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A revised look at pi-electron delocalization in benzene

机译：苯中π电子离域的修订版

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Aromaticity occupies a central place in chemical thought. Aromaticity as such, is not an experimental quantity and no generally acceptable definition has been established to quantify this property. Conventionally, it is associated with conjugated n-systems with very high stability, planarity, delocalization with small bond length alternation, preference for substitution reactions over the addition reactions, and exalted diamagnetic susceptibilities. Aromaticity is associated with cyclic arrays of mobile electrons with favourable symmetries, leading to fully delocalized structures with equal bond lengths. Benzene is a seminal example of aromaticity. Antiaromatic systems are characterized by localized rather than delocalized electronic and geometric structures, #pi#-bonding and #sigma#-#pi# separation in the planar conjugated systems are an integral part of contemporary chemical thinking. The #sigma#-#pi# separation was the basis for the famous Huckel rules which account for the special stability for closed shell 4n+2 #pi#-electron systems and instability for 4n#pi#-electron systems. The stability of cyclopropenyl cation and the apparent instability of cyclobutadiene laid strong foundations to the 4n + 2 rule for aromaticity, and the 4n rule for antiaromaticity. These Huckel rules have become among the most powerful paradigms of chemistry, in general and organic chemistry, in particular. Consequently, aromatic species are associated with highly delocalized z-electronic component, with equal bond lengths, and special stability relative to an open-chain reference. In contrast, antiaromatic systems exhibit bond localizations and their geometries are non-uniform, exhibiting alternating C-C bond lengths, z-delocalization and geometry have thus found a strong link in the aromatic and antiaromatic organic compounds. This led to a 'popular belief' that symmetric geometries, with equal C-C bondlengths, in species such as benzene or allyl are due to the inherent tendency of #pi#-electrons for strong delocalization in the aromatic species. Apparently, these ideas have become so popular and powerful and also an integral part of our thinking in structural organic chemistry, without any concrete proof.
机译：芳香性在化学思想中占据中心位置。如此的芳香性不是实验量，并且尚未建立普遍可接受的定义以量化该性质。常规地，它与具有非常高的稳定性，平面性，具有小的键长变化的离域，比取代反应更优选取代反应以及抗磁化率高的共轭n系统相关。芳香性与具有良好对称性的移动电子的环状阵列相关，从而导致具有相同键长的完全离域结构。苯是芳香性的开创性例子。抗芳香族系统的特征在于局部的而不是局部的电子和几何结构，平面共轭体系中的＃pi＃键和#sigma＃-＃pi＃分离是当代化学思想不可或缺的一部分。 #sigma＃-＃pi＃分离是著名的Huckel规则的基础，该规则解释了封闭壳4n + 2＃pi＃-电子系统的特殊稳定性和4n＃pi＃-电子系统的不稳定性。环丙烯基阳离子的稳定性和环丁二烯的明显不稳定性为芳香性的4n + 2规则和抗芳香性的4n规则奠定了坚实的基础。这些Huckel规则已成为化学领域最强大的范式之一，特别是在一般有机化学领域。因此，芳香族物质与高度离域的z电子组分相关，它们具有相等的键长和相对于开链参考的特殊稳定性。相反，抗芳族体系表现出键的定位，并且它们的几何形状是不均匀的，表现出交替的C-C键长，z-离域和几何形状因此在芳族和抗芳族有机化合物中发现了牢固的联系。这导致了一种“普遍的信念”，即诸如苯或烯丙基的物种中具有相同C-C键长的对称几何形状是由于＃pi＃电子固有的趋向于在芳香族物种中发生强离域作用。显然，这些想法已经变得如此流行和强大，并且在没有任何具体证据的情况下，也是我们在结构有机化学中不可或缺的一部分。

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