Highly efficient 19F heteronuclear decoupling in solid-state NMR spectroscopy using supercycled refocused-CW irradiation
We present heteronuclear 19F refocused CW (rCW) decoupling pulse sequences for solid-state magic-angle-spinning NMR applications. The decoupling sequences have been designed specifically to ensure suppression of the pertinent 13C–19F dipolar coupling interactions while simultaneously suppressing strong anisotropic chemical shift as well as homonuclear 19F–19F dipolar coupling effects as typically present in perfluorated compounds. In an extensive numerical and experimental analysis using a rigid, organic solid as a model compound, it becomes evident that the supercycled rCW schemes markedly improve the decoupling efficiency, leading to substantial enhancements in resolution and sensitivity when compared to previous state-of-the-art methods. Furthermore, considerable gains in robustness toward rf mismatch as well as offset in the radio-frequency carrier frequency are observed, all of which clearly render the new rCW schemes the methods of choice for 19F decoupling in rigid, fluorinated compounds – which is further supported by a Floquet-based theoretical analysis.