How cationic and anionic portions of an imidazolium-based ionic liquid interact with molecular liquids: Insights from density functional theory calculations

Abstract

Intermolecular interactions of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]) as an imidazolium-based ionic liquid, with three molecular liquids (MLs) including dimethyl sulfoxide (DMSO), methanol (MeOH) and acetonitrile (AN) had been examined by density functional theory (DFT) methods. All possible configurations of both cationic ([emim]⋯ML) and anionic ([Tf2N]⋯ML) complexes were considered and investigated. It was found that configuration No. 5 was the most stable configuration of [emim][Tf2N] in all environments while the minimum configurational relative energy of cationic complexes was observed for configuration No. 1. In anionic complexes, it shifted between configuration No.1 and No.3 for different complexes in different environments. Calculated interaction energies showed that replacement any of the [emim][Tf2N] parts with a ML molecule decreased the intermolecular interactions. Among molecular liquids, stronger intermolecular interactions were observed for DMSO which was consistent with experimental results. It was shown that electrostatic interactions, charge transfer energies and hydrogen bond power are responsible for interaction energy changes. In both [emim]⋯ML and [Tf2N]⋯ML complexes, some intermolecular hydrogen bonds were detected using AIM results. Among them, DMSO formed stronger hydrogen bonds than MeOH and AN with both cationic and anionic portions, which was compatible with experimental results about hydrogen bonding power. Our results quantitatively showed, why mixing ionic liquids with molecular liquids is one of the ways to modulate their viscosity.