Calculations of silicooxygen ring vibration frequencies

In the work model calculations of the vibrations of ideally isolated silico oxygen rings (using PM3 method) have been carried out, three-, four-, and s ix-membered rings have been considered. It has been found that that the thr ee-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 b ridging Si-O-Si bonds are;ca. 0.02-0.04 Angstrom shorter than the non-bridg ing Si-O- bonds. Hypothetical IR spectra for all the rings considered have been also calculated. Analysis of these hypothetical spectra leads to the c onclusion that the whole spectrum can be divided into four wavenumbers regi ons, 1200-1100 cm(-1) stretching Si-O(Si) vibrations; 1000-800 cm(-1) stret ching Si-O- vibrations; 800-600 cm(-1); the region in which a band characte ristic 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 memb ers increases cm the 'ring band' shifts to lower wavenumbers: 725 cm(-1) fo r three-membered rings, 650 cm(-1) for four-membered rings and 610 cm(-1) f or six-membered rings. Calculated spectra have been compared with the exper imental spectra of cyclosilicates. They showed good agreement in the 1200-6 00 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 tow ards lower wavenumbers. (C) 1999 Elsevier Science B.V. All rights reserved.