Rotational and Translational Dynamics of N -butyl- N -methylpiperidinium Trifluoromethanesulfonimide Ionic Liquids Studied by NMR and MD Simulations

Translational and rotational dynamics of the room temperature ionic liquid N-butyl-N-methylpiperidinium trifluoromethanesulfonimide, [C ₄mpip] [Tf ₂N], were investigated by ¹H, ¹⁹F, and ¹³C nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulation. ¹H NMR and MD results for the temperature-dependent diffusion coefficients, D, spin-lattice relaxation times, T ₁, and rotational correlation times, τ _c, for each site in the cation of [C ₄mpip][Tf ₂N] are in good agreement. The T ₁ data indicate a long N-CH ₃ group rotation time, comparable to the overall molecular reorientation time. Correlation time comparisons between 1-butyl-3-methylimidazolium RTILs with different anions (Cl ^-, PF ₆ ^-, Tf ₂N ^-) argue against invoking hydrogen bond interactions between the N-CH ₃ hydrogens and the electron rich center of the anion as an explanation of the long N-CH ₃ rotation times. Correlation time comparisons between piperidinium and pyrrolidinium RTILs with a common anion (Tf ₂N ^-) suggest that correlation times for N-CH ₃ rotation are primarily a function of the local structure of the cation.