title: 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(+) (C(2)···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.
authors.affiliation: Department of Chemistry, Imperial College London, London, SW7 2AZ, UK. richard.matthews@imperial.ac.uk p.hunt@imperial.ac.uk.
authors.forename: Richard P
authors.initials: RP
authors.lastname: Matthews
authors.forename: Tom
authors.initials: T
authors.lastname: Welton
authors.forename: Patricia A
authors.initials: PA
authors.lastname: Hunt
mesh_terms: D001665:Binding Sites; D002713:Chlorine / Q000737:chemistry*; D003198:Computer Simulation; D006859:Hydrogen / Q000737:chemistry*; D006860:Hydrogen Bonding; D007093:Imidazoles / Q000737:chemistry*; D052578:Ionic Liquids / Q000737:chemistry*; D008956:Models, Chemical; D062105:Molecular Docking Simulation
chemical_list: D007093:Imidazoles; D052578:Ionic Liquids; D002713:Chlorine; C029899:imidazole; D006859:Hydrogen