Some Crystal Structures

Investigations of the equilibria obtaining in solution have provided information concerning the stoichiometry and stability of the species formed when the beryllium ion is hydrolyzed. Although the identification of the minor species can never be regarded as definitive, there is little doubt that the principal species are Be2(OH)3+ and Be3(OH)3+ in acid solutions and Be(OH)3 and Be(OH)r in strongly basic solutions. Further support for these conclusions is provided by some crystal structures. The structure of [Be3(0H)3(H20)6]... 

The question as to the best formulation of structures and species in some binary fluoride systems was the subject of extensive experimental investigations, involving infrared and Raman spectroscopy in the molten state and in solutions as well as NMR spectroscopy and conductometric and cryoscopic measurements. Some crystal structure studies have also been published. The systems of SeF4 with BF3, SbF5, AsF5, NbF5, and TaFs have been studied recently. 

Table 30 Some Crystal Structure Data for the Bismuth Halides...

The importance of polymorphism in pharmaceuticals cannot be overemphasized. Some crystal structures contain molecules of water or solvents, known as hydrates or solvates, respectively, and they are also called as pseudopolymorphs. Identifying all relevant polymorphs and solvates at an early stage of development for new chemical entities has become a well-accepted concept in pharmaceutical industry. For poorly soluble compounds, understanding their polymorphic behavior is even more important since solubility, crystal shape, dissolution rate, and bioavailability may vary with the polymorphic form. Conversion of a drug substance to a more thermodynamically stable form in the formulation can signiLcantly increase the development cost or even result in product failure. 

DTPA and all DTPA-bisamides reported up to now bind Ln3+ ions in a 8-coordi-nate fashion through the three N atoms of the diethylenetriamine backbone and five carboxylate and/or amide O-atoms, both in the solid and in solution. An inspection of crystal structures shows that diethylenetriamine moieties in these complexes always occur either in the XX or in the 88 conformation [ 1,2]. In these conformations, steric interactions are minimized. The coordination sphere is completed by one water molecule or, in some crystal structures, by the O-atom of a neighboring carboxylate group. In a recently reported low temperature (173 K) X-ray structure of K2 [Yb(DTPA)(H20)] second sphere waters were observed adjacent to the carboxylate oxygens [3]. The inner-sphere water is the most important source of the relaxivity of the corresponding Gd3+ complex, but the second sphere water molecules contribute significantly to it as well [3]. 

Two advanced techniques have been proposed and applied to some crystal structures (Section IV,C), in which aspherical distributions of valence electrons around an atom are directly taken into account in the least-squares calculations. Aspherical atomic form factors are introduced in the least-squares refinement in the first method (29, 38, 80) and multipole parameters describing the aspherical valence distributions are used in the second method (31, 34, 46). 

There are some crystal structures in which further symmetries are present in addition to those prescribed by their three-dimensional space groups. The phenomenon is called hypersymmetry [102], Thus, it refers to symmetry features not included in the system of the 230 three-dimensional space groups. For example, phenol molecules, connected by hydrogen bonds, form spirals with threefold screw axes as indicated in Figure 9-55. This screw axis does not extend, however, to the whole crystal, and it does not occur in the three-dimensional space group characterizing the phenol crystal. 

There are hypersymmetry phenomena in some crystal structures that are characterized by extra symmetry operations applicable to infinite chains of molecules. This kind of hypersymmetry has proved to be more easily detectable and has been reported often in the literature [104],... 

Strong hydrogen bonds can be symmetrical or unsymmetrical. In some crystal structures, the midpoint of the O 0 hydrogen-bonded separation is a crystal-... 

Table 13.2. Distribution of hydrogen-bonding patterns in the crystal structures of pyranoses and pyranosides. (N) indicates neutron diffraction studies the S3 configuration of molecules is D, unless otherwise indicated some crystal structures appear in two groups 00... 

Several [CoL(NH3)5]3+ and m-[CoL(X)(en)2]2+ complexes have been prepared (Table 26) and some crystal structures are available (Table 25). Pentaammine complexes are easily made from [Co(DMSO)(NH3)5](C104)3, and there is no need to use a better leaving group.340,341 The physical properties ( H NMR, visible-UV, 13C NMR, pATa) for both systems (L = imH) have been comprehensively documented.341,342 The acidity of the N3 proton in (65) is enhanced by coordination and the imidazolate anion is readily accessible (pXa 10.0 vs. 14.1 for imH). Several deprotonated bimetallic bridged species have been prepared and structure (66) has been put forward as a model for heterobiometallic protein centres.343 The tetrazole complexes (68) exist predominantly in the anionic form (pA 1-2)351 and preparation normally gives the N(2)-bonded form. However Ellis and Purcell352 have prepared the N(l) isomer by reacting a coordinated nitrile with N3 ion at pH 5 (equation 52). Properties of both isomers are available.351... 

In this section, after introducing some crystal structures, a selection of band structures calculated for different phases of the ET salts will be presented in Sect. 2.3.2. The band structures for some special materials will be presented in Chap. 4 together with the experimentally determined FS. In Sect. 2.3.3 an introduction to the superconducting properties of the 2D materials will be given. 

Some crystal structures of chelate complexes have been reported. An O-acryloyl-lactate-TiCU complex (Fig. 3) [26,27] has rare out-of-plane (Fig. 4) coordination of the acryloyl carbonyl group to the titanium a further study has been conducted [28]. Diethyl phthalate-TiCU [29], l,2-diketone-TiCl4 [25], and achiral [24] or chiral [30] acyloxazolidinone-TiCU complexes have been reported to involve in-plane coordination as shown in Fig. 5. The /S-alkoxyketone-TiCU complex shown in Fig. 6 [31] is characterized by a rare out-of-plane coordination geometry (dihedral bond angle of... 

TABLE lO-I Some Crystal Structure Parameters of Amides and Peptides... 

For simple compounds, p = 30 pm works well when tq is also in pm. Lattice enthalpies are twice as large when charges of 2 and 1 are present, and four times as large when both ions are doubly charged. Madelung constants for some crystal structures are given in Table 7-2. 

Some crystal structures of zinc proteins are available (29) however, information concerning zinc centers in proteins is rather limited. Because of its filled 3d-shell, zinc is not accessible by spectroscopic techniques such as optical absorption and EPR. Zinc sites in proteins may be studied indirectly by spectroscopy Cd substitution and monitoring by Cd NMR spectroscopy or Co substitution and monitoring by UV-visible or circular dichroism are popular procedures. Nevertheless, it should be borne in mind that these surrogates may not be faithful reporters of zinc sites. 

Liquid crystal Some Some crystal structure... 

In Section 7.7, the binding modes revealed by some crystal structures of thrombin-inhibitor complexes were discussed. Inhibitor studies on thrombin have been... 

However, the incorporation of metal cations whose valence is different from that of A1 or P leads to the formation of electronically unsaturated sites, as schematically shown in Figure 3. This addition of aliovalent metal cations into the lattice of AlPO-n generates solid acidity and ion-exchange sites. There are numerous reports on the incorporation of many different metal cations into the lattice of AlPO-n. Table 2 summarizes the reported isomorphous substituted AlPO-n. The family of AlPO-n substituted with metal cations is generally called metal aluminophosphates (MeAPO-n). The typical metal cations substituted into AlPO-n are Li, B, Be, Mg, Ti, Mn, Fe, Co, Zn, Ga, Ge, Si, and As. The Si-substituted AlPO-n is called a silicoaluminophosphate and denoted as SAPO-n, where n also means the framework structure, and it is distinct from the MeAPO-n materials.SAPO-n exhibits both structural diversity and compositional variation. In particular, the crystal structure of SAPO-n is of greatest interest, because the distribution of the Si atom in the framework is quite complicated. Some crystal structures, such as SAPO-40, are only found in SAPO-n and not in AlPO-n or zeolite. The mole... 

Figure 4.2.18 Molecular and some crystal structures of disaccharides.

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