Hydrogen bonding and π–π interactions in imidazolium-chloride ionic liquid clusters

Abstract

A systematic electronic structure analysis of hydrogen bonding (H-bonding), anion–π⁺ and π⁺–π⁺ interactions present in [C₁C₁im]Cl ion-pairs (IPs) and selected [C₁C₁im]₂Cl₂ IP-dimers has been carried out. Interactions have been characterised using a combination of QTAIM, NCIPLOT, NBO and qualitative MO theory. IP-dimers form non-directional charge quadrupolar arrangements due to Coulombic interactions. These are found to associate either as clusters or as loosely associated IP–IP structures. Large conformational changes are found to occur for very little cost in energy, indicating that charge screening is essentially independent of the cation ring orientation. H-bond formation is accompanied by charge transfer and polarisation of the entire [C₁C₁im]⁺ ring. Charge transfer does not follow the same trend for the CHelpG, QTAIM and NBO methods. Weak “stacked” π⁺–π⁺ interactions are stabilised in the presence of anions, which locate between and at the periphery of the rings, novel strongly bent H-bonds are also present. Primary (ring; C–H⋯Cl⁻) H-bonds and anion–π⁺ (C²⋯Cl⁻) interactions are found to decay more rapidly with distance than secondary (aliphatic; C^M–H⋯Cl⁻) H-bonds. This leads to an increase in the relative importance of secondary H-bond interactions in the IP-dimers. Moreover, rotation of the methyl groups within the “stacked” π⁺–π⁺ IP-dimers facilitates the formation of (stronger) linear secondary H-bonds. Thus, compared to isolated IPs, secondary H-bonds may play an increased role within the condensed phase. Overall we find that structural fluidity is facilitated by fluctuating hydrogen bond, π⁺–π⁺ and anion–π⁺ interactions.