Structure of hybrid polyhedral oligomeric silsesquioxane propyl methacrylate oligomers using ion mobility mass spectrometry and molecular mechanics

Ion mobility and molecular modeling methods were used to examine the gas phase conformational properties of polyhedral oligomeric silsesquioxanes (POSS) propyl methacrylate (PMA) oligomers. MALDI was utilized to generate sodiated [(PMA)CP7T8](x)Na+ ions, x = 1, 2, and 3, and their collision cross-sections were measured in helium using ion mobility based methods. Experimental results indicate only one conformer for the Na(+)1-mer and Na(+)3-mer, but two or more conformers for the Na(+)2-mer. Theoretical modeling of the Na(+)1-mer using the AMBER suite of programs indicates only one family of low-energy structures is found, in which the sodium ion binds to the carbonyl oxygen on the PMA and 4 oxygens on one face of the POSS cage. The calculated cross-section of this family agrees very well with the experimental value, with < 2% deviation. For the Na(+)2-mer, theory predicts three separate conformer families based on whether the backbone attachments to the two POSS cages are cis, extended trans (larger), or trans (smaller). The calculated cross-sections agree very well with the two experimental values. For the Na(+)3-mer, theory indicates that a low energy syndiotactic family of structures has an average cross-section consistent with the experimental cross-section, while the isotactic structures are too large by 3%. Modeling shows the nonbonded interactions of the capping Cp groups stabilize POSS cage packing leading to compact structures being lowest in energy. Of importance are the facts that neither the cationizing metal ion nor the nature of the end groups affects the structure of the POSS containing oligomers.