Crystal and molecular structures of tetrathia[7]heterohelicene: racemate and enantiomer

The crystal and molecular structures of the racemate and enantiomer tetrathia[7]heterohelicene have been determined in order to elucidate their molecular geometries and crystal packing modes. The racemate and enantiomer both crystallize from benzene in the orthorhombic form with Z= 4. The former belongs to the space group Pbcn with a= 15.383(2), b= 14.195(2), c= 7.767(1)A, and the latter belongs to the space group P212121 with a= 16.975(2), b= 11.236(1), c= 9.186(1)A. The racemate molecule retains C2 symmetry in the crystal, while the enantiomer lacks this symmetry, primarily because of differences in the dihedral angles between the successive aromatic rings. In the racemate, the molecules are stacked in a column with alternating chirality, while in the enantiomer the molecules are confined orthogonally to each other, resulting in strain. These packing modes are consistent with the observed geometrical deformation of the enantiomer, and provide a clue to the problem of chirality recognition.