Refinement of linear polymer crystal structures determined from electron diffraction data

The reliability of linear polymer structures determined by using electron diffraction data is investigated. The results of n-beam dynamical calculations and kinematic calculations which take account of crystal bending are compared with experimental structure factors for five published structures. In specific cases, both dynamical scattering and crystal bending effects are found to be important. Finally, guidelines are given for obtaining electron diffraction data which are optimal for structure solution.     

Differential inhibition of postnatal brain,spinal cord and body growth by a growth hormone antagonist

Background  

Growth hormone (GH) plays an incompletely understood role in the development of the central nervous system (CNS). In this study, we use transgenic mice expressing a growth hormone antagonist (GHA) to explore the role of GH in regulating postnatal brain, spinal cord and body growth into adulthood. The GHA transgene encodes a protein that inhibits the binding of GH to its receptor, specifically antagonizing the trophic effects of endogenous GH.     

Electron diffraction structure analysis of epitaxially crystallized n-paraffins

Electron diffraction data from a series of paraffins epitaxially crystallized on benzoic acid are used to determine their crystal structures. Even-chain paraffins from n-C₃₂H₆₆ to n-C₆₀H₁₂₂ are shown to crystallize isostructurally as the orthorhombic polymorph described for n-C₃₆H₇₄. Like the even-chain compounds, odd-chain paraffins do not crystallize in their lowest-energy form. A quantitative zonal crystal structure analysis for n-C₃₃H₇₄ suggests that the molecular packing is in the B-polymorphic form proposed earlier by other workers.     

Crystal structure of zeolite MCM-68: a new three-dimensional framework with large pores

The crystal structure of the aluminosilicate MCM-68 was solved from synchrotron powder diffraction data by the program FOCUS. The unit cell framework contains Si100.6Al11.4O224. This material crystallizes in space group P42/mnm, where, after Rietveld refinement, a=18.286(1) A and c=20.208(2) A. A three-dimensional framework is found that contains continuous 12-ring channels and two orthogonal, intersecting, undulating 10-ring channels. Rietveld refinement of the model coordinates optimizes the framework geometry, to match the observed intensity profile by Rwp=0.1371, R(F2)=0.1411. It is not possible to determine the location of approximately 0.84 K+ cations remaining in the unit cell after the material is steamed and then dehydrated. The framework model also successfully predicts observed electron diffraction data in two projections, and the tetragonal projection can be determined independently from these data by direct methods. The calculated density of the framework structure is 1.66 g/cm3, and the T-site framework density is 16.6 T/1000 A3.     

Quantitative analysis of electron diffraction data from epitaxially grown crystals

Epitaxial growth produces oriented crystal projections where a long unit cell axis is parallel to the major crystal face rather than perpendicular to it and thus provides a favorable condition for electron diffraction data collection which minimizes perturbations due to elastic crystal bending. Even though the crystals are not so perfect as those produced by solution growth, the intensity data from them have been useful for the quantitative crystal structure analysis of several molecular organic compounds and linear polymers as discussed in this review.     

Transfer measurements for the Ti+Ni systems at near barrier energies

Large enhancements have been observed in the sub-barrier fusion cross sections for Ti+Ni systems in our previous studies. Coupled channel calculations incorporating couplings to 2⁺ and 3⁻ states failed to explain these enhancements completely. A possibilty of transfer channels contributing to the residual enhancements had been suggested. In order to investigate the role of relevant transfer channels, measurements of one- and two-nucleon transfer were carried out for ^46,48Ti+⁶¹Ni systems. The present paper gives the results of these studies.     

The role of symmetry in building up zeolite frameworks from layered zeolite precursors having ferrierite and CAS layers

Layered zeolite precursors represent an emerging class of materials that expand the zeolite field in a new direction and have already produced new intriguing discoveries. One is a new fundamental insight that a zeolite framework can assemble by two pathways, directly in 3-D and via a layered precursor. Also new types of materials such as pillared and delaminated zeolites have been synthesized. Starting with the discovery of MCM-22 precursor in 1990s, other framework structures have now been obtained by topotactic condensation, including classical zeolites such as sodalite and ferrierite. The recent structural diversity observed with FER and CAS layers, which were found to produce two frameworks each, i.e. FER/CDO and CAS/NSI, respectively, are herein rationalized as resulting from the absence of in-plane mirror symmetry. A systematic treatment is envisioned, such as consideration of translational and pseudo-translational interactions in condensation of elementary shapes into close-packed assemblies, providing a blueprint for a general approach to analyze new precursors in the future. Other already documented instances of structural diversity include the presence of both types of packing in one preparation (CAS and NSI) and incomplete pairing of silanols connecting FER layers in ESR-12.     

P-derived organic cations as structure-directing agents: synthesis of a high-silica zeolite (ITQ-27) with a two-dimensional 12-ring channel system

Recently, efforts have been made to synthesize large-pore, multidimensional zeolite frameworks as a basis for new catalysts to improve various hydrocarbon conversions. A new aluminosilicate zeolite, ITQ-27, has been prepared using the phosphorus-containing structure-directing agent, dimethyldiphenylphosphonium. Its crystal structure was determined in its calcined form by direct methods (FOCUS) on synchrotron powder diffraction data (lambda = 0.8702 A) after the unit cell and space group were determined from tilt electron diffraction experiments on individual microcrystals. The material crystallizes in space group Fmmm, where a = 27.7508(5) A, b = 25.2969(7) A, and c = 13.7923(4) A. The final model, refined by Rietveld methods, comprises seven unique T-sites forming a framework with straight 12-MR channels that are connected by 14-MR openings between them. (Corresponding 12-ring pore dimension is 6.94 A x 6.20 A.) Since access from one 14-MR opening to the next is through the 12-MR channel, the structure is best described as a two-dimensional, 12-MR framework.     

Using FTIR spectroscopy to detect sericin on historic silk

Silks represent some of the most precious ancient and historic textile artefacts in collections worldwide.Their optimum preservation demands an appreciation of their characteristics.One important concern,especially with regard to ancient Chinese silks,is whether the fabrics have been degummed.Silks with remnant sericin gum coating the fibroin fibres would require different conservation protocol.In previous research on aged silks,the presence of sericin has been inferred from amino acid analysis of hydrolysa...