Measuring Molecular Dynamics and Activation Energies for Quaternary Acyclic Ammonium and Cyclic Pyrrolidinium Ionic Liquids Using 14N NMR Spectroscopy

The N-14 NMR spin-lattice (R-1) and spin-spin (R-2) relaxation rates were determined as a function of temperature for a series of tetra-alkyl acyclic ammonium and cyclic pyrrolidinium ionic liquids (ILs). Through the use of the R-2/R-1, ratio method, it was shown that for the majority of these ILs, the reorientational dynamics are not in the extreme narrowing regime, but instead are in the dispersive relaxation regime, thus allowing a unique solution for the correlation time to be determined. The temperature variation of the R-2 relaxation rate, along with the temperature variation of the calculated correlation times, allowed activation energies for the reorientational dynamics to be measured and compared. In addition, these NMR relaxation experiments enabled the N-14 quadrupolar coupling product to be extracted, which revealed surprising temperature dependence. Collectively, the N-14 NMR results allow the impact of cation and anion identity on the local reorientational dynamics of these ILs to be delineated.