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.
Highlights
Ionic liquids (ILs) have found numerous applications as replacements for volatile organic solvents due to their characteristically low vapor pressures, large liquid temperature ranges, good thermal and chemical stabilities, and tunable properties
While the H4 and H5 ring protons exhibit the longest T1s, the 2A and 2B alkyl chain protons had the shortest. This is expected considering that greater flexibility is realized with increasing distance from the ring. This is similar to what we observed for the EMIM TFSA system where the ring protons and alkyl chain methylene groups had the longest and shortest 1H T1s respectively.[27]
The results show a greater average hydrodynamic radius for the TFSA anion by 28% compared to the BMIM cation
Summary
Ionic liquids (ILs) have found numerous applications as replacements for volatile organic solvents due to their characteristically low vapor pressures, large liquid temperature ranges, good thermal and chemical stabilities, and tunable properties. It has been shown that using small asymmetrical cations with larger, fluorous anions can produce low-viscosity ILs13,14 and adjusting the length and structure of the functional groups can affect the melting and glass transition temperatures.[12] As a cation, the 1-alkyl-3-methylimidazolium (CnMIM+) offers significant charge delocalization over its polar head group, as well as molecular asymmetry in its non-polar alkyl side chains. This prevents crystallization, thereby expanding its liquid phase range.[15] As an anion, bis(trifluoromethylsulfonyl)amide (TFSA, [CF3SO2-N-SO2CF3]−) has a low barrier for rotation about the N-S bonds, which results in conformational flexibility that produces ILs with lower viscosity and higher ionic conductivity.[26,27] When combined with CnMIM+ cations, TFSA can yield room temperature ILs with high ionic conductivities.[16]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
