Structural principles

In the crystal structure of Pr2ls there are two crystallographically independent praseodymium sites, Prl and Pr2, which have coordination numbers of 7 and 8 with mean Pr-I distances of 322.0 and 335.3 pm, respectively [19]. These distances are much like the respective Pr-I distances for coordination number 7 in K2PrIs (d= 321.2 pm [20]) and are as expected for coordination number 8 in 

Prl3 (PuBr3-type of structure, for Lal3 d = 334pm (6x), 340 pm (2x) [21]). Pr2ls is a semiconductor which orders antiferromagnetically below Tn = 37(1) K [19, 

In a localized picture, the electronic structure of Pr2ls [23] and also of Pr2Br5 [24] can be understood as if triangular clusters Pr3 contained one electron in a tricentric orbital. These triangles share common corners to chains Pr2/2Pri/i.  

The structural principles of Prl2 can be derived either from 4 nets (Prl2-I) or 3 nets (all other modifications) of iodine atoms that are stacked along a prominent crystallographic direction, in most cases the [001] direction. Between these layers, half of the respective interstices are filled with praseodymium atoms (but see Prl2-V below). Please note that 4 and 3 nets are closely related to each other, it only needs a shear procedure to transform one net to the other (Fig. 4.3). In the iodine layers I-I distances are even shorter in the 4 net (386 in Prl2-I [4]) than in the 3 net of Prl2-IV (426.5 pm [6, 9]). 

Prl2-V is unique. Tetrahedral Pr4 clusters may reasonably be drawn (Fig. 4.6). These are capped on all four faces and have three terminal iodine 

The structure of both cristobalite and tridymite are composed of layers of six-membered rings of Si-0 tetrahedra. The stacking sequence of these layers 

In earlier literature reports, x-ray data of a-based ceramics, the /3-like phase observed in certain silica minerals was explained by a structural model based on disordered Q -tridymite. However, others have suggested that the structure of the stabilized jS-cristobalite-like ceramics is closer to that of a-cristobalite than that of Q -tridymite, based on the 29Si nuclear magnetic resonance (NMR) chemical shifts (Perrota et al 1989). Therefore, in the absence of ED data it is impossible to determine the microstructure of the stabilized jS-cristobalite-like phase. ED and HRTEM have provided details of the ceramic microstructure and NMR has provided information about the environments of silicon atoms in the structure. Infrared spectroscopy views the structure on a molecular level. 

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Janin, J., Chothia, C. Domains in proteins definitions, location and structural principles. Methods Enzymol. 115 420-430, 1985. 

Easters, I., et al. Structural principles of parallel p barrels in proteins. Proc. Natl. Acad. Sci. USA 85 3338-3342, 1988. 

Lesk, A.M., Branden, C.-L, Chothia, C. Structural principles of a/p barrel proteins the packing of the interior of the sheet. Proteins 5 139-148, 1989. 

We can immediately discern from Figure 5.11 that the molecule is divided into two clearly separated domains that seem to be of similar size. For the next step we would need a stereopicture of the model or, much better, a graphics display where we could manipulate the model and look at it from different viewpoints. Here instead we have made a schematic diagram of one domain (Figure 5.12), which is normally not done until the analysis is completed and the structural principles are clear. 

Murzin, A.G. Structural principles for the propeller assembly of p sheets the preference for seven-fold symmetry. Trotei/is 14 191-201, 1992. 

Together with the structural principles established by the Bragg school concerning the many types of silicates, Goldschmidt s ideas were taken further by Linus Pauling in California to establish the modern science of crystal chemistry. A good early overview of the whole field can be found in a book by Evans (1939, 1964). 

The phase relations, stoichiometry and structural chemistry of the metal borates have been extensively studied because of their geochemical implications and technological importance. Borates are known in which the structural unit is mononuclear (1 B atom), bi-, tri-, tetra- or penta-nuclear, or in which there are polydimensional networks including glasses. The main structural principles underlying the bonding in crystalline metal borates are as follows " ... 

The structural principles and reaction chemistry of B-8 compounds have recently been reviewed. This includes not only electron-precise 4-, 5- and 6-membered heterocycles of the types described above, but also electron-deficient polyhedral clusters based on closo-. 

The oxoacids of P are more numerous than those of any other element, and the number of oxoanions and oxo-salts is probably exceeded only by those of Si. Many are of great importance technologically and their derivatives are vitally involved in many biological processes (p. 528). Fortunately, the structural principles covering this extensive array of compounds are very simple and can be stated as follows ... 

It follows from these structural principles that each P atom is 5-covalent. However, the oxidation state of P is 5 only when it is directly bound to 4 O atoms the oxidation state is reduced by 1 each time a P-OH is replaced by a P-P bond and by 2 each time a P-OH is replaced by... 

This by no means exhaustive discussion may serve to indicate the value of the information provided by magnetic data relative to the nature of the chemical bond. The quantum-mechanical rules for electron-pair bonds are essential to the treatment. Much further information is provided when these methods of attack are combined with crystal structure data, a topic which has been almost completely neglected in this paper. It has been found that the rules for electron-pair bonds permit the formulation of a set of structural principles for non-ionic inorganic crystals similar to that for complex ionic crystals the statement of these principles and applications illustrating their use will be the subject of an article to be published in the Zeitschrift fur Kristallographie. 

This physical differentiation of the anions and cations under discussion in regard to size and charge finds expression throughout this paper. Markedly different roles are attributed anions and cations in the construction of a crystal as a result a pronounced distinction between them has been made in the formulation of the structural principles. 

This highhght examines several new hybrid layered structures in the hetero-metallic perrhenate and vanadate families, whereby the late transition-metals are incorporated and their roles probed in the structures of layered solids. From these two families, new structural principles have emerged that not only help us understand key stractural features and correcdy forecast new compositions, but equally, have yielded many surprises (chirality, reduced phases) that show some of the most exciting chemistry is still waiting to be discovered or even imagined ... 

Conrad O, Jansen C, Krebs B (1998) Boron - Sulfur and Boron - Selenium compounds -From unique molecular structural principles to novel polymeric materials. Angew Chem IntEd37 3208-3218... 

The enantioselectivity of the BINOL-Ti(IV)-catalyzed reactions can be interpreted in terms of several fundamental structural principles.42 The aldehyde is coordinated to Ti through an apical position and there is also a 0-HC=0 hydrogen bond involving the formyl group. The most sterically favored approach of the alkene toward the complexed aldehyde then leads to the observed product. Figure 10.2 shows a representation of the complexed aldehyde and the TS structure for the reaction. 

Important structural principles for ionic crystals, which had already been recognized in part by V. Goldschmidt, were summarized by L. Pauling in the following rules. First rule Coordination polyhedra... 

Of the numerous ternary and polynary diamond-like compounds we deal only with those that can be considered as superstructures of zinc blende. A superstructure is a structure that, while having the same structural principle, has an enlarged unit cell. When the unit cell of zinc blende is doubled in one direction (c axis), different kinds of atoms can occupy the doubled number of atomic positions. All the structure types listed in Fig. 12.8 have the tetrahedral coordination of all atoms in common, except for the variants with certain vacant positions. 

This chapter first discusses basic structural principles relating to the main classes of glycoconjugates in order to assist understanding of nematode structures. It then focuses on a selection of parasitic nematodes where detailed structural data on glycoconjugates have been obtained in recent years and where this structural information is starting to provide insights into possible molecular functions. 

Contents Introduction. — Structural Principles. — Synthesis and Reactions of Macromoiecular Compounds. — The Properties of the Individual Macromolecule. — States of Macromoiecular Aggregation. 

B. Electronic Features and Structural Principles of Molecular Main Group Metal Pnictides... 

The great success of the Lewis-structure concept naturally calls attention to exceptional cases in which Lewis-structural principles appear to be violated. An important... 

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