Is Fluorine Really a Halogen Investigation of Halogen Bonds using Steric Charge

Calculated steric charges on the molecular surfaces are used to investigate the halogen bonds formed between some R-X molecules (R= NC, NCCC, CF3CC, FCC, CF3 and CH3, and X=F, Cl and Br) and ammonia, water as well as formaldehyde. It is shown that the formation of halogen-bonded complexes could be regarded as an 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 obtained positive steric charge on the surface of fluorine atom in R-F molecules predicts no halogen bonds for these compounds. Hence, it is suggested that the R-F…Y interactions be referred to “fluorine bond”, instead of halogen bond. This index also successfully determines the direction of halogen bonds for those cases in which the σ-hole theory fails.