Electron paramagnetic resonance of Gd3+ aqua complexes in vitrified aqueous solutions and Gd3+ aqua ions adsorbed on the capillary surface

Abstract

Electron paramagnetic resonance (EPR) spectra of Gd3+ aqua complexes are measured in dilute aqueous solutions of Gd(NO3)3 (C<0.2 M) at room temperature. A partial resolution of the fine structure observed in the spectra is characteristic of solid disordered systems and results in an increase in the effective width of the EPR line with a decrease in the Gd3+ concentration. This phenomenon is explained in terms of adsorption of Gd3+ aqua ions on the surface of the measuring capillaries. The fine structure is revealed in the EPR spectra of Gd(NO3)3 aqueous solutions, namely, the Gd(NO3)3 solutions vitrified at a temperature of 77 K (with an addition of 10–15 vol % glycerol) and Gd(NO3)3 solutions quasi-vitrified at 298 K (with an addition of 70–90 vol % glycerol). Analysis of the EPR spectra demonstrates that these solutions contain two types of aqua complexes with fine structure parameters D1=180 G and D2=580 G. Reasoning from a comparison with x-ray diffraction data, the fine structure parameters D1 and D2 are assigned to higher symmetric eight-coordinate and lower symmetric nine-coordinate Gd3+ aqua complexes, respectively.