The two-component bands at about 4500 and 800 cm −1 in infrared spectra of hydroxyl-containing silicas. Interpretation in terms of Fermi resonance

Abstract

The band at about 4550 cm −1 in the near-infrared (IR) spectra of OH-containing high-surface area silicas is known to resolve into a doublet under certain experimental conditions. The interpretation of this by Tsyganenko [Zh. Fiz. Khim. (Moscow) 56 (1982) 2330], which has received no recognition so far, is advocated and is elaborated as follows: (1) there is Fermi resonance interaction between the SiOH bending fundamental and a doubly excited vibration giving rise to a mixed-state doublet around 800 cm −1, (2) the Fermi doublet propagates in the near IR in combinations with the OH stretching vibration (≈3750 cm −1). From a review of available IR transmission, inelastic neutron scattering (INS), and Raman scattering experimental data and model densities of vibrational states it is inferred that the doubly excited state causing Fermi resonance is likely to be either the first overtone of the bridging-oxygen rocking mode (400 cm −1 in the density of vibrational states) or the binary summation of the bridging-oxygen symmetric stretching in bent Si–O–SiOH units (the D 1 Raman band near 490 cm −1, in the interpretation of Mulder et al.) with the A ′ component of the Si–OH wagging mode (expected at about 300 cm −1). In the near-IR spectra of OH-containing vitreous silica and natural opals, the irregular shape of the band near 4500 cm −1, as argued, has a similar origin.