Calculations of silicooxygen ring vibration frequencies

Abstract

In the work model calculations of the vibrations of ideally isolated silicooxygen rings (using PM3 method) have been carried out. three-, four-, and six-membered rings have been considered. It has been found that that the three-membered silicooxygen rings are flat and practically undeformed showing D 3h symmetry. The rings of higher number of ring members (i.e. n>3) are deformed to some extent. The deformation reveals itself most significantly in the Si–O–Si bond angles distribution. In the case of all the rings the bridging Si–O–Si bonds are ca. 0.02–0.04 Å shorter than the non-bridging Si–O − bonds. Hypothetical IR spectra for all the rings considered have been also calculated. Analysis of these hypothetical spectra leads to the conclusion that the whole spectrum can be divided into four wavenumbers regions, 1200–1100 cm −1 stretching Si–O(Si) vibrations; 1000–800 cm −1 stretching Si–O − vibrations; 800–600 cm −1; the region in which a band characteristic of silicooxygen rings appears, and below 600 cm −1 bending −O–Si–O − and (Si)O–Si–O(Si). It has been also found that as the number of ring members increases the ‘ring band’ shifts to lower wavenumbers: 725 cm −1 for three-membered rings, 650 cm −1 for four-membered rings and 610 cm −1 for six-membered rings. Calculated spectra have been compared with the experimental spectra of cyclosilicates. They showed good agreement in the 1200–600 cm −1 region. In the experimental spectra as well as in the calculated ones, with increasing the number of ring members the ‘ring band’ shifts towards lower wavenumbers.