High-Resolution ¹⁹F MAS NMR Spectroscopy: Structural Disorder and Unusual J Couplings in a Fluorinated Hydroxy-Silicate

High-resolution ¹⁹F magic angle spinning (MAS) NMR spectroscopy is used to study disorder and bonding in a crystalline solid. ¹⁹F MAS NMR reveals four distinct F sites in a 50% fluorine-substituted deuterated hydrous magnesium silicate (clinohumite, 4Mg₂SiO₄·Mg(OD_1−xF_x)₂ with x = 0.5), indicating extensive structural disorder. The four ¹⁹F peaks can be assigned using density functional theory (DFT) calculations of NMR parameters for a number of structural models with a range of possible local F environments generated by F⁻/OH⁻ substitution. These assignments are supported by two-dimensional ¹⁹F double-quantum MAS NMR experiments that correlate F sites based on either spatial proximity (via dipolar couplings) or through-bond connectivity (via scalar, or J, couplings). The observation of ¹⁹F−¹⁹F J couplings is unexpected as the fluorines coordinate Mg atoms and the Mg−F interaction is normally considered to be ionic in character (i.e., there is no formal F−Mg−F covalent bonding arrangement). However, DFT calculations predict significant ¹⁹F−¹⁹F J couplings, and these are in good agreement with the splittings observed in a ¹⁹F J-resolved MAS NMR experiment. The existence of these J couplings is discussed in relation to both the nature of bonding in the solid state and the occurrence of so-called “through-space” ¹⁹F−¹⁹F J couplings in solution. Finally, we note that we have found similar structural disorder and spin−spin interactions in both synthetic and naturally occurring clinohumite samples.