Mining the Cambridge Structural Database for Matched Molecular Crystal Structures: A Systematic Exploration of Isostructurality

The Cambridge Structural Database (CSD) is the world leading collection of small-molecule crystal structures and represents an invaluable resource for crystal engineers. It enables structures to be readily compared and new insights to be gained from the comparison. In order to search the database for pairs of structures that are related by the same chemical transformation, and to systematically investigate the effect of this transformation on crystal packing, a repository of matched molecular crystal structures has been derived from the CSD. This makes it easy to find all pairs of structures differing by the same chemical change or, alternatively, all available chemical modifications to a given CSD entry. Our analysis shows one of the many possible applications of these data. An extensive, yet not exhaustive, exploration of isostructurality across the entire CSD has been carried out with the aim of identifying packing features within crystals that maintain isostructurality. With particular focus on terminal chemical modifications observed between single-component structures with Z′ equal to 1, packing similarity has been calculated with an enhanced version of existing software. Across the entire data set of approximately 125 000 matched molecular pairs, 4% of the pairs were isostructural. Several cases showed an enrichment with respect to this baseline value, and examples have been discussed to illustrate some of the questions which can be asked and how they can be answered using the data set. This will open up avenues of research for the future and increase our understanding of the impact of functional groups on crystal packing.