Structural Investigation of Encapsulated Fluoride in Polyhedral Oligomeric Silsesquioxane Cages Using Ion Mobility Mass Spectrometry and Molecular Mechanics

A new series of encapsulated fluoride polyhedral oligomeric silsesquioxane (POSS) materials, [(CH3)(4)N+][F-@(R8Si8O12)], where R = vinyl, phenyl, styrenyl, trifluoropropyl, nonafluorohexyl, or tridecafluorooctyl, were synthesized by the reaction of tetramethylammonium fluoride with the R8Si8O12 POSS in tetrahydrofuran. Encapsulation of the fluoride was confirmed with F-19 and Si-29 NMR spectroscopy. Ion mobility and molecular modeling methods were used to investigate the gas phase conformational properties of these POSS. Theoretical calculations demonstrate that the binding energy of fluoride to the interior of the POSS cage ranges from 70 to 270 kcal/mol as a function of substituent. Sodiated positive ions of the form H+[F-@R8T8]Na+ (T = SiO3/2, R = styrenyl, phenyl, and vinyl) were examined by MALDI; ESI was used to study the negative ions F-@R8T8 (R = styrenyl, phenyl, vinyl, trifluoropropyl, and nonafluorohexyl). The ion mobilities of these species were measured and used to calculate collision cross sections. These cross sections were compared to X-ray crystal structures and theoretical cross sections obtained from molecular mechanics and dynamics calculations. Experimental cross sections were consistent with all of the known X-ray crystal structures (styrenyl, vinyl, and phenyl POSS species). The experimental cross sections also agreed with the calculated cross sections for each species. As a result of the compact nature of the POSS cages, each sample had only one stable conformation, and only one low-energy family of structures was found for each set of sample calculations.