Preferred intermolecular cation···anion interactions within the [EMIM][DCA] ionic liquid and its interaction with a water co-solvent molecule

Abstract

A theoretical study intended to investigate the preferred geometry arrangement of the cation/anion units within the 1-ethyl-3-methyl-imidazolium dicyanamide ([EMIM][DCA]) ionic liquid (IL) and its geometry arrangement and energetic properties upon interaction with a water co-solvent molecule has been performed for the first time. DFT and MP2 methods have been selected for the investigation in order to compare the performance of the methods. The standard 6-31++G(d,p) basis set was utilised for the geometry optimisation while the 6-311++g(3d2f,3p2d)//6-31++G(d,p) was utilised to estimate the energy and to eliminate the basis set superposition error on the energies. The result of the investigation suggests that the cation/anion units within the ionic liquids are preferably held through a combination of hydrogen bonding and dispersion interactions. This confirmed through the use of energetic and bond distance parameters as well as the H1 NMR analysis of the available geometries. Geometries held solely by hydrogen bonding are minimally preferred. The explicit co-solvent water molecule preferably interact with [EMIM][DCA] in which the cation/anion units are stabilised predominantly by the presence of dispersion interactions. The inclusion of the implicit solvation is associated with significant decrease in the magnitude of the cation⋯anion interaction energy, largely due to the increased separation distance between the units. A comparison of the ionic liquid – co-solvent interaction energy estimated by different calculation methods suggests minimal differences among the methods in the estimated energy values.