Competition between lone pair-π, halogen bond, and hydrogen bond in adducts of water with perhalogenated alkenes C2ClnF4−n (n = 0-4)

- Charge transfer from O lone pair to alkene π* antibonding orbital.
- lp→π is most strongly bound complex type between H2O and perhalogenated alkenes.
- Binding energies larger than 2 kcal/mol, insensitive to identity of halogen atoms.
- Halogen-bonded complexes are slightly less stable, followed by OH⋯X H-bond.

A thorough search of the potential energy surface is carried out for heterodimers of water with C2ClnF4−n. Three different types of interactions are observed. Structures dominated by a lone pair-π interaction have the highest binding energies, and are stabilized by charge transfer from O lone pairs of H2O to the CC π* antibonding orbital of the alkene. Halogen-bonded O⋯Cl complexes are slightly less strongly bound, followed by OH⋯X hydrogen bonds. The replacements of Cl by F atoms have only small effects upon binding energies. Inclusion of vibrational and entropic effects removes the clear energetic superiority of lp-π binding energies. When combined with the observation of several similar geometries for each particular heterodimer type, and a sensitivity to basis set, it would be quite difficult to predict with any degree of certainty the single most stable configuration, even with very high level calculations.