Structural and Spectral Properties of Aqueous Hydrogen Fluoride Studied Using ab Initio Molecular Dynamics

Abstract

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.

Ab Initio Molecular Dynamics Simulation of a Water–Hydrogen Fluoride Equimolar Mixture

“…[19] Nevertheless, the bifluoride ions observed here are spontaneously formed during simulation and no longer a result from the simulation setup. The amount of data collected for the statistics is also larger, since four bifluoride ions were examined here.…”

“…This distance is comparable to the value 2.49-2.50 previously found for the FH···OH 2 QF···HÀOH 2 fluctuating complex in simulations of more-diluted solutions. [19] The first peak is sharp, but the value of g(r) does not go below one after the peak and, therefore, it yields an increasing n(r) (number of nearest neighbours), without a plateau. This situation does not allow a precise estimation of the number of neighbours, since no obvious cutoff can be chosen.…”

“…[18,19] Its stability is held responsible for HF weakness in water at low concentrations. On the contrary, when the acid is strong, HF is assumed to dissociate and the complex FH···OH 2 is supposed to be outnumbered by the complex F···HÀOH 2 .…”

“…[18] and [19], respectively). To build a global picture of this weak/strong transition, the equimolar mixture had to be investigated by the same means.…”

“…[19] How does the stability of this neutral complex evolve in a more ionic environment? Does it finally dissociate?…”