Crystallization data literature

Most of the force fields described in the literature and of interest for us involve potential constants derived more or less by trial-and-error techniques. Starting values for the constants were taken from various sources vibrational spectra, structural data of strain-free compounds (for reference parameters), microwave spectra (32) (rotational barriers), thermodynamic measurements (rotational barriers (33), nonbonded interactions (1)). As a consequence of the incomplete adjustment of force field parameters by trial-and-error methods, a multitude of force fields has emerged whose virtues and shortcomings are difficult to assess, and which depend on the demands of the various authors. In view of this, we shall not discuss numerical values of potential constants derived by trial-and-error methods but rather describe in some detail a least-squares procedure for the systematic optimisation of potential constants which has been developed by Lifson and Warshel some time ago (7 7). Other authors (34, 35) have used least-squares techniques for the optimisation of the parameters of nonbonded interactions from crystal data. Overend and Scherer had previously applied procedures of this kind for determining optimal force constants from vibrational spectroscopic data (36). 

Some common substances for which crystallization data are reported in the literature and in patents are listed in Table 16.4. 

TABLE 16.4. Some Common Substances for which Crystallization Data Are Reported in the Literature and in Patents3... 

The procedure based on the direct use of the g tensor anisotropy and Eq. (2.24) is quite common for S = xh systems, since g values from frozen solutions are easily obtainable. In this case, both the second order Zeeman contributions and possibly the effects of temperature on the g values are neglected. Furthermore, the directions of the molecular axes are arbitrarily assumed unless single-crystal data are available. Attempts are available in the literature regarding low spin cobalt(II) [77] and copper(II) [61]. 

Reference cites the literature from which the crystal data, atomic coordinates, and displacement factors were obtained. In many cases there are multiple refinements of the same zeolitic material, but because of space limitations not all refinements could be included. We would be appreciative if authors and users would inform us of any errors or omissions. A listing of the references for isotypic species can be found in the Atlas of Zeolite Framework Types (Baerlocher, McCusker and Olson (2007)). A list of references to structure analyses of zeolites with different cations, up to 1982, is given in the Compilation of Extra Framework Sites in Zeolites, Mortier (1982). 

Tabic l.l gives those crystal data for the C,S polymorphs that have been obtained using single crystal methods. The literature contains additional unit cell data, based only on powder diffraction evidence. Some of these may be equivalent to ones in Table 1.1, since the unit ceil of a monoclinic or triclinic crystal can be defined in different ways, but some are certainly incorrect. Because only the stronger reflections are recorded, and for other reasons, it is not possible to determine the unit cells of these complex structures reliably by powder methods. The unit cells of the T, Mj and R forms are superficially somewhat different, but all three are geometrically related transformation matrices have been given (12,HI). 

Most papers in the chemical, biological, and material science literature reporting the results of crystal structure analyses include a section (or a footnote) listing the crystal data, which should indicate the quality of the crystal, the form of the experiment, the accuracy of the measurements, and the precision of the results. The journals all have slightly different requirements for the contents of this section the list presented in Table 3 conforms to the most rigorous requirements of the International Union of Crystallography (lUCr) and its journal... 

In addition to these compilations of crystal data in which instances of polymorphism may be recorded, a number of texts on the subject of the solid state properties of organic compounds contain many examples of polymorphism. Since these books are based in part, at least, on work by the authors not published elsewhere, they may be considered as primary literature sources. Particularly noteworthy in this regard are the books by Pfeiffer (1922), Kofler and Kofler (1954), and McCrone (1957). 

The crystal data compilation of Donnay and Ondik (8) does not list the crystal structure of Zrl (cr) but does tabulate both ZrCl and ZrBr as cubic structures. The literature data related to the crystal structure of Zrl is not definitive. Assuming Zrl (cr) also has a cubic structure, the adopted heat capacity values are estimated so as to parallel those for ZrCl and ZrBr. The values below 300 K are calculated in the same manner as for ZrBr (cr) (2). The high temperature heat capacities are obtained graphically. 

The easiest way to access the crystal structure literature is through "Molecular Structures and Dimensions", which provides an up-to-date well indexed bibliography of all organic and organometallic crystal structures that are available (2). This excellent series is the work of the Cambridge Crystallographic Data Centre, established by Professor Olga Kennard in the U.K. 

Reference cites the literature from which the crystal data, atomic coordinates, and temperature factors were obtained. In many cases there are multiple refinements of the same zeolitic material, but because of space limitations not all refinements could be included. We would be appreciative if... 

Examples of data warehouses published in literature are the SPINE [12] and CerBeruS [13] systems. While the SPINE system is focusing on the support and mining of protein crystallization data, the CerBeruS system links data relevant for the SAR analysis of larger data sets such as HTS data. 

CRYSTALLIZATION OF ORGANIC CHEMICALS. All the examples given in this chapter are for inorganic salts. The principles of crystaUization from solution, however, apply equally to organic materials. Walas lists a number of organic compounds for which crystallization data are reported in the literature. Organic compounds are also often purified by melt crystallization as discussed in the next section. 

Magnetic properties of R2Co14B compounds. The values listed under Tc and Ts represent Curie temperatures and spin reorientation temperatures. The values of the saturation magnetization (7S), saturation moment (Ms) and the anisotropy constant (K ) refer to 4.2 K. References to literature data used to obtain these values are indicated in brackets. In most cases single-crystal data were used, in other cases average values obtained on aligned powders were listed. 

Fig. 10 includes some additional data from the literature concerning the binary compounds 7REs03 9Si02 and the isostmctural alkali rare-earth apatites. Most of the crystal data reported show poor agreement of the cell dimensions for the same compound. The differences might depend on the X-ray diffraction technique used, and/or the method of evaluating of the experimental data. However, in most cases e.s.d. s... 

Owing to the nature of hypervalent bonding and the T-shaped geometry of the iodine(ni) center, the formation of six-membered iodine heterocycles is highly unfavorable. Several such compounds have been reported in the literature [328,342-346] however. X-ray structural data on six-membered iodine(III) heterocycles is not available. Moreover, based on the available X-ray single-crystal data for several pseudocyclic six-membered iodine(V) derivatives (Section 2.2.2), it can be expected that these compounds may exist as their noncyclic tautomers [345]. 

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