Alkanes versus Oligosilanes: Conformational Effects on σ-Electron Delocalization
Alkanes versus Oligosilanes: Conformational Effects on σ-Electron Delocalization
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DOI：
10.1021/jacs.1c10616
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发表时间：
2021-12-29
影响因子：
15
通讯作者：
Michl, Josef
中科院分区：
文献类型：
--
作者：
Jovanovic, Milena;Michl, Josef
观察和计算都表明，在前线分子轨道σ-电子离域的程度和构象依赖性是完全不同的烷烃CnH 2n +2和低聚硅烷SinH 2n +2，等排和等电子饱和链由碳或硅原子，分别。我们发现，不同的构象效应可以理解在简单直观的条款。有两种模式的α-电子离域，强构象敏感的骨架离域通过骨架X-X键（共轭和σ-超共轭）和只有弱构象敏感的横向离域通过横向X-H键（σ-超共轭和σ-同共轭）。在烷烃中，这两种模式都是活跃的，并相互补充，导致所有构象的离域。在低聚硅烷，只有骨架离域的孔是重要的前沿轨道，甚至更简单的梯形C模型提供了一个足够直观的描述强的σ-电子离域的构象依赖。最终，这种差异主要是由于碳和氢的电负性相似，而硅的电负性较低，这导致了Si-H键的极化。这种理解已经来自于一个简单的Hiickel级扩展阶梯H模型的无限规则螺旋链的近似代数解的分析，使用一个孔的有效质量作为离域的措施。该模型是从经典的Sandorfy H模型中推导出来的，并通过拟合密度泛函或Hartree-Fock理论的结果来参数化。
Observations and computations both suggest that the extent and the conformational dependence of sigma-electron delocalization in frontier molecular orbitals are quite different in alkanes CnH2n+2 and oligosilanes SinH2n+2, the isosteric and isoelectronic saturated chains built from carbon or silicon atoms, respectively. We find that the different conformational effects can be understood in simple intuitive terms. There are two modes of a-electron delocalization, strongly conformationsensitive skeletal delocalization through backbone X-X bonds (aconjugation and sigma-hyperconjugation) and only weakly conformationsensitive lateral delocalization through lateral X-H bonds (sigma-hyperconjugation and sigma-homoconjugation). In alkanes, both modes are active and complement each other, leading to delocalization in all conformations. In oligosilanes, only skeletal delocalization of holes is important in frontier orbitals, and the even simpler ladder C model provides an adequate intuitive description of the strong conformational dependence of sigma-electron delocalization. Ultimately, the difference is primarily due to the similar electronegativity of carbon and hydrogen as opposed to the lower electronegativity of silicon, which causes a polarization of Si-H bonds. This understanding has been derived from an analysis of approximate algebraic solutions of a simple Hiickel-level extended ladder H model for an infinite regular helical chain, using the effective mass of a hole as a measure of delocalization. This model is derived from the classical Sandorfy H model, and is parametrized by fitting to results of density functional or Hartree-Fock theory.