Abstract: To understand how oxide structures react at the molecular scale, rates of steady oxygen-isotope exchanges were followed in three isostructural molecules of ca 40 atoms as a function of solution composition. These molecules were chosen because the structures in solution are known with complete confidence, yet isotope-exchange reactions can be followed spectroscopically at individual oxygens. The series of molecules differ only in a single Ti(IV) --> substitution in one of the three metal sites, making a series of structures having the stoichiometries: [H_xNb10O₂₈]^(6-x)-, [H_xTiNb9O₂₈]^(7-x)-, and [H_xTi₂Nb8O₂₈]^(8-x)-. As in our previous study of the [H_xNb10O₂₈]^(6-x)- ion, we find that isotope-exchange reactions at particular oxygens cannot be understood without considering dynamics of the entire nanometre-size structure, and the interaction of the entire structure with solution. The rates for all reactive oxygens vary similarly with pH within a single molecule, but the relative importance of the proton- or hydroxide-enhanced pathways for isotopic exchange vary systematically across the series, along with Brönsted acid-base properties, and scale like the charge of the unprotonated structure in solution. The local effect on site reactivities of the Ti(IV) substitution is surprisingly small and is of the same order as that due to changes in the counterions. The extents to which the functional-group reactivities reflect global properties of the molecules is striking and emphasizes the importance of having accurate structural information in simulating geochemical reactions. The broad amphoteric chemistry of the rates resembles other classes of oxide reactions, such as ester hydrolysis and mineral dissolution kinetics.

Villa, E. M., Ohlin C. A., Casey W. H. Adding reactivity to structure 2: Oxygen-isotope-exchange rates in three isostructural oxide ions American Journal of Science , 2010, 310, 629-644.

Department of Chemistry and Department of Geology, University of California, Davis, CA

Abstract The first example of a discrete peroxo-polyoxotitanoniobate is introduced and characterized by x-ray crystallography.

Ohlin, C. A.,[1,2] Villa, E. M.,[1,2] Fettinger, J. C.,[1] Casey, William H.[1,2] The first peroxotitanoniobate cluster - [N(CH₃)₄]₁₀[Ti₁₂Nb₆O₃₈(O₂)₆] Inorg. Chimica Acta, 2010, 363(15), 4405-4407.

1. Department of Chemistry, University of California, Davis, CA.
2. Department of Geology, University of California, Davis, CA.

Abstract Understanding simple oxygen exchange reactions is important to a variety of communities concerned with the chemistry of oxides with water. Limitations in the methods available for studying reactions at these oxide-water interfaces, as well as difficulties in characterizing their structures, have led to the use of polyoxometalates (POMs) as model molecules. POMs are metaloxide ions comprised of group 5 and 6 metals. These ions constitute discrete and often soluble clusters than can be spectroscopically probed with great confidence. In addition, POMs are interesting in their own right owing to their structural and chemical diversity, and are finding an increasing number of applications. We have been investigating the oxygen-isotope exchange kinetics in these ions and aqueous solution by ¹⁷O Nuclear Magnetic Resonance (NMR) to help better recognize what controls molecule-water interface processes on the level of individual oxygen sites. These structures are chosen because the isotope-exchange reactions could be followed separately from dissociation or condensation of the structure.

Villa, E. M., Ohlin, C. A, Casey, W. H..Borate Accelerates Oxygen-Isotope Exchange for Polyoxoniobate Ions in Water Eur. Chem. J. , 2010, 16(29), 8631-8634.

Department of Chemistry and Department of Geology, University of California, Davis, CA

Abstract We compare oxygen-isotope exchange rates for all structural oxygens in three polyoxoniobate ions that differ by systematic metal substitutions of Ti(IV)=>Nb(V). The [H_xNb₁₀O₂₈ ]^(6-x)-, [H_xTiNb₉O₂₈]^(7-x)-, and [H_xTi₂ Nb₈O₂₈]^(8-x)- ions are all isostructural, yet have different Brönsted properties. Rates for sites within a particular molecule in the series differ by at least 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 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 base-enhanced pathways. The proton-enhanced pathway for isotope exchange dominates at most pH conditions for the [H_xTi₂ Nb₈O₂₈]^(8-x)- ion, but the base-enhanced pathways are increasingly important for the [H_xTiNb₉O₂₈]^(7-x)- and [H_xNb₁₀O₂₈]^(6-x)- structures at higher pH. The local effect of Ti(IV) substitution could be assessed by comparing rates for structurally similar oxygens on each side of the [H_xTiNb₉O₂₈]^(7-x)- ion and is surprisingly small. Interestingly, these nanometer-size structures seem to manifest the same general averaged casey +geo amphoteric chemistry that is familiar for other reactions affecting oxides in water, including interface dissolution by proton- and hydroxyl-enhanced pathways.

Villa, E. M., Ohlin, C. A., Casey, W. H. Oxygen-isotope exchange rates for three isostructural polyoxometalate ions J. Am. Chem. Soc., 2010,132(4), 5264-5272.

Department of Chemistry and Department of Geology, University of California, Davis, CA

Abstract Our understanding of mineral and glass dissolution has advanced from simple thermodynamic treatments to models that emphasize adsorbate structures. This evolution was driven by the idea that the best understanding is built at the molecular level. Now, it is clear that the molecular questions cannot be answered uniquely with dissolution experiments. At the surface it is unclear which functional groups are present, how they are arranged, and how they interact with each other and with solutes as the key bonds are activated. An alternative approach has developed whereby reactions are studied with nanometre-sized aqueous oxide ions that serve as models for the more complicated oxide interface. For these ions, establishing the structure is not a research problem in itself, and bond ruptures and dissociations can be followed with much confidence. We review the field from bulk -dissolution kinetics to the new isotope-exchange experiments in large oxide ions.

Ohlin, C. André;[1,2] Villa, Eric M.;[1,2] Rustad, James R.,[2] Casey, William H. [1,2] "Dissolution of insulating oxide materials at the molecular scale ", Nature Mat., 2010, 9, 11-19.

1. Department of Chemistry, University of California, Davis, CA.
2. Department of Geology, University of California, Davis, CA.