Structure of (E)-3,4-dibromotetrahydrothiophene 1,1-dioxide, C4H6Br2SO2, as determined in the gas phase by electron diffraction, in the crystalline phase at 150 K by X-ray diffraction and by ab initio computations

Abstract

The structure of (E)-3,4-dibromotetrahydrothiophene 1, 1-dioxide, C₄H₆SO₂Br₂, has been determined at 150 K by X-ray crystallography and in the gas phase by electron diffraction constrained using bond length and angle differences calculated ab initio. The results show that in both phases the ring adopts the half twist conformation with C₂ symmetry. However, in the solid phase the bromine atoms occupy equatorial positions, whereas in the gas phase the optimum fit is for a mixture of axial and equatorial conformers. These are present in effectively equal amounts [proportion of axial conformer 47.2(23)%], as also predicted by ab initio calculations at the MP2/6-31 G* level. Salient mean structural parameters (r/pm, angles/degrees) for the gas phase (r_a) and the solid phase, respectively, are: r(SO)142.5(1), 143.8(7); r(S–C) 179.0(3), 179.7(18); r(C–C) 152.9(4), 153.6(10); r(C–Br) 194.8(3), 194.7(10); 050117.9(10), 117.7(8); CSC 92.8(7), 96.4(6); CCCC 55.9(10), 57.7(15); BrCCBr (axial)–167.9(18) and (equatorial)–70.8(11), –64.3(20). These values are in good agreement with those obtained in an ab initio(MP2/6-31G* level) study of the molecular geometry.