Vibrational spectroscopy of the ring structures in silicates and siloxanes

In the presented paper an attempt to build a structural model for crystal silicates on the basis of selected siloxane systems has been proposed. Siloxanes and silicates are different chemical compounds arranged as molecular or ionic crystals, respectively. Siloxane group (Si-O bond) is their common feature. It is suggested that as the proportion of the Si-O to Si-C bonds in the siloxane structure grows they become more similar to silicates. In the investigations siloxanes with -CH3 as a terminal group and selected silicates have been chosen as model compounds. Vibrational spectroscopy has been used as an investigation tool. For the selected siloxane molecules infrared spectra have been calculated (HF/6-31G(d) method) and compared with the experimental ones of siloxanes and silicates. It has been found that the spectra corresponding to siloxanes and silicates of similar structures agree fairly well in spite of different character of alkyl group present in siloxanes from that of metal cations in silicates. Based on the spectra similarities visual analysis of siloxanes normal vibrations has been applied to identify the bands in the spectra of silicates. Accuracy of such band identification in the spectra of silicates grows with the increase in the SiO4 tetrahedra connectedeness. It becomes possible from structures composed of secondary tetrahedra (Q2) but similarities are significant starting from the structures with tertiary (Q3) tetrahedra which makes it possible to distinguish them as a separate group of molecular silicates. (C) 2004 Elsevier B.V. All rights reserved.