Vibrational spectroscopy of the double 4-, 6-membered rings in silicates and siloxanes

Siloxanes and silicates are different chemical compounds but they all contain silicooxygen units. Double 6-membered silicooxygen rings are important building units of more complicated silicate structures (silicalites and zeolites). The aim of this work is to understand vibrational spectra of double 6-membered cyclosilicates based on the spectra of selected siloxanes. For the selected siloxane molecules infrared spectra have been calculated (using various terminal groups) by ab initio method and compared with the experimental ones of silicates. Visualization of the calculated normal modes has been used as a tool to define the band type in the spectra of siloxanes and to interpret them in terms of group-characteristic frequencies as tetrahedral, overtetrahedral and ring bands. Similarities between IR spectra of T8 and T12 siloxanes and the spectra of double 4-, 6-membered cyclosilicates allow to identify the bands in the experimental spectra of silicates.