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45086-G3
A Computational Study of the Hydrolysis and Solvation Chemistry of Highly-Charged Metal Cations
The aqueous chemistry of metal cations is of great interest due to their important roles in chemistry, geochemistry and biochemistry. The structures, charges and stabilities of aqueous metal cations and their polynuclear hydrolysis products are crucial for understanding and controlling processes such as their adsorption onto soil/mineral particles; coagulation/precipitation; chemical separations; and interactions with living organisms. Understanding transition and inner transition metal hydrolysis presents a special experimental challenge, due to the complexity and variability of the species formed by these ions in water, as well as radioactivity in some cases. Despite decades of research, triggered by applications ranging from drug design to nuclear technology, many physico-chemical characteristics of these highly-charged ions and their hydrolysis products remain unknown.
In this project, we tackle the characteristics of polynuclear species formed by solvation a IVB group metal cation, Zr4+. In the period September 2006-September 2007, we have worked on determining structures and dynamics of a number of small Zr(IV) polynuclear species. We have completed the planned studies on the tetramer, and have significantly advanced our understanding of the monomer, dimer, and trimer species. We have also taken several new directions in attempting to understand the initial steps in the process of solvation of Zr(IV) ion.
Our focus was on the Zr(IV) tetramer, [Zr4(OH)8(H2O)16]8+, as it is thought to be the major component of the Zr(IV) polymer system in aqueous solution, present as a dominant ionic cluster species with respect to the other Zr(IV) clusters under various experimental conditions. We conducted a combination of ab initio molecular dynamics (AIMD) and quantum mechanical studies in gas-phase and aqueous solution, and related our results to the available experimental data, to provide atom-level information on the behavior of this species in aqueous solution. Our simulations indicate that the tetramer structure is stable on the picosecond time scale in an aqueous environment, represented through 108 water molecules surrounding the tetramer. We find that it is of a planar form, comprising eight-coordinated Zr(IV) ions with antiprism/irregular dodecahedron ligand arrangement. In combination with our studies of the Zr(IV) dimer and trimer clusters, our results provide detailed geometrical information on structural motifs for building zirconium polymers, and suggest a possible polymerization path. The structure determined by us using computational methods agrees well with the available experimental data on this species. Experimental data on the dynamics is not available for comparison.
We have also analyzed the structure and dynamics of a number of possible dimer and trimer structures using a combination of AIMD simulations and conventional quantum mechanical calculations. We have determined the structural characteristics and analyzed the aqueous solution dynamics of the two cluster species (one dimer and one trimer) that remain stable throughout the gas-phase simulations. Our study points to and provides detailed geometrical information for a stable structural motif for building zirconium polymers, the Zr(OH)2Zr bridging unit with 7-8 coordinated Zr ions, which, however, cannot be used to construct a stable structure for the trimer. We find that a stacked trimer, not featuring this motif, is a possible structure, though not a very stable one, shedding new light on this species, and its possible importance in the aqueous chemistry of Zr(IV) ion.
In summary, we have completed the planned Zr(IV) tetramer studies, and made significant advancements with respect to the Zr(IV) dimer and trimer clusters. The gas-phase Zr(IV) monomer studies, as well as studies of solvated Zr(IV) are also underway.
Applications of ab initio molecular dynamics simulations to the problems of heavy metal, highly charged ion solvation are novel and not much investigated, with a potential of providing information that can not be accessed through experiments. Thus, this project presents a challenging and attractive direction for developing my research, as there are many important highly charged ion systems which can be addressed using the approach developed here. Due to the fact that we use several levels of computational chemistry methods to study the system, the project allows that appropriate sub-projects be assigned to undergraduate, graduate and postdoctoral researchers, with the researchers at all three levels being able to provide meaningful contributions to the understanding of the solvation process. So far, two postdoctoral and one undergraduate researcher have been successfully involved in the project. I therefore believe that this project is also important for the purposes of research education.
