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.     

Strongly isolated ferromagnetic layers in poly-trans-mu-dichloro- and poly-trans-mu-dibromobis(1-(2-chloroethyl)-tetrazole-N4)copper(II) complexes

Two new isostructural compounds, dichlorobis(1-(2-chloroethyl)tetrazole)copper(II) (1) and dibromobis(1-(2-chloroethyl)tetrazole)copper(II) (2), have been prepared. The synthesis, characterization, and spectral and magnetic properties as well as the crystal and molecular structures of 1 and 2 have been studied. Both complexes form two-dimensional, distorted square grid planes of copper and halides, distinctly separated by layers of tetrazole ligands. The differential (ac) magnetic susceptibility, chi = (deltaM/deltaH)(T), and magnetization M(H) of both complexes have been studied as a function of temperature and field. The compounds possess a ferromagnetic interaction within the isolated copper-halide layers (J/k(B) = 8.0 K, J/k(B) = 10.2 K, respectively, for the chloride and the bromide, and T(c) = 4.75 K, T(c) = 8.01 K). The magnetic coupling J'/k(B) between the different layers is found to be very weak (|J'/J| 相似文献   

Modellbetrachtungen zum Verständnis unerwarteter Eigenschaften der metalloiden Clusterverbindung [Ga84(N(SiMe3)2)20][Li6Br2(THF)20]·2Toluol

Towards the Understanding of the Unexpected Properties of the Metalloid Cluster Compound [Ga₈₄(N(SiMe₃)₂)₂₀][Li₆Br₂(THF)₂₀]·2Toluol In several short communications we have recently reported on the electrical and superconducting properties of the crystalline title compound 1 which contains anionic Ga₈₄R₂₀‐moieties. Here we present a collection of these results, complemented and interpreted by using DFT‐calculations on model clusters (Ga₈₄(NH₂)₂₀^?). These calculations allow a) a first insight into the dynamics of the Ga₈₄‐moieties (e.g. a rotation of the central Ga₂‐dumbbell) and thus an explanation of the temperature‐dependent Ga‐NMR‐spectra described recently, and b) estimations on the lattice energy of 1 and its resulting unexpected energetic stabilization compared to metallic gallium. A possible contribution of the cations in the electrical conduction mechanism of 1 can also be made feasible with model calculations. The basis for all the results presented is to be found in the “perfect” arrangement of nanoscopic Ga₈₄‐clusters in the crystal. This theoretically predicted condition for superconductivity in a “chain” of identical metal cluster molecules is a requirement which can hardly be realized by means of physical fabrication methods. Therefore, on the one hand the results presented here make for some disillusionment in the field of nanoscience, but on the other hand, especially in the field of synthetic chemistry, they present rewarding challenges for fundamental work in the future.     

Oxidative carbon–carbon coupling. III. Oxidative polymerization of bifunctional arylcyanoacetic esters

Bifunctional arylcyanoacetic esters were oxidatively coupled to high molecular weight, colorless, amorphous polymers, soluble in common organic solvents. Brittle films were obtained by casting or compression molding. Thermal stability of the polymers is poor due to the weak C? C bond formed by oxidative coupling. Radical dissociation-recombination of this bond (evidenced by ESR) results in meso-dl equilibration, lowering the T_g of the polymers.     

69,71Ga NMR studies of a superconducting Ga 84 cluster compound

We present ^69,71Ga-NMR experiments on microcrystalline samples of the recently discovered supramolecular compound Ga ₈₄ [ N ( SiMe ₃ ) ₂ ] ₂₀ Li ₆ Br ₂ ( thf ) ₂₀ ^. 2 toluene, which is composed of ligand-coordinated Ga₈₄ metal clusters, packed together in a fully ordered crystalline matrix. The compound is highly conducting and even shows superconductivity below T _c ~ 7.2 K. Our preliminary results between 10-300 K show a metallic-like behavior: the nuclear spin-lattice relaxation rate T ₁ ^-1 follows the Korringa law ⁶⁹ ( T ₁ T ) ^-1 = 0.36 s ^-1 K ^-1 , but with a relaxation rate approximately three times smaller than in bulk -Ga metal. No quantum-size effects are observed, the Korringa law being followed down to 10 K, whereas the quantum-gaps for individual clusters should amount to ~ 10 ³ K. These results therefore suggest a transport process based on intermolecular charge transfer, similar as in alkali-doped fullerenes and silicon-clathrates.     

Fundamentals of melt infiltration for the preparation of supported metal catalysts. The case of Co/SiO2 for Fischer-Tropsch synthesis

We explored melt infiltration of mesoporous silica supports to prepare supported metal catalysts with high loadings and controllable particle sizes. Melting of Co(NO(3))(2)·6H(2)O in the presence of silica supports was studied in situ with differential scanning calorimetry. The melting point depression of the intraporous phase was used to quantify the degree of pore loading after infiltration. Maximum pore-fillings corresponded to 70-80% of filled pore volume, if the intraporous phase was considered to be crystalline Co(NO(3))(2)·6H(2)O. However, diffraction was absent in XRD both from the ordered mesopores at low scattering angles and from crystalline cobalt nitrate phases at high angles. Hence, an amorphous, lower density, intraporous Co(NO(3))(2)·6H(2)O phase was proposed to fill the pores completely. Equilibration at 60 °C in a closed vessel was essential for successful melt infiltration. In an open crucible, dehydration of the precursor prior to infiltration inhibited homogeneous filling of support particles. The dispersion and distribution of Co(3)O(4) after calcination could be controlled using the same toolbox as for preparation via solution impregnation: confinement and the calcination gas atmosphere. Using ordered mesoporous silica supports as well as an industrial silica gel support, catalysts with Co metal loadings in the range of 10-22 wt % were prepared. The Co(3)O(4) crystallite sizes ranged from 4 to 10 nm and scaled with the support pore diameters. By calcination in N(2), pluglike nanoparticles were obtained that formed aggregates over several pore widths, while calcination in 1% NO/N(2) led to the formation of smaller individual nanoparticles. After reduction, the Co/SiO(2) catalysts showed high activity for the Fischer-Tropsch synthesis, illustrating the applicability of melt infiltration for supported catalyst preparation.     

Metallic susceptibility in giant molecule: Pd561Phen36O200

The giant metal cluster molecule Pd₅₆₁Phen₃₆O _x is the largest member of a series that has recently been prepared by Schmid [1]. Pronounced size effects are expected to show up in the magnetic properties of metal particles of the size of the Pd-561 core of the molecule. We have measured the susceptibility on four different batches of the material, with quite satisfactory reproducibility. There is a low temperature tail that is attributed to small amounts of impurities. The nearly temperature-independent paramagnetism that is observed is most probably due to an exchange enhanced Pauli spin susceptibility and is already close to the bulk value. The absence of size effects, i.e. a drop at low temperatures, is either due to interactions between the clusters or a result of a very small energy level spacing.     

Lubrication, adsorption, and rheology of aqueous polysaccharide solutions

Aqueous lubrication is currently at the forefront of tribological research due to the desire to learn and potentially mimic how nature lubricates biotribological contacts. We focus here on understanding the lubrication properties of naturally occurring polysaccharides in aqueous solution using a combination of tribology, adsorption, and rheology. The polysaccharides include pectin, xanthan gum, gellan, and locus bean gum that are all widely used in food and nonfood applications. They form rheologically complex fluids in aqueous solution that are both shear thinning and elastic, and their normal stress differences at high shear rates are found to be characteristic of semiflexible/rigid molecules. Lubrication is studied using a ball-on-disk tribometer with hydrophobic elastomer surfaces, mimicking biotribological contacts, and the friction coefficient is measured as a function of speed across the boundary, mixed, and hydrodynamic lubrication regimes. The hydrodynamic regime, where the friction coefficient increases with increasing lubricant entrainment speed, is found to depend on the viscosity of the polysaccharide solutions at shear rates of around 10(4) s(-1). The boundary regime, which occurs at the lowest entrainment speeds, depends on the adsorption of polymer to the substrate. In this regime, the friction coefficient for a rough substrate (400 nm rms roughness) is dependent on the dry mass of polymer adsorbed to the surface (obtained from surface plasmon resonance), while for a smooth substrate (10 nm rms roughness) the friction coefficient is strongly dependent on the hydrated wet mass of adsorbed polymer (obtained from quartz crystal microbalance, QCM-D). The mixed regime is dependent on both the adsorbed film properties and lubricant's viscosity at high shear rates. In addition, the entrainment speed where the friction coefficient is a minimum, which corresponds to the transition between the hydrodynamic and mixed regime, correlates linearly with the ratio of the wet mass and viscosity at ～10(4) s(-1) for the smooth surface. These findings are independent of the different polysaccharides used in the study and their different viscoelastic flow properties.