Fragment Symmetry and Chirality

Relatively few molecular structures in the CSD exhibit molecular symmetry. However, many substructural fragments of interest in structure correlation studies are small and symmetric. This fact is recognized in Chapter 2, and various aspects of fragment symmetry are discussed there. We now examine the consequences of fragment symmetry on the search process itself and, hence, on the relative ordering of the Np geometrical parameters recorded for each fragment in the multivariate data matrix G Nf,Np). 

We need to consider enantiomorphs in most multivariate analyses which use crystallographic data. Most of the structures are either (a) non-chiral, or (b) chiral molecules for which the absolute chirality has not been determined. In some cases, however, the 3 D coordinate sets in the CSD do reflect absolute chirality either the absolute stereochemistry has been established explicitly by the X-ray experiment, or it is so well known that we expect the coordinates to represent the correct enantiomorph as, for example, in the case of the pyranose sugars. In these instances we might wish to ignore the enantiomorphs. 

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