Molecular Recognition Of Xylenes By Paraisopropylcalix [4] Arene

It is shown that paraisopropylcalix [4] arene selectively complexes paraxylene from a 1:1:1 mixture of the three xylene isomers. The crystal structures of the complexes of paraisopropylcalix [4] arene with ortho and meta xylenes are isomorphous, crystallizing in the monoclinic group Cc. The complex with paraxylene is tetragonal (group P4/n). In the solid state, the macrocycle loses its 4 symmetry when complexed with ortho or meta xylene. Calculation of intra and intermolecular energies shows that the supramolecular energies are very similar, as are the macrocycle conformation energies of all three complexes. The result is that the phenomenon of molecular recognition observed can either be attributed to the greater number of efficient approaches between paraxylene and the macrocycle, due to its symmetry, in comparison to the other two isomers, or to the higher crystallization rate of the paraxylene complex in the considered medium. These conclusions are in agreement with the fact that crystallization with o- and m-xylene takes place after a much longer time than with p-xylene.