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, g_eff, has been analysed in order to determine Δ², 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 T_v, 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 unchelated Gd(III) were observed. The shifts in resonance field for Gd(III) in these two compounds are due primarily to differences in the magnitude of ZFS. The spectral resolution as a function of frequency exhibits a maximum in the range of our experiments; the resolution disappeared at either higher or lower resonance frequency. Study of ZFS by EPR at multiple high fields offers a new and sensitive route to probe water interactions and chelate dynamics in biologically relevant systems having high spin ions.