Highly efficient 19F heteronuclear decoupling in solid-state NMR spectroscopy using supercycled refocused-CW irradiation

Abstract

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.