Nature and stability of weak halogen bonds in the gas phase: Molecular beam scattering experiments and ab initio charge displacement calculations

摘要

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 H_2O-CF_4 and H_2O-CCl _4 weakly bound complexes. Simultaneous measurements of the collision cross section of water, molecular oxygen, and argon atoms scattered by the same target (CF_4 or CCl_4) have been performed in the thermal energy range. The experimental results show unambiguously that H _2O-CCl_4 is ～0.5 kJ/mol more stable than O _2-CCl_4 and Ar-CCl_4, which is incompatible with a fully noncovalent nature of the interaction. Interestingly, this is not the case for the analogous CF_4 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 H _2O toward CCl_4 and negligible from H_2O toward CF_4.