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ABSTRACT The infrared vibrational modes of sol-gel derived optical silica monoliths depend upon thermal treatments and chemical environments, as shown by Fourier transform infrared spectroscopy (FTIR). A semi-empirical quantum mechanical theory (PM-3 in MOPAC 6.1) is used to analyze the structural changes responsible for the spectral shifts. Optimized structures of 2-member, 3-, 4-, 5- and 6-member rings of SiO 2 are calculated. The force constants for the molecular bonds in the rings are obtained and converted to the associated vibrational spectra for the rings. The peak position of the asymmetric transverse optical (ASITO) mode of the rings shifts from 1070 cm-' for 2-member rings to 1100 cm' for 3-member rings, 1150 cm-1 for 4-member rings, 1140 cm-' for 5-member and 1120 cm' for 6-member rings. The IR data show a 38 cm-' shift of the ASJTO mode as the gel-silica density changes from 1.1 g/cc to 2.2 g/cc. Thus, the intensification and shift of the ASJTO mode in the gel-silica to higher wave-numbers corresponds to a change in the distribution to larger silicate size rings. INTRODUCTION Amorphous silica (a-silica) contains a distribution of Si-O-Si bond distances and angles1 , and a distribution of 2-, 3-, 4-, 5-, 6-membered rings of silica tetrahedra.2 '3 Quantum mechanical calculations of silicate model clusters, using ab initid'4 and semi-empirical' methods, show that the geometries of molecules and the theoretical IR and Raman spectra are in general agreement with experiment. However, the limitations of cluster size in the ab initio calculations 3'4 and the theoretical approximations of the modified neglect of differential overlap (MNDO) method do not yield a complete match with experiment. 5 We have used a later version of a semi-empirical molecular orbital (MO) method, PM-3 (contained in MOPAC 6.1)6, to calculate the optimized geometries and IR spectra of 2-, 3-, 4-, 5-, and 6-membered silica rings.7 Force constants are obtained with PM-3 and converted to a vibrational spectrum using a FORCE, normal mode analysis, calculation within MOPAC, as described by Stewart.8' 9 The results from the force constants calculations of PM-3 include the vibration frequencies and energy contributions (in %) of all pairs of atoms in the molecule. The intensity of the vibrational transitions that are infrared-active are obtained and plotted. The calculated transition lines were fitted with gaussian distributions having a half width of 50 cm-' to make the predicted a-silica spectrum resemble the experimental spectrum more closely.7 The PM-3 calculation yields a distribution of S-O-Si and O-Si-O bond angles7 that are similar to those obtained by Mozzi and Warren' using XRD Pair Function Distribution Analysis of a - S0 2. The PM-3 method also produces vibrational modes7 in the range of 1120-1150 cm-' which match the AS1TO and AS 2TO modes of a - Si0 2, as discussed by Kirk.'"

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Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society

This study compares the calculated vibrational spectra of the various silica ri