Infrared, Raman and inelastic neutron scattering study of phase transitions in trimethyloxosulphonium iodide, (CH 3 ) 3 SOI

Calorimetric (DTA), infrared, Raman and inelastic neutron scattering (INS) measurements have been performed for the trimethyloxosulphonium iodide compounds (CH 3 ) 3 SOI and (CD 3 )SOI over the range 10–450 K. Two solid‐solid phase transitions were found at about 200 and 250 K and the vibrational study showed that only the low‐temperature crystalline form (III) is completely ordered. From a temperature dependence study of the optical and INS spectra it is concluded that methyl group reorintations are mainly involved in the phase (III)⟷phase (II) transition, while the whole molecule reorientations about the three‐fold axis, partly involved in the phase (II)⟷phase (I) transformation, must be much slower than and decoupled from methyl motions. Finally, the potential barrier height against methyl rotations has been estimated to 13.3 kJ mol −1 from the observed torsional frequencies and compares favourably with the activation energies derived from previous NMR experiments (10.9 kJ mol −1 ) and from quasi‐elastic neutron scattering results (10.6 kJ mol −1 ).