Investigating the property and strength of intermolecular interaction in saturated and unsaturated cyclic cations constructed ionic liquids

Abstract In this work, we have investigated the intermolecular interaction and hydrogen bond (H-bond) strength of unsaturated and saturated cations of ionic liquids (ILs) ([EMIM]+, [Epy]+, [Epr]+, and [Epp]+) with Cl-, [BF4]-, [DCA]-, and [OAC]- anions, for the aim of determining how is the influence of the unsaturated and saturated cations on the ion binding. Via molecular dynamic simulations and density functional theory calculations, it is found that saturated [Epr]+ and [Epp]+ manifest the strongest binding energies with Cl- and [OAC]-, where electrostatics effect overwhelms the induction and dispersion effects, as the dominant factor determining ion binding. On the other hand, the resolution of H-bond strength in ion-pair implies that it is stronger in [Epr]+ and [Epp]+ based ILs, especially the strongest when they pair with Cl-. Our results verify that saturated heterocycle-based cations serve as stronger anion binding sites and stronger H-bond donors than unsaturated ones, and that such ILs can be potentially applied in areas with specific demanding with strong ion binding.