Knowledge Bank

The name pseudoaromatic is applied to a class of conjugated unsaturated hydrocarbons, first defined by Craig, which do not have totally symmetrical ground state wave functions computed from the principal valence bond structures for the proposed planar molecules. In general, the proposed pseudoaromatic molecules have defied synthesis. The stability of the symmetrical forms of several pseudoaromatic molecules with respect to a complete set of bond length displacements for each has been given a Huckel approximation molecular orbital description with an iterative computational procedure. The basic assumptions of both Lennard-Jones and of Longuet-Higgins and Salem have been adopted for the resonance integral and sigma framework description. Under both sets of assumptions, it is found that the symmetrical forms of $pentalene1$ and heptalene are unstable with respect to a polyene-like structure corresponding to a principal valence bond structure of the symmetrical molecule. The symmetrical forms of several other pseudoaromatic molecules are computed to be stable but unusually soft with respect to particular displacements of their bond lengths. The computations have also been performed for several well-known aromatic molecules, and comparisons are made with the results for pseudoaromatic molecules.