A Molecular Orbital Model of Gel-Silica Ir Spectra

Abstract

ABSTRACTThe 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 SiO2 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 (AS1TO) mode of the rings shifts from 1070 cm−1 for 2-member rings to 1100 cm−1 for 3-member rings, 1150 cnr1 for 4-member rings, 1140 cm−1 for 5-member and 1120 cm−1 for 6-member rings. The IR data show a 38 cm”1 shift of the AS1TO mode as the gel-silica density changes from 1.1 g/cc to 2.2 g/cc. Thus, the intensification and shift of the AS1TO mode in the gel-silica to higher wave-numbers corresponds to a change in the distribution to larger silicate size rings.