Inter­molecular inter­actions in AST zeolites through ¹⁴N NMR and DFT calculations

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 (C_Q = 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.