Nonpolar-to-Polar Phase Transition of a Chiral Ionic Plastic Crystal and Switch of the Rotation Symmetry

The synthesis of a new ionic plastic crystal, tetraethylammonium-d₂₀ d-10-camphorsulfonate, is reported. The crystal has three solid phases, the structures of which were determined by single-crystal X-ray diffraction (XRD). XRD analysis revealed a phase transition from nonpolar space group P2₁2₁2 to polar space group P2₁ with increasing temperature. The dynamics of the d-10-camphorsulfonate anion and the tetraethylammonium-d₂₀ cation was investigated by variable-temperature ¹H and ²H solid-state NMR spectroscopy. The anion showed swing motion in high-temperature phases, which is in good agreement with the disorder observed in XRD analysis. The cation showed uniaxial rotation even in the low-temperature phase. The rotation axis of the tetrahedral structure showed a transition from the two-fold axis to the three-fold axis with the phase transition.