19F and 13C NMR Signal Assignment and Analysis in a Perfluorinated Ionomer (Nafion) by Two-Dimensional Solid-State NMR

Abstract

The 19F and 13C NMR resonances of the perfluorinated ionomer, Nafion, are assigned to their corresponding chemical groups using two-dimensional (2D) 13C−19F heteronuclear correlation and 19F-exchange NMR experiments under 28 or 30 kHz magic-angle spinning, combined with peak area and relaxation time information. On the basis of these new experimental data, we revise the assignment of more than half of the resolved 19F NMR peaks. In particular, the backbone CF group is shown to resonate at −138 ppm, the side-group CF at −144 ppm, and the SCF2 group, which can be selected by a T2 filter, at −117 ppm. The OCF2 groups resonate slightly downfield from the CF3 fluorines. Deconvolution of the 19F and 13C spectra based on cross sections from the 2D spectra provides the peak widths and positions of all side group and several backbone sites. The inhomogeneous broadening observed in both 13C and 19F NMR spectra for the sites near the backbone CF group reveals static disorder near the branch point, which contrasts with the high conformational order of the rest of the backbone and the mobility of the ends of the side group.