EPR Study of O3 − on Magnesium Oxide

Molecular oxygen reacts with O⁻ on the surface of magnesium oxide to form the ozonide ion, ${\mathrm{O}}_3^{-}$ . The EPR spectrum of this ion was used to verify its formation. Oxygen‐17 was introduced into the molecule either via the O⁻ ion or the O₂ molecule. The spectrum is characterized by g₁=2.0172, g₂=2.0100, and g₃=2.0014, with ${\mathrm{|\; a}}_3^a\text{|=26\hspace{0.17em}}\mathrm{G}{\mathrm{,\; |\; a}}_3^b\text{|=82\hspace{0.17em}}\mathrm{G}$ , and ${\mathrm{|\; a}}_3^c\text{|=65\hspace{0.17em}}\mathrm{G}$ . Here, $a_3^a$ is the hyperfine splitting for the oxygen atom which was originally O⁻. The splitting along the other principal axes was less than 5 G. A model is suggested in which a terminal oxygen of the bent molecule is in the plane of the surface, and the plane of the molecule is perpendicular to the surface. The ozonide ion slowly dissociated at 25° to form ${\mathrm{O}}_2^{-}.$ The oxygen‐17 isotope distribution indicates that the superoxide ion was largely derived from molecular oxygen, rather than the O⁻ ion.