Halogen bonding in mono- and dihydrated halobenzene

Density functional theory calculations were performed on halogen-bonded and hydrogen-bonded systems consisting of a halobenzene (XPh; X = F, Cl, Br, I, At) and one or two water molecules, using the M06-2X density functional with the 6-31+G(d) (for C, H, F, Cl, Br) and aug-cc-pVDZ-PP (for I, At) basis sets. The counterpoise procedure was performed to counteract the effect of basis set superposition error. The results show halogen bonds form in the XPh-H2O system when X > Cl. There is a trend towards stronger halogen bonding as the halogen group is descended, as assessed by interaction energy and X•••O_w internuclear separation (where Ow is the water oxygen). For all XPh-H2O systems hydrogen-bonded systems exist, containing a combination of CH•••O_w and O_wH_w•••X hydrogen bonds. For all systems except X=At the X•••Hw hydrogen-bonding interaction is stronger than the X•••O_w halogen bond. In the XPh-(H₂O)₂ system halogen bonds form only for X > Br. The two water molecules prefer to form a water dimer, either located around the C-H bond (for X = Br, At, and I) or located above the benzene ring (for all halogens). Thus, even in the absence of competing strong interactions, halogen bonds may not form for the lighter halogens due to (i) competition from cooperative weak interactions such as C-H•••O and OH•••X hydrogen bonds, or (ii) if the formation of the halogen bond would preclude the formation of a water dimer.