Novel insight into Clar’s aromatic π-sextets

For a selection of smaller benzenoid hydrocarbons we calculated ring bond orders (RBO) by adding the Pauling bond orders for CC bonds forming individual benzene rings. The RBO show full parallelism with qualitative expectations based on Clar’s intuitive sextet theory. The highest RBO values belong to sextets of a single Clar structure, intermediate values to ‘migrating’ sextets and the smallest values to the ‘empty’ rings, and rings involving essentially single CC bond. This novel quantitative upgrading of the Clar structural formulas of aromatic benzenoid hydrocarbons offers numerical characterization of individual benzene rings as fully aromatic, intermediate, and weakly aromatics.