Crystal structure graphical comparison

Several recent general papers on the properties of quinoxaline N-oxides have reported studies on the crystal structures of quinoxaline 1,4-dioxide,380 its 2,3-dimethyl derivative,380 ethyl 7-chloro-3-methyl-2-quinoxalinecarboxylate 1,4-dioxide,40 and N-(2-hydroxyethyl)-3-methyl-2-quinoxalinecarboxamide 1,4-dioxide 931 the NMR spectral data of quinoxaline 1,4-dioxide for comparison with those of related dioxides 348 the NMR data for biologically active quinoxalinecarboxamide dioxides 381 thermochemical data for several quinoxaline dioxides 183 and polaro-graphic or cyclic voltammetric data for 2,3-disubstituted quinoxaline dioxides.239 894... 

Fortunately, the need to amass coordinates of atoms in reported crystal structures has already been appreciated, and efficient computer-based databases are now available. As a result, it is not necessary to type large sets of numbers into a computer, because they can be derived in computer-ready form from a database. In addition to the ready access to atomic coordinate tables, there are many excellent computer-graphics, geometrical, and statistical programs available for comparisons of structures. In practice, however, it is additionally advantageous to build molecular models either of the ball-and-stick variety or the space-filling variety, if chemical or biochemical insight is required from a comparison of molecular structures observed by crystal structure analyses. 

Correlations of three-dimensional structure with the chemical, physical, and biological properties of a compound require comparisons of bonding geometry, of conformation, and/or of interactions of the compounds with their respective environments in the crystal structure. Lists of results from calculations of bonding geometry can be used, but graphical methods are frequently more effective, and easier for the reader. 

This chapter is intended to provide a unified view of selected aspects of the physical, chemical, and biological properties of the actinide elements. The f block elements have many unique features, and a comparison of the lanthanide and actinide transition series provides valuable insights into the properties of both. Comparative data are presented on the electronic configurations, oxidation states, redox potentials, thermochemical data, crystal structures, and ionic radii of the actinide elements, together with a miscellany of topics related to their environmental and health aspects. Much of this material is assembled in tabular and graphical form to facilitate rapid access. Many of the topics covered in this chapter, and some that are not discussed here, are the subjects of subsequent chapters of this work, and these may be consulted for more comprehensive treatments. This chapter provides a welcome opportunity to discuss the biological and environmental aspects of the actinide elements, subjects that were barely mentioned in the first edition of this work but have assumed great importance in recent times. 

Substances with these low-symmetry structures yield powder patterns which are almost impossible to index by graphical methods, although the patterns of some orthorhombic crystals have been indexed by a combination of graphical and analytical methods. The essential difficulty is the large number of variable parameters involved. In the orthorhombic system there are three such parameters (flf, b, c), in the monoclinic four (a, b, c, P), and in the triclinic six a, b, c, oc, p, y). If the structure is known, patterns of substances in these crystal systems can be indexed by comparison of the observed sin 0 values with those calculated for all possible values of hkl. 

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