Vibrational spectra of D4R and D6R structural units

Calculated vibrational spectra of aluminosilicate D4R and D6R structural units have been derived using ab initio methods. These units constitute the models of secondary building units (SBU) of two important structural zeolite groups. In the calculations, the model of isolated double four- and six-membered rings containing both AlO4 and SiO4 tetrahedra has been used. Based on the results of calculations, normal vibrations of the units have been visualized. This has enabled us to identify the bands in the spectra obtained. In particular, distinguishment of the characteristic ring or pore-opening vibrations occurring in these units has been possible. The effect of Al→Si substitution in tetrahedral positions on the spectra shape has been also analysed. The calculated results obtained have been applied to interpret experimental spectra of selected zeolites. Their frameworks are formed by spatial connection of structural units in three directions. For the interpretation, the spectra of zeolites whose structure is exclusively composed by the connection of the structural units studied have been chosen.