Extended Halogen Bonding between Fully Fluorinated Aromatic Molecules

Halogen bonding is a noncovalent interaction where an electrophilittavon a halogen atom, the so-called sigma-hole, attracts a nucleophilic site on an adjacent molecule. The polarizability of halogens relates to the strength of the sigma-hole, and accordingly the halogen halogen distance becomes shorter in the order of Cl, Br, and I. Fully fluoro-substituted aromatic molecules, on the contrary, are generally believed not to form halogen,bonds due to the absence of a sigma-hole. Here, we study atomic-scale in-plane F-F Contacts with high-resolution force microscopy. Our ab initio calculations show that the attractive dispersion forces can overcome the electrostatic repulsion between the fluorine atoms, While the anisotropic distribution of the negative electrostatic potential leads the directional bond and even changes the gap. The coexistence of these two competing forces results in the formation of a ``windmill`` structure, containing three C-F center dot center dot center dot F bonds among neighboring molecules. While the sigma-hole is absent, the scheme,Of the C-F center dot center dot center dot F bonding has a high similarity to halogen bonding.