Abstract:
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.
