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

Abstract

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 ([C₁mim][FSA]) and 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)amide ([C₁mim][NTf₂]) as samples, the longitudinal and transverse relaxation times (T₁ and T₂) for ¹⁹F and ¹H 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 ¹⁹F and ¹H nuclei, respectively, their dynamics can be independently investigated and the relationships between them can be discussed. For [C₁mim][FSA], the only observed phase changes included melting and crystallisation. The temperature dependences of T₁ and T₂ for ¹⁹F were similar to those of T₁ and T₂ for ¹H, indicating similar dynamics due to the formation of strong anion–cation interactions. For [C₁mim][NTf₂], the T₁ and T₂ values for both ¹⁹F and ¹H discontinuously changed at same temperatures, which were assigned to the crystallisation and melting points. However, the T₁ curves for ¹⁹F and ¹H were different in the crystalline region, suggesting independent dynamics for the anions and cations in [C₁mim][NTf₂]. 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.