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The structure of the silica AST zeolites (octa­deca­sil) synthesized in fluoride medium using tetra­methyl­ammonium (TMA) as the organic structure-directing agent has been reinvestigated using 14N NMR quadrupolar parameters and DFT calculations. The value of the experimental 14N quadrupolar coupling constant (CQ = 27 kHz) is larger than expected for a TMA cation possessing a high degree of motion. The analysis of a DFT-optimized octa­deca­sil cluster along with the comparison between measured and calculated 14N NMR parameters demonstrate the presence of weak C—H...O hydrogen bonds between the TMA in the [46612] cages and the silica skeleton. These inter­molecular inter­actions can be related to the presence of Si...F tetrel bonds within the [46] cages. These new results provide additional information with regard to the formation mechanisms and structure of the octa­deca­sil zeolites.

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