Computational and Theoretical Chemistry Group (CATCO), Department of Chemistry, Southern Methodist University , 3215 Daniel Ave., Dallas, Texas 75275-0314, United States.
J Org Chem. 2016 Oct 21;81(20):9669-9686. doi: 10.1021/acs.joc.6b01761. Epub 2016 Sep 28.
Vibrational frequencies can be measured and calculated with high precision. Therefore, they are excellent tools for analyzing the electronic structure of a molecule. In this connection, the properties of the local vibrational modes of a molecule are best suited. A new procedure is described, which utilizes local CC stretching force constants to derive an aromaticity index (AI) that quantitatively determines the degree of π-delocalization in a cyclic conjugated system. Using Kekulé benzene as a suitable reference, the AIs of 30 mono- and polycyclic conjugated hydrocarbons are calculated. The AI turns out to describe π-delocalization in a balanced way by correctly describing local aromatic units, peripheral, and all-bond delocalization. When comparing the AI with the harmonic oscillator model of AI, the latter is found to exaggerate the antiaromaticity of true and potential 4n π-systems or to wrongly describe local aromaticity. This is a result of a failure of the Badger relationship (the shorter bond is always the stronger bond), which is only a rule and therefore cannot be expected to lead to an accurate description of the bond strength via the bond length. The AI confirms Clar's rule of disjoint benzene units in many cases, but corrects it in those cases where peripheral π-delocalization leads to higher stability. [5]-, [6]-, [7]-Circulene and Kekulene are found to be aromatic systems with varying degree of delocalization. Properties of the local vibrational modes provide an accurate description of π-delocalization and an accurate AI.
振动频率可以被高精度地测量和计算。因此，它们是分析分子电子结构的优秀工具。在这方面，分子局部振动模式的性质最适合。描述了一种新的程序，该程序利用局部 CC 伸缩力常数来推导一个芳香性指数 (AI)，该指数定量地确定了环状共轭体系中 π 离域的程度。使用苯作为合适的参考物，计算了 30 个单环和多环共轭烃的 AI。结果表明，AI 通过正确描述局部芳香单元、外围和全键离域，以平衡的方式描述了 π 离域。当将 AI 与 AI 的谐振子模型进行比较时，发现后者夸大了真实和潜在的 4n π 体系的反芳香性，或者错误地描述了局部芳香性。这是由于 Badger 关系（较短的键总是较强的键）的失败，这只是一个规则，因此不能期望通过键长来准确描述键强度。在许多情况下，AI 证实了 Clar 的不相交苯单元规则，但在那些外围 π 离域导致更高稳定性的情况下，它对其进行了修正。[5]-、[6]-、[7]-轮烯被认为是具有不同离域程度的芳香体系。局部振动模式的性质提供了对 π 离域的准确描述和准确的 AI。
