Department of Physical Chemistry, Collegium Medicum, Nicolaus Copernicus University, Kurpińskiego 5, 85-950 Bydgoszcz, Poland.
Phys Chem Chem Phys. 2011 Jul 28;13(28):12998-3008. doi: 10.1039/c1cp20165d. Epub 2011 Jun 20.
The values of twenty-eight aromaticity indices were estimated for distorted benzene geometries coming from normal modes of thermal vibrations at room temperature. The applied Principal Component Analysis (PCA) revealed that four orthogonal principal components are sufficient for the description of 80% of the population variance. The importance of different aromaticity criteria was confirmed by values of variable loadings. The first principal component has a quite complex composition. Although the highest contribution to PC1 comes from ATI the roles of magnetic susceptibility, REC, EN and A(j) are also non-negligible. The second orthogonal set of variables constituting PC2 comprises PDI and to a lesser extent REC. The importance of θ and EN is emphasized by contributions to PC3. The fourth PC is defined mostly by ΔE. This suggests that geometry alterations encountered during thermal vibrations are associated with changes in electron delocalization and chemical shift. Additionally, electron density changes along normal vibration modes affect also the resonance energy of the ring. Although thermal vibrations of benzene at room temperature do not exceed ±25% and most of analyzed parameters fluctuate within ±10%, the very different molecular properties must be taken into account for proper description of consequences of molecular vibrations on π-electron delocalization in a benzene ring.
在室温下的热振动的正常模式下，对扭曲的苯几何形状的二十八个芳香性指数的值进行了估计。应用主成分分析（PCA）表明，对于 80%的总体方差描述，四个正交的主成分就足够了。通过变量载荷值，确认了不同芳香性标准的重要性。第一主成分具有相当复杂的组成。尽管 ATI 对 PC1 的贡献最大，但磁化率、REC、EN 和 A(j) 的作用也不可忽视。构成 PC2 的第二组正交变量包括 PDI，而 REC 的作用则较小。θ 和 EN 的重要性通过对 PC3 的贡献得到了强调。第四个 PC 主要由ΔE 定义。这表明，在热振动过程中遇到的几何形状改变与电子离域和化学位移的变化有关。此外，沿正常振动模式的电子密度变化也会影响环的共振能。尽管苯在室温下的热振动不超过±25%，并且大多数分析参数在±10%内波动，但必须考虑到非常不同的分子性质，以正确描述分子振动对苯环中π电子离域的影响。
