Lin Xuhui, Wu Wei, Wiberg Kenneth B, Mo Yirong
The State Key Laboratory of Physical Chemistry of Solid Surfaces, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, iChEM, and College of Chemistry and Chemical Engineering , Xiamen University , Xiamen 361005 , China.
Department of Chemistry , Western Michigan University , Kalamazoo , Michigan 49008 , United States.
J Phys Chem A. 2018 Sep 27;122(38):7716-7722. doi: 10.1021/acs.jpca.8b07427. Epub 2018 Sep 14.
Most recently, Wiberg and Rablen examined a few substituted methanes and identified the polar effect, which is associated with the atomic charge at the central carbon, as a possible factor for bond energy changes while the exact role of the hyperconjugation effect is unsettled. In this work, we revisited a series of substituted methanes CHX ( n = 0-4, X= F, Cl, CN, OCH) by explicitly computing the intramolecular electron delocalization energies using the simplest variant of ab initio valence bond (VB) theory, namely the block-localized wave function (BLW) method. This BLW method is designated to derive electron-localized states where intramolecular electron delocalization is "turned off". Computations show that the deactivation of the intramolecular electron delocalization in these molecules only slightly increases the charge (i.e., reduces the atomic population) at the central carbon in all systems. This seems consistent with the polar effect therein as proposed by Wiberg and Rablen. But apart from the stabilizing polar effect, there should be steric (including both Pauli and electrostatic) repulsion among the substituent groups. This is evidenced by the stretched C-X bonds in CX compared with the same bonds in HCX (X = Cl, CN) when the specific n → σ orbital interactions are "turned off", though there is indeed notable polar effect in fluoromethanes considering fluorine is the most electronegative atom. By focusing on one C-X' bond, we show that its involved orbital interactions enhance the bond strength from HC-F' (by 11.6 kcal/mol) to FC-F' (by 18.7 kcal/mol) but weaken from HC-CN' (by 11.6 kcal/mol) to (CN)C-CN' (by 9.3 kcal/mol).
最近，维伯格和拉布伦研究了几种取代甲烷，并确定了与中心碳原子上的原子电荷相关的极性效应，认为这可能是键能变化的一个因素，而超共轭效应的确切作用尚未确定。在这项工作中，我们重新审视了一系列取代甲烷CHXₙ（n = 0 - 4，X = F、Cl、CN、OCH₃），通过使用从头算价键（VB）理论的最简单变体，即块定域波函数（BLW）方法，明确计算分子内电子离域能。这种BLW方法旨在推导分子内电子离域被“关闭”的电子定域态。计算表明，这些分子中分子内电子离域的失活仅略微增加了所有体系中中心碳原子上的电荷（即减少了原子布居）。这似乎与维伯格和拉布伦提出的其中的极性效应一致。但除了稳定的极性效应外，取代基之间应该存在空间（包括泡利和静电）排斥。当特定的n → σ轨道相互作用被“关闭”时，与HCX（X = Cl、CN）中的相同键相比，CX中C - X键的伸长就证明了这一点，尽管考虑到氟是电负性最强的原子，氟甲烷中确实存在显著的极性效应。通过关注一个C - X'键，我们表明其涉及的轨道相互作用增强了从HC - F'（增强11.6 kcal/mol）到FC - F'（增强18.7 kcal/mol）的键强度，但从HC - CN'（减弱11.6 kcal/mol）到(CN)C - CN'（减弱9.3 kcal/mol）则减弱了键强度。
