The C-H...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 (plus sign in circle)C-H...Cl(-) and ( plus sign in circle)C-H...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 (plus sign in circle)C-H...Cl(-) and (plus sign in circle)C-H...Br(-) interactions have some covalent character, especially the (plus sign in circle)C-H...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(C-H) (*) orbital interaction. However, in the Emim2X and Emim3X complexes, the calculated results suggested a dominant electrostatic character for the (plus sign in circle)C-H...Cl(-) and (plus sign in circle)C-H...Br(-) interactions.

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