Evidence for Anion-Anion Interaction in Amino Acid Ionic Liquids Probed by Far-Infrared Spectroscopy

Herein, it is shown that far-infrared spectroscopy is a sensitive probe for detecting not only cation-anion but also anion-anion interaction in imidazolium-based ionic liquids (ILs). The low-frequency spectra of ILs with the same 1-ethyl-3-methylimidazolium cation but different anions are measured, namely the acetate and glycinate anion, whereby the latter forms the simplest amino acid IL (AAIL). It is shown that two glycinate anions form two hydrogen bonds between either the carboxylate and amino groups allowing for attractive anion-anion interaction despite the Coulomb repulsion between the ions of like charge. The spectral signature of the torsional motion of the amine groups ranges between 200 and 350 cm^-1, strongly depending on the binding motif as determined by cation-anion or anion-anion interactions. This is confirmed by density functional theory calculations of frequencies of larger IL clusters exhibiting these characteristic binding motifs. The spectral signatures assigned to the cation-anion interactions are correlated to measured enthalpies of vaporization, confirming a once proposed relation which can be obviously used for predicting thermodynamic properties from spectroscopy for less stable AAILs.