Benzenoid isomers with greatest and smallest Kekulé structure counts

Abstract

In families of benzenoid isomers, the species with the greatest and smallest value of the Kekul? structure count (K) possess, respectively, the greatest and smallest thermodynamic stability and, respectively, the smallest and greates reactivity. In this paper the structure of the isomers with extremal K-values for the benzenoid molecules A = A(a, b, c, d, e) (with the formula C4h+2H2h+4) and B1 = B1(a, b, c, d)& B2 = B2(a, b, c, d) (with formula C4hH2h+2) were determined. The system A is obtained by attaching linear polyacene fragments of length a, b, c, and d to a linear polyacene of length e + 2, so that h = a + b + c + d + e + 2, the number of hexagons of A, is constant. The systems B1 and B2 are obtained by attaching linear polyacene fragments of length a, b, c, and d to pyrene, in two different ways, so that h = a + b + c + d + 4, the number of hexagons of B1 and B2, is constant. The main conclusion is that K is maximal if the parameters a, b, c, d differ by at most one, and that K is minimal if b, c, d are equal to zero.