Abstract

It is shown for a representative set of isomeric benzenoids that pi-electron partitions and signatures can serve for characterizing and ordering benzenoids. Benzenoid signatures (sequences s6 through s1 where the subscripts correspond to numbers of pi electrons in all rings) are obtained by examining the numbers of assigned pi electrons ranging from 6 to 1 for each ring in all resonance structures. The pi-electron partitions and signatures of all 33 non-isoarithmic peri-condensed benzenoid hydrocarbons with eight rings and four contiguous internal carbon atoms allow an ordering of these benzenoids that agrees fairly well with increasing numbers of Kekulé valence structures and Clar sextets. Interestingly, an excellent correlation (R2 > 0.99) is observed between s6 + s5 + s2 + s1 and s4 + s3, and an explanation for this observation is provided: the number P of pi electrons is divided unequally between two components: s34 = s4 + s3 and s1256 = s6 + s5 + s2 + s1 so that s1256 or the quotient s1256/s34 = Q can serve as a new metric for perfect matchings of polyhexes and a criterion for ordering and for evaluating the complexity of isomeric benzenoids quantitatively.

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