Evaporation of tetramers in Sb4n clusters and conditions for the formation of Sb2n+1 clusters

Antimony clusters are produced by the inert gas condensation technique. They are found to be built from Sb₄ units. The fragmentation by evaporation of Sb₄ units is studied as a function of the excess energy in the cluster. By this way the binding energy of the Sb₄ units in the cluster is found to be about 1.5 eV, well below the binding energy of a Sb atom in the bulk and in Sb₄(?3eV). The evolution of ionization potentials of Sb_4n clusters confirms that their structure is probably non metallic. Finally the possible metastable character of this Sb_4n structure is discussed.     

Monte Carlo simulation for the adsorption of symmetric triblock copolymers I. Configuration distribution and density profiles of adsorbed chains

Monte Carlo simulations for the adsorption of symmetric triblock copolymers from a nonselective solvent at a solid-liquid interface have been performed on a lattice model. In simulations, triblock copolymer molecules are modeled as self-avoiding linear chains composed of m segments of A and n segments of B arranged as A_m/2B_nA_m/2. Either segment A or segment B is attractive, while the other is non-attractive to the surface. The microstructure of the adsorbed layers, including the segment-density profiles and the size distribution of loops, tails and trains are presented. The effect of the adsorption energy, the bulk concentration, the chain composition, as well as the chain length on various adsorption properties has been studied. The results have shown that the size distribution of various configurations is dependent of the adsorption energy, the chain composition and the chain length. The mean length of the loops, trains and tails is insensitive to the bulk concentration. The mean length of the trains increases and that of the tails decreases as the adsorption energy and the length of the attractive segments increase. The mean length of the loops for the end-adsorbed copolymers appears a maximum and that for middle-adsorbed copolymers appears a minimum as the length of attractive segments increases. The length of the non-attractive segments affects mostly the size distribution of the tails. The longer the chain is, the larger the tail appears. The mean length of the tails and loops increases linearly as the length of the non-attractive segments increases, but that of the trains approximately is unchanged.     

Tetranuclear copper(II) complex with the heterocyclic azomethine ligand: Crystal structure and magnetic properties

A tetranuclear copper(II) complex based on azomethine, which is the condensation product of 1-phenyl-3-methyl-4-formylpyrazol-5-one with 1,3-diaminopropan-2-ol, is synthesized. The complex includes two different tetranuclear clusters: symmetrical and unsymmetrical. They have a pseudo-cubane structure and are in a ratio of 1 : 2. The quantum-chemical calculation shows that the “unsymmetrical” conformer does not correspond to the local minimum on the molecular potential energy surface. Its existence is thus determined by the crystalline packing effects. According to the results of measurements of the temperature dependence of the magnetic susceptibility, the ground spin state is a singlet caused by the overall antiferromagnetic interaction between the copper ions. Accepting the molar magnetic susceptibility of the complex to be equal to the sum of susceptibilities of the “symmetrical” and “unsymmetrical” clusters and assuming that the spin-Hamiltonian for both clusters includes three exchange parameters, the temperature dependence of the magnetic susceptibility of the complex is satisfactorily described with the following parameters of the model: J _1A = ?178, J _2A = 80, J _3A = 18, J _1B = ?26, J _2B = ?74, J _3B = 46 cm^?1, g _A = g _B = 2.05.     

Photoinduced hydrogen evolution with bisviologen-linked ruthenium(II) complexes and hydrogenase

Bisviologen-linked ruthenium(II) complexes with different methylene chain length between ruthenium complex and viologen, Ru(bpy)₂(dcbpy)C_mV_AC_nV_B(m=2, n=3; m=3, n=4), were synthesized and characterized. From luminescence spectra, the photoexcited state of Ru(bpy)₂(dcbpy) moiety is oxidatively quenched by the bound viologen, and an intramolecular electron transfer occurs. Luminescence lifetime measurements show that the electron transfers from the photoexcited state of ruthenium(II) complex moiety to the bound bisviologen more rapidly than that of monoviologen-linked ruthenium(II) complexes. Ru(bpy)₂(dcbpy)C_mV_AC_nV_B were applied to the photoinduced hydrogen evolution in the system containing nicotinamide-adenine dinucleotide phosphate (reduced form, NADPH), Ru(bpy)₂(dcbpy)C_mV_AC_nV_B and hydrogenase under steady state irradiation. In the case of Ru(bpy)₂(dcbpy)C₃V_AC₄V_B, the efficient photoinduced hydrogen evolution was observed.     

Facial dissociations of water molecules on the outside and inside of armchair single-walled silicon nanotubes: theoretical predictions from multilayer quantum chemical calculations

The possible reaction pathways of dissociative adsorption of a single water molecule on the sidewalls of armchair (n, n) (n?=?4?C10) single-walled silicon nanotubes (SWSiNT) have been investigated by the multilayer models. Both the simplified fragment embedding and ONIOM calculations were carried out to study the diameter dependence of reactivity for the dissociation of water on SWSiNTs. The active fragments with different cluster sizes, such as Si₁₆H₁₀, Si₃₀H₁₆, and Si_10mH_4m (m?=?4?C10), were used for the multilayer calculations. The employment of the medium-sized Si₃₀H₁₆ cluster is able to reach a good balance between the computational efficiency and accuracy for the large-sized reaction system. In comparison with those full B3LYP/LANL2DZ calculations for Si(4,4) and Si(5,5) nanotubes, the approximate multilayered models can give reasonable predictions on the optimized geometries, activation energies, and exothermic energies with significant reduction in computational cost. The external complexes of the dissociative adsorption of H₂O on SWSiNTs were predicted to be more stable than those internal complexes. The convex surfaces of SWSiNTs were also more reactive to H₂O with the smaller activation barrier energies (10?C13?kcal/mol) than those (15?C22?kcal/mol) on the concave side. Both the activation barriers and exothermic energies of dissociative adsorptions of H₂O on the internal (external) sidewalls of armchair SWSiNT were found to be insensitive to the tube curvature. The passivation of the outer surface and the removal of water molecules may be crucial for the experimental preparation of the single-walled silicon nanotubes.     

Adsorption of NO2 on Small Silver Clusters with Copper Impurity: A Density Functional Study

Density functional theory calculations were performed to investigate the structural and energetic properties of NO₂ adsorption on small bimetallic Ag _n Cu _m clusters (n?+?m?≤?5). Generally NO₂ is adsorbed in bridge configuration. The adsorbates prefer Cu sites when both Ag and Cu co-exist in the clusters. The adsorption energies and the dissociation energies of the complex clusters increase as the Cu content increases for the given cluster size. Our calculation suggests that the bimetallic Ag _n Cu _m may react with NO₂ dissociatively by way of Ag atom, Ag₂ or AgCu loss. The N–O vibrational properties of the complex clusters were also discussed and analyzed.     

Conformational behavior of styrene–p-chlorostyrene triblock copolymers in dilute solutions. I. Composition dependence of conformational behavior

Dilute solution properties of (styrene-p-chlorostyrene) triblock copolymers in various solvents were studied over a wide range of molecular weight and composition. Viscosity and osmotic pressure results indicate that the conformational behavior of the B_mA_nB_m and A_mB_nA_m copolymers (A = styrene; B = p-chlorostyrene; m and n denote the number of units) are similar in nonselective solvents such as toluene and 2-butanone, but different in selective solvents such as carbon tetrachloride and cumene. Short-range and long-range interaction parameters of the block copolymers were determined by applying the Stockmayer–Fixman method to viscosity data and also by application of the equation relating the osmotic virial coefficient and the excluded volume. The results show that the unperturbed dimensions of the block copolymers vary linearly with composition, and long-range interaction parameters in nonselective solvents can be expressed by those of the parent homopolymers, the chemical composition, and values of the interaction parameter β_AB between styrene and p-chlorostyrene monomeric units.     

Rules for macropolyhedral boranes

Based on extensive computational studies, rules to derive the thermodynamically most stable macropolyhedral borane for any formula between B_nH_n−4 to B_nH_n+8 were identified. Formally, the macropolyhedral boranes may be obtained by condensing regular convex borane clusters where as many BH₃ moieties are eliminated as vertexes are shared in the macropolyhedral framework. Macropolyhedral boranes consisting of two cluster fragments may be classified according to their general formulae ranging from B_nH_n−4 to B_nH_n+8. For each of these formulae, various structure types are conceivable differing in the number of shared vertexes and in the types of combined cluster fragments. However, for each general formula, only one structure type is known experimentally and this one is also computationally found to be thermodynamically preferred! For each class of macropolyhedral B_nH_m boranes, a preferred number of shared vertexes is identified, and this determines the number of skeletal electron pairs. With this knowledge, the type of fused clusters, i.e. the most favourable framework, may be predicted. The concept of preferred fragments may be applied to even predict the distribution of vertexes among the fused fragments in the thermodynamically most stable isomers. When there is at least one closo fragment it has 12-vertexes. Without any closo fragment the most stable macropolyhedral borane has a nido 10-vertex cluster fragment.     

FTMS studies of mass-selected,large cluster ions produced by direct laser vaporization

A method is described for the production of large cluster ions by direct laser vaporization in a low-pressure FTMS. Production of high-mass carbon cluster ions (C_n⁺; 40 <n < 180) and bismuth-antimony (Bi_xSb_y⁺) cluster ions containing up to five metal atoms are reported. The observed distributions are compared with those obtained previously by both direct laser vaporization and molecular beam sources. Details of the mechanism for formation of these larger cluster ions by direct laser vaporization are discussed. The mass selectivity and long ion residence times obtainable in the FTMS may now be utilized in the study of these cluster ions. Results are presented from a limited study of the ion/molecule reactions and collision induced dissociation of the high-mass carbon cluster ions.     

Univalent metal uranyl carbonates

Optimal procedures were developed for the synthesis of A _4?m ^I B _m ^I [UO₂(CO₃)₃]·nH₂O (A^I = Li, Na, K, Rb, Cs, NH₄, Ag, Tl; B^I = Na; m = 0?1; n = 0?6) compounds. The structure and thermal decomposition of these compounds were studied by X-ray diffraction, precision IR spectroscopy, and thermal analysis. The IR bands were assigned using mathematical simulation based on small-vibration theory. The standard enthalpies of formation of the compounds at 298.15 K were determined for the first time by reaction calorimetry.     

Mischkristalle aus A4B3-Molekülen (A = P,As; B = S,Se)

Mixed Crystals from A₄B₃ Molecules (A = P, As; B = S, Se) The system P₄S₃? P₄Se₃? As₄S₃? As₄Se₃ was investigated by thermal and x-ray methods. Five regions of solid solubility with different crystal structures were found at room temperature. The range of existence can be influenced by the temperature of annealing. All these phases transform into a plastic-crystalline modification with complete solid solubility at higher temperature. A decomposition reaction of the A₄B₃ molecules was observed in the P₄Se₃/As₄Se₃/As₄S₃ part of the system. The molecules decompose into A₄B₄ molecules and an amorphous phase. The existence of all molecules of the type P_nAs_4–nS_mSe_3–m (n = 0–4, m = 0–3) and also As₄S_mSe_4–m (m = 1–3) was verified by mass spectrometric measurements. The thermochemical data of the mixed crystals are determined by the type of the constituent A₄B₃ molecules. The temperature and the entropy of the α–β transition are lower for mixed crystals, formed by substituted molecules, than for those of the same structure, consisting of pure A₄B₃ molecules.     

Comparative DFT study of N2 and no adsorption on vanadium clusters Vn (n = 2–13)

Using gradient‐corrected density functional theory, we have comparatively studied the adsorption properties of diatomic molecules N₂ and NO on vanadium clusters up to 13 atoms. Spontaneous dissociation is found for N₂ adsorbing on V_n with n = 4–6, 12, and for NO with n = 3–12, respectively, whereas for the rest of the clusters, N₂ (NO) molecularly adsorbs on the cluster for all the possible sites. The incoming N₂ retains the magnetism of V_n except for V₂ and V₆ whose moments are quenched from 2 μ_B to zero. Consequently, the moments of V_nN₂ (n = 2–13) show even/odd oscillation between 0 and 1 μ_B. On the adsorption of NO, the magnetic moments of V_n with closed electronic shell are raised to 1 μ_B at n = 4, 8, and 10, and 3 μ_B at n = 12, whereas for open shell clusters, their magnetic moments increase for n = 5 and 9 and decrease for n = 2, 3, 5–7, 11, and 13 by 1 μ_B. These findings are rationalized by combinatory analysis from several aspects, for example, the geometry and stability of bare clusters, charge transfer induced by the adsorption, feature of frontier orbitals, and spin density distribution. © 2012 Wiley Periodicals, Inc.     

Structure and Magnetism of Hybrid Fe and Co Nanoclusters up to N?≤?7 Atoms

The structural and magnetic properties of small gas-phase Fe _m Co _n clusters with m?+?n ranging between 2 and 7 atoms are investigated using spin-polarized density functional theory. For a given cluster size possible compositions are subject to optimization using a variety of initial structures. The geometry, bond lengths, binding energies and magnetization are reported for the lowest energy structures. The results show that a magnetization peak occurs for Fe₄, while for hybrid clusters, switching a cobalt atom with an iron atom increases the cluster’s total magnetization by 1?μ _B . Our structural predictions are generally in agreement with other theoretical results; the origin of the discrepancies arising in some cases is discussed.     

Syntheses and characterization of zero-dimensional molybdoantimonites, A2(Mo4Sb2O18) (A=Y, La, Nd, Sm, Gd and Dy)

Six new isostructural A₂(Mo₄Sb₂O₁₈) (A=Y, La, Nd, Sm, Gd and Dy) compounds have been synthesized by solid-state reactions and characterized by single crystal X-ray diffraction and spectroscopic techniques. They crystallize in C2/c space group with 4 formula units and contain A³⁺ cations and discrete centrosymmetric anionic (Mo₄Sb₂O₁₈)⁶⁻ aggregates, made of tetrahedral MoO₄ and disphenoidal SbO₄ moieties. They exhibit characteristic Sb³⁺ photoluminescence.     

Theory of titration curves: Locations of points of maximum slope on potentiometric heterovalent (“asymmetrical”) precipitation titration curves

By a wholly rigorous general treatment of the potentiometric titration curve representing the titration of B^m+ with A^n- to give the precipitate B_nA_m, where n ≠ m, it is shown on taking into account the effect of dilution that: (1) If $\text{n}\text{m}$ > 1, the inflection point must precede the equivalence point, but there is no inflection point if the initial concentration of the ion titrated is smaller than a certain value, which is determined by the values of m, n, and the solubility product of the precipitate. (2) If $\text{n}\text{m}$ < 1, the inflection point may follow the equivalence point — though not by more than a certain definite amount — or may precede it or coincide with it. In every such case there is a concentration of A^n- that will cause the equivalence point to coincide with an inflection point; this concentration depends on the values of m, n, and the solubility product of the precipitate. (3) Unless the difference between the equivalence and inflection points is negligibly small, the traditional treatment in which dilution is neglected gives seriously erroneous estimates of the location of the inflection point.     

Macro-and microphase separation in nonaqueous solutions of binary multiblock copolymers

In terms of the random phase approximation, a global analysis of the thermodynamic stability of solutions of multiblock copolymers (A_nB_m)_k and (A_nB_m)_kA_n was performed. Phase diagrams for various parameters of the copolymer structure including the macrophase and microphase separation regions were obtained. Critical lines near which this approximation still holds and the microphase separation corresponds to the weak segregation mode were also constructed. The formation of ordered structures with a period of the order of a wavelength of visible light becomes possible under certain conditions. It was shown that the homogeneous state of solutions of multiblock copolymers in certain narrow ranges of values of the parameters breaks down through a simultaneous growth of fluctuations with substantially different wavelengths, a phenomenon which must lead to the appearance of structures ordered at two length scales. The width of such regions increases with an increase in the number of blocks k and a decrease in the degree of polymerization n + m of a block. The potential use of these multiblock-copolymer solutions as photonic crystals is discussed.     

NMR in metal cluster compounds compared to glasses

The field and temperature dependence of the³¹P nuclear spin lattice relaxation rate in the metal cluster compound Ru₅₅(P(t-Bu)₃)₁₂Cl₂₀ follows a power law: 1/T ₁ ∝T ⁿ B ^?m , withn=1.5±0.1 at 3.25 T andn=1.3±0.1 at 6.45 T;m ? 1.4. Such dependences have so far only been observed in inorganic glasses and been attributed to two level systems. The correspondence suggests that the relaxation rate is due to interaction of theP-nuclear moment with electronic spins of stochastically moving charge carriers, which are thought to be responsible for the electrical conductivity through hopping between neigboring cluster molecules.     

Argon Adsorption on Cationic Gold Clusters Aun+ (n ≤ 20)

The interaction of Au_n⁺ (n ≤ 20) clusters with Ar is investigated by combining mass spectrometric experiments and density functional theory calculations. We show that the inert Ar atom forms relatively strong bonds with Au_n⁺. The strength of the bond strongly varies with the cluster size and is governed by a fine interplay between geometry and electronic structure. The chemical bond between Au_n⁺ and Ar involves electron transfer from Ar to Au, and a stronger interaction is found when the Au adsorption site has a higher positive partial charge, which depends on the cluster geometry. Au₁₅⁺ is a peculiar cluster size, which stands out for its much stronger interaction with Ar than its neighbors, signaled by a higher abundance in mass spectra and a larger Ar adsorption energy. This is shown to be a consequence of a low-coordinated Au adsorption site in Au₁₅⁺, which possesses a large positive partial charge.     

DFT study of small aluminum and boron hydrides: isomeric composition and physical properties

The structure, energetics, and physical properties, including rotational constants, characteristic vibrational temperatures, dipole moment, static polarizability, HOMO-LUMO gap, formation enthalpy and collision diameter of different isomeric forms of atomic Al _n H _m and B _n H _m clusters with n = 1..4 and all feasible m numbers are studied within the density functional theory framework. The search of isomer structures has been accomplished using multistep hierarchical algorithm. Temperature dependences of thermodynamic functions (enthalpy, entropy and specific heat capacity) have been calculated both for the individual isomers and for the ensemble of isomers with equilibrium composition for each class of clusters, taking into account the anharmonicity of cluster vibrations and the contribution of excited electronic states. The prospects of the application of small atomic Al _n H _m and B _n H _m clusters as the components of energetic and hydrogen storage materials are also discussed.     

IR double resonance experiments with size selected clusters for identification of isomers

Infrared photodissociation spectra of (CH₃OH) _n clusters (n=2, 3 and 6) and the mixed dimer C₂H₄ · CH₃COCH₃ are presented. The clusters are generated in a supersonic jet expansion and size selected by scattering from a helium atomic beam combined with mass spectrometric detection. Continuous CO₂-lasers are used to vibrationally excite the molecules in the cluster leading to rapid dissociation of the complex. Various dissociation peaks that are found in single-laser dissociation spectra can be assigned unambigously in a pump-probe experiment with two lasers to either different isomers (acetone-ethene dimer) or splitted lines of one isomer (methanol hexamer). For size distributions, the method is able to select contributions of single masses which is demonstrated for mixtures of methanol dimers and trimers.