NUCLEAR MAGNETIC-RELAXATION IN AQUEOUS-SOLUTIONS OF THE GD(HEDTA) COMPLEX

The luminescence decay times and rate constants of terbium complexed with HEDTA (N-(2-hydroxyethyl)ethylenediaminetriacetic acid) are obtained at pH values ranging from pH 1.5 to 12 and used to determine the number of water molecules bound at the inner coordinate sphere of the lanthanide complex. The proton nuclear magnetic relaxation rate, 1/T1, is reported as a function of pH for gadolinium complexed with HEDTA over a range of magnetic field strengths corresponding to proton Larmor frequencies between 0.01 MHz and 30 MHz. The data analysis is largely based on the Solomon, Bloembergen, and Morgan equations after the outer-sphere contribution is taken into account. The number of water molecules in the first coordination sphere was measured by using fluorescence lifetime methods and the assumption that the result is the same for the lanthanide complex of gadolinium and terbium. The parameters obtained from the analysis are consistent with other estimates of interatomic distances and rotational correlation times. Further, the data show that the dimerization of the complex at high pH does not compromise the relaxation efficiency of the metal center, but it increases approximately by a factor of 2. © 1990, American Chemical Society. All rights reserved.