1:2 compounds, structures descriptions

Palm and co-workers, as part of their research regarding the applicability of PSA, selected 20 compounds (Table 16.1) with quite reliable values for the percent fraction absorbed (%FA) in humans that were devoid of, or compensated for, problems such as active transport and efflux mechanisms [2]. The dataset consists of a variety of compounds drugs, drug-like compounds, as well as sugar-like structures. A number of studies based on this dataset using various structural descriptions have been performed [2, 17-22],... 

Principles and types of modular series have been summarized in a book by Ferraris et al. (2004). A hierarchical classification of structure has been introduced as well as the application of modularity to structure description and modelling. The order/disorder theory has been presented as fundamental to developing a systematic theory of polytypism, dealing with structures based on both ordered and disordered stacking of one or more layers. The structures of a great number of compounds (minerals, complex oxides, salts, etc.) have been described and discussed. 

The intent of this chapter is twofold. One is to give brief structural descriptions of many of the copper-oxide superconductors. For in-depth information, the reader is referred to the original publications. The second is to provide detailed crystallographic information (lattice constants, positional and thermal parameters, space groups, etc.), and compositional data on many of the superconductors discussed. Also, calculated x-ray powder diffraction patterns for these same compounds are tabulated. It is hoped that such information will prove useful to the superconductivity researcher. 

Xenon exhibits a rich variety of stereochemistry. Some of the more important compounds of xenon are listed in Table 17.5.1, and their structures are shown in Fig. 17.5.2. The structural description of these compounds depends on whether... 

The first systematic structure analysis of mercury oxosalts, salts containing oxygen atoms that are not part of an acid residue, was recently published. Structural description in terms of oxocentered [OHg4] tetrahedra with nonequivalent Hg bonds linked by both vertices and edges efficiently illustrated the unique structures and interrelationships of these compounds. 

Another frequently quoted criterion for CS phases is that the cations must have a strong preference for octahedral co-ordination. Once again this does not provide an adequate guide line for our purposes. This is clearly illustrated by the behaviour of those oxides and fluorides which possess the rutile structure. Out of this considerable number of compounds, only two or possibly three, Ti02, VO2, and perhaps Cr02 show CS behaviour. The others show little range of stoicheiometry and point defects seem to be a reasonable structural description to account for this range. 

A proposal made recently by Lambert and coworkers [Id] for the nomenclature and structural description of polysilane dendrimers treats the problem of the high redundancy of very complex names of these compounds which possess a high degree of symmetry and might be described better by different ways. 

The method is based on a combination of the DARC system (Description, Acquisition, Retrieval and Computer-aided design) and the PELCO (Perturbation of an Environment Limited Concentric and Ordered) procedure. Moreover, it accounts for the simultaneous representation of all the data set compounds and the population of the compounds structurally contained in them the data set compounds, which are those compounds for which a molecular property has been experimentally evaluated, generate an ordered multidimensional space that constitutes a —> hyperstructure. 

Dimoglo, A.S. (1985) Compositional approach to electronic structure description of chemical compounds, oriented on computer analysis of structure-activityrelationships. Khim-Farm. Zh., 4, 438 44. 

In addition a recent approach to structural description has been reported in which common structural types have been given a three letter designation relating the structure to a well-known solid-state, typically inorganic, compound [6]. For example the diamond-like net constructed from the connection of tetrahedral nodes has 66 topology but is simply described as dia, relating the structure directly... 

When a compound is molecular under normal conditions it is usual to regard it as covalent (although ionic molecules such as NaCl(g) can at be made by vaporizing the solid compounds at high temperatures). When two elements of different electronegativity form a solid compound alternative descriptions may be possible. Consider the compounds BeO and BN. Both form structures in which every atom is surrounded tetrahedrally by four of the other kind (BN also has an alternative structure similar to that of graphite). For BeO this is a plausible structure on ionic... 

Other compounds crystalhzing with a Cdl2 lattice include MgBr2, Mgl2, Cal2, iodides of many J-block metals, and many metal hydroxides including Mg(OH)2 (the mineral brucite) in which the [OH] ions are treated as spheres for the purposes of structural description. 

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