Symmetry and dynamics of FHF⁻ anion in vacuum, in CD₂Cl₂ and in CCl₄. Ab initio MD study of fluctuating solvent–solute hydrogen and halogen bonds

Abstract

FHF^- anion is a classic example of a central-symmetric strongly hydrogen bonded system that has been intensively investigated both experimentally and theoretically. In this paper we focus on solvent effects on symmetry, structure and dynamics of the anion. The FHF^- anion is studied in vacuum, dissolved in CH₂Cl₂ and dissolved in CCl₄ by ab initio molecular dynamics simulations. We show that CH₂Cl₂ molecules form CHF hydrogen bonds with lone pairs of fluorine atoms, while CCl₄ molecules form halogen bonds. These specific non-covalent solvent-solute interactions are cooperatively coupled to the FHF^- hydrogen bonds. The fluctuation of solvent molecules' positions is the driving force changing the FHF^- hydrogen bond geometry. Most of the time, the anion is solvated asymmetrically, which leads to the asymmetric bridging particle position, though the time-averaged D_∞h symmetry is retained. Interestingly, this transient asymmetrization of FHF^- is more pronounced in CCl₄, than in CH₂Cl₂. We show that the ¹H and ¹⁹F NMR chemicals shifts react sensitively to the changes of anion's geometry and the limiting chemical shifts of free solvated FH and F^- are strongly solvent-dependent.