## Abstract

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, ΔG_{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 ΔG_{hyd}^{298}(F^{-}) value of -104.3 ± 0.7 kcal/mol, together with our previously calculated ΔF_{hyd}^{298}(H^{+}) value of -262.4 kcal/mol determined by using the same computational protocol, gives ΔG_{hyd}^{298}(F^{-}) + ΔG_{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.

Original language | English |
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Pages (from-to) | 2020-2029 |

Number of pages | 10 |

Journal | Journal of Physical Chemistry A |

Volume | 108 |

Issue number | 11 |

DOIs | |

State | Published - Mar 18 2004 |

## ASJC Scopus subject areas

- Physical and Theoretical Chemistry