Communication: covalent nature of X⋯H2O (X = F, Cl, and Br) interactions.

Open-shell halogen (X = F, Cl, Br) atoms form entrance-channel complexes with H2O, which play an important role in the X + H2O reactions. To understand their structures and origin of stability, we report an extensive ab initio study of such complexes and contrast them with complexes between H2O and H/O((3)P). Evidence is presented to show that the interaction between a halogen atom and H2O is dominated by a weak but covalent bond, rather than dispersion and/or electrostatic interactions.