3-Dimensional structural characterization of cationized polyhedral oligomeric silsesquioxanes (POSS) with styryl and phenylethyl capping agents

Abstract The 3-dimensional gas-phase conformations of polyhedral oligomeric silsesquioxanes (POSS), R 8 Si 8 O 12 , capped with styryl and phenylethyl substituents (R) and cationized by sodium were examined. MALDI was used to generate sodiated styryl–POSS (Na + Sty 8 T 8 ) and phenylethyl–POSS (Na + PhEt 8 T 8 ) ions and their collision cross-sections in helium were measured using ion mobility-based methods. Five distinct conformers with different collision cross-sections were experimentally observed for Na + Sty 8 T 8 while only one conformer was detected for Na + PhEt 8 T 8 . Theoretical modeling of Na + Sty 8 T 8 , using molecular mechanics/dynamics calculations, predicts three low-energy conformations. In each conformer, the Na + ion binds to four oxygens on one side of the SiO cage and the styryl groups extend away from the cage. However, different numbers of styryl groups “pair” together (forming 2, 3 or 4 pairs), yielding three different conformations. The calculated cross-sections of these conformers match the largest three cross-sections obtained from the ion mobility experiments (∼2% error). If, however, one or two of the styryl groups are rotated so that the phenyl groups are “ cis ” with respect to the Si atom on the cage (i.e., the SiCCC dihedral angle changes from 180 to 0°) two smaller conformers are predicted by theory whose cross-sections match the smallest two values obtained from the ion mobility experiments (1–2% error). Theoretical modeling of Na + PhEt 8 T 8 yields one low-energy conformation in which the Na + ion binds to one oxygen on the SiO cage and is sandwiched between two phenyl groups. The remaining phenylethyl groups fold toward the SiO cage, yielding a significantly more compact structure than Na + Sty 8 T 8 (∼20% smaller cross-section). The calculated cross-section of the predicted Na + PhEt 8 T 8 structure agrees very well with the experimental cross-section obtained from the ion mobility experiments (∼1% error).