Knowledge Bank

The energies and wave functions of the ground state and the lower excited states of benzene, naphthalene, anthracene, napthacene, pentacene and azulene are calculated by the method proposed by R. G. Parr and the $author1$ This computation starts with molecular orbitals derived from the simplified LCAO-MO theory. Interelectronic interaction is included, simplified by the formal neglect of differential overlap between atomic orbitals. The one-center Coulomb integral and the two-center Coulomb integral are obtained semiempirically, theoretical values from Slater orbitals being used for all other Coulomb integrals. The final wave functions and energies are obtained by interaction of the lowest energy configuration and singly excited configurations. Essentially all computations were performed automatically by means of a high-speed digital computer, the IBM EDPM 701, using a highly generalized program. The theory predicts the positions and intensities of the lower U. V. bands. These are in very good agreement with the available experimental results, the positions of the maxima being predicted to within 0.3 e.v. The symmetry assignments for the various singlet and triplet states are discussed. For example, the weakest singlet absorption band of the polyacenes, which is also the lowest in the case of naphthalene, arises from a $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Ag⟶1B3u$ transition (long axis polarization), whereas the lowest triplet state for all the polyacenes has $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}B2u$ symmetry. Certain features of the ground state electronic structure and energy are also investigated in relation to experiment and the conventional simplified LCAO-MO theory.