Aromaticity and antiaromaticity in the cyclic 6π and 4π molecules of carbon and silicon E ₆ H ₆ and E ₄ H ₄ (E = C, Si)

Quantum chemical calculations using density functional theory at the BP86/def2-TZVPP level are reported for the structures and aromaticities of the monocyclic molecules E₆H₆ and E₄H₄ (E = C, Si). The results reveal drastic differences between the carbon and silicon homologues. Benzene (1b) is the global energy minimum on the C₆H₆ PES whereas planar <i>D</i>_6h Si₆H₆ (2d) is not an energy minimum and the <i>D</i>_3d form 2c is higher in energy than the prismane isomer 2a. There is an ubiquitous number of stable phenyl compounds but the only experimentally known Si₆R₆ compound has the structure of the tricyclic species 2b, which is lower in energy than 2c. In sharp contrast, the homologous carbon isomer 1c is more than 120 kcal mol^-1 higher in energy than 1b. The carbon compounds C₆H₆ and C₄H₄ show a characteristic preference for substitution reaction of benzene 1b and for addition reaction of cyclobutadiene 3a. The Si₆H₆ silicon homologue 2c has a weaker preference for substitution reaction than benzene, but also tetrasilacyclobutadiene 4a prefers substitution over the addition reaction. The comparison of the calculated (pseudo) π conjugation of the cyclic compounds and acyclic reference systems suggests aromatic stabilization/destabilization for the carbon systems. The values for the silicon compounds are inconclusive and the separation of σ and π interactions is difficult due to the strong deviation of some silicon systems from planarity. The NICS values are not a reliable indicator for aromatic stabilization due to π conjugation. Chemical bonding models that have been developed and derived for compounds in the first octal series of the periodic table are only suitable to a limited extent for molecules with heavier main group atoms. This comes from the radii of the s/p valence orbitals of the atoms, which are very similar for the first octal row atoms leading to effective sp hybridization. The chemical bonds of the heavier atoms have a much higher p character because the radius is bigger than the valence s orbitals.