Controlled formation of mass-selected Cu-Au core-shell cluster beams

Synthesis of bimetallic clusters is a topic of accelerated interest; their physical and chemical properties are greatly dependent on their composition, size, and structure. The cluster beam technique is widely used for preparation of clusters. However, creating bimetallic clusters with well-controlled composition, size, and structure, especially for larger clusters (>100 atoms), is still a big challenge. Here we demonstrate that not only size and composition but also the structure of bimetallic clusters can be controlled by tuning aggregation parameters.     

双核钨簇合物W2Cl4（OR）4（ROH）2结构的EXAFS研究

我们曾用EXAFS和X射线单晶衍射研究了M_3W_2Cl_9类双钨簇合物的结构,发现簇胳[W_2Cl_9]~(3-)的结构很牢固,改变阳离子M~+的性质和大小,都不会改变其结构。W_2Cl_9(RO)_4(R′OH)_2类簇合物与M_3W_2Cl_9类似,具有一个双钨簇胳。Anderson等曾研究R为甲基或乙基的两个簇合物的单晶结构,正四价的两个钨原子间用双键及两个氧桥键相联接。与M_3W_2Cl_9另一不同处是取代基RO及R′OH与钨原子直接相联,其间还形成氢键,故RO或R′OH对簇胳的影响要大于M_3W_2Cl_9中阳离子M~+。本文合成6种具有不同取代基的W_2Cl_4·(RO)_4(ROH)_2化合物,用EXAFS研究其结构,以期了解不同取代基对簇胳结构的影响。     

Ionization potentials and geometrical structure of pure and oxygenated barium clusters

In this article, we present results obtained on structures of small bare and oxygenated barium clusters produced by the laser vaporization technique. Contrary to the very weak Ba—Ba binding (0.202 eV), the Ba—O binding is rather strong (5.8 eV). As a consequence, extremely small amounts of oxygen in the cluster source are sufficient to induce the nucleation of oxygenated barium clusters. Surprisingly, at least for substoechiometric systems, Ba_nO_m clusters exhibit the same icosahedral structure as bare clusters do. Mass spectra seem to indicate that replacing a barium atom with a strongly polar BaO molecule does not affect the geometrical structure of the cluster. This aspect is more easily understandable if we assume that the oxygen atom takes place inside the structure of the Ba_n cluster without significant distortions. Ab-initio calculations have been undertaken in order to give a satisfactory account of this hypothesis.     

Cluster analysis of single particle mass spectra measured at Flushing, NY

ART-2a and a density based cluster method, density based spatial clustering of application with noise (DBSCAN), have been used for classification of the single particle mass spectra measured at New York City. Using too large of a vigilance factor in ART-2a leads to many similar clusters with overlap, and thus a low vigilance factor was used in this study. The DBSCAN method can identify clusters with complex shapes and various sizes, and representative spectra are chosen to identify different particle types within each cluster. The cluster structure of the single particle mass spectra were examined by DBSCAN. Both methods found that the major clusters were sea salt and anthropogenic combustion emissions. The continua in sulfate, potassium and OC particles were found by DBSCAN and a large cluster was formed, while ART-2a broke it into several small clusters without finding this continuum. A detailed discussion of the cluster analysis results including representative mass spectra, size distributions and temporal behavior will be provided.     

Distribution of interatomic distances in large metallic clusters

Spherically averaged pseudopotential (SAPS) calculations have been done for Mg _n clusters, withn up to 250 within the framework of density functional theory. The electronic structure is computed resorting to the Thomas-Fermi-Dirac-Weizsäcker (TFDW) approximation for the kinetic energy. The equilibrium geometries have been obtained by minimizing the total cluster energy with respect to the atomic positions using the steepest-descent method. The ground state geometries obtained in this way are formed by spherical atomic shells, the number of them increasing with cluster size, up to a number of four for the biggest sizes considered here. An analysis of the distribution of the interatomic distances shows that the more internal is the shell, the more contracted are the interatomic distances. This effect diminishes progressively with increasing cluster size. For the purpose of comparison, similar calculations have been done with Cs _n clusters in the same size range, allowing us to reproduce previous results obtained using a more elaborated density functional technique (Kohn-Sham method). The inhomogeneous contraction of interatomic distances then appears as a general fact for simple metallic clusters and not only for alkaline ones.     

Gold and platinum molecular cluster compounds studied by 197Au Mössbauer Spectroscopy

The technique of ¹⁹⁷Au Mössbauer absorption spectroscopy has been used to study four varieties of the high nuclearity gold molecular cluster compounds abbreviated as Au₅₅, all having a cuboctahedral structure, with 12 PPh₃-ligands or modifications of PPh₃. The technique of emission spectroscopy developed in our laboratory [1] has been applied to four different molecular platinum carbonyl cluster compounds of varying cluster nuclearity, abbreviated as Pt₃₈, Pt₂₆, Pt₂₄, and Pt₁₉ [2]. The four Au55 compounds were studied, both as dry materials and as frozen solutions. The M6ssbauer parameters of the chlorine ligated site of the water soluble version differ strongly due to the effect of Na⁺ on the Au-Cl distances. In the frozen solution, the effect of the ionic charge on this cluster can be clearly seen. The similarity of the spectra of all the Pt compounds, especially the occurrence of a substantial singlet contribution in all spectra, is explained by a coalescing mechanism for the low-nuclearity clusters, induced by the neutron irradiation damage inflicted while preparing the sample as M6ssbauer sources. The observed decrease of the absorption intensity with increasing temperature is evidence for a cluster character of the sample remaining after neutron irradiation.     

Cluster chemical ionization for improved confidence level in sample identification by gas chromatography/mass spectrometry

Upon the supersonic expansion of helium mixed with vapor from an organic solvent (e.g. methanol), various clusters of the solvent with the sample molecules can be formed. As a result of 70 eV electron ionization of these clusters, cluster chemical ionization (cluster CI) mass spectra are obtained. These spectra are characterized by the combination of EI mass spectra of vibrationally cold molecules in the supersonic molecular beam (cold EI) with CI-like appearance of abundant protonated molecules, together with satellite peaks corresponding to protonated or non-protonated clusters of sample compounds with 1-3 solvent molecules. Like CI, cluster CI preferably occurs for polar compounds with high proton affinity. However, in contrast to conventional CI, for non-polar compounds or those with reduced proton affinity the cluster CI mass spectrum converges to that of cold EI. The appearance of a protonated molecule and its solvent cluster peaks, plus the lack of protonation and cluster satellites for prominent EI fragments, enable the unambiguous identification of the molecular ion. In turn, the insertion of the proper molecular ion into the NIST library search of the cold EI mass spectra eliminates those candidates with incorrect molecular mass and thus significantly increases the confidence level in sample identification. Furthermore, molecular mass identification is of prime importance for the analysis of unknown compounds that are absent in the library. Examples are given with emphasis on the cluster CI analysis of carbamate pesticides, high explosives and unknown samples, to demonstrate the usefulness of Supersonic GC/MS (GC/MS with supersonic molecular beam) in the analysis of these thermally labile compounds. Cluster CI is shown to be a practical ionization method, due to its ease-of-use and fast instrumental conversion between EI and cluster CI, which involves the opening of only one valve located at the make-up gas path. The ease-of-use of cluster CI is analogous to that of liquid CI in ion traps with internal ionization, and is in marked contrast to that of CI with most other standard GC/MS systems that require a change of the ion source.     

An improved clusterion-photoelectron coincidence technique for the investigation of the ionisation dynamics of clusters

The introduction of photoion-photoelectron coincidence techniques has made it possible to investigate photoionisation properties of heavy clusters, which are not accessible by conventional mass spectrometry. This technique has been further developed in combination with a zero-volt electron energy analyser and greatly improved in performance. The method has been applied to the investigation of different homogeneous and heterogeneous clusters. This type of cluster experiment requires both a very high resolution and a large dynamic range in order to identify also clusters present in low abundance. As an example, a series of coincidence mass spectra of Xe clusters has been recorded at different wavelengths. Below a photon energy of 11.1 eV, the range of observable clusters shifts to higher cluster sizes with decreasing energy. Appearance potentials and the binding energy of different cluster ions were obtained. Intensity fluctuations, already observed in spectra with electron bombardment ionisation (magic numbers), have also been detected in the coincidence spectra and become most pronounced near the ionisation threshold. This indicates that these fluctuations are caused by the size-dependent stability of the ionic and not the neutral cluster. Furthermore, the threshold size does not change linearly with cluster size. The binding energy per particle seems to change drastically aroundn=13 which indicates the existence of a shell structure in the cluster ion.     

Ion mobility measurements of metal halide clusters

The failure of standard solid state methods to determine the structure of very small clusters has been the starting point to adapt the ion chromatography technique to cluster experiments. A new approach combining an ion drift cell and a time-of-flight mass spectrometer for cluster ion mobility measurements is described. In this publication we concentrate on the experimental set-up and the data analysis starting with the time-of-flight mass spectra. Furthermore, first results concerning relative ion mobilities for cesium iodide and sodium iodide clusters will be shown to demonstrate the feasibility of the new tandem instrument.     

Designed Assembly of Heterometallic Cluster Organic Frameworks Based on Anderson‐Type Polyoxometalate Clusters

A new approach to prepare heterometallic cluster organic frameworks has been developed. The method was employed to link Anderson‐type polyoxometalate (POM) clusters and transition‐metal clusters by using a designed rigid tris(alkoxo) ligand containing a pyridyl group to form a three‐fold interpenetrated anionic diamondoid structure and a 2D anionic layer, respectively. This technique facilitates the integration of the unique inherent properties of Anderson‐type POM clusters and cuprous iodide clusters into one cluster organic framework.     

Ni3Al合金液态与非晶中的原子团簇

采用常温常压分子动力学模拟技术，模拟了液态Ni3Al中原子团簇在快速凝固条件下的演变过程，模型采用的是TB(tight binding)作用势.用偶分布函数、键对和多面体等结构参数来描述快速凝固条件下团簇种类和数量的变化，并将团簇结构可视化.在2 000 K下，液态Ni3Al中团簇数量较少，且都是由缺陷二十面体构成；在4×1013 K•s－1的冷速下，团簇的数量随温度的降低不断增加，且出现完整二十面体团簇，体系最终形成了由二十面体和缺陷二十面体团簇网络所组成的非晶结构.     

A bronze matryoshka: the discrete intermetalloid cluster [Sn@Cu12@Sn20](12-) in the ternary phases A12Cu12Sn21 (A = Na, K)

The synthesis and crystal structure of the first ternary A-Cu-Sn intermetallic phases for the heavier alkali metals A = Na to Cs is reported. The title compounds A(12)Cu(12)Sn(21) show discrete 33-atom intermetalloid Cu-Sn clusters {Sn@Cu(12)@Sn(20)}, which are composed of {Sn(20)} pentagonal dodecahedra surrounding {Cu(12)} icosahedra with single Sn atoms at the center. Na(12)Cu(12)Sn(21) and K(12)Cu(12)Sn(21) were characterized by single-crystal XRD studies, and the successful synthesis of analogous A-Cu-Sn compounds with A = Rb and Cs is deduced from powder XRD data. The isotypic A(12)Cu(12)Sn(21) phases crystallize in the cubic space group Pn ?3m (No. 224), with the Cu-Sn clusters adopting a face centered cubic arrangement. A formal charge of 12- can be assigned to the {Sn@Cu(12)@Sn(20)} cluster unit, and the interpretation of the title compounds as salt-like intermetallic phases featuring discrete anionic intermetalloid [Sn@Cu(12)@Sn(20)](12-) clusters separated by alkali metal cations is supported by electronic structure calculations. For both Na(12)Cu(12)Sn(21) and K(12)Cu(12)Sn(21), DFT band structure calculations (TB-LMTO-ASA) reveal a band gap. The discrete [Sn@Cu(12)@Sn(20)](12-) cluster is analyzed in consideration of the molecular orbitals obtained from hybrid DFT calculations (Gaussian 09) for the cluster anion. The [Sn@Cu(12)@Sn(20)](12-) cluster MOs can be classified with labels indicating the numbers of radial and angular nodes, in the style of spherical shell models of cluster bonding.     

Synchrotron radiation study of chloromethane clusters: effects of polarizability and dipole moment on core level chemical shifts

Free neutral chloromethane clusters have been produced by adiabatic expansion and investigated by means of photoelectron spectroscopy. By studying the differences between the cluster binding energy shifts at the Cl 2p and C 1s thresholds we have shown that antiparallel packing is the dominant local structure of the clusters. This geometry is induced by the polar character of the constituent molecules, and it accords with the structures of chloromethane solids and dimers. Furthermore, results obtained from the analysis of the outer valence levels of the clusters support the suggested local structure. The roles of the polarizability and of the dipole moment of the constituent molecules in the cluster binding energy shifts are discussed in comparison with a recent investigation of bromomethane clusters.     

夹心三明治结构的二十五核银硫簇合物[Ag25(SC6H4Pri)18 (dppp)6](CF3SO3)7·CH3CN：结构表征及光学性能

在本研究中,使用(ⁱPrC₆H₄SAg)_n作为合成银硫纳米簇合物的前驱体。当存在辅助的双膦配体1, 3-双(二苯基膦)丙烷(dppp)及可溶性银盐(CF₃SO₃Ag),在超声合成条件下,获得了一例二十五核银硫簇合物Ag₂₅,并借助X射线单晶衍射技术确定相应的分子结构。该银硫簇合物具有夹心三明治的骨架结构：分别由两个结构相似的圆柱体共享一个七核纯银簇所构成的金属簇平面。固态紫外可见漫反射光谱测试表明该簇合物的能级带隙为2.5 eV,属于半导体材料的范畴。固态发光测试显示,该簇合物在室温下发射绿光。     

Synthese und Molekülstrukturen von amido‐ und imidoverbrückten Clustern elektronenreicher Übergangsmetalle

Synthesis and Structures of Amido‐ and Imidobridged Clusters of Electron‐rich Transition Metals We report in this article the results, which could be obtained within the DFG‐Project “Nitridobrücken zwischen Übergangsmetallen und Hauptgruppenelementen”. Reactions of electron‐rich transition metal compounds with either stannylated or lithiated amine derivatives lead in the presence of phosphines in different organic solvents to the formation of a large amount of nitrogenbridged transition metal clusters. The structures of 1 — 27 have been characterized by single crystal X‐ray‐structure analysis.     

Reinforcement of polystyrene by covalently bonded oxo-titanium clusters

Oxo-metallic clusters are employed as inorganic nanobuilding blocks in order to obtain new organic–inorganic hybrid materials. Nanobuilding blocks are well-defined preformed entities which allow a better control of the inorganic domains for the elaboration of hybrid compounds. The oxo-alcoxo cluster Ti₁₆O₁₆(OEt)₃₂ presents a shell of labile ethoxy groups which can be selectively exchanged with the preservation of the oxo-core in order to introduce polymerizable ligands at the surface of this nanobrick. Three different new clusters, Ti₁₆O₁₆(OEt)_32−x(OPhCHCH₂)_x, have been synthesised, each cluster bears exactly 4, 8 and 16 styrenic groups. These functional clusters were copolymerized with styrene leading to three dimensional networks where the inorganic nano-fillers are covalently linked to the organic polymer. Thus new hybrid materials can be obtained and these nanobricks are good models to correlate the structure of hybrid materials and their physical properties especially their mechanical and thermal properties. The structure of the materials in function of the organic–inorganic ratio and in function of the cluster functionalities was investigated by SAXS, and the formation of the different levels of aggregation is reported.     

表面辅助激光解吸电离-飞行时间质谱中多聚体离子产生规律的研究

表面辅助激光解吸电离质谱(SALDI-MS)已经成为固态、液态样品分析的重要手段,并且分析对象逐渐由生物大分子扩展到小分子.然而,对小分子电离微观反应机理的研究仍处于起步阶段.本研究选择3种分子结构相似的稠环芳烃化合物芘、六苯并苯、红荧烯作为研究对象,考察了这3种化合物激光电离产物的差异,并研究了激光能量对六苯并苯产物离子分布的影响.结果表明,观察到芘、六苯并苯多聚体离子产物,还观测到了六苯并苯失去C2H2的碎片离子峰;而没有观测到明显的红荧烯聚合物离子产物,只观测到了大量失去C6H5的碎片离子峰.最后,对3种不同化合物的激光电离微观机理进行了分析.由于芘、六苯并苯具有平面大π键,分子间的π-π键相互作用是产生多聚体离子的主要原因;而红荧烯的空间位阻削弱了分子间相互作用,从而阻碍了多聚体离子的形成.     

Metalloid aluminum and gallium clusters: element modifications on the molecular scale?

As members of the same group in the periodic table, the industrially significant elements aluminum and gallium exhibit strong similarities in the majority of their compounds. In contrast there are significant differences in the structures of the two elemental forms: Aluminum forms a typical closest-packed metallic structure whereas gallium demonstrates a diversity of molecular bonding principles in its seven structural modifications. It can therefore be expected that differences between Al and Ga compounds will arise when, as for the elemental forms, many metal-metal bonds are formed. To synthesize such cluster compounds, we have developed the following synthesis procedure: Starting from gaseous monohalides at around 1000 degrees C, metastable solutions are generated from which the elements ultimately precipitate by means of a disproportionation reaction at room temperature. On the way to the elemental forms, molecular Al and Ga cluster compounds can be obtained by selection of suitable ligands (protecting groups), in which a core of Al or Ga atoms are protected from the formation of the solid element by a ligand shell. Since the arrangement of atoms in such clusters corresponds to that in the elements, we have designated these clusters as metalloid or elementoid. In accordance with the Greek word [see text] (ideal, prototype), the atomic arrangement in metalloid clusters represents the prototypic or ideal atomic arrangement in the elements at the molecular level. The largest clusters of this type contain 77 Al or 84 Ga atoms and have diameters of up to two nanometers. They hold the world record with respect to the naked metal-atom core for structurally characterized metalloid clusters.     

Particle-swarm structure prediction on clusters

We have developed an efficient method for cluster structure prediction based on the generalization of particle swarm optimization (PSO). A local version of PSO algorithm was implemented to utilize a fine exploration of potential energy surface for a given non-periodic system. We have specifically devised a technique of so-called bond characterization matrix (BCM) to allow the proper measure on the structural similarity. The BCM technique was then employed to eliminate similar structures and define the desirable local search spaces. We find that the introduction of point group symmetries into generation of cluster structures enables structural diversity and apparently avoids the generation of liquid-like (or disordered) clusters for large systems, thus considerably improving the structural search efficiency. We have incorporated Metropolis criterion into our method to further enhance the structural evolution towards low-energy regimes of potential energy surfaces. Our method has been extensively benchmarked on Lennard-Jones clusters with different sizes up to 150 atoms and applied into prediction of new structures of medium-sized Li(n) (n = 20, 40, 58) clusters. High search efficiency was achieved, demonstrating the reliability of the current methodology and its promise as a major method on cluster structure prediction.