180. Adding reactivity to structure 2:
Oxygen-isotope-exchange rates in three isostructural oxide ions

December 6, 2010

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: [HxNb10O28](6-x)-, [HxTiNb9O28](7-x)-, and [HxTi2Nb8O28](8-x)-. As in our previous study of the [HxNb10O28](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.

179. Isotopic Fractionation of
Mg2+(aq), Ca2+(aq), and Fe2+(aq) with Carbonate Minerals

August 25, 2010

Abstract Density functional electronic structure calculations are used to compute the equilibrium constant (the isotope fractionation factor) for 26Mg/24Mg and 44Ca/40Ca isotope exchange between carbonate minerals and uncomplexed divalent aquo ions. The most reliable calculations at the B3LYP/6-311++G(2d,2p) level predict equilibrium constants K, reported as 103ln(K) at 25 deg. C, of -5.3, -1.1, and +1.2 for 26Mg/24Mg exchange between calcite (CaCO3), magnesite (MgCO3), and dolomite (Ca0.5Mg0.5CO3), respectively, and Mg2+(aq), with positive values indicating enrichment of the heavy isotope in the mineral phase. For 44Ca/40Ca exchange between calcite and Ca2+(aq) at 25 deg. C, the calculations predict values of +1.5 for Ca2+(aq) in six-fold coordination and +4.1 for Ca2+(aq) in seven-fold coordination. We find that the reduced partition function ratios can be reliably computed from systems as small as M(CO3)610- and M(H2O)62+ embedded in a set of fixed atoms representing the 2nd shell (and greater) coordination environment. We find that the aqueous cluster representing the aquo ion is much more sensitive to improvements in the basis set than the calculations on the mineral systems, and that fractionation factors should be computed using the best possible basis set for the aquo complex, even if the reduced partition function ratio calculated with the same basis set is not available for the mineral system. The new calculations show that the previous discrepancies between theory and experiment for Fe3+-hematite and Fe2+-siderite fractionations arise from an insufficiently accurate reduced partition function ratio for the Fe3+(aq) and Fe2+(aq) species.

Rustad, J.R., Casey, W. H., Yin, Q.-Z., Bylaska, E. J., Felmy, A. R., Bogatko, S. A., Jackson, V. E., Dixon, D. A. Isotopic Fractionation of Mg2+(aq), Ca2+(aq), and Fe2+(aq) with Carbonate Minerals Geochim. Cosmochim. Acta, 2010, 47(22), 6301-6323

178. The first peroxotitanoniobate cluster
- [N(CH3)4]10[Ti12Nb6O38(O2)6]

June 10, 2010

Abstract Not available.








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

177. 99Tc-MAS-NMR on a Cationic Framework
Material for Trapping TcO4-

May 26, 2010

Abstract Not available.








Ping, Y., Wang, S., Alekseev, E.V., Depmeier, W., Hobbs, D.T., Albrecht- Schmitt, T. E., Phillips, B. L., Casey, W. H. 99Technetium 99 MAS NMR on a cationic framework material that traps TcO4- ions Angew. Chem. Int. Ed., 2010, 49(34), 5975-5977.

176. Borate Accelerates Oxygen-Isotope Exchange
for Polyoxoniobate Ions in Water

May 26, 2010

Abstract 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. Link

175. How the overlapping time scales for
peptide binding and terrace exposure lead to nonlinear step dynamics during growth of calcium oxalate monohydrate

May 21, 2010

Abstract Using in-situ atomic force microscopy (AFM), we investigate the inhibition of calcium oxalate monohydrate (COM) step growth by aspartic acid-rich peptides and find that the magnitude of the effect depends on terrace lifetime. We then derive a time-dependent step-pinning model in which average impurity spacing depends on the terrace lifetime as given by the ratio of step spacing to step speed. We show that the measured variation in step speed is well fit by the model and allows us to extract the characteristic peptide adsorption time. The model also predicts that a crossover in the time scales for impurity adsorption and terrace exposure leads to bistable growth dynamics described mathematically by a catastrophe. We observe this behaviour experimentally both through the sudden drop in step speed to zero upon decrease of supersaturation and through fluctutations in step speed between the two limiting values at the point where the catastrophe occurs. We discuss the model's general applicability to macromolecular modifiers and biomineral phases.

Weaver, M. L., Qiu, S. R., Friddle, R. W., Casey, W. H., De Yoreo, J. J. How the overlapping time scales for peptide binding and terrace exposure lead to nonlinear step dynamics during growth of calcium oxalate monohydrate Crystal growth & design , 2010, 10(7), 2954–2959 . Link

174. EPR evidence for Co(IV) species produced
during water oxidation at neutral pH

April 21, 2010

Abstract Thin film water oxidation catalysts (Co-Pi) prepared by electrodeposition from phosphate electrolyte and Co(II) nitrate have been characterized by electron paramagnetic resonance (EPR) spectroscopy. Co-Pi catalyst films exhibit EPR signals corresponding to populations of both Co(II) and Co(IV). As the deposition voltage is increased into the region where water oxidation prevails, the population of Co(IV) rises and the population of Co(II) decreases. The changes in the redox speciation of the film can also be induced, in part, by prolonged water oxidation catalysis in the absence of additional catalyst deposition. These results provide spectroscopic evidence for the formation of Co(IV) species during water oxidation catalysis at neutral pH.

McAlpin, J. G., Surendranath, Y., Dinca, M., Stich, T., Stoian, S., Casey, W. H., Nocera, D., Britt, R. D. EPR Evidence for Co(IV) Species Produced During Water Oxidation at Neutral pH J. Am. Chem. Soc., 2010,132(20), 6882-6883.

173. Oxygen-isotope exchange rates for three
isostructural polyoxometalate ions

March 8, 2010

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 [HxNb10O28 ](6-x)-, [HxTiNb9O28](7-x)-, and [HxTi2 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 [HxTi2 Nb8O28](8-x)- ion, but the base-enhanced pathways are increasingly important for the [HxTiNb9O28](7-x)- and [HxNb10O28](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 [HxTiNb9O28](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 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.

172. NDTB-1: A supertetrahedral cationic
framework that removes TcO4- from solution

March 7, 2010

Abstract A cubic thorium borate possesses a porous supertetrahedral cationic framework with extraframework borate anions. These anions are readily exchanged with a variety of environmental contaminants (see picture), especially those from the nuclear industry, including chromate and pertechnetate.

Wang, S.; Alekseev, V.; Diwu, J.; Casey, W.H.; Phillips, B.L.; Depmeier, W.; Albrecht-Schmitt, T.E "NDTB-1: A supertetrahedral cationic framework that removes TcO4- from solution ", Angew. Chem. Int. Ed, 2010, 49(6), 1057-1060.

171. Porous Capsules
{(M)M5}12FeIII30 (M = MoVI, WVI): Sphere Surface Supramolecular Chemistry with 20 Ammonium Ions, Related Solution Properties, and Tuning of Magnetic Exchange Interactions

March 7, 2010

Abstract A manifold of hydrogen-bonding options on a highly active, functionalized capsule surface exhibiting 20 crown ether type pores allows the fixation and recognition of 20 ammonium cations (see picture; W green, Fe orange, O red, N blue, H light gray), which are partially released in solution, thereby leading to related equilibria.

Todea, A.M.; Merca, A.; Bögge, H.; Glaser, T.; Pigga, J.M.; Langston, M.L.K.; Liu, T; Prozorov, R.; Luban, M; Schröder, C.; Casey, W.H.; Müller, A. "Porous Capsules {(M)M5}12 FeIII30(M = MoVI, WVI): Sphere Surface Supramolecular Chemistry with 20 Ammonium Ions, Related Solution Properties, and Tuning of Magnetic Exchange Interactions", Angew. Chem. Int. Ed., 2010, 49(3), 514-519.

170. Dissolution of insulating oxide materials at
the molecular scale

March 7, 2010

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

169. Sub-nm AFM imaging of peptide-mineral
interactions: How clustering and competition lead to acceleration, hysteresis and catastrophe

March 7, 2010

Abstract In vitro observations have revealed major effects on the structure, growth, and composition of biomineral phases, including stabilization of amorphous precursors, acceleration and inhibition of kinetics, and alteration of impurity signatures. However, deciphering the mechanistic sources of these effects has been problematic due to a lack of tools to resolve molecular structures on mineral surfaces during growth. Here we report atomic force microscopy investigations using a system designed to maximize resolution while minimizing contact force. By imaging the growth of calcium-oxalate monohydrate under the influence of aspartic-rich peptides at single-molecule resolution, we reveal how the unique interactions of polypeptides with mineral surfaces lead to acceleration, inhibition, and switching of growth between two distinct states. Interaction with the positively charged face of calcium-oxalate monohydrate leads to formation of a peptide film, but the slow adsorption kinetics and gradual relaxation to a well-bound state result in time-dependent effects. These include a positive feedback between peptide adsorption and step inhibition described by a mathematical catastrophe that results in growth hysteresis, characterized by rapid switching from fast to near-zero growth rates for very small reductions in supersaturation. Interactions with the negatively charged face result in formation of peptide clusters that impede step advancement. The result is a competition between accelerated solute attachment and inhibition due to blocking of the steps by the clusters. The findings have implications for control of pathological mineralization and suggest artificial strategies for directing crystallization.

Friddle, R.W.; Weaver, M.L.; Qiu, S.R; Wierzbicki, A.; Casey, W.H.; De Yoreo, J.J. "Sub-nm AFM imaging of peptide-mineral interactions: How clustering and competition lead to acceleration, hysteresis and catastrophe", Proc. Nat. Acad. Sci., Accepted.


168. Influence of explicit hydration waters in
calculating the hydrolysis constants for geochemically relevant metals

March 7, 2010

Abstract The effect of including a second, explicit solvation shell of water molecules is examined on the DFT calculation of a selection of aquo-metal ions’ pKa values (deprotonation constants). Our goal is to gauge the accuracy of how certain cluster approximations and implicit solvation models (PCM or COSMO) affect the results. The ions in this study include: Al3+(aq); Fe3+(aq); Cr3+(aq); Mn3+(aq); Co3+(aq); Ga3+(aq); Fe2+(aq); Mg2+(aq); and Be2+(aq). Overall, we find that experimental pKa constants can be calculated successfully to within 1−2 pH units, provided a consistent hydration structure is maintained for products and reactants.

Wander, M.C.F.; Rustad, J.R. ; Casey, W.H. "Influence of explicit hydration waters in calculating the hydrolysis constants for geochemically relevant metals ", J. Phys Chem. A, 2010 , 114(4), 1917-1925.

About our Research

The Casey laboratory specialises broadly in aqueous chemistry related to the environmental and geological sciences. More ...