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 - (H 2 O) 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 - (H 2 O) 4 cluster is lower in Gibbs free energy than the previously considered most stable structure of F - (H 2 O) 4 . The first ab initio prediction of potential stable hydration structures for F - (H 2 O) 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 - (H 2 O) 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 - (H 2 O) 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, Δ G hyd 298 (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 Δ G hyd 298 (F - ) value of −104.3 ± 0.7 kcal/mol, together with our previously calculated Δ G hyd 298 (H + ) value of −262.4 kcal/mol determined by using the same computational protocol, gives Δ G hyd 298 (F - ) + Δ G hyd 298 (H + ) = −366.7 ± 0.7 kcal/mol in excellent agreement with the value of −366.5 kcal/mol derived from the available experimental data.