Abstract
The bond alternations in pentalenes and heptalenes substituted by electron-withdrawing groups are studied by use of the Pariser–Parr–Pople–type SCF CI MO method and the symmetry rule for substituted conjugated hydrocarbons. The influence of electron-withdrawing substituents on geometrical structures of pentalene and heptalene markedly depends on the attached positions. The 2- and 5-substitutions in pentalene and the 1-, 3-, 5-, 6-, 8-, and 10-substitutions in heptalene increase the amount of positive charge and, at the same time, relax the bond alternations in the parent molecules. This relaxation correlates with the amount of the positive charge in the pentalene and heptalene skeletons. The large amount of positive charge brings about the stabilization of the delocalized structures. The critical values of positive charge for the existence of bond alternations are smaller than the unit positive charge. The substitutions at the other sites in both molecules are anomalous in the sense that they increase the...
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