Abstract
The existence of two polymorphic modifications (α and β) of 1-methylthiolanium iodide was found by means of thermal analysis, X-ray diffraction and infrared absorption spectrometry. The transition point and the enthalpy of transition (ΔHtr) are 78° and 5.4 kJ mol-1, respectively, by differential scanning calorimetry. The transition is enantiotropic. The crystal structure of the α-phase (room temperature) is determined by single-crystal X-ray analysis. The crystal is orthorhombic, space group pnma, with a=10.82, b=6.37, c=11.90 Å, Z=4. The structure was solved by heavy-atom methods and was refined by full-matrix least-squares calculations to R=0.056 for 637 observed reflections. The structure is ionic, composed of I- ions and 1-methylthiolanium ions. The 1-methylthiolanium ion has mirror symmetry and adopts the envelope conformation with the methyl group in axial position. The β-phase was examined by X-ray powder diffraction methods at 100°. The lattice type is face-centered orthorhombic with a=10.97, b=6.72, c=11.63 Å, Z=4. Consequently, the space group is restricted to F222, Fmm2 or Fmmm. The zinc-blende type structure based on F222 is the most plausible one on the basis of the packing of the ions. These results suggest that in the β-phase the 1-methylthiolanium ions have statistical orientations.
