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 CHCl and dissolved in CCl by ab initio molecular dynamics simulations. We show that CHCl molecules form CHF hydrogen bonds with lone pairs of fluorine atoms, while CCl 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 symmetry is retained. Interestingly, this transient asymmetrization of FHF is more pronounced in CCl, than in CHCl. We show that the H andF 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.

Symmetry of Hydrogen Bonds in Two Enols in Solution

“…20 Even the classic case of the bifluoride ion FHFcan be desymmetrized owing to fluctuations of the positions of solvent molecules. 21 Moreover, we concluded that there is no stabilization associated with symmetric, short, or low-barrier H-bonds.…”

Intramolecular Hydrogen Bond Energy and Its Decomposition—O–H∙∙∙O Interactions

“…In the gas phase, the stable linear structure of the (FHF) − anion is characterized by the F•••F distance of 2.304 Å with the H-atom located in its mid-point (infrared diode laser spectroscopy was applied for the analysis of this anion) [73]. However, other studies have shown that the environmental effects in solvent [74] or in crystals [75] may significantly influence this geometry. Particularly, in crystal structures, the D ∞h symmetry is often broken; for the sample of precise determined crystal structures, the range of F•••F distances from 2.194 Å to 2.382 Å is observed, the proton is often moved from the central position, and the system even loses linearity [75].…”

Isotopic-Perturbation NMR Study of Hydrogen-Bond Symmetry in Solution: Temperature Dependence and Comparison of OHO and ODO Hydrogen Bonds

“…17b,18 Indeed, this interpretation was supported by computer simulations, 19 including the case of otherwise symmetric FHF -. 20 Further examples show that asymmetry is also seen in NHN and NHO H-bonds. 21 In summary, we and others have been unable to find evidence for symmetric Hbonds in solution (except at very low temperature, where the solvent becomes well ordered), 22 and with only one recent exception that demonstrates the subtlety of solvation.…”