Classical and Car-Parrinello Molecular Dynamics Simulations of Polyvalent Metal Ions in Water

Abstract: The aqueous solvation of metal ions is one of the long-standing and complex problems in chemistry, with implications for and applications in a broad range of biochemical and electrochemical systems, where water is the all-pervasive medium.This thesis describes computer simulations of Al3+(aq), Fe2+(aq), Fe3+(aq) and Cu2+(aq). Various aspects of the solvation of these polyvalent metal ions in water are addressed, at different levels of theory, using Car-Parrinello molecular dynamics, classical molecular dynamics and quantum-mechanical cluster calculations. Polyvalent metal ions are particularly interesting because of their large influence on the solvent structure, dynamics and thermodynamics, as well as on the properties of the individual solvent molecules. Polyvalent metal ions in aqueous solution also constitute a challenging subject for computer simulations since a sophisticated interaction model is needed to incorporate the large many-body effects. All the ion-water coordination figures in this thesis are octahedral, except in the Cu2+(aq) solution, where the ion is penta-coordinated with four equatorial neighbours in a plane and one axial neighbour located ~0.45 Å further out from the ion. The equatorial ion-water bonds have covalent character, while the axial water molecule is only electrostatically bound. For all the ions, the OD stretching frequencies of the first-shell water molecules are much more downshifted than in liquid water. In the case of Cu2+(aq), however, only the OD frequencies of the equatorial water molecules are downshifted with respect to bulk water whereas the OD frequencies of the axial water molecule are slightly upshifted. Various limitations of the Car-Parrinello molecular dynamics simulations have been explored and compared, such as finite system-size effects and shortcomings in the electronic structure calculations. The Car-Parrinello simulations are found to give reasonable descriptions of the polyvalent metal ions in aqueous solution.