Conformational analysis and vibrational spectroscopic studies of tetraethoxysilane and its hydrolysis products: a DFT prediction

Abstract

The density functional theory was used to carry out a systematic investigation about the conformational and vibrational analyses of tetraethoxysilane (TEOS) and its hydrolysis products (Si(OCH2CH3)4−n(OH)n, n = 1–4). All possible potential energy curve (PEC) scans were performed in different symmetry point groups, D2d, S4, C2v and Cs, for the fully hydrolyzed product of TEOS, Si(OH)4. The main results indicate clear differences in the spectral profile of each molecular system, which can be used for monitoring the sol-gel processes at the hydrolysis stage. The most significative changes take place in 750–1000 cm−1, where mainly the SiO asymmetric stretching and δSiOH angular deformation modes are localized. A significant coupling between SiO and δSiOH is observed for the computed spectra of Si(OH)4 in gas-phase, with more pronounced effect for S4 ad D2d symmetries. For the partial hydrolysis products of TEOS (Si(OCH2CH3)4−n(OH)n, n = 1–3) this coupling is also present and it is evidenced by the fact that both vibrational bands in 750–900 cm−1 (δSiOH) and 900–1000 cm−1 (νSiO) show a great contribution of the SiO bond length displacements as much as of the SiOH bond angle changes.