Relaxation of solvent protons by solute Gd³⁺-chelates, revisited

The magnetic field dependence (NMRD profile) of 1/T₁, of solvent protons in an aqueous solution of Gd(DTPA)²⁻ was remeasured at 5,15,25,30, and 35°C. The data were reanalyzed with the usual low-field theory, using recently published values for T_M, the residence lifetime of the single inner-coordinated waters of solute Gd(DTPA)²⁻. (These T_M values are significantly longer than earlier estimates). Values were obtained for three dynamic parameters: T_R, the rotational relaxation time of solute ions, and T_SO and T_V, the low-field relaxation time of the Gd³⁺ magnetic moment and the related correlation time. These Gd(DTPA)²⁺ values, together with recent results for T_M for Gd(DTPA-BMA) — a nonionic structural analog of Gd(DTPA)²⁻ with an unusually long T_M — were used to calculate NMRD profiles at 5 and 35°C. These profiles agree very well with new data given here for a solution of Gd(DTPA-BMA). This reaffirms the importance of knowing the temperaturedependent values of T_M a priori in order to obtain unambiguous quantitative theoretical analyses of NMRD profiles of chelates of known structure. Additionally, the theory of inner sphere relaxation is extended to high fields, at which the magnetic energy of a solute moment is greater than its thermal energy.