Structure elucidation from crystal

Structure Elucidation from Crystal Powders. For many practical materials, such as polymers and zeolite catalysts, it is impossible to synthesize large crystals. Therefore the structure has to be found from powders. Powder XRD (preferably using synchrotron radiation) and neutron diffraction are the most important techniques, but experiments using other analysis methods like High Resolution Electron Microscopy (HREM) and Electron Diffraction (ED), MAS-NMR and EXAFS can add valuable information (8). 

The importance of alkali metal binding with available 7r-electron density in the formation of CIPs was also demonstrated by Niemeyer in the structural elucidation of the first monomeric non-solvated lithium cuprate, [(2,6-Mcs2(LI L)2CuLi] 450, formed from the reaction of 2 equiv. of (2,6-Mcs2Gf,I L)Li with /-BuOCu in pentane.447 The complex crystallizes as two different independent molecules in which the C-Cu-C angles differ (171.1° and 173.8°) as does the mode of coordination to the Li cations C pso and rf to one pendant Ph in molecule 1, with an additional rf interaction to a second Ph group in molecule 2. In the second molecule, the Li site is 10% occupied by a Cu ion. 

As described above, most solid-state reactions are heterogeneous, in the sense that reactant and product are in different solid phases. In many of these, product crystals first appear as nuclei that grow at the expense of the parent crystal. On the other hand, there are some solid-state reactions that are not accompanied by a phase change and for which, therefore, analogy with a solid-state transformation is not plausible. Such reactions are of particular interest in several respects They make possible conversion of a single crystal of reactant to a single crystal of product they enable study, for example by X-ray diffraction, of the structures of the parent and product molecules as functions of the degree of conversion in more or less constant environments and one can elucidate from them the constraints that the parent crystal imposes both on the reaction pathway and on the conformation of the product. It is in connection with the latter that this subject is of particular interest in the present context. This class of processes has been discussed by Thomas (183). 

The absolute configuration of the stereo centers of vinblastine (1) was determined from the X-ray crystal structure of vincristine (2) methiodide (79,80) in view of the known relationship between 1 and 2. The absolute stereochemistry at C-I8 in vinblastine (1) and related derivatives can also be deduced by means of ORD and CD spectroscopy (81,82). The determination was made possible by the synthesis and structure elucidation of several compounds possessing the unnatural configuration at C-18 (82,84). Because this stereo center controls the relative geometry of the... 

The adduct formed by two lithium atoms with polycondensed aromatic hydrocarbons crystallizes with two solvating molecules of TMEDA. The structure of the crystals derived from naphthalene (73) and anthracene (74) was elucidated by XRD. This arrangement of the unsolvated lithium atoms, in 7 -coordination fashion on the opposite sides of two contiguous rings, was found by MNDO calculations to be the most favorable one for naphthalene, anthracene and phenanthrene (75) . 

For proteins that are members of gene families, such as cytochrome P450, available structural models derived from X-ray crystallography and NMR data allow inference of the structure of newly discovered family members. The inference is based on the sequence variations derived from comparing the reference and new DNA sequences. While a high degree of computational power is needed, predictions based on this approach provide preliminary estimates of protein structure without a huge investment in expression, purification, and crystallization. A full structural elucidation requires several years for each protein, even if it can be crystallized. 

Molecular structure elucidation, principally by single crystal X-ray diffraction, has become almost routine and is now available for the majority of the metal amides presently discussed. In the 1980 book, however, such data were provided for just 112 compounds, 54 of which were for d- and/-block metals and 41 for the Group 13 metal amides. The contrast with the developing situation is illustrated by reference to Group 1 metal amides from four X-ray data sets in 1980 there were more than 200 by the end of 2007. 

While working on a related series of non-polymeric model compounds, we prepared the binaphthyl derivative 35 by the synthetic route depicted in Scheme 3.17 [34, 51] The yield was particularly poor (10%) but, nevertheless, a full characterization of 35 was achieved. The crystal structure of 35 has been elucidated from single crystals obtained from THF [51] and reveals that the molecule adopts a folded conformation in the solid state that is characterized by an almost parallel... 

Over 25 million compounds are known, but only about one percent of them have their crystal structures elucidated by X-ray and neutron diffraction methods. Many inorganic structures are closely related to each other. Sometimes one basic structural type can incorporate several hundred compounds, and new crystalline compounds may arise from the replacement of atoms, deformation of the unit cell, variation of the stacking order, and the presence of additional atoms in interstitial sites. 

The first triarylmethane dyes were synthesized on a stricdy empirical basis in the late 1850s an example is fuchsine, which was prepared from the reaction of vinyl chloride with aniline. Their structural relationship to triphenylmethane was established by Otto and Emil Fischer (5) with the identification of pararosaniline [569-61-9] as 4,4,4,-triaminotriphenylmethane and the structural elucidation of fuchsine. Several different structures have been assigned to the triarylmethane dyes (6—8), but none accounts precisely for the observed spectral characteristics. The triarylmethane dyes are therefore generally considered to be resonance hybrids. However, for convenience, usually only one hybrid is indicated, as shown for crystal violet [548-62-9], Cl Basic Violet 3 (1), for which Amax = 589 nm. 

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