EPR and DFT study of the ethylene reaction with O− radicals on the surface of nanocrystalline MgO

Abstract

Different radical forms of oxygen (O−, O 2 − and O 3 − ) on the surface of nanocrystalline MgO are well known. It was earlier demonstrated that EPR-silent species with properties very similar to those of O− radicals exist on the surface of magnesium oxide in addition to the O− radicals observed by EPR. In this study we characterized the reactivity of these two types of O− radicals in reaction with ethylene. It was demonstrated that this reaction yields different products for observable and unobservable O− radicals. Conventional $${\text{O}}_{{3{\text{C}}}}^{ - }$$ radicals generated by MgO illumination with UV light at room temperature in the presence of oxygen initiate hydrogen atom abstraction from ethylene to form secondary radicals H2C=C− with hyperfine splitting A 1 = 59 G, A 2 = 6 G. The $${\text{O}}_{{4{\text{C}}}}^{ - }$$ radicals not observed directly by EPR were synthesized by MgO illumination in the presence of oxygen at 163 K followed by evacuation at 203 K. They were shown to react with ethylene to form an addition product with two groups of two equivalent protons with isotropic hyperfine constants A 1 = 38 G and A 2 = 23 G. Such radicals were obtained for the first time by reaction of O− radicals with ethylene on the MgO surface. Their concentration was approximately equal to the concentration of [ $${\text{O}}_{{4{\text{C}}}}^{ - }$$ ·O2] complexes observed by EPR before the oxygen desorption. The structures of both radicals were simulated by DFT, and a good match between the experimental and computational results was obtained.