Single-crystals methods

The same paper also described an analogous complex of 1,4-dibromotetra-fluorobenzene, 17-8, whose molecular structure, as obtained by single crystal methods, is shown as Fig. 17. 

The structures of 16 copper(II) complexes of bidentate amidino-O-alkylureas ([CuL jyXj) have been determined by single crystal methods [59-61,63-66]. The range of ligands and of copper(II) salts studied is summarised in Table 2. Although the N-alkylamidino-O-alkylurea derivatives form part of the systematic study by Suksangpanya et al. [59-61] [Cu(LHm)2]Cl2 [66], [Cu(LHm)2]Br2 [63], [Cu(LHe)2] Cl2 [64] and [Cu(LHe)2]Br2 [65] were prepared in independent studies. 

III. The Single-Crystal Method of Studying Surface Reactions. 63... 

X-ray diffraction (XRD) Analytical techniques, which include powder and single-crystal methods, that use X-rays to study the crystalline properties of solid substances. Traditionally, powder XRD has been primarily used to identify crystalline substances in solid samples. Single-crystal methods provide detailed information on the crystal structure and chemistry of a single crystal, such as identification of the space group and the types and distributions of the atoms in the crystal. 

Tabic l.l gives those crystal data for the C,S polymorphs that have been obtained using single crystal methods. The literature contains additional unit cell data, based only on powder diffraction evidence. Some of these may be equivalent to ones in Table 1.1, since the unit ceil of a monoclinic or triclinic crystal can be defined in different ways, but some are certainly incorrect. Because only the stronger reflections are recorded, and for other reasons, it is not possible to determine the unit cells of these complex structures reliably by powder methods. The unit cells of the T, Mj and R forms are superficially somewhat different, but all three are geometrically related transformation matrices have been given (12,HI). 

However, with time the single-crystal method has been... 

Question 7.7 Why are the structures of promethium compounds determined by powder methods and not by single-crystal methods ... 

In theory single crystal methods can be effectively used for structure solution from powder diffraction X-ray data. However, in powder diffraction the number of peaks involved are limited, and hence the data-to-parameter ratio is very small, due to the transfer of the three-dimensional data to one dimension, namely, 29. In spite of the inherent shortcomings, conventional crystallographic methods such as Direct, Patterson and maximum entropy methods have been successfully applied to powder diffraction data. The most popular program, which uses reciprocal space methods for stmcture solution is EXPO. ... 

Type Y and single crystal faujasite (13, 14, 84). Since crystallographic studies are conducted v ith very small amounts of material, especially in the case of the single-crystal method, it is likely that the effective control of the environment for traces of water and other impurities becomes very critical and may affect the position of the cations. 

The 34 ( 8), 44 (19), and 47 (19) types are all now known to be structures with the chabazite framework topology (as determined by single-crystal methods) but prepared with different templates, TEAOH, cyclohexyl ami ne, or N,N-di ethyl-ethanol ami ne, respectively. As initially prepared, there was sufficient variation in the x-ray powder patterns of the as-synthesized forms for different framework topologies to be considered. Once calcined, the three species exhibit essentially identical powder patterns, consistent with the framework topology. Other properties vary substantially among the three species. There still remains the possibility of unique, template mediated, framework siting for the metal in these and other tempiated forms of the chabazite topology. 

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