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.

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.

Polar solvent fluctuations drive proton transfer in hydrogen bonded complexes of carboxylic acid with pyridines: NMR, IR and ab initio MD study.

We study a series of intermolecular hydrogen-bonded 1 : 1 complexes formed by chloroacetic acid with 19 substituted pyridines and one aliphatic amine dissolved in CD2Cl2 at low temperature by 1H and 13C NMR and FTIR spectroscopy. The hydrogen bond geometries in these complexes vary from molecular (O-HN) to zwitterionic (O-H-N+) ones, while NMR spectra show the formation of short strong hydrogen bonds in intermediate cases. Analysis of C[double bond, length as m-dash]O stretching and asymmetric CO2- stretching bands in FTIR spectra reveal the presence of proton tautomerism. On the basis of these data, we construct the overall proton transfer pathway. In addition to that, we also study by use of ab initio molecular dynamics the complex formed by chloroacetic acid with 2-methylpyridine, surrounded by 71 CD2Cl2 molecules, revealing a dual-maximum distribution of hydrogen bond geometries in solution. The analysis of the calculated trajectory shows that the proton jumps between molecular and zwitterionic forms are indeed driven by dipole-dipole solvent-solute interactions, but the primary cause of the jumps is the formation/breaking of weak CHO bonds from solvent molecules to oxygen atoms of the carboxylate group.