Structural characterization of POSS siloxane dimer and trimer

Ion mobility and molecular modeling methods were used to examine the gas-phase conformational properties of POSS (polyhedral oligomeric silsesquioxanes) siloxane oligomers. MALDI generated the sodiated dimers (Cy7T8)(2)O center dot Na+ and (Cp7T8)(2)O center dot Na+ and the trimer ion [Cy7T8O-Cy8Si8O11-OCy7T8] Na+ (T = SiO1.5, Cy = cyclohexyl, and Cp = cyclopentyl); their collision cross sections were measured in helium using ion mobility based methods. Experimental results are consistent with one observed conformer for each of the dimers and the exo-exo isomer for the trimer (formed by retention of configuration at silicon during synthesis). For the cyclopentyl dimer, theory predicts two separate conformer families based on whether the substituents on the POSS cages are staggered (sigma(calc) = 357 angstrom(2)) or eclipsed (sigma(calc) = 365 angstrom(2)); experiment fits the staggered conformation best (sigma(expt) = 355 angstrom(2)). For the cyclohexyl dimer, on the other hand, folding of the Cy groups seems to be important: (sigma(calc) = 402 angstrom(2) vs unfolded sigma(calc) = 421 angstrom(2) or partially folded sigma(calc) = 407 angstrom(2)). Similar to the Cy dimer, for the trimer theory indicates that a low-energy exo-exo family of structures fits the experimental cross section (sigma(expt) = 557 angstrom(2)) only if several Cy groups are folded (sigma(calc) = 557 angstrom(2)) as compared to unfolded structures (average sigma(calc) = 570 angstrom(2)). Modeling shows the nonbonded interactions of the capping Cp and Cy groups stabilize POSS cage packing leading to compact structures being lowest in energy. Modeling also suggests that two POSS cages bonded together by a single oxygen atom gives a benchmark 8.3-8.5 angstrom cage-cage center distance which may be used to evaluate cage packing in other POSS oligomers. X-ray crystal structures at 100 and 298 K for (Cy7T8)(2)O were carried out which show that solid state cage-cage intermolecular distances are perfectly consistent with the modeled POSS-PMA dimer and trimer structures. At room temperature there is considerable unresolved disorder of the cyclohexyl R groups in the X-ray structure due to rapid boat-chair interconversion.