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In order to assess the relative importance and the structure defining role of close packing interactions (molecular shape) and specific directional interactions (e.g. hydrogen bonds, halogen-halogen interactions), a systematic study of the crystal structures of para-disubstituted benzene derivatives was started. The molecular shapes and possible p-p interactions are similar for each of these compounds, while different specific interactions may arise between the various functional groups. 198 unique crystal structures were retrieved from the Cambridge Structural Database [1]. The molecular arrangements in these structures were classified using the Compack algorithm [2], which tests the similarity of the coordination shell of 12-16 molecules packed around a reference molecule. Molecules were assigned to sets so that each molecule in a set had at least one partner in the same set for which a match of 15 molecules with a 20% distance tolerance was found. These sets provide a broad, general classification of the crystal packing arrangements. Since some geometrical distortions were observed between the structures belonging to the same set, another method of classification was also applied. The 198 structures were searched by Conquest for short intermolecular contacts between the benzene rings (with any pair of atoms being closer than the sum of their van der Waals radii + 0.3 Å). The resulting dataset defines the spatial relationship between immediately neighbouring molecules in terms of centroid-centroid distances and interplanar angles. The contacts in the dataset were classified using the cluster analysis program Tanagra [3]. Four clusters representing distinct interaction modes between the aromatic moieties were identified. The occurrence of the different interaction modes between immediate neighbours in the crystal structures correlates with the grouping of the packing arrangements. This correlation confirms the significance of the packing sets deduced from Compack based comparisons. It was found that there are popular arrangements, which are exhibited by compounds having completely different substituents (e.g. OH, SO2Me, Br, B(OH)2, Me), some hydrogen bonding and some hydrophobic. A description of these arrangements will be given, and they will be compared with the packing arrangements of aromatic hydrocarbons [4]. The role of the functional groups in the formation of the patterns will be discussed.

Close packingvsspecific interactions:para-disubstituted benzene derivatives