LOCAL ENVIRONMENT OF CATIONS AND IONOMER MORPHOLOGY FROM N-14 SUPERHYPERFINE INTERACTION - ESR OF CU-2+ IN NAFION ACETONITRILE

The local environment of cupric ions in Nafion salts swollen by CH3CN and CD3CN has been investigated by electron spin resonance (ESR) spectroscopy. The advantage of this method is the ability to detect and analyze superhyperfine (shf) splittings from the nitrogen ligands. The number and relative intensity of these shf signals is a direct indication of the number of 14N ligands closest to the central cation. The results indicate preferential solvation of the cation to the oxygen ligands from the water remaining after one cycle of drying of the membranes to constant weight, even though the water/Cu2+ and CH3CN/Cu2+ molar ratios are approximately 2.5 and 118, respectively. Only after an additional cycle of drying are all four equatorial ligands replaced by four nitrogen ligands. A clear isotope effect on the solvation process is detected for CH3CN and CD3CN, with the latter replacing the water less effectively. Changes in the local environment of the cation over several weeks are detected at room temperature and are thought to arise from the presence of different axial ligands and different degrees of tetrahedral distortions of the tetragonal symmetry of the Cu2+ complexes. © 1990, American Chemical Society. All rights reserved.