Aromaticity of topological isomers of cyclazines and their [14] to [18] annulenes

The structures and magnetic properties of the parent cyclazines [2.2.2], [3.2.2], [3.3.2], [4.2.2], [3.3.3], [4.3.2], their topological isomers and the doubly bridged [14] to [18] annulenes were studied computationally at the B3LYP/6-311+G** level of density functional theory (DFT). The magnetic criteria Λ (magnetic susceptibility exaltations, MSE), ∑NICS (the summation of nucleus-independent chemical shifts), and 1H NMR chemical shifts, geometry-based HOMA (harmonic oscillator model of aromaticity) index and HOMO-LUMO gaps were used to elucidate their aromatic and anti-aromatic properties. The excellent correlation between Λ and the summation of NICS(0) and NICS(1) demonstrates that these two originally proposed indices are still good indicators of the whole aromaticity of these polycyclic ring systems. The computed NMR chemical shifts are also in good agreement with available experimental data. In general, there is a good relationship between different criteria of aromaticity.