Departamento de Química, Universidad de Guanajuato, Gto. México.
Acc Chem Res. 2012 Feb 21;45(2):215-28. doi: 10.1021/ar200117a. Epub 2011 Aug 17.
Aromaticity is indispensable for explaining a variety of chemical behaviors, including reactivity, structural features, relative energetic stabilities, and spectroscopic properties. When interpreted as the spatial delocalization of π-electrons, it represents the driving force for the stabilization of many planar molecular structures. A delocalized electron system is sensitive to an external magnetic field; it responds with an induced magnetic field having a particularly long range. The shape of the induced magnetic field reflects the size and strength of the system of delocalized electrons and can have a large influence on neighboring molecules. In 2004, we proposed using the induced magnetic field as a means of estimating the degree of electron delocalization and aromaticity in planar as well as in nonplanar molecules. We have since tested the method on aromatic, antiaromatic, and nonaromatic compounds, and a refinement now allows the individual treatment of core-, σ-, and π-electrons. In this Account, we describe the use of the induced magnetic field as an analytical probe for electron delocalization and its application to a large series of uncommon molecules. The compounds include borazine; all-metal aromatic systems Al(4)(n-); molecular stars Si(5)Li(n)(6-n); electronically stabilized planar tetracoordinate carbon; planar hypercoordinate atoms inside boron wheels; and planar boron wheels with fluxional internal boron cluster moieties. In all cases, we have observed that planar structures show a high degree of electron delocalization in the π-electrons and, in some examples, also in the σ-framework. Quantitatively, the induced magnetic field has contributions from the entire electronic system of a molecule, but at long range the contributions arising from the delocalized electronic π-system dominate. The induced magnetic field can only indirectly be confirmed by experiment, for example, through intermolecular contributions to NMR chemical shifts. We show that calculating the induced field is a useful method for understanding any planar organic or inorganic system, as it corresponds to the intuitive Pople model for explaining the anomalous proton chemical shifts in aromatic molecules. Indeed, aromatic, antiaromatic, and nonaromatic molecules show differing responses to an external field; that is, they reduce, augment, or do not affect the external field at long range. The induced field can be dissected into different orbital contributions, in the same way that the nucleus-independent chemical shift or the shielding function can be separated into component contributions. The result is a versatile tool that is particularly useful in the analysis of planar, densely packed systems with strong orbital contributions directly atop individual atoms.
芳香性对于解释各种化学行为是不可或缺的，包括反应性、结构特征、相对能量稳定性和光谱性质。当解释为π电子的空间离域时，它代表了许多平面分子结构稳定的驱动力。离域电子系统对外磁场敏感；它会产生一个具有特别长程的感应磁场。感应磁场的形状反映了离域电子系统的大小和强度，并可能对相邻分子产生很大的影响。2004 年，我们提出使用感应磁场作为估计平面和非平面分子中电子离域和芳香性程度的一种手段。此后，我们已经在芳香族、反芳香族和非芳香族化合物上测试了该方法，现在的改进允许对核心、σ-和π-电子进行单独处理。在本报告中，我们描述了使用感应磁场作为分析探针来研究电子离域，并将其应用于一系列不常见的分子。这些化合物包括硼氮烷；全金属芳香族体系 Al(4)(n-)；分子星 Si(5)Li(n)(6-n)；电子稳定的平面四配位碳；硼轮内的平面高次配位原子；以及具有通量内部硼簇部分的平面硼轮。在所有情况下，我们都观察到平面结构在π电子中表现出高度的电子离域，在某些情况下，在σ 骨架中也表现出高度的电子离域。从数量上看，感应磁场来自于分子的整个电子系统，但在长程范围内，来自离域电子π系统的贡献占主导地位。感应磁场只能通过实验间接证实，例如，通过核磁共振化学位移的分子间贡献。我们表明，计算感应场是理解任何平面有机或无机系统的有用方法，因为它对应于解释芳香分子中异常质子化学位移的 Pople 模型。事实上，芳香族、反芳香族和非芳香族分子对外部磁场的响应不同；也就是说，它们在长程上减少、增加或不影响外部磁场。感应磁场可以分解为不同的轨道贡献，就像核独立化学位移或屏蔽函数可以分离成组成贡献一样。其结果是一个功能强大的工具，在分析具有强轨道贡献的平面、密集堆积系统时特别有用，这些轨道贡献直接位于单个原子之上。
