Theoretical characterization of charge-transfer reactions between N2 and O2 molecules and paramagnetic oxygen vacancies on the MgO surface

In this work we have studied computationally, at the Hartree–Fock and density functional levels, the interaction of O2 and N2 molecules with paramagnetic oxygen vacancy (FS+ centers) located at the (1 0 0) MgO surface. We have employed the molecular cluster approach, which describes the adsorption site by means of a limited number of ions embedded in a large array of point charges, as well as the more sophisticated perturbed cluster method, where the selected cluster is self-consistently embedded in the quantum-chemical description of the regular host crystal. We found that, at short distances from the surface, an electron transfer from the surface to the admolecules occurs, with formation of charge-transfer complexes FS2+/N2− or FS2+/O2− that are stabilized by the electrostatic interaction between the charged fragments. However, FS2+/O2− is stable with respect to the dissociation into FS++O2 fragments, while FS2+/N2− results to be a metastable product.