Ab initio calculations for the 2s and 2p core level binding energies of atomic Zn, Zn metal, and Zn containing molecules

Abstract

Quantum chemical ab initio calculations have been performed for the 2s and 2p core level binding energies of atomic Zn, Zn clusters and a few Zn containing molecules. The calculations were performed by means of wave function based methods at different levels of approximation: Koopmans’ theorem, frozen core hole approach, ΔSCF and ΔCASSCF approximations. Scalar relativistic corrections and spin–orbit coupling were included by means of perturbation theory. For atomic Zn, the calculated binding energies for the 2p_1/2 and 2p_3/2 core levels agree within 0.3 eV with experiment; for the 2s level there is a deviation of 3.5 eV which is due to a Coster-Kronig process not included in the present calculations. The calculated chemical shifts for various Zn clusters, from Zn₄ up to Zn₈₇, are decomposed into initial-state and final-state effects. The initial-state effects lead to larger binding energies and converge rapidly with increasing cluster size to shifts of +2.0 and +2.4 eV for 2s and 2p, respectively. The final-state effects lower the binding energies. They converge slowly, roughly proportional to 1/R (R being the cluster radius), to the value for Zn metal. Our final results for the atom-to-metal shifts, −2.7 eV both for 2s and 2p, agree fairly well with the experimental data, −2.9 eV. In the Zn containing molecules, the final-state effects are similar to those in the clusters, increasing slowly with increasing size of the ligand sphere. The initial-state effects, on the other side, depend strongly on the chemical properties of the ligands: They are positive for electron accepting ligands such as methyl and ethyl and in particular CF₃, but negative for the electron donating ligands NH₃ and pyridine.