EPR spectroscopy of MRI-related Gd(III) complexes: Simultaneous analysis of multiple frequency and temperature spectra, including static and transient crystal field effects

For the first time, a very general theoretical method is proposed to interp ret the full electron paramagnetic resonance (EPR) spectra at multiple temp eratures and frequencies in the important case of S-state metal ions comple xed in liquid solution. This method is illustrated by a careful analysis of the measured spectra of two Gd3+ (S = 7/2) complexes. It is shown that the electronic relaxation mechanisms at the origin of the EPR line shape arise from the combined effects of the modulation of the static crystal field by the random Brownian rotation of the complex and of the transient zero-fiel d splitting. A detailed study of the static crystal field mechanism,shows t hat, contrarily to the usual global models involving only second-order term s, the fourth and sixth order terms can play a non-negligible role. The obt ained parameters are well interpreted in the framework of the physics of th e various underlying relaxation processes. A better understanding of these mechanisms is highly valuable since they partly control the efficiency of p aramagnetic metal ions in contrast agents for medical magnetic resonance im aging (MRI).