Stepwise Conformational Disorder in an Ionic Plastic Crystal

Structural changes associated with the phase transitions of choline bis(trifluoromethanesulfonyl)imide, [Chol][NTf₂], were revealed using Raman spectroscopy in situ with differential scanning calorimetry (DSC) measurements. Raman bands probed along the calorimetric measurements identifying the gauche or anti conformers of [Chol] and the transoid or cisoid conformers of [NTf₂]. The gauche conformer of [Chol] and the transoid conformer of [NTf₂] are present in the low-temperature crystal. The disorder in the plastic crystal phase of [Chol][NTf₂] is linked to the conformational flexibility acquired by [Chol], while [NTf₂] preserves the same conformation as in the low-temperature crystal. The decoupling in the conformational dynamics between [Chol] and [NTf₂] in the plastic crystal is found from Raman spectra obtained during heating or cooling DSC measurements. Such separate conformational dynamics persist along the second solid-solid transition seen in the DSC heating curve, with [NTf₂] gaining conformational flexibility only after melting. The low-frequency range was probed by Raman and inelastic neutron scattering (INS) spectroscopies. Owing to the intense quasi-elastic scattering tail in the Raman spectra resulting from fast relaxations, the plastic crystal's low-frequency Raman spectra resemble liquid phase spectra. Based on infrared spectra in the high-frequency range related to the ν(OH) stretching mode, the cation-cation hydrogen bond structural motif, which is alleged to exist in the liquid phase yet absent in the low-temperature crystal, is found to be present already in the plastic crystal phase of [Chol][NTf₂].