Multi-frequency EPR determination of zero field splitting of high spin species in liquids: Gd(III) chelates in water

Multi-frequency EPR spectroscopy at 9.5, 35, 94, and 249 GHz has been employed to investigate the zero field splitting (ZFS) of high spin ions in liquids. In particular, experiments are reported on aqueous solutions of DTPA and DOTA chelates of Gd(III), and on the uncomplexed ion, which are relevant to the effectiveness of paramagnetic contrast agents for magnetic resonance imaging (MRI). The field dependence of the centroid of the resonance line, characterized by an effective g factor, geff, has been analysed in order to determine δ1, the trace of the square of the ZFS matrix. Analysis of the variation in transverse electron spin relaxation (T 2e) with experimental frequency provides yet another route to measure δ2 from EPR data. This analysis also gives δv, a correlation time describing the time-dependent ZFS effect. The ZFS parameters so obtained agree well with results obtained by the analysis of proton nuclear magnetic relaxation dispersion. At 94 GHz, partially resolved spectra from chelated and unc...