Department of Chemistry, University of Wisconsin, Madison Wisconsin 53706, USA.
J Am Chem Soc. 2013 Apr 17;135(15):5762-7. doi: 10.1021/ja312222k. Epub 2013 Apr 9.
The paradoxically low basicity (despite high anionicity) of oxygen in the characteristic Si-O-Si linkages of silicone polymers is investigated with hybrid density functional and natural bond orbital (NBO) computational methods, extending a previous study of idealized disiloxane and dimethyl ether parent species to fully methylated derivatives that more faithfully model the silicone polymers of industrial and environmental importance. Despite the complicating distortions of the sterically crowded di-t-butyl ether "analog", the physical picture of enhanced hyperconjugative (resonance-type) delocalization in Si-O vs C-O bonding is essentially preserved (and indeed accentuated) in permethylated species. NBO-based orbital overlap diagrams are employed in conjunction with structural, hybridization, and polarity descriptors to illustrate the subtle phase-matching relationships that confer superior enthalpic and entropic stability (and low basicity) on permethylated Si-O-Si linkages. Our results challenge both ionic models of Si-O bonding and conventional electrostatic-type models of H-bonding and acid-base reactivity.
采用杂化密度泛函和自然键轨道（NBO）计算方法研究了硅氧烷聚合物中 Si-O-Si 键合特征的氧的反常低碱性（尽管阴离子性高），将先前对理想化的二硅氧烷和二甲醚母体物种的研究扩展到更真实地模拟工业和环境重要性的硅氧烷聚合物的全甲基化衍生物。尽管空间位阻拥挤的二叔丁基醚“类似物”的复杂扭曲，Si-O 与 C-O 键合中增强的超共轭（共振型）离域的物理图像在全甲基化物种中基本保持不变（实际上更加突出）。NBO 基轨道重叠图与结构、杂化和极性描述符结合使用，说明了赋予全甲基化 Si-O-Si 键合优异焓和熵稳定性（低碱性）的微妙的相位匹配关系。我们的结果挑战了 Si-O 键的离子模型和传统的静电型氢键和酸碱反应性模型。
