Neutron diffraction studies three-dimensional structure

The structure of LiIn(CH3)4 (56) (76) is a three-dimensional network. Each lithium atom is surrounded by a tetrahedral array of carbon atoms. The structure of LiB(CH3)4 (57) (79) consists of planar sheets of lithium atoms bridged by tetramethylboron groups. A unique feature is the presence of both linear and bent Li—C—B units. A neutron diffraction study of LiB(CH3)4 (79) illustrates the Li—H—C interactions present in... 

In polymer science, an understanding of the relationship between microscopic structure and macroscopic properties is essential for an intelligent design of new improved materials. In this section, we present some case studies that illustrate the role which solid-state NMR can play with regard to the determination of the microscopic structure. It is first necessary to consider how solid-state NMR relates to other methods for structural determination, in particular the established scattering methods. For solids, for which a single crystal of suitable size can be obtained, the ability of X-ray or neutron diffraction methods to determine the complete three-dimensional structure with atomic resolution cannot be matched. 

It is emphasized that accurate interatomic distances are a prerequisite to achieving the three-dimensional structure of peptides, proteins and macromolecules. A careful evaluation of the following several points is the most important step to obtaining reliable interatomic distances by the REDOR experiment, although they have not always been taken seriously into account in the early papers. In practice, it is advisable to employ a standard sample such as [1- C, N]glycine [18], whose C-N interatomic distance is determined to be 2.48 A by a neutron diffraction study, to check that the... 

The crystal and molecular structure of xenon difluoride, Xep2, has been determined by a three-dimensional neutron diffraction study. The crystals are tetragonal with a = 4.315, c = 6.990 A. The space group is lA/mmm and there are two molecules in the unit cell. Special positions of four-fold and two-fold multiplicity are ... 

Both X-ray and neutron fiber diffraction (as well as electron microscopy) techniques have been applied to filamentous viruses, for which the prospect of three-dimensional crystals is poor. By combining neutron and X-ray fiber diffraction, NMR, circular dichroism, and Raman and infrared spectroscopies, an atomic model for the filamentous bacteriophage Pfl has been derived (Liu and Day, 1994). Other studies concerning Pfl have relied on purely X-ray fiber diffraction data, together with molecular modeling, to provide detailed filament structures (Pederson et at, 2001 Welsh et at, 1998a,b, 2000). Eiber diffraction was also used to solve the structure of the rodlike helical tobacco mosaic virus (TMV), where all of the coat protein and three genomic nucleotides... 

The protein-solvent interface was studied in an explicit solvent environment of 3182 water molecules by MD simulations performed on metmyoglobin [31].Both the structure and dynamics of the hydrated surface of myoglobin are similar to that obtained by experimental methods calculating three-dimensional density distributions, temperature factors and occupancy weights of the solvent molecules. On the basis of trajectories they identified multiple solvation layers around the protein surface including more than 500 hydration sites. Properties of theoretically calculated hydration clusters were compared to that obtained from neutron and X-ray data. This study indicates that the simulation unified the hydration picture provided by X-ray and neutron diffraction experiments. 

Another experimental technique to study the self-association of phenols is to investigate how molecules of phenols pack together in the crystalline state. This type of analysis is made possible by the availability of the computer-based CSD. The CSD contains unit-cell dimensions of more than 230,000 (April 2001 release) three-dimensional crystal-structure determinations that have been studied by X-ray or neutron diffraction. Each crystal structure is identified by a unique six-letter code, called its REFCOD, with an additional two digits for duplicate structures and measurements. 

The first complete investigation of the a and P polymorphs of lead azide was performed by Miles [42]. Later the crystal structure of Pb(N3)2 was been studied by many other investigators Pfefferkorn [44], Azaroff [45], and Hattori and McCrone [46] for a- and j3-Pb(N3)2 Glen [47], Saha [48], and Choi and Boutin [49] for a-Pb(N3)2 Lamnevik and Soderquist [50, 51] for a, 3,7, and 6 lead azide. The crystal structure of a-Pb(N3)2, as described above, was determined by Choi and Boutin [49] by using three-dimensional neutron-diffraction data, but the crystal structures of the jS, 7, and 6 forms are not yet known. It is found that the P form transforms gradually to the a form in water [45] and also transforms irreversibly to the a form at about 160°C [50]. 

Zeolites form another class of materials useful for fundamental studies . As mentioned earlier, zeolites are microporous silica-aluminates with micropores of dimensions comparable to organic molecules. The materials are unique, because these micropores are determined by the three-dimensional crystallographic structure of the material and catalytic events occur at the interphase of zeolite micropore and zeolite lattice. As a result the catalytically active sites are well defined. Zeolites are used in practice in the acidic form or promoted with metal or sulfide particles. High Resolution Electron Microscopy, Neutron Diffraction and Solid State NMR are techniques that arc applied for structural characterization and to study the behaviour of chemisorbed molecules. 

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