45086-G3
A Computational Study of the Hydrolysis and Solvation Chemistry of Highly-Charged Metal Cations
Understanding chemistry and physico-chemical properties of aqueous solutions of inorganic ions is of crucial importance for a variety of chemical problems in both basic and applied science. Solutions of a number of highly charged metal cations present an experimental challenge, due to the polymerization that occurs to different extents depending on pH, aging, temperature, and concentration. Computational chemistry methods provide a tool for circumventing some of these problems, and have been shown to contribute important information about solution characteristics. We focus on solvation of the Zr(IV) ion, due to a wide range of zirconium applications that call for a better understanding of its characteristics. In the period September 2007-September 2008, 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 dimer and trimer species. We have also started investigating another important Zr(IV) polynuclear species, the hexamer, and continued the studies on the monomer species. We conducted a systematic analysis of a number of possible dimer and trimer structures, by varying the chemical nature of the oxygen-containing bridges, as well as the terminal groups. We used 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 there are several dimer and trimer structures which are stable on the picosecond time scale in an aqueous environment, represented through 49 and 77 water molecules surrounding the dimer and the trimer, respectively. We find that several forms of the dimer, with different combinations of O and OH bridges are possible, whereas the H2O bridges cause the structure to fall apart rapidly. In aqueous solution, the tested dimer structure is stable on the order of 10 ps at 300 K. The two Zr(IV) ions of the dimer in aqueous solution are 7 and 8-coordinated. The arrangement of the terminal water molecules and OH groups within the monomer units is either square antiprism or pentagonal bipyramid, depending on the coordination, and the spatial relationship between the two units is such that water molecules are staggered. From our simulations, the trimer species appears to be stable only if in a stacked form, where the third monomer unit is “stacked” on top of a dimer, as opposed to the linear trimer, which falls apart during the initial stages of the CPMD simulation. In addition, all the attempted stacked trimers exhibit an oscillation of one of the monomer units with respect to the other two, on a picosecond scale. Thus, we postulate that the stacked trimer might persist only on a short time scale upon its formation, which might be the reason for conflicting experimental reports regarding its existence. Also, our current hexamer simulations indicate that the trimer might be a transient species in the process of hexamer formation. In summary, we have completed the planned Zr(IV) dimer and trimer studies. The Zr(IV) monomer and hexamer studies are underway. Since starting the independent research career, my research has developed in two directions: investigations of inorganic ion solvation and biomimetic foldamer design. Support from ACS PRF grant has been crucial for establishing the former direction. Importantly, ab initio molecular dynamics simulations have not been utilized much to solve the problems of heavy metal, highly charged ion solvation. The results I have obtained so far on solvation of Zr(IV) ion have demonstrated that this methodology can be successfully utilized for such problems, and thus has established an important area of research for me. This project presents a challenging and attractive direction for my further career development, as there are many more 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.
