No abstract

For the past decade, the emerging class of porous metal-organic frameworks has been becoming one of the most promising materials for the construction of extralarge pore networks in view of potential applications in catalysis, separation and gas storage. The knowledge of the atomic arrangements in these crystalline compounds is a key point for the understanding of the chemical and physical properties. Their crystal size limits the use of single-crystal diffraction analysis, and synchrotron radiation facilities may allow for the analysis of tiny crystals. We present here a microdiffraction set-up for the collection of Bragg intensities, which pushes down the limit to the micrometre scale by using a microfocused X-ray beam of 1 mum. We report the structure determination of a new porous metal-organic-framework-type aluminium trimesate (MIL-110) from a single crystal of a few micrometres length, showing very weak scattering factors owing to the composition of the framework (light elements) and very low density. Its structure is built up from a honeycomb-like network with hexagonal 16 A channels, involving the connection of octahedrally coordinated aluminium octameric motifs with the trimesate ligands. Solid-state NMR (27Al,13C,1H) and molecular modelling are finally considered for the structural characterization.

The three-dimensional structure of A-amylose crystals, as a model of the crystal domains of A-starch granules, was revised using synchrotron radiation microdiffraction data collected from individual micron-sized single crystals. The resulting datasets allowed a determination of the structure with conventional X-ray structure determination techniques normally used for small molecules and not for polymers. Whereas the gross features of this improved structure do not differ extensively from previous structure determination, the high resolution of the diffraction diagrams, which is unusual for a crystalline polymer, allowed the resolution of important new fine details. These include a distortion of the amylose double helices resulting from the occurrence of two intracrystalline molecules of water and a tight network of hydrogen bonds involving each of the primary and secondary hydroxyl groups of the glucosyl moieties. Pairs of water molecules are located in discrete pockets that do not interfere with one another. In addition, the refinement of the new structure indicates a "parallel-down" organization of the amylose molecules within the unit cell as opposed to the previous "parallel-up" model. This new feature indicates that within the crystals, the nonreducing ends of the amylose molecules are oriented toward the c-axis direction of the unit cell. The description of this geometry is important to correlate the crystallography of the granules of A-starch with their ultrastructure and their mode of biosynthesis.Here, we present for the first time the resolution of the structure of a polymer crystal from a full X-ray dataset collected on micron-sized polymer single crystals using synchrotron radiation microdiffraction. This achievement is a substantial advance, which opens the way to many more studies since the technique of growing polymer and biopolymer single crystals is well established.3