Hydrogen bonding between 1-ethyl-3-methyl-imidazolium dicyanamide ionic liquid and selected co-solvents with varying polarity: A DFT study

Abstract

A theoretical study was performed to investigate the solute-solvent interactions between 1-ethyl-3-methyl-imidazolium ([EMIM]+) dicyanamide ([DCA]-) ionic liquid (IL) and each of the three traditional molecular solvents, namely, water, methanol and chloroform, to elucidate the geometry of the stable IL⋯co-solvent structures as well as the interaction strength at different possible sites. The study was performed by utilising a simplified structure approach in which one solvent molecule interacts with the IL at different possible interaction sites. Geometry optimisations were performed by using the density functional theory (DFT) method in combination with the 6-31++G(d,p) basis set; the energy was improved by using the B3LYP/6-311++G(3df,3pd)// B3LYP/6-31++G(d,p) approach to eliminate the basis set superposition error on the energies. The results of the study show that IL forms a strong interaction with the co-solvent at nearly all possible interaction sites. The C–H⋯N intermolecular interactions between the cation and the anion units of the IL are weakened when the solvent molecule interacts simultaneously with the hydrogen bond donor involved between the cation and the anion units of the IL. The polar solvents (methanol and water) interact more strongly with the IL than the nonpolar chloroform solvent. The results presented herein have fundamentally advanced the understanding of the specific structural, energetic and electronic changes induced on [EMIM]DCA cation–anion units by the selected co-solvents and provided a detailed description of the interaction patterns between [EMIM]DCA and the selected co-solvent molecules. Such information, which had not previously been provided, could serve as a basis for further studies on derivatives of [EMIM]DCA with the selected co-solvents or with other solvents not considered in this work.