Strain-induced yielding in bubble clusters

We study how shearing clusters of two or four bubbles induces bubble separation or topological rearrangement. The critical deformation at which this yielding occurs is measured as a function of shear rate, liquid composition, and liquid content in the cluster. We establish a geometrical yield criterion in the quasistatic case on the basis of these experimental data as well as simulations. In the dynamic regime, the deformation where the cluster yields increases with the strain rate, and we derive a scaling law describing this phenomenon based on the dynamical inertial rupture of the liquid meniscus linking the two bubbles. Our experiments show that the same scaling law applies to two- and four-bubble clusters.     

Polymer-protected bimetallic clusters with various alloying structures

Colloidal dispersions of polymer-protected bimetallic cluster can be prepared by refluxing the alcoholic solutions of two kinds of metal ions in the presence of poly(N-vinyl-2-pyrrolidone). The bimetallic clusters thus prepared had higher activity as catalysts than the corresponding monometallic clusters. The structure of bimetallic clusters, analyzed by an Extended X-ray Absorption Fine Structure(EXAFS) technique as well as ultraviolet-visible(Uv-vis) absorption spectra, depends on the preparation method. Some of them have a special core-shell structure. The order of the tendency to be a core is estimated to be Au>Pt>Pd>Rh. This order could be explained by the redox potential of the corresponding metal ion and the coordination ability of the metal to polymer.     

Packing of protein structures in clusters with magic numbers

Recently we have proposed a model for folding proteins into packed’ clusters’. We have constructed a local homology measure for protein fold classes by projecting consecutively secondary structures onto a lattice. Taking into account hydrophobic forces we have found a mechanism for formation of clusters containing magic numbers of secondary structures and multipla of these clusters. A scheme for the relation between the sequence information and the native fold is given. We have performed a statistical analysis of available protein structures and found agreement with the predicted preferred abundances. In this paper we demonstrate that the results are robust to variations in the coordination number of the model.     

The influence of solvent and ligands on characters of ZnS clusters

According to zinc blende and wurtzite structures of ZnS nanocrystals, four clusters (Zn₃S₃, Zn₄S₄, (Zn₃S₃)₂, and (Zn₃S₃)₃), were investigated at B3LYP/Lanl2dz theoretical level. In simultaneous consideration of the influence of solvent and ligands, we calculated their Raman and absorption peaks, which are agreement with experimentally reported results. The calculated Raman spectra of Zn₃S₃, Zn₄S₄, (Zn₃S₃)₂, and (Zn₃S₃)₃ are in the range of 260–310 cm⁻¹. During the calculation of absorption spectra, time-dependent density-functional theory (TDDFT) is employed. We have found an obvious blue-shift in the calculated wavelengths of the absorption peaks after consideration of the solvent. In solvent environment, the wavelength of absorption peak shifts to red with the increase of the atomic numbers from Zn₃S₃, to (Zn₃S₃)₂ and (Zn₃S₃)₃ clusters, which is induced by the quantum size effect. Since the sizes of the current calculated clusters are much smaller than the experimentally reported nano-sized ZnS nanocrystals, the calculated wavelengths of absorption peak of the four clusters are shorter than the nano-sized ZnS nanocrystals. Through the analysis of S–Zn–ligand structures, we speculate that the main influence of ligands comes from thiol of ligand because all S–Zn–ligand structures have similar Wiberg Bond Index (WBI) values, absorption spectra, and bond length in theory.     

Gas-phase structures of neutral silicon clusters

Vibrational spectra of neutral silicon clusters Si(n), in the size range of n = 6-10 and for n = 15, have been measured in the gas phase by two fundamentally different IR spectroscopic methods. Silicon clusters composed of 8, 9, and 15 atoms have been studied by IR multiple photon dissociation spectroscopy of a cluster-xenon complex, while clusters containing 6, 7, 9, and 10 atoms have been studied by a tunable IR-UV two-color ionization scheme. Comparison of both methods is possible for the Si(9) cluster. By using density functional theory, an identification of the experimentally observed neutral cluster structures is possible, and the effect of charge on the structure of neutrals and cations, which have been previously studied via IR multiple photon dissociation, can be investigated. Whereas the structures of small clusters are based on bipyramidal motifs, a trigonal prism as central unit is found in larger clusters. Bond weakening due to the loss of an electron leads to a major structural change between neutral and cationic Si(8).     

Electronic structures of exohedral lanthanide-C60 clusters

We have studied the electronic structures of several gas phase exohedral lanthanide (Ln)-C(60) clusters, Ln(n)C(60) (Ln=Pr, Ho, Tb, Tm, Eu, and Yb) with n=1-4, by photoionization spectroscopy of the neutrals and photoelectron spectroscopy of their anions. Both of the spectroscopic analyses reveal that most of the Ln atoms preferably take +3 oxidation states, while Eu atoms alone assume +2 oxidation states, and that C(60) accepts up to twelve donated electrons in Ln(n)C(60). An additional photoionization examination of the oxygen atom mixing into the Ln(n)C(60) clusters demonstrated that each oxygen atom reduces two electrons from C(60). This result implies that the number of accepted electrons in C(60) can be varied by a suitable choice of the number of Ln atoms and O atoms.     

General principles governing structures of small clusters

General rules which govern electronic and geometric structures of small clusters are formulated, and their validity is documented with the results of the MRD - CI investigations for Li _n , BeLi _k , Be _l (n=2?14,k=2?6,l=2?13) as well as on IIa and IVa tetramers. The MRD - CI results are compared with investigations performed with other methods.     

Studies on structures of lipid A-monophosphate clusters

Single crystalline clusters of lipid A-monophosphate were grown from organic dispersions containing 5-15% (v/v) water at various volume fractions, φ, and temperatures. The morphology of the single lipid A-monophosphate crystals was either rhombohedral or hexagonal. The hexagonal crystals were needlelike or cylindrical in shape, with the long dimension parallel to the c axis of the unit cell. The crystalline clusters were studied using electron microscopy and x-ray powder diffraction. Employing molecular location methods following a Rietveld refinement and whole-pattern refinement revealed two monoclinic crystal structures in the space groups P2(1) and C2, both converged with R(F) = 0.179. The two monoclinic crystal structures were packing (hydrocarbon chains) and conformational (sugar) polymorphs. Neither of these two structures had been encountered previously. Only intramolecular hydrogen bonding was observed for the polymorphs, which were located between the amide and the carboxyl groups. Another crystalline structure was found in the volume-fraction range 2.00 × 10(-3) ≤ φ ≤ 2.50 × 10(-3), which displayed hexagonal symmetry. The hexagonal symmetry of the self-assembled lipid A-monophosphate crystalline phase might be reconciled with the monoclinic symmetry found at low-volume-fractions. Therefore, lowering the symmetry from cubic, i.e., Ia 3d, to rhombohedral R 3 m, and finally to the monoclinic space group C2 was acceptable if the lipid A-monophosphate anion was completely orientationally ordered.     

Benchmark structures and binding energies of small water clusters with anharmonicity corrections

For (H(2)O)(n) where n = 1-10, we used a scheme combining molecular dynamics sampling with high level ab initio calculations to locate the global and many low lying local minima for each cluster. For each isomer, we extrapolated the RI-MP2 energies to their complete basis set limit, included a CCSD(T) correction using a smaller basis set and added finite temperature corrections within the rigid-rotor-harmonic-oscillator (RRHO) model using scaled and unscaled harmonic vibrational frequencies. The vibrational scaling factors were determined specifically for water clusters by comparing harmonic frequencies with VPT2 fundamental frequencies. We find the CCSD(T) correction to the RI-MP2 binding energy to be small (<1%) but still important in determining accurate conformational energies. Anharmonic corrections are found to be non-negligble; they do not alter the energetic ordering of isomers, but they do lower the free energies of formation of the water clusters by as much as 4 kcal/mol at 298.15 K.     

Electronic structures and state densities for faujasite clusters

MINDO/3 and CNDO/2 methods have been applied to clusters simulating the structure of faujasite, with the inclusion of from 6 to 12 aluminum-oxygen and silicon-oxygen tetrahedra. A study has been made of how the integral and orbital electron-state densities vary with the dimensions of the zeolite fragment, the boundary conditions, and the calculation method. The fine structure in the cation state density peaks for aluminum in the valency band is only slightly dependent on the boundary conditions and the cluster dimensions, while the differences between the MINDO/3 and CNDO/2 results amount to a systematic shift in the peaks together with slight changes in the fine structure. On the other hand, the boundary conditions have a considerable effect on the orbital state density profiles for the oxygen ions in the upper valency quasiband for the faujasite clusters.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 2, pp. 210–216, March–April, 1986.     

Isomeric structures and structural transformations in small clusters

The temperature dependence of energies of the isomers of a seven-particle system is studied with a view toward understanding ergodicity problems in Monte Carlo simulations. It is found that the phase space of particles in a cluster is not ergodic at lower temperatures.     

Binary clusters: homogeneous alloys and nucleus-shell structures

Extinction spectra of Ag_x-Au_(1?x)-alloy particles embedded in glass are compared to spectra of gold coated silver clusters and silver coated gold clusters. It is shown, that the optical extinction of alloy particles is described by the Mie theory applying the homogeneous dielectric function \(\hat \varepsilon \) (ω,R, x) measured by Paquet. The absorption of core-shell clusters was calculated deriving an extension of the Mie theory to spheres with arbitrary numbers of shells of arbitrary materials. Comparison to measured spectra points to s-electron motion in the clusters with only slight scattering at the interior interface. The appearance of two distinct Mie peaks however proves the existence of the sharp Ag-Au interface. The extended Mie formalism was also applied to a multilayer system consisting of sodium and a dielectric substance as an example for a spherical hetero structure.     

Cyclic water clusters in tape-like and cage-like structures

Controlling the ratio of 2,2'-bpy to benzene-1,3,5-tricarboxylic acid produces two interesting complexes, namely [Co(2,2'-bpy)?]?(SO?)?8.5H?O (1) and [Cu?(BTCA) (2,2'-bpy)?] (OH)?(2,2'-bpy)?.??14H?O (2) (H?BTCA = benzene-1,3,5-tricarboxylic acid, 2,2'-bpy = 2,2'-bipyridine). We report the structural evidence in the solid state of discrete lamellar water cluster conformations. These units are found to act as supramolecular glue in the aggregation of cobalt (II) or copper (II) complexes to give three dimensional cage-like networks through hydrogen-bonding. It is interesting that the structure of complex 1 contains a 3D negatively charged cage.     

Ordered structures of defect clusters in gadolinium-doped ceria

The nano-domain, with short-range ordered structure, has been widely observed in rare-earth-doped ceria. Atomistic simulation has been employed to investigate the ordering structure of the nano-domain, as a result of aggregation and segregation of dopant cations and the associated oxygen vacancies in gadolinium-doped ceria. It is found that the binding energy of defect cluster increases as a function of cluster size, which provides the intrinsic driving force for the defect cluster growth. However, the ordered structures of the defect clusters are different from the chain model as previously reported. Adjacent oxygen vacancies prefer to locate along <110>/2 lattice vector, which results in a unique stable structure (isosceles triangle) formation. Such isosceles triangle structure can act as the smallest unit of cluster growth to form a symmetric dumbbell structure. This unique dumbbell structure is hence considered as a building block for the development of larger defect clusters, leading to nano-domain formation in rare-earth-doped ceria.     

IR-photodissociation of size selected molecular clusters and their structures

Infrared photodissociation spectra of (CH₃NH₂) _n clusters were measured fromn=2 ton=6 near the monomer absorption of the C-N stretching mode at 1044 cm^?1 using a cw-CO₂ laser. The clusters were size-selected by scattering from a helium beam. The spectrum of cold dimers shows a red (1038 cm^?1) and a blue (1048 cm^?1) shifted peak which is attributed to the non-equivalent position of the C-N in the open dimer structure. The larger clusters exhibit only one peak between 1045.4 cm^?1 and 1046.0 cm^?1 caused by the equivalent position of the C-N in the cyclic structures of the larger clusters. Structure calculations confirm these results. Secondly, the mixed complexes C₂H₄-CH₃COCH₃ and C₂H₄-(CH₃COCH₃)₂ were investigated. The dimer spectrum, measured around the monomer frequency of the out-of-plane bending mode of C₂H₄ at 949 cm^?1, shows two peaks at 946.2 cm^?1 and 961.3 cm^?1. This splitting is attributed to two different isomers that are found in configuration calculations. A similar behaviour is found for the trimer.     

簇外环化镍十硼烷簇合物的合成与结构研究

研究了[NiCl~2(PPh~3)~2],B~1~0H~1~0^2^-与硫代苯甲酸的反应,得到四个簇合物,其中三个簇合物[(PPh~3)(PhCOS)~2Ni·B~1~0H~1~0]·0.5C~6H~1~4(1),[(PhCOS)~2NiB~1~0H~8(PPh~3)](2),[(PhCOS)~3NiB~1~0H~7(PPh~3)](3)。通过单晶X射线衍射进行了结构研究。三个簇合物均为十一顶巢式构型,并分别存在两个、两个、三个簇外环化的五元环,具有三个环的簇合物至今未见其它文献报道。结构分析表明：簇外环化可以增强Ni－B之间的成键作用。     

Electronic structures of electron-rich octahedrally condensed transition-metal chalcogenide clusters

The electronic structures of some electron-rich octahedrally condensed transition-metal chalcogenide clusters are analyzed with the aid of extended Hückel and density functional molecular orbital calculations. A simple orbital approach is developed to analyze the electron counts of these clusters, which do not obey any existing electron-counting rules. Different electron counts are allowed, depending upon the nature of the metal. Optimal counts are discussed. Metal-metal bonding is generally weak in these species. Consequently, their structural arrangements are mainly governed by metal-ligand interactions.