Structural Characterization of POSS Siloxane Dimer and Trimer (PREPRINT)
Abstract
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)2O Na+and (Cp7T8)2O 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 (exp 2)) or eclipsed (Sigma calc = 365 (EXP 2)); experiment fits the staggered conformation best (sigma expt = 355 (EXP 2)). For the cyclohexyl dimer, on the other hand, folding of the Cy groups seems to be important: (Sigma expt = 402 (EXP 2) vs unfolded Sigma calc = 421 (EXP 2) or folded Sigma calc = 407 (EXP 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 (EXP 2)) only if several Cy-groups are folded (Sigma calc = 557 (EXP 2)) as compared to unfolded structures (avg. Sigma calc = 570 (EXP 2)). 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.
Document Details
- Document Type
- Technical Report
- Publication Date
- Nov 14, 2005
- Accession Number
- ADA449111
Entities
People
- Ashwani Vij
- Connie Mitchell
- Joseph Schwab
- Mike Bowers
- Stan Anderson
- Timothy Haddad
Organizations
- Air Force Research Laboratory