Location of the fluoride ion in tetrapropylammonium fluoride silicalite-1 determined by H-1/F-19/Si-29 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 MEI top ology, has been unambiguously determined using solid-state NMR experiments alone. With the H-1 --> Si-29 CP-INADEQUATE experiment, the 12 peaks in the highly resolved Si-29 MAS NMR spectrum of [TPA]-F-[Si-MFI] were assigned. Using these peak assignments it was possible to perform H-1/F-19/Si-29 trip le 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-r elated" Si-9 sites, making them equivalent on the NMR time scale. The impor tance of this result and the general applicability of the approach are disc ussed.