Hydration of the Fluoride Anion. Structures and Absolute Hydration Free Energy from First-Principles Electronic Structure Calculations
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
We have performed a series of first-principles electronic structure calculations to determine the most stable structures of F-(H2O)n clusters (n = 4, 8, 12, and 16) and to determine 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 calculations provide the first ab initio prediction of potential stable hydration structures for F-(H2O)n clusters (n = 8, 12, and 16). The energetic results reveal an important trend in the relative stability of the hydration structures from n = 8 to n = 12 and to n = 16: the tetrahedrally coordinated fluoride anion hydration structure becomes more and more stable as compared to the other hydration structures with a pyramidal coordination, i.e., a surface ion cluster state. This suggests that with increasing n, the fluoride anion will be internally solvated in large enough F-(H2O)n clusters. This provides 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 leading to the result that the most stable structure in solution is not necessarily the most stable structure in gas phase. In particular, 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 the solution. The absolute hydration free energy of fluoride anion in aqueous solution, ΔGhyd298(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 same approach recently used to calculate the absolute hydration free energy of the proton and of the hydroxide anion. The predicted ΔGhyd298(F-) value of –104.3 ± 0.7 kcal/mol, together with our previously calculated ΔGhyd298(H+) value of –262.4 kcal/mol determined by using the same computational protocol, gives ΔGhyd298(F-) + ΔGhyd298(H+) = –366.7 ± 0.7 kcal/mol in excellent agreement with the value of –366.5 kcal/mol derived from the available experimental data.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 949161
- Report Number(s):
- PNNL-14387
- Journal Information:
- Journal of Physical Chemistry. C, Vol. 108, Issue 11; ISSN 1932-7447
- Publisher:
- American Chemical Society
- Country of Publication:
- United States
- Language:
- English
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