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

摘要

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 [4~6 6~12] 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.