Anion and cation dynamics of sulfonylamide-based ionic liquids and the solid-liquid transitions

Some of the important factors that characterise room-temperature ionic liquids (RTILs) are the variety of conformations adopted by the constituent ions and their flexibility. Using 1,3-dimethylimidazolium bis(fluorosulfonyl)amide ([C 1 mim][FSA]) and 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)amide ([C 1 mim][NTf 2 ]) as samples, the longitudinal and transverse relaxation times ( T 1 and T 2 ) for 19 F and 1 H were determined as a function of temperature and were correlated with the dynamics of the phase behaviours of the two RTILs. Because the anions and cations in the two compounds have 19 F and 1 H nuclei, respectively, their dynamics can be independently investigated and the relationships between them can be discussed. For [C 1 mim][FSA], the only observed phase changes included melting and crystallisation. The temperature dependences of T 1 and T 2 for 19 F were similar to those of T 1 and T 2 for 1 H, indicating similar dynamics due to the formation of strong anion-cation interactions. For [C 1 mim][NTf 2 ], the T 1 and T 2 values for both 19 F and 1 H discontinuously changed at same temperatures, which were assigned to the crystallisation and melting points. However, the T 1 curves for 19 F and 1 H were different in the crystalline region, suggesting independent dynamics for the anions and cations in [C 1 mim][NTf 2 ]. In the crystalline state for each salt, the cation dynamics was distinctly separated into the framework movement of the imidazolium ring and the movement of the methyl groups, while the anion dynamics was characterised by the movement of the entire anion. The influence of the crystal structure on the dynamics of each salt was also considered. Temperature dependences of 19 F T 1 and T 2 values for [C 1 mim][NTf 2 ] and dynamics of the anions.