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.

聚寡倍半硅氧烷（Polyoligosilsesquioxanes, POSS）是一类庞大的硅氧笼状分子家族，其直径介于1~2 nm，可被视为完美的单分散二氧化硅纳米颗粒。POSS可通过在表面连接多种功能配体的方式进行合成。

本文报道了对最简单的POSS纳米颗粒之一——八甲基-POSS（化学式为Si₈O₁₂(CH₃)₈，其中心Si₈O₁₂笼状结构被8个甲基配体环绕）的晶体结构与配体动力学的全面研究结果。中子粉末衍射数据表明，该体系存在显著依赖于温度的甲基旋转动力学行为。

研究团队采用拉曼散射与非弹性中子散射技术测定了该样品的振动光谱，并将实验测量结果与密度泛函理论（density functional theory）计算的预测值进行了对比。尤为关键的是，非弹性中子散射光谱观测到了甲基扭转振动的基频跃迁与第一泛频跃迁；在该分子的理想八面体对称构型下，此类跃迁在拉曼与红外光谱中均为禁阻跃迁。研究人员借助这些跃迁的能量值，计算得到了甲基扭转的势垒高度。

通过准弹性非相干中子散射技术对甲基动力学行为的直接测量，研究人员获得了氢原子的跳跃距离以及甲基旋转的活化能。综上，上述一系列研究结果完整呈现了该POSS分子的结构与配体动力学细节。