NMR studies of chemical exchange from aqueous metal clusters: Establishing a structure-reactivity connection for complex geochemical systems

In order to understand contaminate adsorption at the surface of metal oxy-hydroxides, this dissertation presents ligand substitution and proton transfer kinetic data for model compounds that have functional groups (eta-H 2O, mu-OH, mu3-O) similar to those found on clay minerals. Because the most reactive site on any given mineral surface are the waters bound to surface metals (eta-H2O), uncovering the kinetic and mechanistic details of water substitution and proton transfer is critical to understanding the timescales for adsorption reactions.; Unlike mineral surfaces, the aqueous molecular clusters used in this study have well-defined structures and contain inert metal ions that are suitable for spectroscopic investigation (e.g. NMR). Because the overarching purpose of this study is to understand how structure dictates reactivity, the studies presented herein examine how changing the coordinated ligands and the transition metal ion influences reaction rates and substitution mechanisms in three oxo-centered trinuclear clusters. The data presented in Chapters 3 and 4 show that by altering the inductive ability of the carboxylate-bridging-ligands, reaction rates span three orders of magnitude (k298 K = 10-2-10-5 s-1) yet the mechanism of substitution does not deviate from dissociative (Id /D). Based on the data presented in Chapter 5, however, the identity of the group 6 transition metal ion induces a mechanistic changeover from dissociative (D) to associative (Ia) but yet the rates are within an order of magnitude (k298K ∼10 -6 s-1). In the final chapters, rates of proton transfer from the coordinated waters on two molecular clusters (Chapters 6 and 7) were investigated. The results from these studies showed that exchange rates were similar to those for simple hexaaqua ions (milliseconds) despite considerable differences in structure, Bronsted acidity of the bound waters and average charge on the metal ions.; Using these results, we were able to establish several structure-reactivity correlations, which will hopefully allow us to extrapolate these results to more complicated inaccessible geochemical systems.