The influence of fluorine spin-diffusion on <SUP>13</SUP>C solid-state NMR line shapes of CF₃ groups

Abstract

Indirect spin-spin couplings ("J-couplings") lead to well-known multiplet patterns in nuclear magnetic resonance (NMR) spectra that are also observable in non-decoupled solid-state NMR spectra, if the J-coupling constant exceeds the linewidth. Such J-multiplet line shapes in the solid state might however be affected by spin diffusion (SD) on the passive nuclei. When the SD rate constant is fast compared to the J-coupling constant, the multiplet resolution can be lost due to a so-called "self-decoupling" mechanism as has been already reported in the context of decoupling and for proton SD in solid adamantane. We herein report on the influence of 19F SD on 13C-detected solid-state NMR spectra of a small organic molecule bearing a trifluoromethyl group. The target compound is the chiral alpha-(trifluoromethyl)lactic acid (TFLA). Enantiopure phases ((R) or (S), respectively) of TFLA are composed of homochiral dimers whereas the racemic phase consists of heterochiral dimers in the solid state. Despite their structural similarity, the 13C line shapes of the CF3 group in cross-polarization spectra recorded at slow to medium magic-angle spinning (MAS) frequencies - in the range between 14.0 kHz and 60.0 kHz - differ substantially. By combining experimental observations, analytical calculations based on the Bloch-McConnell equations, and numerical spin-dynamics simulations, we demonstrate that differences in the 19F SD rate constant between racemic and enantiopure TFLA-phases significantly affect the respective solid-state 13C NMR spectral line shapes. Slowing down SD by increasing the MAS frequency restores the quartet line shape for both phases of TFLA.