Structural Characterization of POSS Siloxane Dimer and Trimer (POSTPRINT)
CALIFORNIA UNIV SANTA BARBARA DEPT OF CHEMISTRY AND BIOCHEMISTRY
Pagination or Media Count:
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 Cy7T8sub2OdotNa and Cp7T8sub2OdotNa, 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 or eclipsed experiment fits the staggered conformation best. For the cyclohexyl dimer, on the other hand, folding of the Cy groups seems to be important. Similar to the Cy dimer, for the trimer theory indicates that a low energy exo-exo family of structures fits the experimental cross-section only if several Cy-groups are folded as compared to unfolded structures. Modeling shows the non-bonded 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 A cage-cage center distance which may be used to evaluate cage packing in other POSS oligomers.
- Industrial Chemistry and Chemical Processing
- Polymer Chemistry