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- Panasci, Adele F., et al.
(författare)
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Cooperation between bound waters and hydroxyls in controlling isotope-exchange rates
- 2012
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Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037 .- 1872-9533. ; 78, s. 18-27
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Tidskriftsartikel (refereegranskat)abstract
- Mineral oxides differ from aqueous ions in that the bound water molecules are usually attached to different metal centers, or vicinal, and thus separated from one another. In contrast, for most monomeric ions used to establish kinetic reactivity trends, such as octahedral aquo ions (e.g., Al(H(2)O)(6)(3+)), the bound waters are closely packed, or geminal. Because of this structural difference, the existing literature about ligand substitution in monomer ions may be a poor guide to the reactions of geochemical interest. To understand how coordination of the reactive functional groups might affect the rates of simple water-exchange reactions, we synthesized two structurally similar Rh(III) complexes, [Rh(phen)(2)(H(2)O)(2)](3+) [1] and [Rh(phen)(2)(H(2)O)Cl](2+) [2] where (phen) = 1,10-phenanthroline. Complex [1] has two adjacent, geminal, bound waters in the inner-coordination sphere and [2] has a single bound water adjacent to a bound chloride ion. We employed Rh(III) as a trivalent metal rather than a more geochemically relevant metal like Fe(III) or Al(III) to slow the rate of reaction, which makes possible measurement of the rates of isotopic substitution by simple mass spectrometry. We prepared isotopically pure versions of the molecules, dissolved them into isotopically dissimilar water, and measured the rates of exchange from the extents of (18)O and (16)O exchange at the bound waters. The pH dependency of rates differ enormously between the two complexes. Pseudo-first-order rate coefficients at 298 K for water exchanges from the fully protonated molecules are close: k(0)(298) = 5 x 10(-8) (+/- 0.5 x 10(-8)) s(-1) for [1] and k(0)(298) = 2.5 x 10(-9)(+/- 1 x 10(-9)) for [2]. Enthalpy and entropy activation parameters (Delta H(double dagger) and Delta S(double dagger)) were measured to be 119(+/- 3) kJ mol(-1), and 14(+/- 1) J mol(-1) K(-1), respectively for [1]. The corresponding parameters for the mono-aquo complex, [2], are 132(+/- 3) kJ mol(-1) and 41.5(+/- 2) J mol(-1) K(-1). Rates increase by many orders of magnitude upon deprotonation of one of the bound waters in complex [1] because of the close proximity of a transferable proton that can convert the bound hydroxyl to a bound water. This interconversion allows the oxygen to exchange as a bound water, rather than as a bound hydroxyl, which is slow at near-neutral pH conditions. (C) 2011 Elsevier Ltd. All rights reserved.
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- Panasci, Adele F., et al.
(författare)
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Rates of Water Exchange on the [Fe-4(OH)(2)(hpdta)(2)(H2O)(4)](0) Molecule and Its Implications for Geochemistry
- 2012
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 51:12, s. 6731-6738
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Tidskriftsartikel (refereegranskat)abstract
- The ammonium salt of [Fe4O(OH)(hpdta)(2)(H2O)(4)](-) is soluble and makes a monospecific solution of [Fe-4(OH)(2)(hpdta)(2)(H2O)(4)](0)(aq) in acidic solutions (hpdta = 2-hydroxypropane-1,3-diamino-N,N,N’,W-tetraacetate). This tetramer is a diprotic acid with pK(a1) estimated at 5.7 +/- 0.2 and pK(a2) = 8.8(5) +/- 0.2. In the pH region below pK(a1), the molecule is stable in solution and O-17 NMR line widths can be interpreted using the Swift-Connick equations to acquire rates of ligand substitution at the four isolated bound water sites. Averaging five measurements at pH < 5, where contribution from the less-reactive conjugate base are minimal, we estimate: k(ex)(298) = 8.1 (+/- 2.6) X 10(5) s(-1), Delta H-double dagger = 46 (+/- 4.6) kJ mol(-1), Delta S-double dagger = 22 (+/- 18) J mol(-1) K-1, and Delta V-double dagger = +1.85 (+/- 0.2) cm(3) mol(-1) for waters bound to the fully protonated, neutral molecule. Regressing the experimental rate coefficients versus 1/[H+] to account for the small pH variation in rate yields a similar value of k(ex)(298) = 8.3 (+/- 0.8) X 10(5) s(-1). These rates are similar to 10(4) times faster than those of the [Fe(OH2)(6)](3+) ion (k(ex)(298) = 1.6 X 10(2) s(-1)) but are about an order of magnitude slower than other studied aminocarboxylate complexes, although these complexes have seven-coordinated Fe(III), not six as in the [Fe-4(OH)(2)(hpdta)(2)(H2O)(4)](0)(aq) molecule. As pH approaches pK(a1), the rates decrease and a compensatory relation is evident between the experimental Delta H-double dagger and Delta S-double dagger values. Such variation cannot be caused by enthalpy from the deprotonation reaction and is not well understood. A correlation between < Fe-III-OH2 > bond lengths and the logarithm of k(ex)(298) is geochemically important because it could be used to estimate rate coefficients for geochemical materials for which only DFT calculations are possible. This molecule is the only neutral, oxo-bridged Fe(III) multimer for which rate data are available.
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