C-13-F-19 DIPOLAR DEPHASING IN MONOFLUORINATED ORGANIC-SUBSTANCES - CHARACTERIZATION BY H-1-C-13-F-19 TRIPLE-RESONANCE C-13 CP/MAS AND F-19 MAS NMR

The conventional solid-state CP/MAS 13C NMR spectra of nonpolymeric monofluorinated organic substances exhibit dramatic homogeneous broadening of resonances of those carbons in close spatial proximity to the magnetic dipole moment of the 19F nucleus. Resonances of carbons directly bonded to 19F can broaden beyond recognition. 13C-19F dipolar dephasing is the 13CT2 relaxation pathway and is promoted by efficient 19F magnetization diffusion. 19F and 13CT2 values have been measured directly to delineate these processes. The principal broadening occurs through the 13C-19F dipolar interaction and has an r-3 carbon-fluorine internuclear distance dependence. The 13C-19F dipolar-dephased 13C spectrum edits the full 1H, 19F doubly di-polar-decoupled spectrum, using the strength of the 13C-19F di-polar interaction as the editing criteria. The 13C-19F dipolar-dephasing experiment constitutes a highly selective method for fluoride functional group determination under high-resolution conditions. The coincident application of resonant 19F and 1H dipolar decoupling diminishes relaxation broadening in the 13C spectrum. 19F decoupling offsets of 10-15 kHz can result in 13C linewidths that are an order of magnitude larger than those measured with optimum decoupling. This dependence and the large 19F chemical-shift range of organofluorine resonances prevent the uniform suppression or elimination of broadening in substances containing multiple, widely separated 19F resonances. Concurrent 1H and 19F decoupling must be an integral feature of high-resolution CP/MAS 13C experiments on fluorinated organic materials performed at routine (<6 kHz) MAS speeds. © 1993 Academic Press. All rights reserved.