Reinvestigation of homoaromaticity of cyclohepta-1,3,5-triene

A reinvestigation of the homoaromaticity of cyclohepta-1,3,5-triene has been done at the DFT (B3LYP/6-31+G*) level. The determination of the aromatic stabilization energies based on the hypohomodesmotic reactions gives either too high value (−19.58 kcal mol−1 using ethane as the hydrogen donor molecule) or too low values (−0.46 kcal mol−1 using cycloheptane or −4.27 kcal mol−1 using cyclopentane molecules as the hydrogen donor molecules), as the ring strains on the two sides of the reactions are not compensated. The isomerisation reaction of methylenecyclohepta-1,3-diene to 1-methylcyclohepta-1,3,5-triene gives a value of −10.68 kcal mol−1 as the isomerisation energy. Investigation of the valence tautomerization of cyclohepta-1,3,5-triene to norcaradiene and back to cyclohepta-1,3,5-triene reveals that a dynamic equilibrium exists between the two valence tautomers due to small activation energy barriers, which causes strong interactions between the π orbitals of the triene moiety and the HCCH2CHσ orbitals. The NBO analysis of the two valence tautomers and the transition structure involved therein support these interactions. The Frontier Molecular Orbitals of cyclohepta-1,3,5-triene, norcaradiene and the transition structure depict unambiguously the formation of the annular electron cloud, which confers homoaromatic character on cyclohepta-1,3,5-triene.