Ray Investigations of Crystal Structures

At present therefore, the details of most atomic structures must be discovered indirectly. The experimental material for the purpose is the X-ray diffraction pattern. Electron diffraction patterns and neutron diffraction patterns are similar, and have been used for the same purpose but the great majority of investigations of crystal structure are based on X-ray diffraction patterns. The interpretation of these diffraction patterns falls into two stages—first, the determination of the shape and dimensions of the unit cell (see Chapter II), and second, the discovery of the positions of the atoms in the unit cell. 

When X-rays are passed through a crystal of sodium chloride, for example, you get a pattern of spots called a diffraction pattern (Figure 3.15b). This pattern can be recorded on photographic film and used to work out how the ions or atoms are arranged in the crystal. Crystals give particular diffraction patterns depending on their structure, and this makes X-ray diffraction a particularly powerful technique in the investigation of crystal structures. 

X-ray investigations of the structure of benzene began in 1923 when Broome took the first X-ray powder photographs of the molecule. Later, Cox (1928) determined the cell dimensions and space group and showed that the molecule was at least centrosymmetric. The development of the work on the benzene structure has been reviewed by Cox (1958). A more detailed paper on the crystal structure of benzene at — 3°C (Cox et al., 1958) has established that the benzene molecule does not deviate significantly from the 6jmmm symmetry predicted by chemical theory, the maximum deviation of the carbon atoms from the mean molecular plane being 0-0013 A. 

Preliminary X-ray investigations of crystals of 9,10-dihydro-anthracene (66) (Iball, 1938) showed that the most likely space group was P2X which, with two molecules in the unit cell, gave no indication of the molecular symmetry. A non-planar conformation for 9,10-dihydroanthracene has been established by Ferrier and Iball (1954). Their crystal structure analysis, using two-dimensional Fourier methods, shows clearly that the molecule is not planar but is bent about the line joining the carbon atoms 9 and 10. Each half of the molecule appears to be planar, the two halves being inclined to each other at approximately 145°. 

There is no essential difference between conventional X-rays and synchrotron X-rays with respect to their use in the investigation of crystal structure. However, the brilliance of synchrotron X-rays (which is currently some ten orders of magnitude greater than that of conventional X-rays), combined with the use of position-sensitive detectors, allows very rapid... 

The coordination theory and the principles governing coordinated structures provide the foundation for an interpretation of the structure of the complex silicates and other complex ionic crystals which may ultimately lead to the understanding of the nature and the explanation of the properties of these interesting substances. This will be achieved completely only after the investigation of the structures of many crystals with x-rays. To illustrate the clarification introduced by the new conception the following by no means exhaustive examples are discussed. 

Many of the investigated mesogenic compounds show solid state polymorphism. In order to obtain useful information about the arrangement of the molecules in the mesophase from the X-ray data of the single crystals, it is important to investigate the crystal structure of those solid phase which transforms into the liquid crystalline phase. For instance, only the crystal structures of the low temperature solid phases of the compounds MBBA [138, 139], MHPOBC [159], and T15 [81] could be determined, but the... 

Structural aspects were discussed, but not heavily, in the first edition. The complexity of new compounds (and macromolecules) now being investigated has driven many of the technological advances in X-ray crystallographic data collection and structure solution over the last two decades. Small-molecule (m.w. < 1,000 g mol-1) structure determinations are now routinely carried out, and Co complexes constitute a significant proportion of these. Indeed, the incorporation of crystal structures in most papers reporting new synthetic coordination chemistry is now a standard feature much more so than at the time of CCC(1987) (Figure 1). Inevitably, most of the new compounds described herein have been the subject of crystal structure determinations, rather... 

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