What is so special about benzene? A comparison of selected carbon and silicon isomers E₆H₆ (E = C, Si)

Quantum chemical calculations are reported for the structures of cyclic molecules E₆H₆ (E = C, Si). The equilibrium geometries and relative energies of selected C₆H₆ and Si₆H₆ isomers reveal drastic differences between the carbon and silicon homologues. Benzene (1C) is the global energy minimum on the C₆H₆ potential energy surface (PES). In contrast, planar D_6h Si₆H₆ (1Si) is not an energy minimum and the nonplanar structure 1Si' with a chair conformation is higher in energy than the prismane isomer 2Si, which is the energetically lowest-lying minimum on the (SiH)₆ PES where all vertices are SiH. In sharp contrast, the prismane isomer of carbon 2C is more than 110 kcal mol^-1 higher in energy than 1C. There are also several isomers on the Si₆H₆ PES with bare Si atoms as vertices, which are even lower in energy than the prismane form 2Si. Substituted homologues of silaprismane 2Si and isomeric cyclic forms of Si₆R₆ with bare Si atoms as vertices are experimentally known. The analysis of the bonding situation shows that the different stabilities of the carbon and silicon species can be traced back to the structural preferences of the carbon and silicon skeletons. The non-planar silicon structures are energetically favoured by the formation of relatively strong σ bonds, whereas the carbon compounds encounter strong Pauli repulsion in the congested isomers. The big differences between the structures and energies of the carbon and silicon compounds come from the radii of the (n)s and (n)p valence orbitals of the atoms. The 2s and 2p AOs of first octal-row atoms have nearly the same radii, which leads to effective hybridization but to strong Pauli repulsion between neighbouring bonds. The radii of the valence (n)p AOs of the heavier main-group atoms where n > 2 are bigger than the (n)s valence orbitals, which induces less Pauli repulsion in congested structures. There is a paradoxical role of the π bond strength in the relative stability of isomeric structures of molecules in the first octal row of the periodic table and heavier homologues. On the one hand, the percentage contribution of the π bond strength in heavier main group compounds is nearly the same or it is even higher than in isostructural isomers of the first octal series. But the non-planar isomers of heavier main group compounds often have lower energy than the planar structures, as they are subject to less Pauli repulsion, unlike the non-planar isomers of the homologues of the first octal row. The key energy component, which is behind the particular stability of benzene, is Pauli repulsion, which is an essential component of the interatomic interactions that form covalent bonds.