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

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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.