Impact of Size and Electronegativity of Halide Anions on Hydrogen Bonds and Properties of 1-Ethyl-3-methylimidazolium-Based Ionic Liquids

Abstract

The effect of the anion size and electronegativity of halide-based anions (Cl^-, Br^-, I^-, and BF₄^-) on the interionic interaction in 1-ethyl-3-methylimidazolium-based ionic liquids (ILs) C₂mim X (X = Cl, Br, I, and BF₄) is studied by a combined approach of experiments (Raman, IR, UV-vis spectroscopy) and quantum chemical calculations. The fingerprint region of the Raman spectra of these C₂mim X ion-pairs provides evidence of the presence of the conformational isomerism in the alkyl chain of the C₂mim⁺ cation. The Raman and IR bands of the imidazolium C₂-H stretch vibration for C₂mim X (X = Cl, Br, I, and BF₄) were noticeably blue-shifted with the systematic change in size of anions and the electronegativity. The observed blue shift in the C₂-H stretch vibration follows the order C₂mim BF₄ > C₂mim I > C₂mim Br > C₂mim Cl, which essentially indicates the strong hydrogen bonding in the C₂mim Cl ion-pair. DFT calculations predict at least four configurations for the cation-anion interaction. On the basis of relative optimized energies and basis-set-superposition-error (BSSE) corrected binding energies for all ion-pair configurations, the most active site for the anion interaction was found at the C₂H position of the cation. Besides information about the C₂H position, our DFT results give insights into the anion interaction with the ethyl and methyl chain of the cation, which was also confirmed experimentally [ Chem. Commun. 2015 , 51 , 3193 ]. The anion interaction at the C₂H site of the cation favors a planar geometry in C₂mim X for X = Cl, Br, and I; however, for BF₄, the system prefers a nonplanar geometry where the anion is located over the imidazolium ring. TD-DFT results were used to analyze the observed UV-vis absorption spectra in a more adequate way giving insights into the electronic structure of the ILs. Overall, a reasonable correlation between the observed and the DFT-predicted results is established.