Near-Hartree-Fock assessment of reorganization effects in ionic states of acetylene

Abstract

Calculations on acetylene 2Πu and 2Πg ions and the 1Σg neutral ground state using a series of wavefunctions with increasing basis set size converge to an estimated Hartree-Fock vertical ionization energy near 10.0 eV and an estimated Hartree-Fock vertical electron affinity near −4.2 eV. The calculations reveal that the near-Hartree-Fock σ orbitals change strongly with the state of ionization leading to σ-electron energy errors (for transferring ground state σ orbitals to the ions) of − 4.5 eV for the cation and + 3.2 eV for the anion. Each carbon atom is calculated to lose only about 0.34 e upon π-electron ionization and to gain about 0.42 e on π-electron capture. Consequently there is poor transferability of the ground state HF σ core to the ionic states so that π-electron energy changes of + 3.2 eV for the cation and − 3.8 eV for the anion are incurred. The error incurred by transferring π orbitals from the ground state to the ions with frozen σ core is −0.8 eV for the cation and − 0.35 eV for the anion. The σ- and π-orbital reorganization effects largely cancel on going to the ions producing gross reorganization energy changes of − 1.3 eV in the cation and − 0.55 eV in the anion. Comparison with C(sp2)→C+ and ethylene π-electron ionization indicate that aside from extensive σ-π cancellation reorganization effects are not transferable between these species. At least double zeta level basis sets are necessary for physically meaningful descriptions of ionic processes.