Hydrogen Bonding versus π-π Stacking Interactions in Imidazolium-Oxalatoborate Ionic Liquid

Intermolecular features like hydrogen bonding and pi-type interactions play pivotal roles in stabilizing molecular structures in ionic liquids with planar rings and hydrogen-bond donors and acceptors. However, the delicate interplay among these interactions is complicated and depends on specific ion types. In this work, ab initio molecular dynamics simulations were performed to reveal competitive and cooperative characteristics among hydrogen bonding and pi-type interactions in a typical imidazolium oxalatoborate ionic liquid. Imidazolium rings take preferential on-top parallel orientations, leading to their particular pi-pi stacking distributions at short distances. Intermolecular interactions between imidazolium and oxalato rings are manifested by short-range on-top parallel orientations and in-plane hydrogen bonding interactions, promoting their parallel displaced offset stacking arrangements. However, on an intermediate distance scale, attractive Coulombic interactions between imidazolium and oxalato rings dominate and contribute to their perpendicular orientations. Spatial coordination patterns between intermolecular oxalato rings are balanced by repulsive electrostatic interactions and steric hindrance effects, leading to their tilted orientations in local environments.