The chemical nature of the ⊕C-H•••X- (X=Cl or Br) interaction in imidazolium halide ionic liquids -: art. no. 044504

The C-H center dot center dot center dot X (X=Cl or Br) interaction is traditionally characterized as a relatively weak interaction. However, this interaction becomes very strong in the imidazolium-based halide ionic liquids [J. Phys. Chem. 123, 174501 (2005)]. This strong interaction had been attributed to the electrostatic interaction between the imidazolium cation and the halide anion. In this paper, the chemical nature of the C-circle plus-H center dot center dot center dot Cl- and C-circle plus-H center dot center dot center dot Br- interactions is investigated by atoms in molecules (AIM) and natural bond orbital (NBO) analyses. The AIM calculations indicate that in the EmimX complexes, the C-circle plus-H center dot center dot center dot Cl- and C-circle plus-H center dot center dot center dot Br- interactions have some covalent character, especially the C-circle plus-H center dot center dot center dot Cl- interaction. Mulliken, ChelpG charge, and natural bond orbital population analyses for these two kinds of interactions indicate that the charge transfer is important in the interaction of the cation with the anion. In addition, the NBO analysis demonstrated that the stabilization energy is due to an n ->sigma(center dot)(C-H) orbital interaction. However, in the Emim2X and Emim3X complexes, the calculated results suggested a dominant electrostatic character for the C-circle plus-H center dot Cl- and C-circle plus-H center dot Br- interactions. (c) 2006 American Institute of Physics.