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 gen eral composition [M-x](x+)[H8Si8-xAlxO12](x-), where Si and Al occupy tetra hedral 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 vibr ations of Linde type A (LTA) zeolites. The normal vibrations that arise fro m configurations with different Si,Al arrangements are calculated by the B3 LYP method. Displacement vectors are used to define the ring-opening vibrat ions as synchronized movement of the tetrahedral and oxygen atoms. The vect ors 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 n ormal mode interpretation of Si-rich analogues of the LTA structure. The ri ng-opening vibrations of the cluster models are correlated to the LTA frame work modes via the higher symmetry group D-4h, which describes the entirely siliceous tetragonal prism, H8Si8O12 Synchronized oxygen atom displacement s are part of all ring-opening modes.