Nuclear magnetic resonance study of the relationship between phase behaviour and reorientational and transrational dynamics in N,N-diethylpyrrolidinium bis(fluorosulfonyl)amide with a plastic crystal phase

Abstract

We investigated the phase transition dynamics between the ordered crystal, plastic crystal (PC), and liquid phases of N,N-diethylpyrrolidinium bis(fluorosulfonyl)amide by measuring the temperature dependences of spin–lattice relaxation time (T₁) and spin–spin relaxation time (T₂) for ¹H and ¹⁹F using low-frequency pulsed nuclear magnetic resonance. T₁ and T₂ are sensitive to the reorientational and translational dynamics of ions, respectively. Because H and F atoms are exclusively present in the cation and anion, respectively, their dynamic behaviours can be investigated separately. The temperature-dependent T₁ curves for ¹H (or ¹⁹F) in the PC and liquid phases were smoothly connected at the melting point, indicating similar rotational motions for the cations (or anions) in both phases. In the PC phase, distinct T₂ components were observed for both ¹H and ¹⁹F, reflecting heterogeneous dynamics due to cooperative translational motion of the cations and anions. These dynamics are attributed to the presence of core and surface phases within each PC crystallite. At the melting point, the T₂ values of the surface phase transitioned smoothly to those of the liquid phase.