Formation of Superoxo Species by Interaction of O2 with Na Atoms Deposited on MgO Powders: A Combined Continuous-Wave EPR (CW-EPR), Hyperfine Sublevel Correlation (HYSCORE) and DFT Study

The formation of O2− radical anions by contact of O2 molecules with a Na pre-covered MgO surface is studied by a combined EPR and quantum chemical approach. Na atoms deposited on polycrystalline MgO samples are brought into contact with O2. The typical EPR signal of isolated Na atoms disappears when the reaction with O2 takes place and new paramagnetic species are observed, which are attributed to different surface-stabilised O2− radicals. Hyperfine sublevel correlation (HYSCORE) spectroscopy allows the superhyperfine interaction tensor of O2−Na+ species to be determined, demonstrating the direct coordination of the O2− adsorbate to surface Na+ cations. DFT calculations enable the structural details of the formed species to be determined. Matrix-isolated alkali superoxides are used as a standard to enable comparison of the formed species, revealing important and unexpected contributions of the MgO matrix in determining the electronic structure of the surface-stabilised Na+O2− complexes.