## Abstract

In this paper we present a quantum-mechanical study of anions in water clusters, X^{-} (H_{2}O)_{n} (X=Cl, Br, I, and n=1-6). Molecular orbital calculations at the self-consistent field (SCF) level and at the second-order Møller-Plesset (MP2) level were performed using extended basis sets. Full structural optimization was conducted at the MP2 level for R=1 and at the SCF level for n=2-6. The energies and charge distribution of X ^{-} (H_{2}O) were calculated at the MP2 level, while the energies of the X^{-}(H_{2}O)_{n} (n=2-6) clusters were calculated at the MP2 level using the SCF optimized geometry. Calculations of total and sequential enthalpies of hydration and for the vertical ionization potentials were conducted for X-(H_{2}O), the hydrogen bonded and linear isomers of X^{-}(H_{2}O)_{2}, the pyramidal structure of X^{-}(H_{2}O)^{3}, and the interior and surface isomers of X^{-}(H_{2}O)_{n}, n=4-6. The calculated hydration enthalpies account well for their experimental size dependence for n=1-6. However, the isomer specificity of the hydration enthalpies is reflected by a small energy difference (δ=1-5 kcal mol^{-1}) between the surface and interior isomers at a fixed n, precluding the assignment of structural isomers on the basis of ground-state energetics. The cluster size dependence and isomer specificity of the calculated vertical ionization potentials in conjunction with experimental data provide a diagnostic tool for the structural assignment of isomers and for the distinction between surface and interior structures. The central prediction emerging from the structure-energetic relations based on cluster size dependence and isomer specificity of vertical ionization potentials, is the prevalence of surface structures for Cl^{-}(H_{2}O)_{n} (n=2-6), Br ^{-}(H_{2}O)_{n} (n=2-6), and I^{-}(H _{2}O)_{n} (n=2-5), while a "transition" from surface to interior structure may be exhibited for I^{-}(H_{2}O) _{6}.

Original language | English |
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Pages (from-to) | 2851-2864 |

Number of pages | 14 |

Journal | The Journal of Chemical Physics |

Volume | 100 |

Issue number | 4 |

DOIs | |

State | Published - 1994 |