Institute of Nuclear Chemistry and Technology, 16 Dorodna Street, 03-195 Warsaw, Poland.
J Chem Inf Model. 2023 Dec 25;63(24):7744-7754. doi: 10.1021/acs.jcim.3c01551. Epub 2023 Dec 6.
The article shows that the definition of the HOMA index of geometrical aromaticity satisfies the axioms of a similarity function between the examined and benzene ring. Consequently, for purely mathematical reasons, the index works exceptionally well as an index of aromaticity: it expresses a geometric similarity to the archetypal aromatic benzene. Thus, if the molecule is geometrically similar to benzene, then it is also chemically similar, and therefore, it is aromatic. However, the similarity property legitimizes using the HOMA-like indices to express similarity to molecules other than benzene, whether cyclic or linear and existing or hypothetical. The paper demonstrates an example of HOMA-similarity to cyclohexane, which expresses a (relaxed)-saturicity property not accompanied by strong structural strains or steric hindrances. Further, it is also shown that the HOMA index can evaluate the properties of whole molecules, such as 25 unbranched catacondensed isomers of hexacene. The index exhibits a significant quadratic correlation with the total energy differences of planar isomers from which the nonplanar ones deviate. Moreover, the HOMA index of hexacene isomers significantly correlates with the Kekulé count connected to the resonance energy in the Hückel approximation. As a result, the study shows that the HOMA index can be used not only for aromaticity analyses but also as a general chemical descriptor applicable to rings, chains, composed molecular moieties, or even whole molecules.
本文表明，HOMA 指数的几何芳香度定义满足被研究物与苯环之间相似函数的公理。因此，纯粹出于数学原因，该指数作为芳香度指数表现异常出色：它表达了与典型芳香苯的几何相似性。因此，如果分子在几何上与苯相似，那么它在化学上也相似，因此它是芳香的。然而，相似性属性使我们可以合理地使用类似于 HOMA 的指数来表达与除苯以外的分子的相似性，无论是环状的还是线状的，无论是存在的还是假设的。本文展示了 HOMA 与环己烷相似性的一个例子，它表达了一种（放松的）饱和性质，而不会伴随着强烈的结构应变或空间位阻。此外，还表明 HOMA 指数可以评估整个分子的性质，例如二十六元并五苯的 25 个无支链全并五苯异构体。该指数与偏离平面异构体的平面异构体之间的总能量差呈显著二次相关。此外，六并五苯异构体的 HOMA 指数与与 Hückel 近似中的共振能相关的凯库勒计数显著相关。结果表明，该研究表明 HOMA 指数不仅可用于芳香性分析，还可作为适用于环、链、组成分子部分甚至整个分子的通用化学描述符。
