Assignment of the vibrational spectra of the parent polysilsesquioxane (POSS): Octahydridosilasequioxane, H8Si8O12

Abstract

Polysilsesquioxanes (POSS) are molecules with the empirical formula (RSiO1.5)n where R is a hydrogen atom or hydroxyl or an organic moiety such as an alkyl, alkene, acrylate or epoxide. The silicon atoms occupy the corners of a cube and oxygen atoms are located on the edges, the versatility of silsesquioxanes arises from the vacant fourth position of silicon. The choice of substituent enables a wide variety of properties to be engineered in a straightforward manner. The parent POSS is octasilsesquioxane, H8Si8O12, with R=H and n=8. The present work employs periodic density functional theory calculations in conjunction with all the available vibrational (infrared, Raman and inelastic neutron scattering) spectra to generate a complete assignment of all the modes of the parent POSS octasilsesquioxane and some of its isotopomers for both the free, (Oh), molecule and the solid state material (C3i site symmetry) including the forbidden and very weak modes. The latter are of interest because in less symmetrical silsesquioxanes, these modes will be activated.