Mobius and Huckel molecular orbitals arising from C=C=C components in annulene rings

Following our previous suggestion that C=C=C units can induce Mobius characteristics in cyclic annulenes, we present a more detailed analysis of the molecular orbitals generated by the interaction of such units with the pi orbitals of the annulene. For cycloheptatetraene (1) these take the form of two occupied Huckel orbitals with b symmetry, two occupied Mobius orbitals with a symmetry which can be sub-classified as having catenane or rotaxane forms, together with a more ambiguous pi -like orbital generated from the sigma system. These latter C-H interactions can be reduced by studying instead the perfluorinated analogues. Computed NICS values revealed a diatropic (aromatic) ring current for perfluorocycloheptetraene. A series comprising cyclic (C=C=C)(n), n = 1-4 in which the phase inversions are co-operative, revealed that for n = 1, 3 the rings were aromatic, but anti-aromatic for n = 2, 4, resulting in a generalisation that 4n + 2 occupancy of Mobius orbitals is required for aromaticity. An isomer of n = 2 (6) in which the chirality of the C=C=C units is opposed revealed aromatic behaviour, which is discussed in relation to two other isomers 4 (p-benzyne) and 5 (bicyclo[2.2.0]hexa-1,3,5-triene). For n = 4 a symmetry lowering Jahn-Teller-like istortion occurs, we think in analogous fashion to that observed for singlet cyclobutadiene. Further work is required to generalise the aromaticity of these Mobius-like molecules.