Hetero-Halogen<middle dot><middle dot><middle dot>Halogen Interactions

Geometry-corrected statistical analysis and MP2 calculations have been performed on C-Cl<middle dot><middle dot><middle dot>F-C, C-Br<middle dot><middle dot><middle dot>F-C, and C-Br<middle dot><middle dot><middle dot>Cl-C hetero-halogen<middle dot><middle dot><middle dot>halogen interactions to understand their preferred geometries and the propensity of these interactions with respect to the corresponding homo-halogen<middle dot><middle dot><middle dot>halogen interactions. It has been shown that the propensities of the hetero-halogen<middle dot><middle dot><middle dot>halogen interactions are higher than the corresponding individual homo-halogen<middle dot><middle dot><middle dot>halogen interactions. Type-2 geometry (angles around the two interacting halogens are similar to 180 degrees and similar to 90 degrees) is found to have a clear preference over Type-1 geometry (angles around the two interacting halogens are more or less the same) in the case of interactions between heavier halogens, C-Br<middle dot><middle dot><middle dot>Cl-C. The preference for linearity around the heavier and more polarizable Br atom is slightly higher than that around the relatively lighter and harder Cl atom. In the cases of C-Cl<middle dot><middle dot><middle dot>F-C and C-Br<middle dot><middle dot><middle dot>F-C interactions, when the angle CFX > angle CXF (X = Cl or Br), Type-1 is slightly more preferable over Type-2; however, no significant preference for Type-1 or Type-2 geometry is observed when angle CFX < angle CXF. In both of these interactions, the preference for linearity is observed around the heavier halogen, but not so clear a preference is found around the F atom. MP2 calculations suggest that the most stable geometry is Type-2 (angle CXHXL = 180 degrees and angle CXLXH = 90 degrees where X-H and X-L are the heavier and lighter halogens respectively), and the order of interactions energy is C-Cl<middle dot><middle dot><middle dot>F-C < C-Br<middle dot><middle dot><middle dot>F-C < C-Br<middle dot><middle dot><middle dot>Cl-C.