Li Hung-Chih, Lee Cheng-Ying, McCabe Clare, Striolo Alberto, Neurock Matthew
Department of Chemical Engineering, Vanderbilt University, Nashville, TN 37235, USA.
J Phys Chem A. 2007 May 10;111(18):3577-84. doi: 10.1021/jp0672757. Epub 2007 Apr 19.
Ab initio quantum mechanical calculations have been performed to establish the potentials for alkyl-substituted polyhedral oligomeric silsesquioxane (POSS) monomers RxH8-x(SiO1.5)8. More specifically, we have examined the unsubstituted POSS (SiO1.5H)8 cage as well as linear and cyclic alkyl-substituted cages where one of the terminating hydrogen atoms is replaced by a hydrocarbon group, that is, R1H7(SiO1.5)8. The results for the minimum-energy configurations indicate that the presence of the linear hydrocarbon chains and cyclic intermediates have very little effect on the structure of the POSS cage. Although the POSS monomeric cage does influence the partial charges of the first few carbon atoms covalently bound to the POSS monomer, its effect on the structural properties of the alkyl chain is small. Differences arise, however, for cyclic alkyl substitutents bound to the POSS cage due to the repulsive interactions between the POSS cage and bulkier cyclic intermediates that result upon rotation of the Si-C-C-C dihedral angles. The interatomic potentials for these rotational, or torsional, terms need to be modified slightly in order to appropriately simulate sterically hindered substitutents on the cage. Our results suggest that combining an atomistic force field independently developed to describe silsesquioxanes with an independent atomistic model developed to describe hydrocarbon chains can be used in classical molecular simulation studies of most alkyl-silsesquioxanes. This avoids the need to develop specific force fields for each substituted POSS cage studied and opens up the possibility of using molecular simulation to probe the thermodynamic and structural properties of these unique nanoscale building blocks.
已进行从头算量子力学计算，以确定烷基取代的多面体低聚倍半硅氧烷（POSS）单体RxH8-x(SiO1.5)8的势能。更具体地说，我们研究了未取代的POSS（SiO1.5H）8笼以及线性和环状烷基取代的笼，其中一个末端氢原子被烃基取代，即R1H7(SiO1.5)8。最低能量构型的结果表明，线性烃链和环状中间体的存在对POSS笼的结构影响很小。虽然POSS单体笼确实会影响与POSS单体共价结合的前几个碳原子的部分电荷，但其对烷基链结构性质的影响很小。然而，由于POSS笼与较大的环状中间体之间的排斥相互作用，当Si-C-C-C二面角旋转时会产生这种相互作用，因此与POSS笼结合的环状烷基取代基会出现差异。为了适当地模拟笼上的空间位阻取代基，这些旋转或扭转项的原子间势能需要稍微修改。我们的结果表明，将独立开发的用于描述倍半硅氧烷的原子力场与独立开发的用于描述烃链的原子模型相结合，可用于大多数烷基倍半硅氧烷的经典分子模拟研究。这避免了为每个研究的取代POSS笼开发特定力场的需要，并开辟了使用分子模拟来探究这些独特纳米级构建块的热力学和结构性质的可能性。
