Application of ion mobility to the gas-phase conformational analysis of polyhedral oligomeric silsesquioxanes (POSS)

Ion mobility experiments and molecular modeling calculations were used to investigate the gas-phase conformational properties of various polyhedral oligomeric silsesquioxanes (POSS) cationized by sodium. POSS, (RSiO3/2)(n), has a rigid Si-O cage with organic substituents attached to each Si atom. Na+POSS ions were formed by electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) and their collision cross-sections were measured in helium using ion mobility methods. Calculated cross-sections of theoretical models of the POSS ions, generated by molecular mechanics (MM) calculations, were compared to experiment for conformational identification. The calculations predict that the Na+ ion remains outside of the Si-O cage and binds to one to two oxygen atoms in the cage or interacts with two neighboring organic substituents. Cross-sections of X-ray structures were also compared to the experimental and theoretical data to determine if any changes occur to POSS in the gas phase (compared to the condensed phase) and to provide a check for the geometries predicted by theory (which used untested Si parameters). Several types of POSS compounds were investigated that had different Si-O cage sizes (Si6O9, Si8O12, Si10O15, Si12O18, ...) and different "R" substituents such as cyclohexyl, cyclopropyl, vinyl, and phenyl groups. Experimental, theoretical, and X-ray cross-sections differed by <2% for each POSS compound.