Comparison of the C-H⋯O bonding in two crystalline phases of 1,4-di-thiane 1,1,4,4-tetra-oxide

The crystal structures of two crystalline phases of 1,4-di-thiane 1,1,4,4-tetra-oxide, C₄H₈O₄S₂, have been determined in order to examine the nature of possible inter-molecular hydrogen bonds. Phase 1 is monoclinic, space group C2/m, with unit-cell dimensions of a = 9.073 (8), b = 7.077 (6), c = 5.597 (5) Å and β = 105.89 (1)°. The mol-ecule adopts 2/m symmetry and all of the mol-ecules are related by translation and thus have the same orientation. Phase 2 is also monoclinic but in space group P2₁/n with unit-cell dimensions of a = 7.1305 (5), b = 5.7245 (4), c = 8.3760 (6) Å and β = 91.138 (2)°. In this phase, the mol-ecule sits on an inversion center and the mol-ecules within the unit cell adopt quite different orientations. In both phases, examination of the potential C-H⋯O hydrogen bonds around each of the independent oxygen atoms (one axial and the other equatorial) shows the general O⋯H patterns to be quite similar with each oxygen atom in contact with four neighboring H atoms, and each H atom contacting two neighboring O atoms. While none of the H⋯O contacts is particularly short (all are greater than 2.5 Å), each mol-ecule has 32 such contacts that form an extensive inter-molecular network. A ¹H NMR spectrum of the compound dissolved in DMSO shows a singlet of 8H at δ 3.677 which indicates that the C-H bonds are only moderately polarized by the single adjacent -SO₂- moiety: strongly polarized C-H bonds have δ values in the 5-6 range [Li & Sammes (1983 ▸). J. Chem. Soc. Perkin Trans. 1, pp. 1303-1309]. The phase 1 crystal studied was non-merohedrally twinned.