A computational study of the molecular and crystal structure and selected physical properties of octahydridosilasequioxane, (Si2O3H2)4. II. Vibrational analysis

A computational study of octahydridosilasequioxane, Si8O12H8, as a free molecule and when embedded in the unit cell R-3, Z=3, showed that the point group of the free molecule is indeed Oh, but that its crystal symmetry is reduced to C3i. Since the molecular and site-group symmetries influence the vibrational structure of a molecule, a full computational vibrational analysis of the isolated molecule and when embedded in the crystal lattice, is reported here. The analysis of the free molecular spectra given here agrees with that of its experimental infra-red (IR)-spectra and allows the assignment of all the vibrational modes, while the computed phonon dispersion of the crystal confirms the assignment of the internal vibrational modes of the molecule in the crystal. The computed and experimental IR spectra as well as Raman spectra show no indication of serious vibrational intermolecular coupling owing to the presence of multiple molecules in the unit cell. This may be the result of a weak intermolecular vibrational coupling in the solid state, which may feature in the low-frequency modes.