The Origin of Aromaticity: Important Role of the Sigma Framework in Benzene

The physical nature of aromaticity is addressed at a high ab initio level. It is conclusively shown that the extrinsic aromatic stabilization energy of benzene E (ease) B , estimated relative to its linear polyene counterpart(s), is very well‐reproduced at the Hartree–Fock (HF) level. This is a consequence of the fact that the contributions arising from the zero‐point vibrational energy (ZPVE) and electron correlation are rather small. More specifically, they yield together 2.0 kcal mol −1 to the destabilization of benzene. A careful scrutiny of the HF energies by virial theorem shows further that the kinetic energies of the σ and π electrons E ( T ) ${{{\sigma \hfill \atop HF\hfill}}}$ and E ( T ) ${{{{\rm \pi} \hfill \atop HF\hfill}}}$ are strictly additive in the gauge linear zig‐zag polyenes, which also holds for their sum E t (T) HF . This finding has the important corollary that E (ease) B is little dependent on the choice of the homodesmic reactions involving zig‐zag polyenes. A detailed physical analysis of the σ‐ and π‐electron contributions to extrinsic aromaticity requires explicit introduction of the potential energy terms V ne , V ee , and V nn , which signify Coulomb interactions between the electrons and the nuclei. The V ee term involves repulsive interaction V ${{{{\rm \sigma} {\rm \pi} \hfill \atop ee\hfill}}}$ between the σ and π electrons, which cannot be unequivocally resolved into σ and π contributions. The same holds for the V nn energy, which implicitly depends on the electron density distribution via the Born–Oppenheimer (BO) potential energy surface. Several possibilities for partitioning V ${{{{\rm \sigma} {\rm \pi} \hfill \atop ee\hfill}}}$ and V nn terms into σ and π components are examined. It is argued that the stockholder principle is the most realistic, which strongly indicates that E (ease) B is a result of favorable σ ‐framework interactions. In contrast, the π‐electron framework prefers the open‐chain linear polyenes.