Casey Laboratory

Abstract The chem. of polyoxometalate ions is richly explored, but mostly via synthesis and exptl. studies. Less emphasis has been placed on developing a robust computational understanding of their aq. dynamics. In this work, we utilize both a previously created force-field model and ab initio mol. dynamics to explore the solvation structure of [Nb10O28]aq 6-. We examine characteristic behaviors of both cluster-water interactions and intra-cluster bond strengths. We show that cluster-water interactions are dictated by electrostatic interactions, which in turn are dictated by the shape of the cluster; ultimately reflecting a quantitatification of the steric shielding to the cluster's oxygen sites. We also show that bond strengths within the cluster do not correlate with oxygen reactivities, lending credence to previous suggestions that oxygen exchange for decaniobate clusters occurs via intermediates that form by significant structural reorganizations.

Hanken, Benjamin E.; Asta, Mark; Casey, William H. Computational characterization of the internal bonding and solvation structure for [Nb10O28]6- Physical Chemistry Chemical Physics, 2013.15(48), 20929-20936.

Abstract The aq.-mineral D44/40Casolid-solution fractionation factors were measured for equilibrating solns. and a set of inorg. solids that trap solvated calcium in six, seven and eight coordination as an intact solvated ion in the lattice. The measured values were then compared to D44/40Casolid-solution values predicted from electronic-structure calcns. and vibrational frequencies of mol. models of the solvated ions and their solid hydrates. Exptl., the D44/40Casolid-solution values differed by ~2‰ for crystals of six- and seven-hydrated calcium, and by about ~2‰ between crystals having seven- and eighthydrated calcium. In comparison, the calcns. predict D44/40Casolid-solution values between the sixfold- and eightfoldcoordinated aquo ions of 5.4‰. Calcns. predict 2.6‰ fractionation between the sixfold- and sevenfold-coordinated aquo ions. Stirred and unstirred expts. gave similar results in most, but not all cases. In general, measured isotopic fractionations compare well with the predictions from quantum mechanics and vibrational anal. Isotopic fractionation is directly correlated with coordination no. We consider this to be a well-poised test of predictions.

Colla, Christopher A.; Wimpenny, Joshua; Yin, Qing-Zhu; Rustad, James R.; Casey, William H. Calcium-isotope fractionation between solution and solids with six, seven or eight oxygens bound to Ca(II) Geochimica et Cosmochimica Acta, 2013.121, 363-373.

Abstract The electrochem. water-oxidn. reaction usually requires a catalyst to reduce the overpotential and Earth-abundant catalysts, like MnO2, are attracting much attention. Here we use chemometric anal., EPR and UV-Vis spectroscopies to track Mn(II) and MnO4 - byproducts to the reaction of a MnO2 film in the presence of cerium(IV) ammonium nitrate. Permanganate ion is involved in at least two key reactions: it may oxidize water to O2 or can combine with Mn(II) to remake MnO2 solid. We propose mechanisms for water oxidn. and present a self-healing process for this reaction.

Najafpour, Mohammad Mahdi; Kompany-Zareh, Mohsen; Zahraei, Ali; Jafarian Sedigh, Davood; Jaccard, Hugues; Khoshkam, Maryam; Britt, R. David; Casey, William H.Mechanism, decomposition pathway and new evidence for self-healing of manganese oxides as efficient water oxidizing catalysts: new insights Dalton Transactions, 2013.42(40), 14603-14611.

Son, Jung-Ho; Casey, William H. A decatungstate-type polyoxoniobate with centered manganese: [H2MnIVNb10O32]8- as a soluble tetramethylammonium salt Dalton Transactions, 2013.42(37), 13339-13342.

Abstract The authors report high-frequency (up to 219 GHz) and correspondingly high-field ESR spectra and x-ray crystal structure of [Co4O4(pyridine)4(acetate)4]+ ([Co4O4(py)4(OAc)4]+) that serves as a structural and spectroscopic model of cobalt-oxide films that are capable of oxidizing water. These results are, in large part, consistent with those from the authors' earlier, lower-frequency study and serve to benchmark future high-field studies on paramagnetic states of the catalyst film.

Stich, Troy A.; Krzystek, J.; Mercado, Brandon Q.; McAlpin, J. Gregory; Ohlin, C. Andre; Olmstead, Marilyn M.; Casey, William H.; David Britt, R. Structural insights into [Co4O4(C5H5N)4(CH3CO2)4]+, a rare Co(IV)-containing cuboidal complex Polyhedron, 2013.64, 304-307.

Abstract N/A

Johnson, Rene L.; Ohlin, C. Andre; Pellegrini, Kristi; Burns, Peter C.; Casey, William H. Dynamics of a Nanometer-Sized Uranyl Cluster in Solution Angewandte Chemie, International Edition, 2013.52(29), 7464-7467.

Abstract Iron- and nickel-substituted decaniobates, TMA6[H2FeIIINb9O28]·14H2O (1) and TMA6[H3NiIINb9O28]·17H2O (2), were hydrothermally synthesized as tetramethylammonium salts and the structures were detd. by X-ray crystallog. The highly sol. title compds. were characterized by ESI-MS, FT-IR and UV-Vis titrn.

Son, Jung-Ho; Ohlin, C. Andre; Casey, William H. Highly Soluble Iron- and Nickel-Substituted Decaniobates with Tetramethylammonium Countercations Dalton Transactions, 2013.42(21), 7529-7533.

Abstract Previous measurements show that calcium manganese oxide nanoparticles are better water oxidn. catalysts than binary manganese oxides (Mn3O4, Mn2O3, and MnO2). The probable reasons for such enhancement involve a combination of factors: The calcium manganese oxide materials have a layered structure with considerable thermodn. stability and a high surface area, their low surface energy suggests relatively loose binding of H2O on the internal and external surfaces, and they possess mixed-valent manganese with internal oxidn. enthalpy independent of the Mn3+/Mn4+ ratio and much smaller in magnitude than the Mn2O3-MnO2 couple. These factors enhance catalytic ability by providing easy access for solutes and water to active sites and facile electron transfer between manganese in different oxidn. states.

Birkner, Nancy; Nayeri, Sara; Pashaei, Babak; Najafpour, Mohammad Mahdi; Casey, William H.; Navrotsky, Alexandra Energetic basis of catalytic activity of layered nanophase calcium manganese oxides for water oxidation Proceedings of the National Academy of Sciences, 2013.110(22), 8801-8806.

Abstract A water-sol. tetramethylammonium (TMA) salt of a novel Keggin-type polyoxoniobate was isolated as TMA9[PV2Nb12O42].19H2O (1). This species contains a central phosphorus site and two capping vanadyl sites. Previously only a single example of a phosphorus-contg. polyoxoniobate, [(PO2)3PNb9O34]15-, was known, which is a lacunary Keggin ion decorated with three PO2 units. However, that cluster was isolated as an insol. structure consisting of chains linked by sodium counterions. In contrast, the [PV2Nb12O42]9- cluster in 1 is stable over a wide pH range, as evident by 31P and 51V NMR, UV/visible spectroscopy, and ESI-MS spectrometry. The ease of substitution of phosphate into the central tetrahedral position suggests that other oxoanions can be similarly substituted, promising a richer set of structures in this class.

Son, Jung-Ho; Ohlin, C. Andre; Johnson, Rene L.; Yu, Ping; Casey, William H. A Soluble Phosphorus-Centered Keggin Polyoxoniobate with Bicapping Vanadyl Groups Chemistry - A European Journal, 2013, 19(16), 5191-5197.

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Casey, William H. Mineral oxides, dissolution of Encyclopedia of Surface and Colloid Science (2nd Edition), 2012, 3982-3991.