Structural and spectral properties of aqueous hydrogen fluoride studied using ab initio molecular dynamics

Aqueous solutions of deuterated hydrofluoric acid (DF) have been studied at three concentrations using ab initio molecular dynamics (AIMD). At low concentration, DF was found to form a strongly bound complex, dynamically fluctuating between F-D⋯D₂O and F^-⋯D₃O⁺ structures. The coordination number of molecular DF is as low as 2. The fluctuation behavior was observed also at higher concentrations, although in that case 40% of the DFs can be considered as dissociated into F^- ions rather than being a part of F^-⋯D₃O⁺ complexes. No FDF ions formed spontaenously at ambient temperature. One such complex was built, and it was stable for the whole simulation time (6 ps). The increasing acidity as a function of concentration is suggested to derive partly from the increasing stabilization of F^- when its surroundings become more ionic. The F^- ion can be also stabilized by HF molecule, which results in a bifluoride ion. The high DF concentration also lowers all coordination numbers and compresses the first solvation shell of heavy atoms (O and F), compared to low concentration or bulk water. Vibrational spectra obtained from the velocity autocorrelation functions agree with experimental results from spectroscopy. The increased intermolecular forces together with the weakening of the intramolecular ones can also be seen in the vibrational power spectra.