Homonuclear dipolar recoupling under ultra-fast magic-angle spinning: Probing 19F-19F proximities by solid-state NMR

We describe dipolar recoupling methods that accomplish. at high magic-angle spinning (MAS) frequencies, the excitation of double-quantum (DQ) coherences between spin-1/2 nuclei We employ rotor-synchronized symmetry-based pulse sequences which are either gamma-encoded or non-gamma-encoded The sensitivity and the robustness to both chemical-shift anisotropy and offset are examined. We also compare different techniques to avoid signal folding in the indirect dimension of two-dimensional double-quantum <-> single-quantum (DQ-SQ) spectra This comprehensive analysis results in the identification of satisfactory conditions for dipolar F-19-F-19 recoupling at high magnetic fields and high MAS frequencies The utility of these recoupling methods is demonstrated with high-resolution DQ-SQ NMR spectra, which allow probing F-19-F-19 proximities in powered fluoroluminates (C) 2009 Elsevier Inc All rights reserved