A novel approach in analyzing aromaticity by homo- and isostructural reactions:: An ab initio study of fluorobenzenes
A novel approach in analyzing aromaticity by homo- and isostructural reactions:: An ab initio study of fluorobenzenes
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DOI：
10.1021/jp0532871
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发表时间：
2005-11-24
影响因子：
2.9
通讯作者：
Müller, T
中科院分区：
文献类型：
--
作者：
Baric, D;Kovacevic, B;Müller, T
用Hartree-Fock（HF）和MP2模型研究了氟取代对苯稳定性的影响。结果表明，基于开链锯齿型多烯的同键反应是不令人满意的。离析物和产物的分子内相互作用的比较表明，它们不是很平衡的。因此，在讨论芳香性时，这些反应应该被放弃。同结构反应是探索芳香性的一个更好的工具，它使用环状单烯和二烯作为参考模型化合物。它们的重原子被强制采取平面几何形状，以实现sigma/pi分离。HF/cc-pVTZ计算表明，苯的外在芳香性B Δ E（Ease）（B）同时来自σ和π贡献。如果采用股东能量分配方案，它们分别为-14.8和-23.1千卡/摩尔。这一结果意味着σ-和π-骨架都有助于B的芳族稳定化，后者更重要。总芳族稳定化Δ E（容易）（B）为-37.9千卡/摩尔。Schleyer的茚-异茚异构化方法也有力地表明，决定苯部分的芳香稳定性的决定性因素是π-电子骨架。如果1，3-环己二烯被用作规范化合物，则外部芳香性的起源被确定为σ-和π-电子的核电子吸引力增加。此外，通过使用一个系统的同构反应，它最终证明，氟苯表现出显着的取代基效应的加和性，就多取代苯的稳定性而言。这个加和性规则是如此精确，以至于它能够描绘氟排斥和芳香性缺陷Δ E（AD）。似乎Δ E（AD）值在下一个最近（邻位）位置处的连续氟取代后增加，从而使得多氟代苯不太稳定。
The influence of fluorine substitutions on the stability of benzene is examined by using the Hartree-Fock (HF) and MP2 models. It is conclusively demonstrated that homodesmotic reactions based on the open-chain zigzag polyenes are unsatisfactory. A comparison of the intramolecular interactions of educts and products shows that they are not well balanced. Hence, these reactions should be abandoned in discussing aromaticity. A much better vehicle for exploring aromaticity is provided by homostructural reactions, which employ cyclic monoene and diene as reference model compounds. Their heavy atoms are enforced to assume planar geometries to enable sigma/pi separation. The HF/cc-pVTZ calculations show that extrinsic aromaticity of benzene B Delta E(ease)(B) arises both from the sigma- and pi-contributions. They are -14.8 and -23.1 in kcal/mol, respectively, if the stockholder energy partitioning scheme is employed. This result implies that both the sigma- and pi-frameworks contribute to the aromatic stabilization of B, the latter being more important. The total aromatic stabilization Delta E(ease)(B) is -37.9 kcal/mol. Schleyer's indene-isoindene isomerization approach also strongly indicates that the decisive factor in determining the aromatic stability of the benzene moiety is the pi-electron framework. The origin of extrinsic aromaticity is identified as the increased nuclear-electron attraction of both sigma- and pi-electrons, if 1,3-cyclohexadiene is used as a gauge compound. Further, by using a system of isostructural reactions, it is conclusively demonstrated that fluorobenzenes exhibit a remarkable additivity of the substituent effects, as far as the stability of multiply substituted benzenes is concerned. This additivity rule is so accurate that it enables delineation of the fluorine repulsions and the aromaticity defect Delta E(AD). It appears that the Delta E(AD) values increase upon sequential fluorine substitution at the next nearest (vicinal) position thus making multiply fluorinated benzenes less stable.