Solid-state F-19 MAS, F-19 CRAMPS, and F-19 -> C-13 CP/MAS NMR study of anamorphous perfluoropolymer

Solid-state F-19 high-speed MAS,F-19 CRAMPS, and F-19 --> C-13 CP/MAS NMR s pectra have been investigated for an amorphous fluoropolymer (CYTOP) having a five-membered heteroring in the main chain. In the F-19 MAS spectra, spi nning at 35 kHz is necessary to ensure no spectral overlap of spinning side bands (SSB) with the isotropic signals. The integrated areas obtained are c onsistent with the molecular structure, and the signal assignments were con firmed using shielding calculations by the GIAO method at a density functio nal level of theory, B3LYP/6-31G(d). A F-19 CRAMPS spectrum measured at a s pinning speed of 6 kHz gives slightly narrower half-height widths despite S SB overlap. All fluorine signals show monoexponential decays in the F-19 sp in-lock experiment with the same T-1 rho(F), indicating the structure is mo nophasic. Two fluorines connected to the same carbon give separate peaks at ambient temperature, which reflects the magnetic inequivalence at the axia l 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 spect ra of CYTOP were measured at a spinning speed of 6 kHz. The evolutions of t he signal intensities in the F-19 --> C-13 variable contact time CP experim ents are adequately described by the quantum mechanical master equation app lied 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 rel ating to the heteronuclear polarization transfer between F-19 and C-13 spin s, those of the homonuclear effective spin-diffusion among fluorines and th e 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.