Raman and ab initio study of the conformational isomerism in the 1‐ethyl‐3‐methyl‐imidazolium bis(trifluoromethanesulfonyl)imide ionic liquid

The calculated and experimental Raman spectra of the (EMI + )TFSI − ionic liquid, where EMI + is the 1‐ethyl‐3‐methylimidazolium cation and TFSI − the bis(trifluoromethanesulfonyl)imide anion, have been investigated for a better understanding of the EMI + and TFSI − conformational isomerism as a function of temperature. Characteristic Raman lines of the planar (p) and non‐planar (np) EMI + conformers are identified using the reference (EMI + )Br − salt. The anion conformer of C 2 symmetry is confirmed to be more stable than the cis ( C 1 ) one by 4.5 ± 0.2 kJ mol −1 . At room temperature, the population of trans ( C 2 ) anions and np cations is 75 ± 2% and 87 ± 4%, respectively. Fast cooling quenches a metastable glassy phase composed of mainly C 2 anion conformers and p cation conformers, whereas slow cooling gives a crystalline phase composed of C 1 anion conformers and of np cation conformers. Copyright © 2006 John Wiley & Sons, Ltd.