Vibrational modes of double four-member rings of oxygen-bridged silicon and aluminum atoms: A DFT study

The vibrational modes of double four-member ring fragments (D4R) of the general composition [M-x](x+)[H8Si8-xAlxO12](x-), where Si and Al occupy tetrahedral sites, 0 less than or equal to x less than or equal to 4, and M+ is a monovalent charge-compensating cation, are assigned to the framework vibrations of Linde type A (LTA) zeolites. The normal vibrations that arise from configurations with different Si,Al arrangements are calculated by the B3LYP method. Displacement vectors are used to define the ring-opening vibrations as synchronized movement of the tetrahedral and oxygen atoms. The vectors are oriented radially to the ring center and parallel to the ring plane. The fragment model Na4H8Si4Al4O12 With D-2d symmetry is selected for the interpretation of the infrared and Raman active fundamentals of zeolite 4A. Lower symmetry configurations are found to be appropriate models for the normal mode interpretation of Si-rich analogues of the LTA structure. The ring-opening vibrations of the cluster models are correlated to the LTA framework modes via the higher symmetry group D-4h, which describes the entirely siliceous tetragonal prism, H8Si8O12 Synchronized oxygen atom displacements are part of all ring-opening modes.