Structural description

The objective of topological nomenclature is not only to accurately describe a structural arrangement but also to inform the reader of the overall structure in terms of its components. It is often noted that the casual reader of a paper that uses topological descriptions of coordination frameworks does not appreciate the subtleties of topological nomenclature. Thus alternative approaches have been described. Structure with high connectivity can be particularly difficult to appreciate and thus an alternative approach based on the interconnectivity of two-dimensional nets, or subnet tectons, has been described [7]. This approach relies on the basis that many simple two-dimensional nets are readily visualised (such as simple square-based 44 nets, or honeycomb arrangements, 63 nets) and many highly connected nets are built from the intersection of these relatively simple nets. This approach works for all structures reported to date and may prove valuable for complex and intricate structures, notably those with connectivity over six. 

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 

Masciocchi, S. Galli, A. Sironi, Comments Inorg. Chem., 2005, 26, 1. 

57 See http //www.nottingham.ac.uk/ pczaib2/growcrvs.htm and references therein. 

Structural Inorganic Chemistry, 5th edn., Clarendon Press, Oxford, 1984 A. F. Wells, Three-Dimensional Nets and Polyhedra, John Wiley and Sons, New York, 1977. 

In comparison, dicarboxylic acids are colorless solids with melting points at about 100°C. All these acids form somewhat soluble metal salts and esters, of which the latter are adequately volatile for gas chromatography analysis. They also have spectral absorption properties that make them suitable for HPLC analysis (Gomis, 1992 Hsiao and Siebert, 1999). 

The strengths of weak adds are measured on the pfCa scale. The p/ ., value is the pH at which the acid and anion concentrations are equal (Table 2.1). The smaller the number on this scale is, the stronger the acid. In solution, weak acids do not fully dissociate into ions, but form equilibria between unchanged acid molecules and their respective charged anions and protons  

---

This is the first monograph that was devoted to structure, chemistry and use of zeolites. It reviews zeolite synthesis to 1973, gives a detailed structural description of synthetic and mineral zeolites, illustrates their physical properties and describes applications. 

The ROSDAL (Representation of Organic Structures Description Arranged Linearly) syntax was developed by S. Welford, J. Barnard, and M.F. Lynch in 1985 for the Beilstein Institute. This line notation was intended to transmit structural information between the user and the Beilstein DIALOG system (Beilstein-Ohlme) during database retrieval queries and structure displays. This exchange of structure information by the ROSDAL ASCII character string is very fast. 

Sccondaiy structure description of secondary structure HELIX, SHELL, J URN... 

L. B. Kiel, L. H. Hall, Molecular Structure Description The Electrotopological State Academic Press, San Diego (1999). 

If listed toxic chemicals are present equal to or above the de minimis cut-off level, your supplier must identify the specific components as they appear in Table II and provide their percentage composition by weight in the mixture or product, tf your supplier maintains that the identity ot a toxic chemical is a trade secret, a generic identity that is structurally descriptive must be supplied on the notice. A maximum concentration level must be provided if your supplier contends that chemical composition information is a trade secret. In either case, you do not need to make a trade secret claim on behalf of your supplier (unless you consider your use of the proprietary mixture a trade secret). On Form R, identify the toxic chemical you are reporting according to its generic name provided in the notification. (See the instructions for Part III, Section 2 on page 18 for more information.) tf the listed chemical is present below the de minimis level, no notification is required. 

A microscopic description characterizes the structure of the pores. The objective of a pore-structure analysis is to provide a description that relates to the macroscopic or bulk flow properties. The major bulk properties that need to be correlated with pore description or characterization are the four basic parameters porosity, permeability, tortuosity and connectivity. In studying different samples of the same medium, it becomes apparent that the number of pore sizes, shapes, orientations and interconnections are enormous. Due to this complexity, pore-structure description is most often a statistical distribution of apparent pore sizes. This distribution is apparent because to convert measurements to pore sizes one must resort to models that provide average or model pore sizes. A common approach to defining a characteristic pore size distribution is to model the porous medium as a bundle of straight cylindrical or rectangular capillaries (refer to Figure 2). The diameters of the model capillaries are defined on the basis of a convenient distribution function. 

Species type Structure Description Example Bond angles... 

Further improvements on the previously discussed models were proposed in the latest model for y - and e - Mn02 by Chabre and Pannetier [12, 43, 44], Starting from De Wolff s model they developed a structural description of manganese dioxides that accounts for the scattering function of all y - and e - Mn02 materials and provides a method of characterizing them quantitatively in terms of structural defects. All y — and e - Mn02 samples can be described on the basis of an ideal ramsdellite lattice affected by two kinds of defects ... 

Structural genomics is the systematic effort to gain a complete structural description of a defined set of molecules, ultimately for an organism s entire proteome. Structural genomics projects apply X-ray crystallography and NMR spectroscopy in a high-throughput manner. 

It is important to note that, for important sub-cases of case /), which will be discussed in more detail in Sect. 2.4, there is a low extent of disorder entropy effects, if any, are small and changes of the lattice dimensions are absent or small. These particular disordered forms are not considered as mesomorphic. In such cases, the limiting models which are fully ordered or fully disordered may be designated respectively as ordered or disordered crystalline modifications, if their consideration is useful for the structural description of a polymeric material. Note... 

Evain M, Brec R (1992) A New Approach to Structural Description of Complex Polyhedra Containing Polychalcogenide Anions. 79 277-306... 

Kier LB, Hall LH. Molecular structure description the electrotopological state. San Diego Academic Press, 1999. 

Rousseeuw [4]. Massart and Kaufman [5] and Bratchell [6] wrote specifically for chemometricians. Massart and Kaufman s book contains many examples, relevant to chemometrics, including the meteorite example [7]. More recent examples concern classification, for instance according to structural descriptions for toxicity testing [8] or in connection with combinatorial chemistry [9], according to chemical... 

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],... 

According to the simple one-electron electronic structure description, excitation into this strong a -> a band should lead to a reduction of the metal-metal bond order to zero resulting in homolytic cleavage into M(CO) units. While condensed phase experiments did at first appear to agree with this prediction (1.14.35-39). more recent time-resolved studies have painted a... 

NBOs 3-5 are the three fluorine lone pairs ( LP ). As shown by the occupancies and hybrid composition, these lone pairs are inequivalent. LP(1) is the s-rich sigma-type sp0 26 lone pair (nF(cr) 79% s character), directed along the bond axis. LP(2) andLP(3) are the p-rich pi-type lone pairs (nF(7t) and nF(rf) 99.97% p-character), perpendicular to the bond axis. The lone pairs have occupancies slightly less than 2.000 00 (due to weak delocalization into Rydberg orbitals of the adjacent H), but overall, the correspondence with the elementary Lewis-structure description is excellent. 

The accuracy of this NBO Lewis-structure description can be quantified in terms of... 

Table 3.1 compares key bonding and ionicity descriptors for the NBO Lewis-structure representations of the five AF diatomic fluorides in this series. From this table it can be seen that the single-term natural Lewis-structure description is practically exact for all species (with %p 99.9% for A = F, Cl, Br, H, and only a... 

How does this NBO description of A—F bonding compare with the classical valence-bond (VB) picture 14 Although it is evident that the NBO Lewis-structure description is very VB-like in its emphasis on localized, transferable electron-pair bonds and lone pairs of the chemist s Lewis diagram, there are important differences in mathematical detail. 

Distance-dependent changes in hybridization Finally, with the diatomic NBO configurations established by Tables 3.15 and 3.17, the only remaining unknown parameter of the NBO Lewis-structure description is the hybridization X of hCT, Eq. (3.84a). For values of R around Rc. we can approximate the fractional s character of hCT by... 

It must be emphasized that the formulation of three-center T-bonds provides a qualitative (not merely incremental) improvement in the accuracy of the natural Lewis-structure description of diborane. Because a three-center orbital is intrinsically more mathematically flexible than a two-center orbital, the description of any molecule is seemingly improved by employing three-center in place of two-center NBOs. However, for most non-boron molecules this improvement would be quite negligible (e.g., less than 0.1% for ethane, whose two-center Lewis-structure... 

---