Fully computational lead-development of fluoro-olefin polymerization catalysts

Coordinative polymerization of fluorinated olefins is still a challenging task. We analyzed the catalytic properties of diimido chromium VI compounds computationally by density functional methods. It was found that reactivity predictions depend strongly on the density functional chosen for the computations. Therefore, DFT calculations were calibrated to high level wave function theory calculations. Then geometrical parameters, which can be tuned by a ligand, were scanned to elucidate their influence on the catalytic activity. Having arrived at an optimal parameter set, we randomly assembled chromium complexes from a fragments database and automatically checked for their activities. This led to the proposal of three exemplary catalyst candidates.     

Interconversion study in 1,4-substituted six-membered cyclohexane-type rings. Structure and dynamics of trans-1,4-dibromo-1,4-dicyanocyclohexane

Cyclohexane is an extremely flexible molecule that oscillates, at room temperature, between two clearly distinct and extreme conformations that cannot be distinguished at room temperature; so much so that the NMR spectrum is a single line that includes all 12 protons be they axial or equatorial. This raises the interesting question as to what happens when there are equal substituents at the 1 and 4 carbon atoms of the ring. Therefore substitution in the 1,4-positions in the cyclohexane ring has been the subject of considerable interest because some form of interconversion between extreme conformations could lead to the existence of a rather unusual behavior. To study this problem, the interconversion in (di- or tetra-1,4)-substituted six-membered cyclohexane-type rings, trans-1,4-dibromo-1,4-dicyanocyclohexane, was found to be a particularly suitable candidate. Although X-ray diffraction studies on the crystalline solid found the molecule to be centrosymmetric, it still shows a significant dipole moment μ in solution, as determined with a procedure that leads to the vapor phase values of μ. Furthermore, the low magnetic field proton NMR spectrum at ambient temperature appears as a single line, a situation that changes with increasing field intensity and different solvents. Both these effects are attributed to dynamics, because small distortions can easily disrupt the exact cancellation of the individual dipoles (which are quite strong) associated with each end of the molecule. The molecule can exist in two forms, with both the bromines in an axial geometry or both in an equatorial position. Interconversion between these forms is observed, as in the parent cyclohexane. The single NMR line observed at low magnetic fields is due to fast exchange and requires that the two forms have roughly equal populations. Spectra obtained at low temperature confirm this, and variable-temperature studies allow measurement of the rates, leading to an enthalpy of activation of 62 kJ mol(-1). More details of the interconversion are provided by some new calculation methods. Even for a relatively small molecule like this, calculation of a full potential energy surface is prohibitive. However, methods are now available to follow the molecule along the reaction coordinate in quite an efficient way. The results of these calculations lead to an extremely detailed picture of chair-chair interconversion in a di- and tetrasubstituted six-membered ring of the cyclohexane family.     

Kinetics of iron–copper sulphides oxidation in relation to protohistoric copper smelting

This article deals with one specific step of the copper extractive metallurgy process: the roasting of iron–copper sulphides. It aims at shedding light on an archaeological issue: the reconstruction of the copper extractive metallurgy processes during protohistory (IV^e–II^e millennium BC). Experimental simulations are performed at laboratory scale by modelizing the conditions of protohistoric furnaces. Kinetic of roasting is studied by thermogravimetry combined with the physico-chemical analysis of synthetic products. The influence of two parameters is studied: (i) the temperature (773, 973 and 1173 K) and (ii) the granularity of the roasted ores (1 mm and 100 μm). In each case, the chemical mechanism governing the oxidation of iron copper sulphide is proposed. Apart from one extreme case (∅ = 1 mm; T = 773 K), it is showed that kinetic is controlled by the transport of molecular oxygen (O₂) from the gas to the grain surface. Moreover, we prove that, in some cases where the diffusivity of gaseous oxygen is low, roasting can be accelerated by the presence of an oxide, which constitute an in-situ source of oxygen. Theses experiments support the hypothesis that such a technique could have allowed a roasting process where iron and sulfur were removed by the solid oxygen instead of the gaseous oxygen. These results allow to validate a one-step copper smelting process starting from sulphidic ores, and to identify the experimental parameters of this process.     

Balanced minimum covers of a finite set

In this paper a method of enumeration for n-balanced, labelled, minimum covers of a finite set of m indistinguishable elements is developed. The method is then used to tabulate the number of such covers for small values of m and n by means of a generating function.     

Nachweis und colorimetrische Bestimmungsmethoden des Molybd?ns

Ohne Zusammenfassung     

Surface analysis of steel sheets with GDOS and EPMA

Summary In the field of steel sheet characterization, chemical surface analysis plays a very important role. Surface composition effects the surface properties of the final product. At Voest-Alpine Stahl Linz the analysis techniques GDOS and EPMA are routinely used to characterize the steel sheet surface. Some fundamental experiments are here described for the optimization of GDOS depth resolution. In the documentation for a semiquantitative approach for steel sheet analysis, investigations and experience about minimum detection limits are presented. Self developed methods for concentration mapping and thickness mapping are described in the field of electron probe microanalysis; Also an EPMA depth profiling method is highlighted. Finally practical examples demonstrate the wide application range of the analysis methods.     

Synthese und Kristallstrukturbestimmung von Dimethyldi-(N-Methylimidazolium)siliciumbromid

Synthesis and Crystal Structure Determination of Dimethyldi-(N-Methylimidazolium)silicon Bromide The reaction of dibromodimethylsilane with n-methylimidazole (NMI) leads to a 1:2 compound stable at room temperature. The reaction was carried out at room temperature and colorless, moisture sensitive crystals were obtained by sublimation. The addition compound crystallizes in the orthorhombic space group Pcam (No. 57) with lattice constants a = 1 110 96(8) pm, b = 1 142.3(2) pm and c = 1 238.9(3) pm. For 1 317 independent reflections, measured at 21°C, the structure could be refined to R = 0.040 and R_w = 0.041.