Abstract: Predicting the reactivity of actinide elements in nature is among the most pressing concerns in environmental geochemistry, yet even some basic measures of reactivity are not well known. Included among these are the mechanisms of ligand exchange in simple complexes. These data are important because the reaction dynamics can be compared to computer simulations to gain confidence for cases where experiments are impossible. Among the key parameters used to describe ligand-exchange mechanisms are activation volumes, which derive from the pressure dependence of the reaction rates. These activation volumes are interpreted to indicate the extents to which the incoming ligand can influence the activated state. In this sense, the UO₂(CO₃)₃^4-(aq) ion is a particularly compelling system because rates of carbonate exchange are apparently independent of free carbonate concentrations.

Johnson, Rene L.;[1] Harley, Stephen J.;[1,2] Ohlin, C. André;[1,3] Panasci, Adele F.;[1] Casey, William H. [1] Multinuclear NMR Study of the Pressure Dependence for Carbonate Exchange in the UO₂(CO₃)₃^4-(aq) Ion Chem. Phys. Chem. 2011, 12(16), 2903-2906. Link.

1. Department of Chemistry, and Department of Geology, University of California, Davis, CA.
2. Energetic Materials Division, Lawrence Livermore National Lab., Livermore, CA
3. School of Chemistry, Monash University, Vic 3800, Australia.

Abstract: Multifrequency EPR spectroscopy and electronic structure calculations were performed on [Co₄O₄(C₅H₅N)₄(CH₃CO₂)₄]⁺ (1+), a cobalt tetramer with total electron spin S = 1/2 and formal cobalt oxidation states III, III, III, and IV. The cuboidal arrangement of its cobalt and oxygen atoms is similar to that of proposed structures for the molecular cobaltate clusters of the Co-Pi water-oxidizing catalyst. The Davies ENDOR spectrum is well-modeled using a single class of hyperfine-coupled ⁵⁹Co nuclei with a modestly strong interaction (principal elements of the hyperfine tensor are equal to [-20(±2), 77(±1), -5(±15)] MHz). Mims ¹H ENDOR spectra of 1+ with selectively deuterated pyridine ligands confirm that the amount of unpaired spin on the cobalt-bonding partner is significantly reduced from unity. Multifrequency ¹⁴N ESEEM spectra (acquired at 9.5 and 34.0 GHz) indicate that four nearly equivalent nitrogen nuclei are coupled to the electron spin. Cumulatively, our EPR spectroscopic findings indicate that the unpaired spin is delocalized almost equally across the eight core atoms, a finding corroborated by results from DFT calculations. Each octahedrally coordinated cobalt ion is forced into a low-spin electron configuration by the anionic oxo and carboxylato ligands and a fractional electron hole is localized on each metal center in a Co 3dxz,yz-based molecular orbital. Mixed-valence theory was employed to calculate ligand field parameters for this essentially [Co^+3.125₄O₄] system. Comparing the EPR spectrum of 1+ with that of the catalyst film allows us to draw conclusions about the electronic structure of this water-oxidation catalyst.

McAlpin, J. Gregory;[1] Stich, Troy A.;[1] Ohlin, C. André;[1,2,3] Surendranath, Yogesh;[4] Nocera, Daniel;[4] Casey, William H.;[1,2] Britt, R. David [1] Electronic Structure Description of a [Co(III)₃Co(IV)O₄] Cluster: A Model for the Paramagnetic Intermediate in Cobalt-catalyzed Water Oxidation J. Am. Chem. Soc. , 2011, 133(39), 15444-15452. Link.

1. Department of Chemistry, University of California, Davis, CA.
2. Department of Geology, University of California, Davis, CA.
3. School of Chemistry, Monash University, Vic 3800, Australia.
4. Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts.

Abstract: Signal analysis in Nuclear Magnetic Resonance spectroscopy is among the most powerful methods to quantify reaction rates in aqueous solutions. To this end, the Swift- Connick approximations to the Bloch-McConnell equations have been used extensively to estimate rate parameters for elementary reactions. The method is primarily used for ¹⁷O-NMR in aqueous solutions, but the list of geochemically relevant nuclei that can be used is long, and includes ²⁹Si, ²⁷Al, ¹⁹F, ¹³C and many others of particular interest to geochemists. Here we review the derivation of both the Swift-Connick and Bloch-McConnell equations and emphasize assumptions and quirks. For example, the equations were derived for CW-NMR, but are used with modern pulse FT-NMR and can be applied to systems that have exchange rates that are shorter than the lifetime of a typical pulse. The method requires a dilute solution where the minor reacting species contributes a neglible amount of total magnetization. We evaluate the sensitivity of results to this dilute-solution requirement and also highlight the need for chemically well- defined systems if reliable data are to be obtained. The limitations in using longitudinal relaxation to estimate reaction rate parameters are discussed. Finally, we provide examples of the application of the method, including ligand exchanges from aqua ions and hydrolysis complexes, that emphasize its flexibility. Once the basic requirements of the Swift-Connick method are met, it allows geochemists to establish rates of elementary reactions. Reactions at this scale lend themselves well to methods of computational simulation and could provide key tests of accuracy.

Harley, Stephen J.; Ohlin, C. André; Casey, William H. Geochemical kinetics via the Swift-Connick equations and solution NMR Geochim. Cosmochim. Acta 2011, 75(13), 3711-3752. Link

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

Abstract: Rates of oxygen-isotope exchange were measured in a tetrasiliconiobate ion in order to better understand how large oxide ions interact with water. The molecule has 19 non-equivalent oxygen sites and is sufficiently complex to evaluate hypotheses derived from our previous work on smaller clusters. We want to examine the extent to which individual oxygens react independently with particular attention given to the order of protonation of the various oxygen sites as the pH decreases from 13 to 6. As in our previous work, we find that the set of oxygen sites reacts at rates that vary over ca 10⁴ across the molecule between pH 6 and 13 but with similar pH dependencies. There is NMR evidence of an intra- or intermolecular reaction at pH 7 where new peaks began to slowly form without losing the ¹⁷O isotopic tag and at pH less-or-equal-to 6 these new peaks formed rapidly. The oxygens bonded to silicon atoms began to isotopically exchange at pH 9 and below. The ¹⁷O NMR peak positions also vary considerably with pH for some, but not all, non-equivalent oxygen sites. This variation could be only partly accounted by electronic calculations, which indicate that oxygens should shift similarly upon protonation. Instead, we see that some sites change enormously with pH while other, similarly coordinated oxygens are less affected, suggesting that either some protons are exchanging so rapidly that the oxygen sites are seeing an averaged charge, or that counterions are modulating the ffect of the coordinated protons.

Johnson, Rene L.;[1] Villa, Eric M;[1,2] Ohlin, C. André;[1,3] Rustad, James R.;[3,4] Casey, William H.[1,3] ¹⁷O NMR and computational study of a tetrasiliconiobate ion, [H_2+xSi₄Nb₁₆O₅₆]^(14-x)- Chem. Eur. J. , 2011, 17(34), 9359-9367. Link

1. Department of Chemistry, University of California, Davis, CA
2. Department of Geological Chemistry, Notre Dame University, South Bend, IN
3. Department of Geology, University of California, Davis.
4. Science and Technology Division, Corning Inc., Corning, NY

Abstract: N/A

Harley, S. J., Ohlin, C. A., Johnson, R. L., Panasci, A., Casey W. H. The pressure dependence of oxygen-isotope-exchange rates between solution and apical oxygens on the UO₂(OH)₄^2- ion. Angew. Chem. Int. Ed. , 2011, 50(19), 4467-4469. Link

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

Abstract: Polyoxometalate ions are being used as ligands in water-oxidation processes related to solar energy production. An important step in these reactions must be the association and dissociation of water from the catalytic sites, the rates of which are unknown. Here we report the exchange rates of water ligated to Co(II) atoms in two polyoxotungstate sandwich molecules using the ¹⁷O-NMR-based Swift-Connick method. The compounds were the [Co₄(H₂O)₂(B-a-PW₉O₃₄)₂]^10- and the larger abba−[Co₄(H₂O)₂(P₂W₁₅O₅₆)₂]^16- ions, each with two waters boundtrans to one another in a Co(II) sandwich between the tungstate ligands. The clusters, in both solid and solution state, were characterized by a range of methods, including NMR, ESI-MS, EXAFS, EPR, FT-IR, UV-Vis and potentiometry. For [Co₄(H₂O)₂(B-a-PW₉O₃₄)₂]^10- at pH=5.4, we estimate: k₂₉₈ = 1.55+/-0.3^.10⁶ s^-1, ΔH = 39.8+/-0.4 kJ mol^-1, ΔS = +7.1+/-1.2 J mol^-1 K^-1 and ΔV = 5.6 +/- 1.6 cm^3.mol^-1. For the Wells-Dawson sandwich cluster (abba−[Co₄(H₂O)₂(P₂W₁₅O₅₆)₂]^16-) at pH=5.54, we find: k₂₉₈ = 1.62+/-0.3^.10⁶ s^-1, ΔH =27.6+/-0.4 kJ mol^-1 ΔS = -33+/-1.3 J mol^-1 K^-1 and ΔV = 2.2 +/- 1.4 cm^{3 .}mol^-1 at pH=5.2. The molecules are clearly stable and monospecific in slightly acidic solutions but dissociate in strongly acidic solutions. This dissociation is detectable via EPR spectroscopy as S=3/2 Co(II) species (such as as the [Co(H₂O)₆]²⁺ monomer ion) and by the significant reduction of the Co-Co vector in the XAS spectra.

C. A. Ohlin,[1,2] S. J. Harley,[1,2] J. G. McAlpin,[1,2] R. K. Hocking,[3] B. Q. Mercado,[1] R. Johnson,[1,2] E. M. Villa,[1,4] M. K. Fidler,[4] M. M. Olmstead,[1] L. Spiccia,[3] R. D. Britt,[1] and W. H. Casey[1,2] Rates of water exchange for two cobalt(II) heteropolyoxotungstate compounds in aqueous solution Chemistry - a European Journal , 2011, 17(16), 4408-4417. Link

1. Department of Chemistry, University of California, Davis, CA
2. Department of Geology, University of California, Davis.
3. School of Chemistry, Australian Centre of Excellence for Electromaterials Science and Monash Centre for Synchrotron Science, Monash University, Victoria
4. Present address: Department of Civil Engineering and Geosciences, University of Notre Dame, Notre Dame