Molecular Structure and Interactions in the Ionic Liquid 1-Ethyl-3-methylimidazolium Trifluoromethanesulfonate

Quantum chemical theory (DFT and MP2) and vibrational spectroscopy (ATR-IR and Raman) were employed to investigate the electronic structure and molecular interactions in the room-temperature ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate. Various possible conformers of a cation-anion pair based on their molecular interactions were simulated in the gas phase. All the different theoretical (MP2, B3LYP, and the dispersion-corrected wB97XD) methods assume the same ion-pair conformation for the lowest energy state. Basis set superimpose error (BSSE) correction was also introduced by using the counterpoise method. Strong C-H···O interactions between the most acidic hydrogen atom of the cation imidazole ring (C2H) and the oxygen atom of the anion were predicted where the anion is located at the top of (C2H). In this case, methyl and alkyl groups also interact with the anion in the form of a C-H···O hydrogen bond. Interestingly, the dispersion-corrected methodology neglects the C4/C5-H···O and C-H···F interaction in the ion-pair calculations. The theoretical results were compared with the experimental observations from Raman scattering and ATR-IR absorption spectroscopy, and the predictions of the molecular interactions in the vibrational spectra were discussed. The wavenumber shifts of the characteristic vibrations relative to the free cation and anion are explained by estimating the geometric parameters as well as the difference in the natural bond orbital (NBO) charge density.