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

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) C(2)mim 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 C(2)mim X ion-pairs provides evidence of the presence of the conformational isomerism in the alkyl chain of the C(2)mim(+) cation. The Raman and IR bands of the imidazolium C-2-H stretch vibration for C(2)mim 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 C-2-H stretch vibration follows the order C(2)mim BF4 > C(2)mim I > C(2)mim Br > C(2)mim CI, which essentially indicates the strong hydrogen bonding in the C(2)mim 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 C(2)mim 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.