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  • Lanthanides  (1)
  • MRI  (1)
  • computer simulations  (1)
  • 1
    Digitale Medien
    Digitale Medien
    Mechanisms of Water Exchange between Lanthanide(III) Aqua Ions [Ln(H2O)n]3+ and bulk water: A Molecular Dynamics Simulation Approach Including High-Pressure Effects (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 285-294 
    Details
    ISSN: 0947-6539
    Schlagwort(e): computer simulations ; high-pressure chemistry ; lanthanide complexes ; ligand exchange ; mechanistic studies ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We studied the microscopic mechanisms of the water exchange reaction between the hydration shells of lanthanide(III) ions (Ln = Nd, Sm, Yb) and bulk water by means of molecular dynamics simulations. In contrast to the residence time of a water molecule in the first hydration shell (τres (1st shell) = 1577, 170 and 410 ps for Nd3+, Sm3+ and Yb3+, respectively), that in the second hydration shell is nearly independent of the type of the cation and amounts to 12-18 ps. Along the lanthanide series a change in the coordination number from 9 to 8 is coupled to a changeover in the water exchange mechanism. The observed water exchange events on the [Nd(H2O)9]3+ aqua ion follow a dissociatively activated Id mechanism via an eightfold-coordinated transition state of square antiprismatic geometry. The lifetime of the transitory square antiprism varies between virtually 0 and 10 ps. The assignment of an Id mechanism (instead of a limiting D mechanism) is supported by the existence of a preferential arrangement between the exchanging water molecules (1800) and by the fact that the calculated average activation volume ΔV≠ = + 4.5 cm3 mol-1 is clearly smaller than the estimated activation volume ΔV≠lim ≈ΔV0 = + 7.2 cm3 mol-1 for a limiting D process. In the case of Sm3+ a ninth water molecule exchanges frequently between the first hydration shell and the bulk and maintains the coordination equilibrium between a [Sm(H2O)8]3+ and a [Sm(H2O)9]3+ aqua ion. The resulting trajectory pattern of incoming and leaving water molecules is an alternation of elimination and addition reactions and cannot be classified into the scheme of D, I or A mechanisms for substitution processes. The reaction volume ΔV0 for the coordination equilibrium [Sm(H2O)8]3+ + H2O → [Sm(H2O)9]3+ can be evaluated consistently both by a thermodynamic and a geometric approach. The observed exchange events for [Yb(H2O)8]3+ exhibit the characteristics of an Ia mechanism. The water exchange takes place via a transition-state geometry close to that of a tricapped trigonal prism and involves a slightly negative activation volume.
    Zusätzliches Material: 13 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020309
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
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  • 2
    Digitale Medien
    Digitale Medien
    17O-NMR, EPR and NMRD Characterization of [Gd(DTPA-BMEA)(H2O)]: A Neutral MRI Contrast Agent (1998)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1998 (1998), S. 2017-2021 
    Details
    ISSN: 1434-1948
    Schlagwort(e): MRI ; GdIII complexes ; NMR spectroscopy ; NMRD ; Lanthanides ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A study including variable-temperature and -pressure, multiple-field 17O NMR, EPR and NMRD has been performed on the MRI contrast agent, [Gd(DTPA-BMEA)(H2O)]. The water exchange rate [kex298 = (0.39 ± 0.02) × 106 s-1] and the activation volume (ΔV≠ = +7.4 ± 0.4 cm3 mol-1), hence the mechanism, are identical to those for [Gd(DTPA-BMA)(H2O)]. The longer rotational correlation time of [Gd(DTPA-BMEA)(H2O)], as obtained from a global analysis of 17O-NMR, EPR and NMRD data, and compared to that of [Gd(DTPA-BMA)(H2O)], can be explained by water molecules hydrogen-bonded to the ether oxygen atoms of the ligand side chain.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
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