H-Bonding structures of 1-ethyl-3-methylimidazolium trifluoroacetate: a vibrational spectroscopic study

Abstract

We have investigated the H-bonding structure of the ionic liquid (IL) 1-ethyl-3-methylimidazolium trifluoroacetate ([Emim:TFA]) via linear and two-dimensional infrared (2DIR) spectroscopy. We directly probed the asymmetric carbonyl stretching mode of the trifluoroacetate anion rather than using an extrinsic solute molecule as a vibrational probe, the more common approach in 2DIR studies of ILs. The CO asymmetric stretching mode exhibits multimodal character in both linear IR and 2DIR measurements, indicating structural inhomogeneity in the H-bonding of TFA. Solvent-dependent linear IR spectra of dilute Emim:TFA in two protic (D₂O and MeOH-d₄) and two aprotic (DMSO-d₆ and acetonitrile) solvents, comparisons with NaTFA spectra in these same solvents, and DFT calculations of Emim:TFA and small, solvated ion clusters were used to characterize the H-bonding structures present in the neat ionic liquid. The most prominent IR feature near 1690 cm⁻¹ originates from hydrogen bonding between TFA and the C2H and C4/5H imidazolium ring protons. Calculations of the two different types of H-bonding interactions, C2H (and C6H, C7H)–TFA and C4H (and/or C5H)–TFA, indicate that their frequencies are different by several wavenumbers. Higher frequency features (>1695 cm⁻¹) are associated with those triple ion structures and higher aggregates where the TFA makes a weak hydrogen bond to Emim⁺. The appearance of cross-peak features in the time-zero 2DIR spectra indicates the presence of intermolecular coupling between the CO stretching modes of TFA anions, which can be expected for such pure IL systems. When diluted in polar aprotic solvents, Emim:TFA exists predominantly in ion pairs, while in polar protic solvents solvent-separated ions are the dominant species. The existence of significant ionic aggregation is only visible in the neat ionic liquid.