Möbius and Hückel molecular orbitals arising from CCC components in annulene rings

Abstract

Following our previous suggestion that CCC units can induce Möbius characteristics in cyclic annulenes, we present a more detailed analysis of the molecular orbitals generated by the interaction of such units with the π orbitals of the annulene. For cycloheptatetraene (1) these take the form of two occupied Hückel orbitals with b symmetry, two occupied Möbius orbitals with a symmetry which can be sub-classified as having catenane or rotaxane forms, together with a more ambiguous π-like orbital generated from the σ 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 (CCC)_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 Möbius orbitals is required for aromaticity. An isomer of n = 2 (6) in which the chirality of the CCC 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 distortion occurs, we think in analogous fashion to that observed for singlet cyclobutadiene. Further work is required to generalise the aromaticity of these Möbius-like molecules.