Beugungsuntersuchungen mit Synchrotron-Röntgen- und Neutronenstrahlen in der Festkörperchemie

Hydrodeoxygenation of Guaiacol over Ceria-Zirconia Catalysts

The hydrodeoxygenation of guaiacol is investigated over bulk ceria and ceria-zirconia catalysts with different elemental compositions. The reactions are performed in a flow reactor at 1 atm and 275-400 °C. The primary products are phenol and catechol, whereas cresol and benzene are formed as secondary products. No products with hydrogenated rings are formed. The highest conversion of guaiacol is achieved over a catalyst containing 60 mol % CeO2 and 40 mol % ZrO2 . Pseudo-first-order activation energies of 97-114 kJ mol(-1) are observed over the mixed metal oxide catalysts. None of the catalysts show significant deactivation during 72 h on stream. The important physicochemical properties of the catalysts are characterized by X-ray diffraction (XRD), temperature-programmed reduction, titration of oxygen vacancies, and temperature-programmed desorption of ammonia. On the basis of these experimental results, the reasons for the observed reactivity trends are identified.

Determination of the distribution of elements with similar electron counts: a practical guide for resonant X-ray scattering

This article attempts to present straightforward and easy-to-understand guidelines for the determination of element distribution in compounds lacking X-ray scattering contrast because they have similar electron counts. Different sources of anomalous dispersion correction terms (especially Δf′ values) are compared with respect to their suitability, reliability and quality. Values from databases are compared with Δfvalues calculated from fluorescence spectra and those refined from single-crystal diffraction data, using both reference crystals without scattering contrast problems and crystals containing elements with similar electron counts. The number of data sets required to determine reliably the element distribution and the optimum wavelengths to be used are discussed. Joint multiple data set refinements are suitable for the refinement of multiply mixed occupancies of elements lacking scattering contrast. The most straightforward method of obtaining Δf′ values depends on the complexity of the problem to be solved and the precision required.

Structure Determination of Homoleptic AuI, AgI, and CuI Aryl/Alkylethynyl Coordination Polymers by X-ray Powder Diffraction

This article describes the structure determination of five homoleptic d(10) metal-aryl/alkylacetylides [RC triple bond CM] (M=Cu, R=tBu 1, nPr 2, Ph 3; R=Ph, M=Ag 4; Au 5) by using X-ray single-crystal and powder diffraction. Complex 1.C6H6 reveals an unusual Cu20 catenane cluster structure that has various types of tBuC triple bond C-->Cu coordination modes. By using this single-crystal structure as a starting model for subsequent Rietveld refinement of X-ray powder diffraction data, the structure of the powder synthesized from CuI and tBuC triple bond CH was found to have the same structure as 1. Complex 2 has an extended sheet structure consisting of discrete zig-zag Cu4 subunits connected through bridging nPrC triple bond C groups. Complex 3 forms an infinite chain structure with extended Cu-Cu ladders (Cu-Cu=2.49(4)-2.83(2) A). The silver(I) congener 4 is iso-structural to 3 (average Ag-Ag distance 3.11 A), whereas the gold(I) analogue 5 forms a Au...Au honeycomb network with PhC triple bond C pillars (Au-Au=2.98(1)-3.26(1) A). Solid-state properties including photoluminescence, nu(C triple bond C) stretching frequencies and thermal stability of these polymeric systems are discussed in the context of the determined structures.

Contemporary Advances in the Use of Powder X-Ray Diffraction for Structure Determination

Many crystalline solids cannot be prepared in the form of single crystals of sufficient size and/or quality for investigation using single-crystal X-ray diffraction techniques, and the opportunity to carry out structure determination using powder diffraction data is therefore essential to understand the structural properties of such materials. Although the refinement stage of the structure determination process can be carried out fairly routinely from powder diffraction data using the Rietveld profile refinement technique, solving crystal structures directly from powder data is associated with several intrinsic difficulties. Nevertheless, substantial progress has been made in recent years in the scope and potential of techniques in this field. This article aims to highlight the types of structural problems for which structure determination may now be tackled directly from powder diffraction data, and contemporary applications across several chemical disciplines are presented. A brief survey of the underlying methodologies is given, with some emphasis on recently developed techniques for carrying out the structure-solution stage of the structure-determination process.

13 C NMR studies of insulin. Part I-Spectral assignments

Natural abundance 75 MHz ¹³ C NMR spectral assignments are reported for bovine and porcine zinc insulin in solution. A large number of protein resonances are well resolved, and approximately 80% of these have been assigned to either residue types or to specific sites within the protein. Assignment techniques included consideration of free amino acid or peptide shifts pH studies and comparison of sequence and spectral differences between bovine and porcine insulin, in addition to the use of NMR relaxation times. The DEPT spectral editing technique was also found to be particularly valuable as an assignment aid. This technique allows subspectra containing only CH, CH₂ or CH₃ carbon types to be generated. The method also produces signal enhancement relative to broad band decoupled ¹³ C NMR spectra of large proteins which generally have reduced nuclear Overhauser enhancements.