Po(IV) hydration: a quantum chemical study.

This work presents a theoretical study on the hydration of Po(IV) in solution. Three points have been addressed: (i) the level of calculation needed to properly describe the system under study, (ii) the hydration number of Po(IV), and (iii) the nature of the polonium-water bonding. The condensed medium effects have been included by means of a continuum solvation model, thus different [Po(H(2)O)(n)](4+) hydrates were embedded in a cavity surrounded by a polarizable dielectric medium. Among the quantum-mechanical calculation levels here considered, the MPW1PW91 functional was shown to be the most suitable, allowing a proper description of the Po-H(2)O interactions at affordable cost. The hydration number of Po(IV) was found to be between 8 and 9. This value is ruled by a dynamic equilibrium involving the octa- and ennea-hydrates, although the 7-fold coordination cannot be completely excluded. The hydration free energy of Po(IV) is estimated to be around -1480 kcal/mol. The Po-H(2)O bonding is dominated by strong electrostatic contributions although a small covalent contribution is responsible for the peculiar arrangement adopted by the smaller hydrates (n < or = 5). A natural bond order (NBO) analysis of the hydrate wave functions shows that the covalent bond involves the empty 6p orbitals of the polonium ion and one lone pair on the oxygen atom of the water molecule. A parallel investigation to the hydrate study, where the polonium ion was replaced by a tetravalent point charge plus a repulsion potential, was carried out. These results allowed a detailed examination of the electrostatic and nonelectrostatic contributions to the polonium hydrate formation.