19F/29Si distance determination and heteronuclear spin counting under fast magic-angle spinning in fluoride-containing octadecasil

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19 F/ 29 Si rotational-echo double-resonance (REDOR), θ -REDOR and Hartmann–Hahn cross-polarization (CP) NMR techniques have been applied under fast magic-angle spinning (MAS) to a powder sample of fluoride-containing octadecasil. The internuclear distance of the 19 F– 29 Si spin pairs formed by the silicons in the D4R units (T-1 site) and the fluoride anions deduced directly from the CP and REDOR measurements is in very good agreement with the X-ray crystal structure. Moreover, heteronuclear spin-counting θ -REDOR experiments unambiguously show that the 19 F and the T-1 29 Si nuclei form fairly isolated spin pairs, whereas multiple-spin behaviour is clearly evidenced at the T-2 site.