CATION SOLVATION IN NAFION CU2+ SWOLLEN BY ACETONITRILE FROM MULTIFREQUENCY ESR AND SIMULATIONS

The gradual replacement of oxygen ligands around Cu2+ with nitrogen ligands in Nafion soaked with acetonitrile has been deduced from ESR spectra at L-, S-, C-, and X-bands and from computer simulations. Results suggest that after one cycle of drying and soaking of the membranes with acetonitrile, Cu2+ has two N-14 ligands in CH3CN and none in CD3CN. The isotope effect is assigned to the difference in the solvation energies of the sulfonic groups and of cations with deuteriated and protiated solvents and is similar to the recently published preferential solvation of Nafion salts by H2O compared with D2O. After two cycles of drying and soaking with the solvents, all four oxygen ligands in the equatorial plane of Cu2+ are replaced by nitrogen ligands. The components of the N-14 superhyperfine tensor have been deduced, taking into consideration the apparent tensor values calculated from the analysis of the splitting from parallel and perpendicular signals of Cu-63. Multifrequency ESR spectra allow determination of all ESR parameters: Spectra at X-band are very important for deducing the g-tensor components; spectra at C-band are critical for the determination of the ratio delta-g/delta-A of the distribution parameters due to strain; the greater resolution for the signals corresponding to m(I) = -3/2 and -1/2 at C-band, to m(I) = -3/2 at X-band, and to m(I) = -1/2 at S-band is crucial for determining the many parameters involved in the simulation of experimental results. We emphasize the importance of ESR at frequencies lower than the usual X-band, for the analysis of disordered systems, in the presence of strain.