Raman Spectroscopic Study, DFT Calculations and MD Simulations on the Conformational Isomerism of N-Alkyl-N-methylpyrrolidinium Bis-(trifluoromethanesulfonyl) Amide Ionic Liquids

The conformational behaviors of N-alkyl-N-methylpyrrolidinium bis-(trifluoromethanesulfonyl) amide ionic liquids (alkyl; propyl and butyl, [P_1n][TFSA]; n = 3 and 4) were studied by Raman spectroscopy in the frequency range of 200−1700 cm⁻¹ at different temperatures. Observed Raman spectra in the frequency range 870−960 cm⁻¹ for [P₁₃][TFSA] and at 860−950 cm⁻¹ for [P₁₄][TFSA] depend on the temperature, indicating that pseudo rotational isomerization of the pyrrolidinium ring exists in the ionic liquids. DFT calculations revealed that the pseudo rotational potential energy surfaces for P₁₃⁺ and P₁₄⁺ ions were similar to each other, i.e., the e6 isomer is the global minimum, whereas the three other isomers e1, e4, and e5 are ca. 3 kJ mol⁻¹ higher in energy. Optimized geometries with no imaginary frequency were successfully obtained for the e6, e1, and e4 isomers. For both cations, the theoretical Raman spectra of the e6 isomers reproduce well the observed data. To explain their observed Raman spectra in a reasonable way, it is necessary to consider one or more species as predicted by DFT calculations, i.e., the e4 isomer of P₁₃⁺ rather than the e1, or the e1 isomer of P₁₄⁺ rather than the e4. In addition, the torsion energy potentials of the alkyl chains of the cations were scanned by DFT calculations. It turns out that the alkyl chains of the cations prefer all trans conformations. It should be emphasized that the alkyl chains of the pyrrolidinium cations show remarkably different conformational behaviors comparing with those of the imidazolium. The isomerization enthalpies Δ_isoH° from the e6 to the e4 isomer of P₁₃⁺ and to e1 of P₁₄⁺ were reasonably estimated from the temperature dependence of Raman spectra based on our proposed assignments to be 2.9 kJ mol⁻¹ for P₁₃⁺ and 4.2 kJ mol⁻¹ for P₁₄⁺, respectively. Thus evaluated experimental Δ_isoH° values, which may contain some uncertainties, are in agreement with those predicted by DFT calculations and MD simulations suggesting that pseudo rotational isomerization equilibria are established in the examined N-alkyl-N-methylpyrrolidinium ionic liquids. The conformational behavior of TFSA⁻ was also investigated. The Δ_isoH° from the trans (trifluoromethyl groups on opposite sides of the S−N−S plane) to the cis isomer were evaluated to be 4.2 kJ mol⁻¹ for [P₁₃][TFSA] and 3.5 kJ mol⁻¹ for [P₁₄][TFSA], respectively, which are similar to that for the 1-ethyl-3methylimidazolium ionic liquid.