Oxygen-17 NMR and EPR studies of water exchange from the first coordination sphere of gadolinium(III) aquoion and gadolinium(III) propylenediaminetetra-acetate

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Abstract
The 17O NMR relaxation times, T1 and T2, for water in aqueous solutions of Gd(ClO4)3 and GdPDTA− and the EPR linewidths for the same complexes have been measured. These data are analyzed in terms of the Solomon‐Bloembergen‐Morgan equations for relaxation of a nucleus near a paramagnetic ion and McLachlan’s theory for average EPR relaxation times. The analysis results in a consistent set of physical parameters for the electron relaxation as well as kinetic constants for the water exchange from the complexes. The rate constant for water exchange at 298 K from the Gd(III) aquoion is 10.6×108 s−1 and for the remaining waters in the GdPDTA− complex is 3.3×108 s−1. These rates are higher than the previously published lower limits for water exchange from lanthanide aquoions. A comparison of these results with the rates of formation of other gadolinium complexes leads to the conclusion that the rate of water departure is not the rate limiting step in lanthanide complexation reactions with polydentate ligands.

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