Ab initio molecular dynamics simulation of a water-hydrogen fluoride equimolar mixture

Hydrogen fluoride and water can be mixed in any proportion. The resulting solutions have unique acidic properties. In particular, hydrogen fluoride undergoes a weak-to-strong acidity transition with increasing concentration of HF To supplement the knowledge already obtained on dilute or moderately concentrated solutions and gas-phase aggregates, an equimolar mixture is studied here by Car-Parrinello molecular dynamics. The natures of the ions and of the complexes formed in the equimolar liquid were determined. Specifically, H3O+, H5O2+, FH-OH2, and HF2- were spontaneously obtained while only hydronium and fluoride ions pre-exist in the equimolar crystal. The behaviour of the proton in the equimolar liquid was compared with mixtures of other proportions simulated previously in an attempt to relate proton dynamics to acidity. In the same way, the behaviour of HF2- was also examined. In this case, proton localization and transfer appeared to be driven by the fluctuating environment of the solvated ion.