Charge Distributions in Positive Ions and Ionization Energies of Conjugated Hydrocarbons

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Abstract
The π-electron ionization energies of conjugated hydrocarbons and charge distributions of the resulting positive ions are examined utilizing closed and open shell SCF theory with deformation of σ and π orbitals. Ionization energies: Koopmans' theorem is shown to be invalid for conjugated hydrocarbons to the extent of about 2–4 ev, the major breakdown occurring through the Σ and Π deformation effects and not through reminimization of the ionic configuration. It is shown that the sigma description is not the same for both the ground and ionized states. A first-order correction to the Σ-framework energy of the ion can be obtained by removing a π electron from a single carbon atom. Very good agreement is obtained between the calculated and observed onization potentials (average discrepancy 0.1–0.2 ev). Justification is given for the so-called ``ω technique'' for the calculation of ionization energies by the Hückel procedure in that this method contains deformation corrections which approximate those derived in this work. Charge distributions: Both deformation effects and reminimization of the ionized state are important in consideration of the charge distribution in the ions. It is shown that certain sites of negative charge density are predicted in positive ions by the reminimization procedure, and that this is both a reasonable and necessary consequence due to the form of the ionic Hartree-Fock Hamiltonian. The wave functions, and consequently the charge distribution, calculated by the ω technique, are shown to be poor approximations of the ionized state Hartree-Fock eigenfunctions.

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