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 emplo yed to investigate the zero field splitting (ZFS) of high spin ions in liqu ids. In particular, experiments are reported on aqueous solutions of DTPA a nd DOTA chelates of Gd(III), and on the uncomplexed ion, which are relevant to the effectiveness of paramagnetic contrast agents for magnetic resonanc e imaging (MRI). The field dependence of the centroid of the resonance line , characterized by an effective g factor, g(eff), has been analysed in orde r to determine Delta(2), the trace of the square of the ZFS matrix. Analysi s of the variation in transverse electron spin relaxation (T-2e) with exper imental frequency provides yet another route to measure Delta(2) from EPR d ata. This analysis also gives tau(v), a correlation time describing the tim e-dependent ZFS effect, The ZFS parameters so obtained agree well with resu lts obtained by the analysis of proton nuclear magnetic relaxation dispersi on. At 94GHz, partially resolved spectra from chelated and unchelated Gd(II I) were observed. The shifts in resonance field for Gd(III) in these two co mpounds are due primarily to differences in the magnitude of ZFS. The spect ral resolution as a function of frequency exhibits a maximum in the range o f our experiments; the resolution disappeared at either higher or lower res onance frequency. Study of ZFS by EPR at multiple high fields offers a new and sensitive route to probe water interactions and chelate dynamics in bio logically relevant systems having high spin ions.