Pulsed ENDOR study of water coordination to Gd3+ complexes in orientationally disordered systems

In orientationally disordered systems, the ENDOR (electron-nuclear double resonance) spectra of high-spin ions having weak crystal field interaction (cfi) contain nuclear transition lines (often overlapped) that belong to several different electron spin manifolds, and the transition lines for each manifold are distorted by the effect of the cfi. In this work, we have shown that, although the latter distortions can be quite considerable in a general case, the statistical distributions of the cfi parameters and orientations of the cfi axes in glassy samples result in ENDOR line shapes similar to the usual powder pattern determined by the hyperfine interaction only, which greatly simplifies their analysis. We have also shown that the two-dimensional Mims ENDOR technique can be used to disentangle the nuclear transitions that belong to different electron spin manifolds. The results of the analysis were applied to study the Gd3+ aquo complex and the Gd3+-based MRI contrast agent GdHPDO3A in frozen glassy water/methanol solutions. The average distance between Gd and protons of the water ligands was found to be about 3.1 Angstrom for both complexes.