Preparation and crystal structure of a quadruply bonded dimolybdenum complex with trans-crotonate ligands

A quadruply bonded dimolybdenum complex with trans-crotonate ligands, Mo₂(O₂CC₃H₅)₄, was synthesized and characterized by UV–VIS, IR, and NMR spectra. The crystal structure was determined by an X-ray single crystal diffraction analysis. The title complex (C₁₆H₂₀O₈Mo₂, M _w = 532.2) crystallized in the triclinic space group P-1 with the following crystallographic parameters: a = 10.236(2) ?, b = 10.393(2) ?, c = 10.524(2) ?, α = 89.59(1)°, β = 73.34(1)°, γ = 70.78(2)°, V = 1008.1(3) ?³, Z = 2, D _c = 1.753 Mg m⁻³, μ(Mo Kα = 0.71073 ?, F(000) = 528, and final R ₁ = 0.041, wR ₂ = 0.114 for observed reflections 2889 (I > 2σ(I)). The Mo₂ ⁴⁺ unit was surrounded by four trans-crotonate ligands resulting in a paddle wheel structure with one disordered trans-crotonate ligand. Molecules are bonded together by means of interactions consisting of the donation of lone pairs of electron on carboxylate O atoms on one molecule to the Mo atom on an adjacent complex. A new pattern of intermolecular bonding is observed.     

Water desorption isotherms of cellulose-acetate membranes

 The water desorption isotherms are determined in three cellulose acetate membranes with different acetyl content as a function of p/p ₀ at 10–40 °C. The partition coefficients (adsorbed water over water pressure) show a minimum at p/p ₀=0.5–0.6. This indicates a two energy mechanism. The agreement of our results with the BET adsorption isotherms only till p/p ₀<0.3 shows that a two energy adsorption mechanism is valid only for small water contents, probably one hydrate layer and a second more liquid-like water layer. At large p/p ₀, the adsorbed water becomes more and more liquid like by polarization of the hydrogen bonds. The heat of desorption is larger than the vaporization heat of water ΔH _vap(H₂O). It decreases with increasing water content asymptotically to ΔH _vap(H₂O). The cause may be a larger van der Waals interaction of the hydrate layer due to coordination numbers larger than 4.4 as in liquid water. Additionally, we found a hole adsorption process by sorbing unpolar solvents. The water and methonal adsorption are 100 times larger due to a swelling mechanism depending on the number of acetyl groups in the membranes. The amounts of n-alcohols sorbed decrease with their chain length. Received: 25 April 1997 Accepted: 10 June 1997     

Highly efficient olefin oxidation catalysts based on regular nano-particles of titanium-containing mesoporous molecular sieves

Ti-MCM-41 and Ti-MCM-48 are successfully prepared by one step synthesis and post-synthesis at room temperature. The samples have been characterized by X-ray diffraction, SEM, TEM, nitrogen adsorption/desorption isotherms and UV-Vis spectroscopy. The results demonstrate the existence of an ordered mesoporous structure. The Ti species are grafted into the pure silica sieve frameworks without any evidence of the formation of non-skeleton anatase. The post-synthesis Ti-MCM-41 regular nano-particles display a higher conversion and selectivity as compared to that for both the post-synthesis Ti-MCM-48 and Ti-MCM-41 with ordinary particles, in the epoxidation of cis-cyclooctene with aqueous H(2)O(2) as an oxidant. The enhanced catalytic activity may be attributed to the existence of a higher number of isolated tetracoordinated Ti active sites and regular nano-particles, which are conducive to the affinity of the reactants with active sites.     

Variation of Dipole Moment Function of O-H Bond of t-Butanol with the Nature of Solvent and Its Temperature

The intermolecular influence on dipole moment function is evaluated for O-H bond of t-butanol in different nonpolar solvents at temperatures ranging from 10° to 60°C employing IR band intensities of fundamental and first overtone bands. Two sets of dipole moment derivatives have been calculated corresponding to – and +- combinations of the transition moment matrix elements R₁₀ and R₂₀, the values for ++ and -+ combinations are equal in magnitude to those for - and +- combinations, respectively and opposite in signs. In general the dipole moment derivatives increase on lowering the temperature as well as with increasing molecular interactions with the solvent molecules. Dipole moment plots with dimensionless coordinate ξ [=(r-r_e)/r_e, where r and r_e are internuclear distances during vibration and at equilibrium, respectively] are reported for various systems considered. It is found that for +- combination the dipole moment maximum shifts to higher internuclear distances when polarisation of the solute molecules is increased by lowering of temperature or increase in molecular interactions between solute and solvent molecules. A reverse trend is observed for – combination.- The OH band of t-butanol vapor has been measured.     

On the response of a two-level quantum system to a class of time-dependent quasiperiodic perturbations

We study analytically the response of a two-level quantum system to a certain class of time-dependent quasiperiodic perturbations generated by a Fibonacci sequence. We show that the quasi-energy spectrum (Fourier transform of the evolution operator) generically is not a denumerable sum of delta functions. Hence the response is not quasiperiodic. Several numerical investigations (Poincaré sections, polarization fluctuation, etc.) suggest an intermediate kind of behavior between quasiperiodic and chaotic.     

Massenspektrometrische Multielementanalyse von Carrara-Marmor

Zusammenfassung Für Vergleichszwecke wurden Spurenelemente in Carrara-Marmor nach Überführung des Carbonats in ein Oxid bzw. Fluorid bestimmt. Unter Verwendung von experimentell ermittelten relativen Empfindlichkeitsfaktoren für Oxid- bzw. Fluorid-Systeme wurden Konzentrationen für die Elemente Mg, Al, Ti, V, Cr, Mn, Fe, Ni, Cu, As, Rb, Sr, Y, Ba und Ce gefunden, die vom gemeinsamen Mittelwert der Oxid- und Fluorid-Bestimmungen um < 20% abweichen.Vortrag anläßlich der Tagung Spurenanalyse, 2. bis 5. April 1973 in Erlangen.     

Progress in analytical application of an inductively coupled plasma/mass spectrometer system

Summary A method for the determination of elements in liquids using inductively coupled plasma mass spectrometry (ICP-MS) is described. The different possibilities and problems together with instrumental parameters are reviewed. After the dissolution step samples are analyzed directly without any preconcentration. Detection limits for 53 elements have been tested and are found to be less than 0.1 ng/ml. Monoisotopic elements can be detected at levels less than 0.05 ng/ml. Interference problems have been studied and hints are given to predict molecular species using algorithm implemented in software packages. The application to trace element analysis in the geological field is demonstrated with a basalt reference sample to verify figures of merit for this method. Accuracy is checked with a NBS SRM steel sample.

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Erfahrungen und Möglichkeiten mit der Anwendung eines induktiv gekoppelten Plasma/Massenspektrometer Systems

     

Dynamical diversity and metastability in a hindered granular column near jamming

Granular media jam into a panoply of metastable states. The way in which these states are achieved depends on the nature of local and global constraints on grains; here we investigate this issue by means of a non-equilibrium stochastic model of a hindered granular column near its jamming limit. Grains feel the constraints of grains above and below them differently, depending on their position. A rich phase diagram with four dynamical phases (ballistic, activated, logarithmic and glassy) is revealed. The statistics of the jamming time and of the metastable states reached as attractors of the zero-temperature dynamics is investigated in each of these phases. Of particular interest is the glassy phase, where intermittency and a strong deviation from Edwards' flatness are manifest.     

Intermolecular effects on OH-vibration bands

The shifts of alcoholic OH-stretching bands by changes of temperature, pressure or transition from vapour to CCl₄-solutions or to argon matrices are measured. All shifts can be discussed together based on the intermolecular interactions and the density-distance-change. The intermolecular interactions of van der Waals forces can be represented in approximation by the critical temperature -or its square root in mixtures-.The shifts of monomeric OH bands is smaller in comparison to H-bonded ones. The differences of shifts caused by H-bonds at different conditions like vapour, CCl₄-solution or Ar matrix are elucidated by this paper.