Location of the Fluoride Ion in Tetrapropylammonium Fluoride Silicalite-1 Determined by ¹H/¹⁹F/²⁹Si Triple Resonance CP, REDOR, and TEDOR NMR Experiments

The location of the fluoride ion in tetrapropylammonium fluoride silicalite-1 ([TPA]-F-[Si-MFI]), an as-synthesized siliceous zeolite with the MFI topology, has been unambiguously determined using solid-state NMR experiments alone. With the 1 H→ 29 Si CP-INADEQUATE experiment, the 12 peaks in the highly resolved 29 Si MAS NMR spectrum of [TPA]-F-[Si-MFI] were assigned. Using these peak assignments it was possible to perform 1 H/ 19 F/ 29 Si triple resonance CP, REDOR, and TEDOR experiments to measure F−Si distances and thus locate the fluoride ion. It is covalently bonded to Si-9 in the [4 1 5 2 6 2 ] cage of the zeolite framework and exchanges between two “mirror-related” Si-9 sites, making them equivalent on the NMR time scale. The importance of this result and the general applicability of the approach are discussed.