Hydration of the fluoride anion: Structures and absolute hydration free energy from first-principles electronic structure calculations

A series of first-principles electronic structure calculations have been performed to determine the most stable structures of F^-(H₂O)_n clusters (n = 4, 8, 12, and 16) and the hydration free energy of fluoride anion (F^-). The calculated results show that a new, tetrahedrally coordinated fluoride anion hydration structure F^-(H₂O)₄ cluster is lower in Gibbs free energy than the previously considered most stable structure of F^-(H₂O)₄. The first ab initio prediction of potential stable hydration structures for F^-(H₂O)_n clusters (n = 8, 12, and 16) are given. The energetic results show that the tetrahedrally coordinated fluoride anion hydration structure becomes more stable as compared to the other hydration structures with a pyramidal coordination, i.e., a surface ion cluster state, as the cluster size increases from n = 8 to n = 12 to n = 16. This suggests that, with increasing n, the fluoride anion will be internally solvated in large enough F^-(H₂O)_n clusters. These results provide insight into the transition from the hydration structure found in small gas-phase hydrated-anion clusters to the hydration structure observed in aqueous solution. The calculated results show that, for a given n, the bulk solvent effects can qualitatively change the relative thermodynamic stability of different possible isomers of F^-(H₂O)_n clusters and the most stable structure in solution is not necessarily the most stable structure in the gas phase. When n = 16, a pyramidally coordinated fluoride anion hydration structure is the most stable structure in the gas phase, whereas a tetrahedrally coordinated fluoride anion hydration structure has the lowest free energy in solution. The absolute hydration free energy of fluoride anion in aqueous solution, ΔG_hyd²⁹⁸(F^-), is predicted to be -104.3 ± 0.7 kcal/mol by using a reliable computational protocol of first-principles solvation-included electronic structure calculations. The predicted ΔG_hyd²⁹⁸(F^-) value of -104.3 ± 0.7 kcal/mol, together with our previously calculated ΔF_hyd²⁹⁸(H⁺) value of -262.4 kcal/mol determined by using the same computational protocol, gives ΔG_hyd²⁹⁸(F^-) + ΔG_hyd²⁹⁸(H⁺) = -366.7 ± 0.7 kcal/mol in excellent agreement with the value of -366.5 kcal/mol derived from the available experimental data.