Acta Crystallographica Section B

Structural Science

Volume 56, Part 6 (December 2000)

research papers

Acta Cryst. (2000). B56, 1011-1017 [ doi:10.1107/S0108768100010636 ]

Crystallographic and molecular mechanics investigation of an order-disorder transition and dimorphism in 5H,10H-dithiolo[2,3-b]-2,5-benzodithiocine-2-thione

Z. H. Chohan, W. T. A. Harrison, R. A. Howie, B. F. Milne and J. L. Wardell

Abstract: Single-crystal X-ray structures are presented for three forms of 5H,10H-dithiolo[2,3-b]-2,5-benzodithiocine-2-thione. The [alpha] (at 150 K) and [alpha] ' (at ambient) forms are very similar and differ only in the presence of crystallographic m symmetry in the molecules of [alpha] ', which is absent in the case of [alpha] . This pair is related by an order-disorder transition. The [beta] phase (also determined at 150 K) has a different structure in terms of the molecular packing from either of the other two and therefore constitutes a true polymorph. Molecular mechanics calculations indicated that the most stable CHCl₃-solvated conformations for the title compound were a pair of twisted U-shaped enantiomers, U_R and U_L, i.e. similar to the arrangements found in the [alpha] and [beta] phases, with the low-lying saddle point between them corresponding to the situation in the [alpha] ' phase. These calculations also indicated that the most stable CHCl₃-solvated conformation for the related dibromo-5H,10H-dithiolo[2,3-b]-2,5-benzodithiocine-2-thione was Z-shaped, in agreement with the crystal structure determined earlier for its DMSO solvate [Wang et al. (1998). Synthesis, pp. 1615-1618].

Keywords: Molecular mechanics; Order-disorder; Dimorphism.