Theoretical characterization of charge-transfer reactions between N-2 and O-2 molecules and paramagnetic oxygen vacancies on the MgO surface

In this work we have studied computationally, at the Hartree-Fock and densi ty functional levels, the interaction of O-2 and N-2 molecules with paramag netic oxygen vacancy (F-s(+) centers) located at the (100) MgO surface. We have employed the molecular cluster approach, which describes the adsorptio n site by means of a limited number of ions embedded in a large array of po int charges, as well as the more sophisticated perturbed cluster method, wh ere the selected cluster is self-consistently embedded in the quantum-chemi cal description of the regular host crystal. We found that, at short distan ces from the surface, an electron transfer from the surface to the admolecu les occurs, with formation of charge-transfer complexes F-S(2+)/N-2(-) or F -S(2+)/O-2(-) that are stabilized by the electrostatic interaction between the charged fragments. However, F-S(2+)/O-2(-), is stable with respect to t he dissociation into F-S(+) + O-2 fragments, while F-S(2+)/N-2(-) results t o be a metastable product. (C) 2001 Elsevier Science B.V. All rights reserv ed.