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

John M. Griffin, Jonathan R. Yates, Andrew J. Berry, Stephen Wimperis, Sharon E. Ashbrook

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77 Citations (Scopus)

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.

Original languageEnglish
Pages (from-to)15651-15660
Number of pages10
JournalJournal of the American Chemical Society
Volume132
Issue number44
DOIs
Publication statusPublished - 10 Nov 2010

Keywords

  • SOLID-STATE NMR
  • MAGNETIC-RESONANCE-SPECTROSCOPY
  • 1ST-PRINCIPLES CALCULATIONS
  • MULTIPLE-QUANTUM
  • MAGNESIUM SILICATES
  • NEUTRON-DIFFRACTION
  • CHEMICAL-SHIFT
  • POWDER NEUTRON
  • O-17
  • MULTINUCLEAR

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