Comparison of intermolecular interactions in two phases of MeNSOF₂

MeNSOF2 was crystallised by laser-assisted zone refinement at 150 K. The structure obtained by X-ray diffraction initially on cooling to 140 K was triclinic, P-̄, but this transformed slowly to a second phase, which was monoclinic C2/c. Bond distances and angles are similar to equivalent interactions in (MeN)2 SF2 and SO2 F2. Intermolecular interactions in both phases are dominated by dispersion, though electrostatics are also important in all the most energetic contacts. Though no interatomic contacts fall within the sums of van der Waals radii of the contacting atoms, PIXEL calculations indicate that some intermolecular energies are similar to medium-strength hydrogen bonds. The monoclinic phase is denser than the triclinic phase, having stronger dispersion interactions. PIXEL and DFT calculations indicate that the two phases are energetically very finely balanced, but phase I becomes competitive at higher temperature on account of the entropy advantage of its ‘looser’ structure. DFT phonon energy calculations suggest that the mechanism of the transition may involve coupling between acoustic and low energy optical phonons.