3-dimensional structural characterization of cationized polyhedral oligomeric silsesquioxanes (POSS) with styryl and phenylethyl capping agents
3-dimensional structural characterization of cationized polyhedral oligomeric silsesquioxanes (POSS) with styryl and phenylethyl capping agents
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DOI：
10.1016/s1387-3806(03)00068-x
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发表时间：
2003-05-15
影响因子：
1.8
通讯作者：
Bowers, MT
中科院分区：
文献类型：
--
作者：
Baker, ES;Gidden, J;Bowers, MT
研究了以苯乙烯基和苯乙基取代基（R）封端并经钠离子化的多面体低聚倍半硅氧烷（POSS）R8 Si 8 O 12的三维气相构象。利用MALDI制备了钠化苯乙烯基POSS（Na（+）Sty（8）T（8））和苯乙基POSS（Na+ PhEt 8 T8）离子，并利用离子迁移率法测量了它们在氦气中的碰撞截面。实验观察到Na（+）Sty（8）T（8）的五种不同碰撞截面的构象，而Na+ PhEt 8 T8只检测到一种构象. Na（+）Sty（8）T（8）的理论模型，使用分子力学/动力学计算，预测了三种低能构象。在每个构象异构体中，Na+离子与Si-O笼一侧的四个氧结合，而苯乙烯基则远离笼延伸。然而，不同数量的苯乙烯基基团“配对”在一起（形成2、3或4对），产生三种不同的构象。这些构象的计算截面与离子迁移率实验中得到的最大三个截面相匹配（误差约为2%）。然而，如果一个或两个苯乙烯基旋转，使得苯基相对于笼上的Si原子是“顺式”的（即，Si-C=C-C二面角从180度变化到0度），通过理论预测了两个较小的构象，其横截面与从离子迁移率实验获得的最小两个值相匹配（1-2%误差）。Na+ PhEt 8 T8的理论建模产生一种低能构象，其中Na+离子与Si-O笼上的一个氧结合，并夹在两个苯基之间。其余的苯乙基向Si-O笼折叠，产生比Na（+）Sty（8）T（8）明显更紧密的结构（类似于小20%的横截面）。预测的Na+ PhEt 8 T8结构的计算横截面与离子迁移率实验获得的实验横截面非常一致（误差接近1%）。(C)2003 Elsevier Science B. V.保留所有权利。
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(+)Sty(8)T(8)) 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(+)Sty(8)T(8) while only one conformer was detected for Na+PhEt8T8. 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 (similar to2% 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 0degrees) 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(+)Sty(8)T(8) (similar to20% 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 (similar to1% error). (C) 2003 Elsevier Science B.V. All rights reserved.