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(4)mpip]-[Tf2N], were investigated by H-1, F-19, and C-13 nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulation. NMR and MD results for the temperature-dependent diffusion coefficients, D, spin-lattice relaxation times, T-1, and rotational correlation times, tau(c), for each site in the cation of [C(4)mpip][Tf2N] are in good agreement. The T-1 data indicate a long N-CH3 group rotation time, comparable to the overall molecular reorientation time. Correlation time comparisons between 1-butyl-3-methylimidazolium RTILs with different anions (Cl-, PF6-, Tf2N-) argue against invoking hydrogen bond interactions between the N-CH3 hydrogens and the electron rich center of the anion as an explanation of the long N-CH3 rotation times. Correlation time comparisons between piperidinium and pyrrolidinium RTILs with a common anion (Tf2N-) suggest that correlation times for N-CH3 rotation are primarily a function of the local structure of the cation.