Structure and Dynamics of Octamethyl-POSS Nanoparticles

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• 作者：Niina Jalarvo ; Olivier Gourdon ; Georg Ehlers ; Madhusudan Tyagi ; Sanat K. Kumar ; Kerwin D. Dobbs ; Robert J. Smalley ; William E. Guise ; Anibal Ramirez-Cuesta ; Christoph Wildgruber ; Michael K. Crawford
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：March 13, 2014
• 年：2014
• 卷：118
• 期：10
• 页码：5579-5592
• 全文大小：610K
• 年卷期：v.118,no.10(March 13, 2014)
• ISSN：1932-7455

文摘

Polyoligosilsesquioxanes (POSS) are a large family of Si鈥揙 cage molecules that have diameters of 1鈥? 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.