| Abstract
| - Ab initio quantum mechanical calculations have been performed to establish the potentials for alkyl-substitutedpolyhedral oligomeric silsesquioxane (POSS) monomers RxH8-x(SiO1.5)8. More specifically, we have examinedthe unsubstituted POSS (SiO1.5H)8 cage as well as linear and cyclic alkyl-substituted cages where one of theterminating hydrogen atoms is replaced by a hydrocarbon group, that is, R1H7(SiO1.5)8. The results for theminimum-energy configurations indicate that the presence of the linear hydrocarbon chains and cyclicintermediates have very little effect on the structure of the POSS cage. Although the POSS monomeric cagedoes influence the partial charges of the first few carbon atoms covalently bound to the POSS monomer, itseffect on the structural properties of the alkyl chain is small. Differences arise, however, for cyclic alkylsubstitutents bound to the POSS cage due to the repulsive interactions between the POSS cage and bulkiercyclic intermediates that result upon rotation of the Si−C−C−C dihedral angles. The interatomic potentialsfor these rotational, or torsional, terms need to be modified slightly in order to appropriately simulate stericallyhindered substitutents on the cage. Our results suggest that combining an atomistic force field independentlydeveloped to describe silsesquioxanes with an independent atomistic model developed to describe hydrocarbonchains can be used in classical molecular simulation studies of most alkyl-silsesquioxanes. This avoids theneed to develop specific force fields for each substituted POSS cage studied and opens up the possibility ofusing molecular simulation to probe the thermodynamic and structural properties of these unique nanoscalebuilding blocks.
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