Role of supramolecular synthons in the formation of the supramolecular architecture of molecular crystals revisited from an energetic viewpoint.

Analysis of the strengths and directionality of intermolecular interactions in the crystals containing only one type of supramolecular synthon allows the suggestion of a general classification of molecular crystals depending on type of their basic structural motifs. All crystals may be divided on four classes namely (I) crystals with isotropic packing of the building units; (II) columnar crystals where the basic structural motif (BSM) is a chain/column; (III) layered crystals with layers as the BSM; (IV) columnar-layered crystals containing chains/columns as the primary basic structural motif and layers as the secondary BSM. Taking into account the participation of different supramolecular synthons in the formation of different levels of the organization of molecular crystals, they may be considered as basic (responsible for the formation of molecular complexes as building units of crystals), primary, secondary and auxiliary, which are involved in the agglomeration of molecules in primary or secondary basic structural motifs or in the packing of these motifs, respectively. The ranking of supramolecular synthons depends on values of energies of intermolecular interactions and it is individual for each crystal.