Original Scientific Article Ring Currents and the PCP Rule

Abstract. According to the recently discovered PCP rule, the intensity of cyclic conjugation in the five-membered rings of polycyclic conjugated hydrocarbons related to acenaphthylene and fluoranthene in-creases with the number of phenyl-cyclopentadienyl (PCP) fragments present in the molecule. The validi-ty of this regularity was first observed by studying the energy effects of cyclic conjugation in the five-membered rings of these systems. In order to show that the PCP rule is not an artifact of the method em-ployed, it is necessary to confirm its validity by other approaches. We now show that the PCP rule is in perfect agreement with calculated topological π-electron ring-currents, ostensibly an independent, quantit-ative, theoretical – albeit, it is here argued, likewise also graph-theoretical – measure of cyclic conjuga-tion in the individual rings of polycyclic π-electron systems. In addition, we show that the molecular-structure dependency of the ring currents in acenaphthylene and fluoranthene congeners also agrees with other, earlier-established, regularities for cyclic conjugation