Polymorphic compounds in the Cambridge Structural Database

The Cambridge Structural Database (CSD) is the repository for the results obtained from the X-ray crystal structure analysis of organic and organometallic compounds 

An example of the caution which must be exercised in performing such searches and the numbers obtained was given by Gavezzotti and Filippini (1995). The search was defined for organic compounds (containing only C, H, N, O, F, Cl, or S) and for which the crystal structures of more than one polymorphic form had been determined. 

A total of 163 clusters were obtained, where a cluster is a group of polymorphic crystal structures of the same compound. Of the 163 clusters, 147 contained two structures, 13 had three, and three had four structures. The authors note that these numbers are first evidence of the high frequency of polymorphism in organic crystals , although the number of clusters is a relatively small percentage of the entries in the database. The number of these clusters is probably more a measure of certain authors interest in the particular polymorphic system in question. A more realistic measure (although certainly not precise because of the caveats mentioned above) of the frequency of polymorphism in these compounds wouid be the fraction of compounds in the database known to be polymorphic, whether multiple structures have been done or not. 

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