Sigma and Pi Changes in Valence States of Pi-Electron Theory and One-Center Coulomb Repulsion Parameters

Abstract

Individual contributions to the total energy change ΔE of the reaction 2C→C++C− are calculated with valence states appropriate to π-electron systems. Methods for the calculation of the open-shell Hartree—Fock (HF) and correlation energies of valence states are given. The HF orbitals entering the main part of ΔE, the one-center repulsion integral γpp, are those of the 1s2η32pz2 valence state of C−. They yield a γpp=12.72 eV, already remarkably close to the semiempirical ``γpp'' of Pariser—Parr theory needed to reproduce, for example, the electronic spectrum of benzene. Both the σ and π orbitals and core correlations of each carbon species change strongly with the state of ionization. The energy of σ HF orbital changes in the reaction is 3.16 eV, of π orbital changes−4.19 eV. Such effects may not cancel out between the different electronic states of an organic molecule, though their net effect on the ΔE of the carbon reaction is −1.0 eV. The rest of ΔE is the 2pz2 correlation, also −1.0 eV. A method is given for the prediction of other ΔE's (i.e., the Pariser—Parr ``γpp'') for heteroatoms. The values obtained are 12.91 eV for pyridine-type nitrogen, 16.47 eV for pyrolle-type nitrogen, 15.38 eV for carbonyl oxygen and 18.85 eV for furan-type oxygen.