Shape mimicry as a design tool in crystal engineering

The pseudosymmetric chiral sulphoxide 1 was designed with two segments of nearly identical shape but with significantly different electron- donor/acceptor properties. Based on the known high statistical preference for organics to pack in one of the centrosymmetric space groups, formation of molecular crystals of enantiomerically pure 1 was predicted to occur with pseudocentrosymmetry, with 1 also playing the role of its enantiomer and packing as would the racemate. Such a packing motif would lack true centrosymmetry, resulting in a polar axis for the crystal and net additivity of the vectors from nitrogen to sulphur (the direction of polarizability for the molecules). Enantiomeric sulphoxide 1 does form molecular crystals with false centrosymmetry, mimicking P2,/c, and with a substantial net directionality of polarizability vectors. In contrast, enantiomeric sulphoxides 3 and 4 form molecular crystals where the vectors from nitrogen to sulphur in neighbouring molecules are essentially opposed.