Can the Theoretical Fitting of the Proton‐Nuclear‐Magnetic‐Relaxation‐Dispersion (Proton NMRD) Curves of Paramagnetic Complexes Be Improved by Independent Measurement of Their Self‐Diffusion Coefficients?

The self‐diffusion ( D c ) coefficients of various lanthanum(III) diamagnetic analogues of open‐chain and macrocyclic complexes of gadolinium used as MRI contrast agents were determined in dilute aqueous solutions (3–31 m M ) by pulsed‐field‐gradient (PFG) high‐resolution 1 H‐NMR spectroscopy. The self‐diffusion coefficient of H 2 O ( D w ) was obtained for the same samples to derive the relative diffusion constant, a parameter involved in the outersphere paramagnetic‐relaxation mechanism. The results agree with an averaged relative diffusion constant of 2.5 (±0.1)×10 −9 and of 3.3 (±0.1)×10 −9 m 2 s −1 at 25 and 37°, respectively, for 'small' contrast agents ( M r 500–750 g/mol), and with the value of bulk H 2 O (2.2×10 −9 and 2.9×10 −9 m 2 s −1 at 25° and at 37°, respectively) for larger complexes. The use of the measured values of D c for the theoretical fitting of proton NMRD curves of gadolinium complexes shows that the rotational correlation times ( τ R ) are very close to those already reported. However, differences in the electronic relaxation time (τ SO ) at very low field and in the correlation time ( τ V ) related to electronic relaxation were found.