Chapter published in Annual Reports on NMR Spectroscopy

17O NMR as a Tool in Discrete Metal Oxide Cluster Chemistry

in Annual Reports on NMR Spectroscopy. Link.

C. André Ohlin, William H. Casey

Abstract: This chapter covers recent developments in 17O NMR spectroscopy as applied to discrete metal oxide clusters, particularly in the context of their use as models in geochemistry and catalysis. Dynamic 17O NMR methods based on the McConnell–Bloch equations are explored in depth, and recent advances are reviewed. High-pressure NMR methods are also discussed and reviewed, as are recent developments in the use of density functional theory in the computation of 17O NMR shifts in polyoxometalates. The emphasis of the chapter is on the new developments that promise to reinvigorate 17O NMR as a central tool in the study of aqueous chemical kinetics, with the most urgent challenges being understanding the rates of isotopic substitution into bridging oxygens in clusters.

Paper on fractionation accepted by Geochimica et Cosmochimica Acta

Computational Prediction of Mg-Isotope Fractionation Between Aqueous [Mg(OH2)6]2+ and Brucite

Geochim. Cosmochim. Acta., Accepted. Link.

Christopher A. Colla, W. H. Casey, C. André Ohlin

Abstract: The fractionation factor in the magnesium-isotope fractionation between aqueous solutions of magnesium and brucite remarkably changes sign with increasing temperature, as uncovered by recent experiments.  To understand this behavior, the Reduced Partition Function Ratios and isotopic fractionation factors (Δ26/24Mgbrucite-Mg(aq)) are calculated using molecular models of aqueous [Mg(OH2)6]2+ and the mineral brucite at increasing levels of density functional theory.  The calculations were carried out on the [Mg(OH2)6]2+·12H2O cluster, along with different Pauling-bond-strength-conserving models of the mineral lattice of brucite. Three conclusions were reached: i) all of the calculations overestimate <Mg-O> bond distances in the aqua ion complex relative to Tutton’s salts; ii) the calculations predict that brucite at 298.15 K is always enriched in the heavy isotope, in contrast with experimental observations; iii) the temperature dependencies of Wimpenny et al. (2014) and Li et al. (2014) could only be achieved by fixing the <Mg-O> bond distances in the [Mg(OH2)6]2+·12H2O cluster to values close to those observed in crystals that trap the hydrated ion. Read more.