Nature and Stability of Weak Halogen Bonds in the Gas Phase: Molecular Beam Scattering Experiments and Ab Initio Charge Displacement Calculations

Molecular beam scattering experiments are presented for the first time in order to characterize the nature and strength of the intermolecular interaction of the gas phase H2O-CF4 and H2O-CCl4 weakly bound complexes. Simultaneous measurements of the collision cross section of water, molecular oxygen, and argon atoms scattered by the same target (CF4 or CCl4) have been performed in the thermal energy range. The experimental results show unambiguously that H2O-CCl4 is similar to 0.5 kJ/mol more stable than O-2-CCl4 and Ar-CCl4, which is incompatible with a fully noncovalent nature of the interaction. Interestingly, this is not the case for the analogous CF4 complexes which all show the same binding energy, within the experimental uncertainty. These results are supported by state-of-the-art ab initio calculation of the dimer structures and of the charge displacement upon the formation of the complex. The focus is on the possible stabilization effects due to halogen bond formation and on the extent of the related charge transfer. The latter has been shown to be large from H2O toward CCl4 and negligible from H2O toward CF4.