Effect of Intermolecular Hydrogen-Bonded Motifs on Packing Pattern Populations

The Box Model of crystal packing has shown that unit cells with low “external” surface area for a
given volume are more prevalent in the Cambridge Structural Database than structures with unit cells of a large
surface area. However, high-surface-area cells exist, and a possible explanation is the presence of strong, structure-directing intermolecular interactions. A study of structures belonging to P21/c has been undertaken in an attempt
to understand the role of intermolecular interactions within the context of the Box Model. From an examination of
the distribution of randomly selected structures over the packing patterns, it is shown that the energetic interaction
which relates the largest faces of the molecules is important. When a strong, symmetry-demanding intermolecular
motif is present in the structure, for example a carboxylic acid dimer motif or a CONH chain motif, the distribution
of structures over the packing patterns changes. These changes can be rationalized in terms of the symmetry of
interactions between the groups required to build the motif and the orientation of the group with respect to the
large face of the molecule. However, the presence of hydrogen-bonded motifs does not appear to greatly perturb the
principal tenet of the Box Model:  minimum surface area is preferred.