Actual and virtual structures in molecular crystals

The sole crystal structure generally observed for a chemical compound (actual crystal structure) corresponds to only one of the many minima present in the crystal energy landscape of that compound. A challenging problem of crystallography is to explain why the many other possible crystal structures (in terms of energy and density) present in the landscape (virtual structures) are not observed as polymorphs. In the present paper, with specific reference to semicarbazides of substituted benzoic acids, the concept of actual and virtual structures is extended and it is proven that optimization of a molecule in a different crystal packing of another molecule (interchangeability of molecules and packings) can lead to low minima of the lattice energy and to a high lattice density also in the case of molecules significantly different in shape (para/meta substituted compounds). This suggests that a crystal structure can be viewed as a sort of elastic system that can be deformed to fit molecules of different electronic character and shape. Surprisingly, in many cases, this deformation is accomplished at low cost of energy and density. Our results also suggest that the small number of polymorphs cannot be explained only by inspection of the crystal energy landscape, which would otherwise suggest many, but also, and perhaps mostly, by the analysis of the initial steps of crystal formation even before nucleation, when many different molecular clusters corresponding to different packings are in competition with each other.