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What is So Special About Benzene? A Comparison of Selected Carbon and Silicon Isomers E6H6 (E = C; Si)
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DOI：10.1039/D6CP00587J.png)
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Abstract
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Quantum chemical calculations using density functional theory at the BP86/def2-TZVPP level and ab initio methods CCSD(T) and MP2 are reported for the structures of the monocyclic molecules E6H6 (E = C; Si). The structures of C6H6 and Si6H6 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 D6h 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 global energy minimum on the Si6H6 PES. In sharp contrast; the homologous carbon isomer 2C is more than 120 kcal/mol higher in energy than 1b. There is a tricyclic isomer of Si6H6 with a different substitution pattern 3Si; which is more stable than 1Si’ and which is only 7.3 kcal/mol higher in energy than 2Si. Substituted homologues Si6R6 of 2Si and the tricyclic 3Si 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. The reason for the different structures and reactivity between molecules of atoms from the first octal series and heavier main group elements lies mainly in the strength of the σ bonds and the associated Pauli repulsion; which is an essential component of the interatomic interactions.
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