Structure and Dynamics of Octamethyl-POSS Nanoparticles

Polyoligosilsesquioxanes (POSS) are
a large family of Si–O
cage molecules that have diameters of 1–2 nm and can be viewed
as perfectly monodisperse silica nanoparticles. POSS can be synthesized
with a wide variety of functional ligands attached to their surfaces.
Here we report the results of a comprehensive study of the crystal
structure and ligand dynamics of one of the simplest POSS nanoparticles,
octamethyl-POSS or Si8O12(CH3)8, where the central Si8O12 cage is surrounded
by eight methyl ligands. Neutron powder diffraction data highlight
the presence of strongly temperature-dependent methyl group rotational
dynamics. Vibrational spectra were measured using Raman and inelastic
neutron scattering techniques, and the results of the measurements
were compared with the predictions of density functional theory calculations.
In particular, the inelastic neutron scattering spectra show the fundamental
and first overtone transitions of the methyl torsional vibrations;
these transitions are forbidden in both Raman and infrared spectroscopy
for the molecule with its ideal octahedral symmetry. The energies
of these transitions are used to determine the height of the torsional
energy barrier. Direct measurements of the methyl group dynamics using
quasielastic incoherent neutron scattering provide the hydrogen atom
jump distance and the activation energy for rotation of the methyl
groups. Together these results provide a detailed picture of the structure
and ligand dynamics of this POSS molecule.