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

A systematic electronic structure analysis of hydrogen bonding (H-bonding), anion-pi(+) and pi(+)-pi(+) interactions present in [C(1)C(1)im]Cl ion-pairs (IPs) and selected [C(1)C(1)im](2)Cl-2 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 [C(1)C(1)im](+) ring. Charge transfer does not follow the same trend for the CHelpG, QTAIM and NBO methods. Weak "stacked'' pi(+)-pi(+) 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 center dot center dot center dot Cl-) H-bonds and anion-p + (C-2 center dot center dot center dot Cl-) interactions are found to decay more rapidly with distance than secondary (aliphatic; C-M-H center dot center dot center dot 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'' pi(+)-pi(+) 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, pi(+)-pi(+) and anion-pi(+) interactions.