CONFORMATIONAL STUDIES ON [16]ANES4 - STRUCTURES OF ALPHA-[16]ANES4 AND BETA-[16]ANES4 ([16]ANES4=1,5,9,13-TETRATHIACYCLOHEXADECANE)

Three morphologies - acicular (alpha), lamellar (beta) and columnar (gamma) are observed for crystals of [16]aneS4 (1,5,9,13-tetrathiacyclohexadecane). The absolute structures of the alpha and beta forms have been determined: the molecular structures are essentially the same but the two forms differ in their crystal packing. Alpha-[16]aneS, crystallizes in the orthorhombic space group Pbc2(1) while beta-[16]aneS4 crystallizes in the monoclinic space group P2(1). The conformation of the molecules is unusual: whereas the other tetrathia macrocycles [12]aneS4 and [14]aneS4 have exclusively exo S atoms, in [16]aneS4 only two lie in exo positions and this structural feature is related to the chemical properties of the macrocycle. Molecular mechanics calculations have been carried out on selected conformers of [16]aneS4 and the results compared with the observed crystal structures and with the hydrocarbon analogue, C16H32. Gamma-[16]aneS4 appears to crystallize in the orthorhombic space group Fdd2 but it suffers from twinning and no structural information could be obtained.