| Abstract
| - In the search for highly efficient magnetic resonance imaging contrast agents, polyamino polypyridinecarboxylate complexes of Gd3+ have shown unusual properties with both very rapid and very slow electronspin relaxation in solution observed by electron paramagnetic resonance. Since the relationship between themolecular structure and the electron spin properties remains quite obscure at this point, detailed studies ofsuch complexes may offer useful clues for the design of Gd3+ compounds with tailored electronic features.Furthermore, the availability of very high-frequency EPR spectrometers based on quasi-optical componentsprovides us with an opportunity to test the existing relaxation theories at increasingly high magnetic fieldsand observation frequencies. We present a detailed EPR study of two gadolinium polyamino polypyridinecarboxylate complexes, [Gd(tpaen)]- and [Gd(bpatcn)(H2O)], in liquid aqueous solutions at multipletemperatures and frequencies between 9.5 and 325 GHz. We analyze the results using the model of randomzero-field splitting modulations through Brownian rotation and molecular deformations. We consider theeffect of concentration on the line width, as well as the possible existence of an additional g-tensor modulationrelaxation mechanism and its possible impact on future experiments. We use 17O NMR to characterize thewater exchange rate on [Gd(bpatcn)(H2O)] and find it to be slow (∼0.6 × 106 s-1).
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