Does Fluorine Participate in Halogen Bonding?

When R is sufficiently electron withdrawing, the fluorine in the RF molecules could interact with electron donors (e.g., ammonia) and form a noncovalent bond (FN). Although these interactions are usually categorized as halogen bonding, our studies show that there are fundamental differences between these interactions and halogen bonds. Although the anisotropic distribution of electronic charge around a halogen is responsible for halogen bond formations, the electronic charge around the fluorine in these molecules is spherical. According to source function analysis, F is the sink of electron density at the FN BCP, whereas other halogens are the source. In contrast to halogen bonds, the FN interactions cannot be regarded as lump-hole interactions; there is no hole in the valence shell charge concentration (VSCC) of fluorine. Although the quadruple moment of Cl and Br is mainly responsible for the existence of sigma-holes, it is negligibly small in the fluorine. Here, the atomic dipole moment of F plays a stabilizing role in the formation of FN bonds. Interacting quantum atoms (IQA) analysis indicates that the interaction between halogen and nitrogen in the halogen bonds is attractive, whereas it is repulsive in the FN interactions. Virial-based atomic energies show that the fluorine, in contrast to Cl and Br, stabilize upon complex formation. According to these differences, it seems that the FN interactions should be referred to as fluorine bond instead of halogen bond.