Correlations between Topological Ring-Currents, π-Electron Partitions, and Six-Centre Delocalisation Indices in Benzenoid Hydrocarbons

Comparison is made between three different indices that characterise the individual rings of a wide range of condensed benzenoid hydrocarbons. Two of these ("pi-electron partitions" and the six-centre delocalisation-indices that have been called "Delta(6)-values") have been introduced only recently as potential indicators of what might be called "local aromaticity", whilst the third ("topological pi-electron ring-currents") was suggested as a possible discriminator in this regard nearly fifty years ago. Whilst linear correlations between ring currents and pi-electron partitions within certain restricted classes of ring types are good (with correlation coefficients of up to 0.998), agreement between the two indices over all classes of ring types is poor. Predictions arising from a consideration of pi-electron partitions and Delta(6)-values seem, on the other hand, to be in somewhat better accord. It is therefore concluded that, despite its superficially intuitive appeal, the ring-current index is out of step both with pi-electron partitions and Delta(6)-values as a general indicator of so-called "local aromaticity". (doi: 10.5562/cca1846)