The complete crystal structure is nowhere preexisting as such. However, its design plan is already decoded in its constituent molecules. Crystal engineering in sensus strictus means finding an algorithm for a predictable and controllable long-range molecular organization resulting in a well-defined 3D crystal structure with potentially useful physical and chemical properties. However, for the molecular organizations in 3D arrays, the exact atom positions are of no more significance, than are the exact electrons, protons and neutrons positions for the atom organizations in molecules (where only the cooperative characteristics like electron densities and nuclei positions are important for the molecular self-identity) Therefore, at present time, crystal engineering is focused mainly on establishing the relationships between molecular and supramolecualr levels, very much like those already established between the atomic and molecular levels.

In this presentation the engineering problem is reduced to a formation of basic building units with inherent recognition properties suitable for a predictable organization into monolayers and bilayers. The uronium benzenesulfonate unit (NH₂COH⁺.C₆H₅SO_3– ) is a novel structural tecton [1] (called also module [2] or synthon [3]), formed through a very strong asymmetric O- H...O hydrogen bond. The tecton is further iteratively self-assembled following hydrogen bond directors and symmetry constrains in order to establish polar monolayers. The benzene rings pointing out of the hydrogen-bonded surface are forced edge-to-face by C– H... p interactions. The interspace between two polar layers is ruled by p ...p interactions dictating face-to-face interdigitation of aromatic portions belonging to two adjacent monolayers, which results into a lamella formation. The juxtaposition of lamellas is effected by long-range ionic interactions between sulfonate portions which protrude the protonated urea portions. The final crystal structure is monoclinic (C2/c) with cell parameters a = 24.430Å , b = 9.810Å , c = 8.390Å , b = 109.85^o, Z = 8.

1. M. W. Husseini and A. De Cian, Chem. Commun., 7, (1998), 727.
2. V. A. Russell and M. D. Ward, J. Chem. Mater., 8, (1996), 1654.
3. G. R. Desiraju, Angew. Chem. Int. Ed. Engl., 31, (1995), 2311.