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

Abstract

A new series of encapsulated fluoride polyhedral oligomeric silsesquioxane (POSS) materials, [(CH3)4N+][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 19F and 29Si 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 sect...