Structure, hydrogen bond dynamics and phase transition in a model ionic liquid electrolyte

We show that solid-state NMR spectroscopy is a suitable method for characterizing the structure, hydrogen bond dynamics and phase transition behavior in protic ionic liquids (PILs). Deuteron line shape and spin relaxation time analysis provide a description of the structural and dynamical heterogeneity in the solid state of the model PIL triethyl ammonium bis(trifluoromethanesulfonyl)amide [TEA][NTf2]. Therein, we observed two deuteron quadrupole coupling constant for the ND bond of the TEA cation, indicating differently strong hydrogen bonds to the nitrogen and oxygen atoms of the NTf2 anion, as we could confirm by DFT calculations. The transition processes in the dynamically heterogeneous phase are characterized by two standard molar enthalpies and thus different stages of melting. We provide geometry, rates and energetics of the cation in the solid and liquid states of the PIL. Comparison with PILs having stronger interacting anions shows higher enthalpy change between the solid and liquid states, lower activation barriers of tumbling motion and higher amplitude of librational motion for the TEA cation in the presence of the weakly interacting anion NTf2. We provide reasonable relations between microscopic and macroscopic properties, as is relevant for any kind of application.

Automated summary

We demonstrate that solid-state NMR spectroscopy is an effective tool for studying the structure, hydrogen bond dynamics, and phase transition behavior of protic ionic liquids (PILs). By analyzing the deuteron line shape and spin relaxation time, we obtain information about the structural and dynamical heterogeneity of the model PIL triethyl ammonium bis(trifluoromethanesulfonyl)amide [TEA][NTf2] in the solid state. The presence of different quadrupole coupling constants for the ND bond of the TEA cation indicates varying hydrogen bond strengths to the nitrogen and oxygen atoms of the NTf2 anion. We also provide insights into the transition processes and energetics of the solid and liquid states of the PIL.