Nuclear Magnetic Resonance Study on Relationship between Phase Behavior and Rotational and Translational Dynamics of an Ionic Liquid with a Plastic Crystal Phase: N-Butyl-N-methylpiperidinium Hexafluorophosphate

Abstract

Abstract N-butyl-N-methylpiperidinium hexafluorophosphate, [C1C4pip]PF6, is an ionic liquid with a plastic crystal (PC) phase. To investigate the dynamics of the phase transition between crystalline, plastic crystalline, and liquid phases of [C1C4pip]PF6, we measured the temperature dependences of longitudinal and transverse relaxation times (T1 and T2, respectively) for 1H and 19F using low-frequency pulse nuclear magnetic resonance (NMR) methods. T1 and T2 changes are sensitive to the rotational and translational dynamics of ions, respectively. Since H and F atoms are only present in the cation and anion, respectively, the dynamic behaviors of each ionic molecule can be investigated separately. In the phase transition between the PC and liquid phase, the temperature-dependent curves of 1H-T1 were smoothly connected, indicating the same rotational motion for the cation occurred in both phases. The curves of 19F-T1 yielded the same conclusion for the rotational motion of the anion in both phases. The temperature-dependent curves of 1H-T2 and 19F-T2 jumped abruptly from phase to phase, indicating different modes of translational motion in each phase. We observed the appearance of a translationally mobile component in both the PC phase and the crystalline phase. This was concluded to be surface or interfacial melting.