reaction mechanism. The results indicate that the conversion of e(-) and CO(2)(•-) to O(2)(•-) deactivates highly reactive species and may actually reduce oxidative stress. For reactions of (H(2)O)(n)(•-) with CH(3)SH as well as CO(2)(•-)(H(2)O)(n) with CH(3)SSCH(3), the reaction products in the gas phase are different from those reported in the literature from pulse radiolysis studies. This observation is rationalized with the reduced cage effect in reactions of gas-phase clusters.

Nonmagnetic and magnetic thiolate-protected Au25superatoms on Cu(111), Ag(111), and Au(111) surfaces

Geometry, electronic structure, and magnetic properties of methylthiolate-stabilized Au${}_{25}$L${}_{18}$ and MnAu${}_{24}$L${}_{18}$ (L = SCH${}_{3}$) clusters adsorbed on noble-metal (111) surfaces have been investigated by using spin-polarized density functional theory computations. The interaction between the cluster and the surface is found to be mediated by charge transfer mainly from or into the ligand monolayer. The electronic properties of the 13-atom metal core remain in all cases rather undisturbed as compared to the isolated clusters in gas phase. The Au${}_{25}$L${}_{18}$ cluster retains a clear highest occupied molecular orbital/lowest unoccupied molecular orbital energy gap in the range of 0.7 to 1.0 eV depending on the surface. The ligand layer is able to decouple the electronic structure of the magnetic MnAu${}_{24}$L${}_{18}$ cluster from the Au(111) surface, retaining a high local spin moment of close to 5 ${\ensuremath{\mu}}_{B}$ arising from the spin-polarized Mn($3d$) electrons. These computations imply that the thiolate monolayer-protected gold clusters may be used as promising building blocks with tunable energy gaps, tunneling barriers, and magnetic moments for applications in the area of electron and/or spin transport.

Interactions of oligomers of organic polyethers with histidine amino acid

Knowledge on noncovalent intermolecular interactions of organic polyethers with amino acids is essential to gain a better understanding on how polymers assemble in organic nanoparticles which are promising for drug delivery and cryoprotection. The main objective of the present study was to determine how polyethers assemble around ionizable amino acids such as histidine. Electrospray mass spectrometry was applied to probe the interactions in model systems consisting of polyethylene glycol PEG-400 or oxyethylated glycerol OEG-5 and amino acid histidine hydrochloride. Molecular dynamics simulation was utilized to visualize the structure of complexes of polyether oligomers with histidine in different charge states. Stable gas-phase clusters composed of polyether oligomers (PEG(n), OEG(n)) with protonated histidine--PEG(n)•His•H(+), OEG(n)•His•H(+), OEG(n)•OEG(m)•His•H(+) and chlorine counterion--PEG(n)•Cl(-), OEG(n)•Cl(-), were observed under electrospray conditions. Molecular dynamics simulation of representative polyether-histidine complexes revealed the stabilization of oligomers by multiple hydrogen and coordination bonds whereby charged groups are wrapped by the polymeric chains. The self-organization of polyether chains around the protonated imidazole group of histidine was revealed. This finding should be considered when modelling a pegylated protein structure and polyether-based organic nanoparticles.

Modeling the hydration of mono-atomic anions from the gas phase to the bulk phase: The case of the halide ions F−, Cl−, and Br−

In this work, we investigate the hydration of the halide ions fluoride, chloride, and bromide using classical molecular dynamics simulations at the 10 ns scale and based on a polarizable force-field approach, which treats explicitly the cooperative bond character of strong hydrogen bond networks. We have carried out a thorough analysis of the ab initio data at the MP2 or CCSD(T) level concerning anion/water clusters in gas phase to adjust the force-field parameters. In particular, we consider the anion static polarizabilities computed in gas phase using large atomic basis sets including additional diffuse functions. The information extracted from trajectories in solution shows well structured first hydration shells formed of 6.7, 7.0, and 7.6 water molecules at about 2.78 Å, 3.15 Å, and 3.36 Å for fluoride, chloride, and bromide, respectively. These results are in excellent agreement with the latest neutron- and x-ray diffraction studies. In addition, our model reproduces several other properties of halide ions in solution, such as diffusion coefficients, description of hydration processes, and exchange reactions. Moreover, it is also able to reproduce the electrostatic properties of the anions in solution (in terms of anion dipole moment) as reported by recent ab initio quantum simulations. All the results show the ability of the proposed model in predicting data, as well as the need of accounting explicitly for the cooperative character of strong hydrogen bonds to reproduce ab initio potential energy surfaces in a mean square sense and to build up a reliable force field.