Matrix ENDOR of ion-exchange systems

Electron nuclear double resonance (ENDOR) has been investigated in sulfocation exchangers, containing free radicals stabilized in polymeric matrix or Cu²⁺ and (VO)²⁺ as counterions. It was shown that the ENDOR signal is mainly due to electron-nuclear dipole-dipole interactions between the unpaired electron and nuclei of polymeric matrix or hydrogen atoms of water molecules which hydrate the charge groups. In order to quantitatively describe the ENDOR line shape and intensity, the theory of matrix ENDOR is developed. The correctness of this theory was tested by comparing the temperature dependence of spin-lattice relaxation times calculated from ENDOR line intensities with the corresponding dependence obtained from stationary saturation electron spin resonance spectra. A good agreement was observed in the temperature range from 200 to 350 K. The structural parameters of surroundings of paramagnetic ions Cu²⁺ and (VO)²⁺, which include four coordinated spheres on the distance from 0.3 to 1.2 nm, were calculated. The motional parameters, correlation time and activation energy of mobile protons were also determined. It is concluded that the activation processes of water self-diffusion and proton exchange take place at high temperature, whereas the proton tunneling transfer is possible at low temperature.