A study of the aromaticity of thia[7]circulene isomers

Abstract

In this study, the TRE (topological resonance energy) method was used to determine the degree of aromaticity of [7]circulene and its isomers with two or more carbon atoms on benzene rings replaced by sulphur atoms. The aromaticity based on the TRE index has been compared with that reported by Gauge using the magnetically induced current (GIMIC) method. According to the GIMIC method, thia[7]circulene isomers must be antiaromatic with a negative ring-current susceptibility (I tot). However, they are aromatic with positive TREs. Thus, I tot values cannot always be used as indicators of global aromaticity. By the calculation of the local aromaticity indices, such as BRE (bond resonance energy) and CRE (circuit resonance energy), we explored the mechanism behind the decreased aromaticity for thia[7]circulene isomers. BRE and CRE results demonstrated that the local aromaticity of benzene unit(s) is dominant, and the contribution of the thiophene unit is small in the thia[7]circulene isomers. The Hückel–London model was used to estimate the intensity of ring currents. We found that general trends of the local aromaticity or antiaromaticity obtained from BRE and CRE indices can also be reproduced properly with the diatropic or paratropic ring current direction calculated from the Hückel–London model.