An Analysis of the Vibrational Characteristics of Zeolites Using Factor Group Methods

Abstract

The complete group theoretical representations for the normal modes of twelve high-symmetry zeolite crystal structures (including sodalite, offretite, and zeolite rho, as well as A, X, Y, and L-type zeolites) have been determined by factor group analysis. Comparisons of the predicted number of Raman and infrared active vibrations were made with existing experimental data in cases where such data were available. The prospects for conducting detailed analyses of the vibrational dynamics of zeolite framework structures based on the factor-group-derived representations were also examined. Much of the accepted intuitive understanding of zeolite vibrational spectra is confirmed by the findings of this work: e.g., that the number of predicted Raman and infrared frquencies (1) is consistently far greater than the number actually observed and (2) depends more on the level of symmetry of the zeolite crystal than on the number of atoms in the Bravais unit cell. Tight overlapping of frequencies, particularly those that are primarily bond stretching in nature, is to be expected.