Ab Initio Molecular Dynamics Study of a Mixture of HF(aq) and HCl(aq)^†

We have studied a mixture of HF and HCl molecules in water using Car−Parrinello ab initio molecular dynamics (CPMD). We have done simulations with 1 HF and 3 HCl molecules, 3 HF and 4 HCl, 6 HF and 8 HCl (6/8 simulation), and 14 HF molecules. All simulations consist of 32 molecules, and they were 10−96 ps long. The HF dissociation probability was around 30%, and HCl's was more than 90%. The solvation of the HF molecule was much better than the solvation of HCl. The solvation environment of F, both the F^- ion and the F in HF, did not depend much on the acids concentration, whereas the Cl coordination numbers were rather sensitive to the concentration. In the 6/8 simulation, all XH−Y (X, Y = F, Cl) type molecules were observed and the FH−F was the most probable. In general, the molecular structures in mixed aqueous acid systems were similar to the pure HF(aq) and HCl(aq) systems.