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

摘要

Translational and rotational dynamics of the room temperature ionic liquid N-butyl-N-mcthylpipendimum tilfluoromethanesulfonimide, [C4mpip]-[Tf2N], were investigated by ~1H, ~(19)F, and ~(13)C nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulation. ~1H NMR and MD results for the tempeiature dependent diffusion coefficients, D, spin-lattice relaxation times, T,, and rotational correlation times, τ_c, for each site in the cation of [C4mpip][Tf2N] are in good agreement. The T1 data indicate a long N-CH3 group rotation time, comparable to the overall molecular reorientation time. Correlation time comparisons between 1 -butyl-3-methyhmidazolium 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-CH, 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.