Chloro/methyl interchange

One of the classic paradigms of crystal engineering is the notion of chloro/methyl interchange, where it is observed that pairs of molecules differing only by interchange of a chloro substituent for a methyl group can adopt identical crystal structures [34]. One question that we might reasonably expect to answer by consultation of the CSD is how often is isostructurality observed for pairs of chloro/methyl substituted molecules The conceptual steps required to answer such a question are as follows ... 

Figure 1.8 Steps in the GRX procedure for the chloro/methyl interchange example. Separate lists of structures containing Cl or CHj substituents are obtained using ConQuest, and the connectivity and chemical formula of the parent residue is derived for all structures. The chemical formula is compared in an initial rapid step, then full connectivity matching is undertaken only for pairs that match at the first stage. Reproduced from [35] by permission of the International Union of Crystallography...

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