Topological Ring-Currents and Clar Sextets in Fully Benzenoid Hydrocarbons I. Structures with Fewer than 18 Rings.

It is well-known that the very special family of conjugated systems described as "fully benzenoid hydrocarbons" have a unique Clar structure that causes all of their rings to be classified as either "full" or "empty". In this work, π-electron ring-currents in more than two dozen such structures are calculated by means of the quasi-graph-theoretical, parameter-free Hückel-London-Pople-McWeeny (HLPM) method. In the case of benzenoid hydrocarbons, the calculated values of such "topological" ring-currents depend on only the carbon-carbon connectivity of the system in question. It is found that, until a certain fully benzenoid hydrocarbon comprising 17 rings is examined, all ring currents in full rings are seen to be larger in intensity than the ring current in benzene, while such currents in empty rings are smaller than the benzene value. There is an indication that this distinction might break down when "giant" fully benzenoid structures are considered. These conclusions are consistent with chemical intuition, with earlier studies examining Clar aromatic sextets by means of NICS calculations, and with previous qualitative ab initio predictions of the patterns of π-electron current-flow in the fully benzenoid hydrocarbons.