Edge Effects in Benzenoid and Total Resonant Sextet Benzenoid Hydrocarbons and Clar's Sextet Principle

Because the effect of a benzenoid perimeter edge on a sextet pattern is of paramount importance, trends in the degree of sextet aromaticity are determined. The spectacular structural isomorphism between the total set of benzenoid hydrocarbons and its strain-free total resonant sextet (TRS) subset as described herein is an unequivocal demonstration of the validity of the Clar sextet principle. Aromaticity and the Clar's sextet principle are intricately related which form the basis of this isomorphism. Hexagons on the edge of a polycyclic benzenoid hydrocarbon tend to have higher spin density, have a higher probability of sextet residence, be more reactive to substitution reactions, and have a perimeter topology described by four distinct parameters by means of a very simple relation. It is shown that because the location of Clar sextets in TRS benzenoids is totally fixed, the relative order of ring aromaticity of the peripheral benzene hexagons can be predicted from their topology. Using Bosanac and Gutman's energy effect (ef) and Aihara's bond resonance energy, we show for the first time that the relative order of decreasing ring aromaticity for perimeter edge hexagons in any specific TRS benzenoid is quarto > trio > duo > solo > empty. TRS benzenoid hydrocarbons are those isomers in benzenoids having their number of formula carbons divisible by six and a maximum number of perimeter bay regions (eta(o)) given by eta(0)(TRS) = (1/2)N-H(sextet) - 3 + F. N-H(sextet) is the number of formula hydrogens and F is the number of perimeter fjords (triplet of adjacent bay regions); for strain-free TRS benzenoids, F = 0.