Can Fluorine form Halogen Bond? Investigation of Halogen Bonds through Steric Charge

Calculated steric charges (q(s) (r) = -1/4 pi del(2) upsilon(s)(r), where upsilon(s) is steric potential) on the molecular surfaces are used to investigate the non-covalent interactions formed between some R-X molecules (R= NC, NCCC, CF3CC, FCC, CF3, CH3, and X=F, Cl, Br) and ammonia, water as well as formaldehyde. It is shown that the formation of halogen-bonded complexes could be regarded as the interaction between a negative steric charge (steric potential depletion) on the surface of halogen atom and a positive steric charge (steric potential accumulation) region of an electron-donor site. But the evaluated positive steric charges on the surface of fluorine atoms in R-F molecules predict no halogen bonds for these compounds. Therefore, it is suggested that the R< C- >F center dot center dot center dot Y interactions should be renamed to fluorine bond, instead of halogen bond. The evaluated steric charges also successfully determine both the strength and direction of halogen bonds for those cases in which the sigma-hole concept fails.