Interplay of Different Moieties in the Binary System 1‑Ethyl-3-methylimidazolium Trifluoromethanesulfonate/Water Studied by Raman Spectroscopy and Density Functional Theory Calculations

The present work reports new insights into specific interactions in aqueous solutions of the ionic liquid (IL) 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (C2mimTfO). A systematic investigation based on a combination of Raman spectroscopy and density functional theory (DFT) calculations shows evidence of self-encapsulation of the ionic moiety. Raman spectroscopy reveals preferred interactions between water molecules and the TfO– anions. The comparison of the experimental results with dispersion-corrected DFT calculations, which yield the predictions of the possible conformers of the cation–water, anion–water, and cation–anion–water structures, strongly supports the hypotheses of site-selective IL/water interactions. The obtained results allow for a detailed discussion of the nature and strength of the molecular interactions. It is shown that the TfO– anion establishes a preferred interaction with water, whereas the vibrational band at 3118 cm–1 for C–H motion at the C(2) position, the most acidic site for cation and anion interaction, does not indicate any specific energy shift, when adding water to the IL. This finding gives evidence for a self-protective microstructure of the molecules of C2mimTfO in an aqueous environment. In contrast to other ILs reported in the literature, there is no evidence of an increasing cation–anion distance in the IL ion-pair when increasing the water content. Instead, the C2mimTfO molecules undergo a perfect rearrangement, allowing interactions at other molecular sites with higher selectivity. A direct exposure to water at the cation–anion interacting site (C(2) position) is avoided. Ultimately, we show that clusters of ion-pair dimers solvated with water exhibit a more stable geometry compared to the hydrated single ion-pairs, and our calculations correctly reproduce the experimental findings