| Abstract
| - The analysis of experimental relaxometric profiles of paramagnetic complexes is usually performed using theSolomon−Bloembergen−Morgan (SBM) theory. The SBM theory is not valid for slowly rotating systemswhen the electronic levels are split at zero field, in which case a modified theory developed in Florenceshould be used. However, for many interesting systems, including Gd-based contrast agents for MRI, theelectron spin relaxation is rather close to the Redfield limit, where none of these approaches is valid. In thepresent paper, the SBM theory and modified versions of the Florence model are compared against a generaltheory valid beyond the Redfield limit (the so-called slow-motion theory). Significant differences are foundalready for the cases where the electron spin relaxation is in a regime near the Redfield limit, but still withinit. Indeed, the values of the parameters describing the electron spin relaxation are underestimated for theSBM theory relative to those used in calculating the slow-motion profiles. The present results are relevant forthe interpretation of relaxometric profiles of paramagnetic complexes and proteins, and for the interpretationof the behavior of contrast agents used in MRI, and should be taken into account when planning theimprovement of the relaxometric properties for the next generation of contrast agents based on theoreticalpredictions.
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