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Combined Solid State NMR and X-ray Diffraction Investigation of the Local Structure of the Five-Coordinate Silicon in Fluoride-Containing As-Synthesized STF Zeolite

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journal contribution
posted on 11.06.2002, 00:00 by Colin A. Fyfe, Darren H. Brouwer, Andrew R. Lewis, Luis A. Villaescusa, Russell E. Morris
The local structure of the [SiO4/2F]- unit in fluoride-containing as-synthesized STF zeolite has been experimentally determined by a combination of solid-state NMR and microcrystal X-ray diffraction to be very close to trigonal bipyramidal. Because the fluoride ions are disordered over two sites, the resulting local structure of the [SiO4/2F]- unit from a conventional XRD refinement is an average between tetrahedral SiO4/2 and five-coordinate [SiO4/2F]-, giving an apparent F−Si distance longer than expected. The correct F−Si distance was determined by slow spinning MAS and fast spinning 19F/29Si CP and REDOR solid-state NMR experiments and found to be between 1.72 and 1.79 Å. In light of this, the X-ray structure was re-refined, including the disorder at Si3. The resulting local structure of the [SiO4/2F]- unit was very close to trigonal bipyramidal with a F−Si distance of 1.744 (6) Å, in agreement with the NMR results and the prediction of Density Functional Theory calculations. In addition, further evidence for the existence of a covalent F−Si bond is provided by a 19F→29Si refocused INEPT experiment. The resonance for the five-coordinate species at −147.5 ppm in the 29Si spectrum is a doublet due to the 19F/29Si J-coupling of 165 Hz. The peaks in this doublet have remarkably different effective chemical shift anisotropies due to the interplay of the CSA, dipolar coupling, and J-coupling tensors. The distortions from tetrahedral geometry of the neighboring silicon atoms to the five-coordinate Si3 atom are manifested in increased δaniso values. This information, along with F−Si distances measured by 19F→29Si CP experiments, makes it possible to assign half of the 29Si resonances to unique tetrahedral sites. As well as determining the local geometry of the [SiO4/2F]- unit, the work presented here demonstrates the complementarity of the solid-state NMR and X-ray diffraction techniques and the advantages of using them together.