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

The 3-dimensional gas-phase conformations of polyhedral oligomeric silsesquioxanes (POSS), R8Si8O12, capped with styryl and phenylethyl substituents (R) and cationized by sodium were examined. MALDI was used to generate sodiated styryl–POSS (Na+Sty8T8) and phenylethyl–POSS (Na+PhEt8T8) 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+Sty8T8 while only one conformer was detected for Na+PhEt8T8. Theoretical modeling of Na+Sty8T8, 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+PhEt8T8 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+Sty8T8 (∼20% smaller cross-section). The calculated cross-section of the predicted Na+PhEt8T8 structure agrees very well with the experimental cross-section obtained from the ion mobility experiments (∼1% error). © 2003 Elsevier Science B.V. All rights reserved.