C. V. Chandran, G. Hempel, T.
F-19-decoupling of half-integer spin quadrupolar nuclei in solid-state NMR: Application of frequency-swept decoupling methods
Solid State Nucl. Mag. 40, 84-87 (2011).
In solid-state NMR studies of minerals and ion conductors, quadrupolar nuclei like Li-7, Na-23 or Cs-133 are frequently situated in close proximity to
fluorine, so that application of F-19 decoupling is beneficial for spectral resolution. Here, we compare the decoupling efficiency of various multi-pulse
decoupling sequences by acquiring F-19-decoupled Na-23-NMR spectra of cryolite (Na3AlF6). Whereas the MAS spectrum is only marginally affected by application of F-19 decoupling,
the 3Q-filtered Na-23 signal is very sensitive to it, as the de-phasing caused by the dipolar interaction between sodium and fluorine is three-fold magnified. Experimentally,
we find that at moderate MAS speeds, the decoupling efficiencies of the frequency-swept decoupling schemes SWf-TPPM and SWf-SPINAL are significantly better than the conventional TPPM and SPINAL sequences.
The frequency-swept sequences are therefore the methods of choice for efficient decoupling of quadrupolar nuclei with half-integer spin from fluorine.