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

We have investigated the H-bonding structure of the ionic liquid (IL) 1-ethyl-3-methylimidazolium trifluoroacetate ([Emim:TFA]) <i>via</i> 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 CO 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 CO 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.