Rates of ligand exchange between >FeIII-OH2 functional groups on a nanometer-sized aqueous cluster and bulk solution

Variable-temperature 17O NMR experiments were conducted on the nanometer-sized Keplerate Mo72Fe30 cluster, with the stoichiometry [Mo72Fe30O252(CH3COO)12[Mo2O7(H2O)]2[H2Mo2O8(H2O)](H2O)91]. approximately 150H2O. This molecule contains on its surface 30 Fe(H2O) groups forming a well-defined icosidodecahedron, and we estimated the rates of exchange of the isolated >FeIII-OH2 waters with bulk aqueous solution. Both longitudinal and transverse 17O-relaxation times were measured, as well as chemical shifts, and these parameters were then fit to the Swift-Connick equations in order to obtain the rate parameters. Correspondingly, we estimate: k(ex)298 = 6.7(+/-0.8) x 106 s-1, which is about a factor of approximately 4 x 104 times larger than the corresponding rate coefficient for the Fe(OH2)63+ ion of k(ex)298 = 1.6 x 102 s-1 (Grant and Jordan, 1981; Inorg. Chem. 20, 55-60) and DeltaH and DeltaS are 26.3 +/- 0.6 kJ mol-1 and -26 +/- 0.9 J mol-1 K-1, respectively. High-pressure 17O NMR experiments were also conducted, but the cluster decomposed slightly under pressure, which precluded confident quantitative estimation of the DeltaV. However, the increase in the reduced transverse-relaxation time with pressure suggests a dissociative character, such as a D or Id mechanism. The enhanced reactivity of waters on the Mo72Fe30 cluster is associated with an increase in the FeIII-OH2 bond length in the solid state of approximately 0.1 A relative to the Fe(OH2)63+ ion, suggesting that a correlation exists between the FeIII-OH2 bond length and k(ex)298. Although there are only few high-spin Fe(III) complexes where both exchange rates and structural data are available, these few seem to support a general correlation.     

Quadrature Formulas for the Wiener Measure

We present a new method for the approximation of Wiener integrals and provide an explicit error bound for a class of smooth integrands. The purely deterministic algorithm is a sequence of quadrature formulas for the Wiener measure, where the knots are piecewise linear functions. It uses ideas of Smolyak, as well as the multiscale decomposition of the Wiener measure due to Lévy and Ciesielski. For the class we obtain n()max(1, 2⁻⁴), where n() is the number of integrand evaluations needed to guarantee an error at most for f .     

Challenges in polypropylene by chemical modification

Radical reactions of i-PP are a well known technical process for the chemical degradation to increase the flowability of the i-PP melt. By decreasing the temperature and increasing the life time of the PP-radicals, a process to synthesize long chain branched i-PP, was developed. The long chain branched-i-PP allows to introduce the i-PP in processing technologies as blow moulding film technology or foaming technology. The mechanism will be discussed. The radical grafting of Polypropylene (PP) becomes more important for the developing of PP-alloys with extended properties. Methylmethacrylate and styrene were polymerized and grafted in PP at low temperatures in solid state. Grafting and polymerizing in solid state means solving the monomers in the PP-powder directly from the reactor without contacting with oxygen (air). The reaction is started by the thermal decomposition of a peroxide. The reactivity of the primary radicals from the peroxide and the transfer reactions of the polymer radical of the PMMA or PS influences the amount of the grafted polymer. The solubility of the monomers and the peroxide in the amorphous i-PP-phase was measured. The grafting yield and the dispersity of the second polymer depends on the solubility and dispersity of peroxide and monomers in the PP-powder particles.     

The effect of MgO on the thermodynamics of the Fe/Fe-redox equilibrium and the incorporation of iron in soda-magnesia-aluminosilicate melts

Melts with the basic compositions 10Na₂O · 10MgO · xAl₂O₃ · (80−x)SiO₂ (x=0, 5, 10, 15 and 20), 10Na₂O · xMgO · 10Al₂O₃ · (80−x)SiO₂ (x=5, 10, 15 and 20) and xNa₂O · 10MgO · 10Al₂O₃ · (80−x)SiO₂ (x=5, 10 and 15) all doped with 0.25 mol% Fe₂O₃ were studied using square-wave voltammetry. The temperatures applied were in the range of 1000–1600 °C. The square-wave voltammograms recorded show peaks caused by the reduction of Fe³⁺ to Fe²⁺. The attributed peak potentials measured decreased linearly with decreasing temperatures. Increasing the MgO-concentration led to more negative peak potentials. Introducing alumina in the melt first resulted in less negative peak potentials. If the molar Al₂O₃-concentration is equal to that of Na₂O (=10 mol%) the peak potentials are least negative. Further increase of the Al₂O₃-concentration led to more negative peak potentials. The variation of the Na₂O-concentration led to a maximum in the peak potentials at an Na₂O-concentration of 10 mol%. An empirical formula which allows the calculation of standard potentials from the chemical composition is proposed. Furthermore, a structural explanation for the effect of the chemical composition is given. Especially, the incorporation of Al₂O₃ as AlO₄⁻-tetrahedra at [Al₂O₃] < [Na₂O] and as network modifier at larger concentrations was structurally explained by the similarities of Fe²⁺ and Mg²⁺, with respect to cation radii and metal–oxygen bond lengths.