Competitive pi interactions and hydrogen bonding within imidazolium ionic liquids

In this paper we have explored the structural and energetic landscape of potential π+–π+ stacked motifs, hydrogen-bonding arrangements and anion–π+ interactions for gas-phase ion pair (IP) conformers and IP-dimers of 1,3-dimethylimidazolium chloride, [C1C1im]Cl. We classify cation–cation ring stacking as an electron deficient π+–π+ interaction, and a competitive anion on-top IP motif as an anion–donor π+–acceptor interaction. 21 stable IP-dimers have been obtained within an energy range of 0–126 kJ mol−1. The structures have been found to exhibit a complex interplay of structural features. We have found that low energy IP-dimers are not necessarily formed from the lowest energy IP conformers. The sampled range of IP-dimers exhibits new structural forms that cannot be recovered by examining the ion-pairs alone, moreover the IP-dimers are recovering additional key features of the local liquid structure. Including dispersion is shown to impact both the relative energy ordering and the geometry of the IPs and IP-dimers, however the impact is found to be subtle and dependent on the underlying functional.