Intermolecular interactions in AST zeolites through N 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 N NMR quadrupolar parameters and DFT calculations. The value of the experimental N quadrupolar coupling constant (C = 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 N NMR parameters demonstrate the presence of weak C-H...O hydrogen bonds between the TMA in the [46] cages and the silica skeleton. These intermolecular interactions can be related to the presence of Si...F tetrel bonds within the [4] cages. These new results provide additional information with regard to the formation mechanisms and structure of the octadecasil zeolites.