Bulkier anions versus hydrogen bonding in imidazolium ionic liquids: Stationary point analysis

Abstract

Hydrogen bonding is an omnipresent phenomenon in room-temperature ionic liquids (RTILs). The presence or absence of the hydrogen bond drastically alternates self-diffusion, shear viscosity, phase transition points, and other physicochemical properties of a pure substance. For electrochemical applications, the presence of cation-anion hydrogen bonding is undesirable because the latter suppresses ionic mobility. In the present paper, we investigate the implication of removing the hydrogen…fluorine interionic attraction in the imidazolium organic borates, being the strongest non-covalent interaction in the [C₂IM][BF₄] salt. Suppression of fluorine interaction centers cancels H-bonding. Furthermore, it changes the most thermodynamically preferable orientation of the cation in the vicinity of the anion. The most acidic hydrogen atom of the imidazole ring remains the paramount electrophilic center of the cation. However, it does not give rise to strong electrostatically driven coordination patterns. The reported structural parameters and electronic density distributions characterize the new regularities of ion-ion coupling. The reported new insights may inspire synthetic efforts to design novel RTILs exhibiting lower electrostatic energies and adjusted solvation properties.