Solid-state 19F MAS, 19F CRAMPS, and 19F → 13C CP/MAS NMR study of an amorphous perfluoropolymer

Solid-state F-19 high-speed MAS,F-19 CRAMPS, and F-19 --> C-13 CP/MAS NMR spectra have been investigated for an amorphous fluoropolymer (CYTOP) having a five-membered heteroring in the main chain. In the F-19 MAS spectra, spinning at 35 kHz is necessary to ensure no spectral overlap of spinning sidebands (SSB) with the isotropic signals. The integrated areas obtained are consistent with the molecular structure, and the signal assignments were confirmed using shielding calculations by the GIAO method at a density functional level of theory, B3LYP/6-31G(d). A F-19 CRAMPS spectrum measured at a spinning speed of 6 kHz gives slightly narrower half-height widths despite SSB overlap. All fluorine signals show monoexponential decays in the F-19 spin-lock experiment with the same T-1 rho(F), indicating the structure is monophasic. Two fluorines connected to the same carbon give separate peaks at ambient temperature, which reflects the magnetic inequivalence at the axial and equatorial positions of the heteroring. However, they start to merge at 140 degreesC nominal temperature, which is explained by a conformational exchange caused by vigorous motion of the ring. F-19 --> C-13 CP/MAS spectra of CYTOP were measured at a spinning speed of 6 kHz. The evolutions of the signal intensities in the F-19 --> C-13 variable contact time CP experiments are adequately described by the quantum mechanical master equation applied to the three-spin CF2 subsystem for the main peak and to the two-spin CF subsystems for the subpeak. The C-F distances and the time constants relating to the heteronuclear polarization transfer between F-19 and C-13 spins, those of the homonuclear effective spin-diffusion among fluorines and the decay or damping of the coherences, were obtained. In addition, the time constants of homonuclear spin diffusion among fluorines were independently measured by the C-13-detected F-19 spin-diffusion experiment.