Abstract How do solids react with water? When some minerals and glasses dissolve, ion exchange and hydrolysis remove the labile metals and leave a less-reactive substructure near the surface. This substructure, or leached layer, can be thin or extend to hundreds of Ångströms.
Casey, W. H. Glass and Mineral corrosion: Dynamics and durability, Nature materials, 2008,7(12), 930-932.
Abstract Peroxoniobate species are found whenever niobium oxides are employed as photocatalysts or used to sequester radionuclides. The reactions of [Nb6O19]8- and [Nb 10O28]6- with hydrogen peroxide each behave differently, and the first example of a peroxopolyoxoniobate species, [N(CH3)4]5[H3Nb6O13 (O(I)2)6] . 9.5 H2O, was structurally characterized (see picture; Nb gray, O red, H white).
Ohlin, C.A., Villa, E.M., Fettinger, J. C., Casey, W. H. Distinctly different reactivities of two similar polyoxoniobates with hydrogen peroxide, Angew. Chem. Int. Ed., 2008,47(43), 8251-8254.
Abstract We examine oxygen-isotope exchanges in a nanometer-size oxide molecule in water and, separately, both its rates of dissociation and molecular products. This molecule, the decaniobate ion ([HxNb10O28](6-x)-), is at the same size scale as geochemically interesting features on minerals, such as surface polymers and kink sites on growth steps, although it is structurally quite dissimilar. Unlike mineral surface structures, however, we have complete confidence in the aqueous structure of this molecule and it yields a clear spectroscopic signature as it reacts. We thus can follow proton-enhanced isotope exchanges and base-induced dissociation in unprecedented detail and clarity. The results are surprising and require new thinking about geochemical reactions at the molecular scale. For example, base-induced dissociation of the molecule, which is unprotonated, causes rates of oxygen-isotope exchanges of all structural oxygens to accelerate dramatically. Similarly, protonation of the molecule causes sets of oxygens to react, although protonation is limited. In general, all reactions are via concerted motions of many atoms and the reactivities vary as though the entire structure was responding to changes in solution composition. The site reactivities could not be inferred from the stable structure of the decaniobate molecule because so much of the structure is involved in each exchange event. Thus, computational models must be structurally faithful to an extraordinary degree, and inherently dynamic, or they will miss the essential chemistry.
Villa,E.M., Ohlin, C. A., Balogh, E., Anderson, T. M., Nyman, M. D., Casey, W. H. Adding reactivity to structure - reaction dynamics in a nanometer-size oxide ion in water, Am. J. Sci.,2008, 308, 942-953.Link
Abstract Magnesium is an essential nutrient, which activates more enzymes than any other mineral element and, thus, plays an important role in biogeochemical cycles. With three stable isotopes naturally abundant (24Mg, 78.992%; 25Mg, 10.003%; 26Mg, 11.005%), magnesium stable isotope fractionation may provide insights into these cycles. Here, we detail for the first time the magnesium stable -isotope distribution in a higher plant, wheat (Triticum aestivum L.), during its growth cycle. Wheat plants were grown in a limiting nutrient supply hydroponically, some being left to mature through senescence and others detopped at maturity for collection of exudates. Measurements of the magnesium isotopic composition of chlorophylls, seeds, shoots, roots, leaves, exudates, and the limiting nutrient solution over time show that the plant appears to establish an isotopic equilibrium with the nutrient available to it and that the plant (in particular, the seeds and exudates) becomes enriched in the heavy isotopes of magnesium in a mass-dependent relationship as the plant reaches maturity. The preference of the plants for heavy magnesium isotopes suggests that a difference might exist in the bioavailable magnesium of agricultural and natural soils due to the periodic removal of heavy magnesium isotopes by harvest.
Black, J. R., Epstein, E., Rains, W. D., Yin, Q-Z, Casey, W. H. Magnesium-isotope fractionation during plant growth: implications for the terrestial magnesium cycle, Environ. Sci. Techn. 2008, 42(21), 7831-7836.
Abstract An extraordinarily useful cluster to detail reaction pathways in water might have been obtained in form of [N(CH3)4]10[Ti12Nb6O44], a polyoxometalate with a central cavity. This compound is soluble in a range of solvents and has been characterized by ESI mass spectrometry and X-ray crystallography (see picture).
Ohlin, C.A. , Villa, E.M., Fettinger, J. C., Casey, W. H. The [Ti12Nb6O44 ]-10 ion - A new type of polyoxometalate structure, Angew. Chem. Int. Ed. 2008 , 47(30), 5634-5646.
Abstract The nanometer-size title cluster is unique in that it reacts slowly enough in water that one can simultaneously observe steady-state oxygen-isotope exchanges and dissociation pathways (see scheme; O red, Nb green), leading to conceptual advances in both geochemistry and polyoxometalate chemistry.
Villa,E. M.,Ohlin, C. A., Balogh,E., Anderson, T. M., Nyman, M. D., Casey, W. H. Reaction dynamics of the decaniobate ion [HxNb10O28](6-x)- in water, Angew. Chem. Int. Ed., 2008, 47(26), 4844-4846.
Abstract Not available.
Balogh, E., Casey, W.H. High-pressure 17O NMR studies on some aqueous polyoxoions in water, Progress Nuclear Magn. Res. Spectr. 2008, 53, 193-207.
Abstract We have simulated exchange of inner-sphere and bulk water molecules for different sizes of Al3+(aq) clusters, Al(H2O)63+ + nH2O for n = 0, 1, 6, or 12, with ab initio and molecular dynamics simulations, in order to understand how robust the ab initio method is for identifying hydrolytic reaction pathways of particular importance to geochemistry. In contrast to many interfacial reactions, this particular elementary reaction is particularly simple and well-constrained by experiment. Nevertheless, we find that a rich array of parallel reaction pathways depend sensitively on the details of the solvation sphere and structure and that larger clusters are not necessarily better. Inner-sphere water exchange in Al3+(aq) may occur through two Langford−Gray dissociative pathways, one in which the incoming and outgoing waters are cis, the other in which they are trans to one another. A large majority of exchanges in the molecular dynamics simulations occurred via the trans mechanism, in contrast to the predictions of the ab initio method. In Al(H2O)63+ + H2O, the cis mechanism has a transition state of 84.3 kJ/mol, which is in good agreement with previous experimental and ab initio results, while the trans mechanism has only a saddle point with two negative frequencies, not a transition state, at 89.7 kJ/mol. In addition to the exchange mechanisms, dissociation pathways could be identified that were considerably lower in energy than experiment and varied considerably between 60 and 100 kJ/mol, depending on the particular geometry and cluster size, with no clear relation between the two. Ab initio calculations using large clusters with full second coordination spheres (n = 12) were unable to find dissociation or exchange transition states because the network of hydrogen bonds in the second coordination sphere was too rigid to accommodate the outgoing inner -sphere water. Our results indicate that caution should surround ab initio simulation of complicated dynamic processes such as hydrolysis, ion exchange, and interfacial reactions that involve several steps. Dynamic methods of simulation need to accompany static methods such as ab initio calculation, and it is best to consider simulated pathways as hypotheses to be tested experimentally rather than definitive properties of the reaction.
Evans, J.R., Rustad, J.R., Casey, W.H. Calculating geochemical reaction pathways - Exploration of the inner -sphere water exchange mechanism in Al(H2O)63+(aq) + n H2O with ab initio calculations and molecular dynamics. J. Phys. Chem. A2008, 112(17), 4125-4140.
The Casey laboratory specialises broadly in aqueous chemistry related to the environmental and geological sciences. More ...