Investigation of Dynamics in BMIM TFSA Ionic Liquid through Variable Temperature and Pressure NMR Relaxometry and Diffusometry

Abstract

A comprehensive variable temperature, pressure and frequency multinuclear (¹H, ²H, and ¹⁹F) magnetic resonance study was undertaken on selectively deuterated 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (BMIM TFSA) ionic liquid isotopologues. This study builds on our earlier investigation of the effects of increasing alkyl chain length on diffusion and dynamics in imidazolium-based TFSA ionic liquids. Fast field cycling ¹H T₁ data revealed multiple modes of motion. Through calculation of diffusion coefficient (D) values and activation energies, the low- and high-field regimes were assigned to the translational and reorientation dynamics respectively. Variable-pressure ²H T₁ measurements reveal site-dependent interactions in the cation with strengths in the order MD₃ > CD₃ > CD₂, indicating dissimilarities in the electric field gradients along the alkyl chain, with the CD₂ sites having the largest gradient. Additionally, the α saturation effect in T₁ vs. P was observed for all three sites, suggesting significant reduction of the short-range rapid reorientational dynamics. This reduction was also deduced from the variable pressure ¹H T₁ data, which showed an approach to saturation for both the methyl and butyl group terminal methyl sites. Pressure-dependent D measurements show independent motions for both cations and anions, with the cations having greater D values over the entire pressure range.