Influence of thermal vibrations on aromaticity of benzene

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.