Computational study of 19F NMR spectra of double four ring-containing Si/Ge-zeolites

Abstract

19F NMR chemical shifts are calculated in order to study the F- environment in double four ring (D4R) containing Si/Ge-zeolites. The calculations with the DFT/CSGT/B3PW91 methodology yielded an agreement within 2 ppm with respect to the experimental peaks corresponding to the D4R units containing 8Si0Ge, 7Si1Ge and 0Si8Ge of the octadecasil zeolite. The optimisation of the 7Si1Ge-, 6Si2Ge-, 5Si3Ge- and 4Si4Ge-D4R unite with DFT/B3LYP methodology shows that a covalent Ge-F bond is formed and therefore a Ge atom in the D4R is pentacoordinated. The displacement of the fluoride ion towards a Ge atom in the Ge-containing D4R units locates four Si/Ge atoms in the close vicinity of the F- and this makes possible a rationalization of the 19F NMR signals in groups according to the number of Si (n) and Ge (m) atoms in the nearest F- environment, F-Si nGem (where n+m = 4). Thus, the calculated chemical shifts show that higher values are observed when the number of Ge atoms in the nearest F- environment increases. © 2006 Wiley-VCH Verlag GmbH & Co. KGaA.