Predicting Structures from Formulae

The skeletal electron contributions made by the transition metal units listed in Table 3.5 assume in all cases that the metal atom uses all nine of its valence shell orbitals, that is, that these are 18-electron systems in the parlance of transition metal chemists. It is well known, however, that some transition metals (such as palladium and platinum) form stable 16-electron rather than 

8 Dodecahedron C2B4H4Me2SnCoCp B4H4(CoCp)4 B4H4(NiCp)4 

9 Tricapped trigonal prism C2B6H8ML, [ML, = CoCp, Mn(CO)j-, Pt(PMe3)2] C2B5H7(CoCp)2 [CBvHsCoCp]- [Co(ti5-C2B6H8)2]- 

10 Bicapped square antiprism C2B7H9C0CP C2B7H7Me2Fe(CO)3 C2B6H8(CoCp)2 [Co(riCC2B7H9)2] CB7H8CoCpNiCp [B9H9NiCp]- 

11 Octadecahedron C2B8HioML [ML, = CoCp, IrH(PPh3)2] [Co(riCC2B8Hio)2] C2B7H9(CoCp)2 [CB9H10C0CP]- 

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Table 14.2 Predicted formulae and reported chalcophosphate structures from Eq. (1).

All the orthovanadates with zircon structure and formula LnV04 where Ln = Ce, Pr, Nd, Sm, Eu, and Gd, which have been studied up to now, have very similar spectra. The vs vibration which is predicted to spUt into two bands from the site symmetry treatment appears in the IR as one strong, broad band with a weak shoulder. On the other hand, Vi appears as a sharp band. CaCr04, which has the same structure, possesses a very similar spectrum. 

So far the emphasis has been on solving the crystal structure from the knowledge of the unit cell and ionic content. The motivation for this work is to provide an automated procedure to help determine or solve the crystal structure of new compounds that are synthesised in a powder form. Of course the methods developed can generate other structural topologies and perhaps a new, yet to be synthesised, crystal structure. However, the task has been to solve a particular structure and so one might extract more information from the experimental data to aid the prediction process (e.g. use of symmetry elements). Thus, the number of unwanted possible (meta)stable structures, or polymorphs, that could be generated may be reduced. In this section, the emphasis is on finding all the important polymorphs for a particular chemical formula. 

A recent study by Vetter et al. [225] showed that 11 polychlorinated bornanes were abundant in different seal species. The most important persistent 7 polychlorinated bornanes with their lUPAC name, different abbreviations, chemical structure, molecular formula, n-octanol/water partition coefficient (log Kqw) and predicted bio concentration factors (BCF and BCFl) in fish are compiled in Table 13. The predicted BCFl values of hepta-, octa- and nonachlorobornanes are between 600,000 and 71,000,000, and the predicted BCF values of these congeners in fish with 5 % lipid range from ca. 32,000 to 3,500,000. Furthermore, in Table 13 the BCF and BCFl values of two polychlorinated bornane congeners (Parlar No. 26 and No. 50) are included, which were calculated by the authors from the measured concentrations in zooplankton and different fish species and the water of a Canadian fresh water lake [226]. It is obvious that the BCF values of the chlorinated bornanes calculated from concentrations in aquatic organisms and water from the environment are by a factor between 1 and ca. 70 greater than the BCFs predicted from the log values. This can be explained in part by bio accumulation. 

From our knowledge of the electronic structure of their atoms, we can predict a reasonable formula for the simplest molecule that might be formed by a metal combined with a nonmetal. For example, we can recognize the univalence of both potassium and chlorine atoms and predict that the simplest molecule will have the molecular formula, KCl. However, a single bond in such a molecule would use only one electron from each atom, leaving unused both vacant orbitals on the potassium and electron pairs on the chlorine. 

Connectivity and stereoisomerism give chemists a way to uniquely differentiate one molecular structure from another. The molecular formula C4H9Br, for instance, represents five different substances (see Figure 4). Predictably, although there is only one compound for each of the connectivities designated 1-bromobutane, 2-bromo-2-methylpropane, and... 

The most obvious feature of these C chemical shifts is that the closer the carbon is to the electronegative chlorine the more deshielded it is Peak assignments will not always be this easy but the correspondence with electronegativity is so pronounced that spec trum simulators are available that allow reliable prediction of chemical shifts from structural formulas These simulators are based on arithmetic formulas that combine experimentally derived chemical shift increments for the various structural units within a molecule... 

Predicting the Structures of Simple Organic Molecules from Their Formulas... 

Software to predict the properties of formulated products is made more powerful by a recursive procedure which can use formulas stored in files as raw materials. Particular care must be taken with program flow control and data structures for the recursion to be effective. This paper illustrates these issues using an example derived from a working formulation system for coatings development. 

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