Multiple-Frequency and Variable-Temperature EPR Study of Gadolinium(III) Complexes with Polyaminocarboxylates: Analysis and Comparison of the Magnetically Dilute Powder and the Frozen-Solution Spectra

An electron paramagnetic resonance (EPR) study of glasses and magnetically dilute powders of [Gd(DTPA)(H2O)](2-), [Gd(DOTA)(H2O)](-), and macromolecular gadolinate(1-) complexes P792 was carried out at the X- and Q-bands and at 240 GHz (DTPA = diethylenetriaminepentaacetato; DOTA = 1.4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetato). The results show that the zero-field splitting (ZFS) parameters for these complexes are quite different in a powder as compared to the frozen aqueous solution. In several complexes, an inversion of the sign of the axial component D of the zero field splitting is observed, indicating a significant structural change. In contrary to what was expected, powder samples obtained by lyophilization do not allow a more precise determination of the static ZFS parameters. The results obtained in glasses are more relevant to the problem of electron spin relaxation in aqueous solution than those obtained from powders.