| The reactions of oxides in water, including the interactions that occur at the mineral-water interface, are of broad interest but are not well understood. Molecular-scale experiments are the only way to gain insight into how these reactions proceed because these clusters link well to computational chemistry.Herein the oxygen-isotope exchange rates and overall reactivities for all structural oxygens in three polyoxoniobate ions that differ by systematic metal substitutions of Ti(IV) => Nb(V) are compared. The [HxNb 10O28](6-x)-, [HxTiNb 9O28](7-x)-, and [HxTi2Nb 8O28](8-x)- ions are isostructural, yet have different Bronsted properties. Rates for sites within a particular molecule in the series differ by over 104, but the relative reactivities of the oxygen sites rank in nearly the same relative order for all ions in the series. Within a single ion, most, if not all, structural oxygens exhibit rates of isotopic exchange that vary similarly with pH, indicating that each structure responds as a whole to changes in pH.Across the series of molecules, however, the pH dependencies for isotope exchanges and dissociation are distinctly different, reflecting different contributions from proton- or hydroxide-enhanced pathways. The proton-enhanced pathway for isotope exchange dominates at most pH conditions for the [H xTi2Nb8O28](8-x)- ion, but the hydroxide-enhanced pathways are increasingly important for the [H xTiNb9O28](7-x)- and [HxNb 10O28](6-x)- ions at higher pH. The local effect of Ti(IV) substitution can also be assessed by comparing rates for structurally similar oxygens on each side of the [HxTiNb9 O28](7-x)- ion and is surprisingly small.Interestingly, these nanometer-size structures seem to manifest the same general averaged amphoteric chemistry that is familiar for other reactions affecting oxides in water, including interface dissolution by proton- and hydroxide-enhanced pathways. |
