Hydration of uranyl (UO22+) cation and its nitrate ion and 18-crown-6 adducts studied by molecular dynamics simulations

Molecular dynamics simulations have been performed first on the UO[sub 2][sup 2+] cation in a box of water molecules in order to investigate the sensitivity of hydration pattern to the electrostatic models used. The coordination number of water to the U and O atoms is not very sensitive to the charges on these atoms. With the different charges, UO[sub 2][sup 2+] is surrounded by five water molecules in the equatorial plane. It bends slightly and the U-O bond lengthens, due to strong coordination of water molecules in the first shell. However, free energy changes upon mutation of one electrostatic model into the other demonstrate the high dependence of hydration energies to the electrostatic distribution in the solute. A van der Waals parameter, 1.6 A, is proposed for the U atom in water. The UO[sub 2][sup 2+], 2NO[sub 3][sup [minus]] salt simulated in water remains as an intimate ion pair with bis(monodentate) coordination and three water molecules in the first shell. In contrast, an inclusion complex of UO[sub 2][sup 2+] with 18-crown-6 displays dissociative behavior to form a second sphere complex, in agreement with experimental evidence. 78 refs., 7 figs., 7 tabs.