The substituent effect on benzene dications
The substituent effect on benzene dications
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DOI：
10.1039/c3cp54089h
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发表时间：
2014-01-01
影响因子：
3.3
通讯作者：
Bickelhaupt, F. Matthias
中科院分区：
文献类型：
--
作者：
Palusiak, Marcin;Domagala, Malgorzata;Bickelhaupt, F. Matthias
最近有人假设，苯环及其4 n + 2 π电子类似物是抵抗的取代基效应，由于这样的系统往往保持其离域的字符。因此，4 n π-电子双阳离子形式的苯应该出现较低的抗取代基效应，与它的母体中性分子相比。出于这个原因，取代对苯的双阳离子形式的影响进行了彻底的研究和单和双取代（帕拉和间位型）的后果进行了评估，通过几个参数，包括各种芳香性指数和取代基效应稳定能（SESE）参数。结果表明，与中性苯不同，它的双阳离子形式对取代反应更为敏感。然而，双阳离子苯本身，作为一个部分，具有显着的赤字的电子，将被视为一个强吸电子中心，从而在一个合作的方式与供电子取代基和反合作的方式与吸电子取代基相互作用。苯的单重态和三重态双阳离子结构之间存在明显的差异，三重态结构的离域性更强，因此对取代基效应的敏感性比单重态结构低。最后，帕拉和间位型取代的研究，发现二取代的双阳离子苯表现出显着不同的行为，从中性苯。虽然在二阳离子苯中可以发现帕拉和间位取代之间的差异，但导致这种观察结果的机制与中性苯中存在的机制不同。最后，它是如何以及为什么双电离的苯降低其芳香性的单线态双阳离子，而芳香性基本上是保守的三线态双阳离子。上述研究结果表明，在苯的带电类似物的情况下，芳香性指数可能会产生误导，并应非常谨慎地使用。
It was recently postulated that the benzene ring and its 4n + 2 pi-electron analogues are resistant to the substituent effect due to the fact that such systems tend to retain their delocalized character. Therefore, the 4n pi-electron dicationic form of benzene should appear to be less resistant to the substituent effect, as compared with its parent neutral molecule. For this reason the effect of substitution on the dicationic form of benzene was thoroughly investigated and the consequences of single and double substitution (of para- and meta-type) were assessed by means of several parameters, including various aromaticity indices and the Substituent Effect Stabilization Energy (SESE) parameter. It is shown that, distinct from neutral benzene, its dicationic form is much more sensitive to the substitution. However, the dicationic benzene itself, as a moiety with a significant deficit of electrons, will be considered as a strongly electron-withdrawing centre, thus interacting in a cooperative way with electron-donating substituents and in an anticooperative way with electron-withdrawing substituents. Clear differences between singlet-and triplet-state dicationic forms of benzene were also found. Triplet state structures seem to be significantly more delocalized, and as a consequence less sensitive to the substituent effect than the singlet state structures. Finally, the para- and meta-type substitution was investigated and it was found that the disubstituted dicationic benzene exhibits significantly different behaviour from that of neutral benzene. Although the difference between para- and meta-substitution can be found for dicationic benzene, the mechanism responsible for such an observation is different from that present in neutral benzene. Finally, it is shown how and why double ionization of benzene reduces its aromatic character in the singlet dication whereas aromaticity is essentially conserved in the triplet dication. The above findings highlight that in the case of charged analogues of benzene the aromaticity indices can be misleading and are to be used with great precaution.