Understanding the magnitude and origin of bidentate charge-assisted halogen bonds of halo-perfluorocarbons and halo-hydrocarbons with halide anions

Bidentate charge-assisted halogen bonds between halo-perfluorocarbons and halide ions play crucial roles in determining the formation of double-helix systems that display an ample phase behavior. This paper presents an ab initio investigation of several trimeric complexes of halo-perfluorocarbons and halo-hydrocarbons with halide anions. In all cases, the intermolecular distances are shown to be much less than the sums of van der Waals radii of involved atoms and the interactions are essentially linear. Bifurcated halogen bonding energies, calculated at the MP2/lanl2DZ^* level, vary over a range from -1.91 to -30.07 kcal/mol. The interactions are comparable to, or even prevail over, conventional hydrogen bonds. Computations described herein also unveil that halo-perfluorocarbons form significantly stronger halogen bonds with anions relative to corresponding halo-hydrocarbons. These results agree with those of the quantum theory of atoms in molecules, since bond critical points are found for these halogen bonds. Finally, natural bond orbital analysis has been applied to provide more insights into the nature of bidentate charge-assisted halogen bonds.