A REGULARITY FOR CYCLIC CONJUGATION IN ACENAPHYHYLENE, FLUORANTHENE AND THEIR CONGENERS

Acenaphthylene, fluoranthene and their congeners are compounds that, in view of their chemical and physical properties, belong among benzenoid hydrocarbons. However, because of the presence of a five-membered ring, in all chemical-graph-theoretical considerations of benzenoid systems, these species have been disregarded. Continuing our recent studies of the title compounds, we report here on a peculiar characteristic of the pattern of their cyclic conjugation, named “the phenyl-cyclopentadienyl rule” (PCP rule): if a benzene ring is connected with the (unique) five-membered ring by exactly one carbon-carbon bond (which we refer to as a PCP constellation), then the cyclic conjugation in the five-membered ring significantly increases. If there are several PCP constellations, then the intensity of cyclic conjugation is additionally increased.