High-resolution 19F MAS NMR spectroscopy: Structural disorder and unusual J couplings in a fluorinated hydroxy-silicate

Abstract

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(OD1-xFx)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.