Superoxide ion encaged in nanoporous crystal 12CaO-7Al₂O ₃y continuous wave and pulsed electron paramagnetic resonance

Continuous wave and pulsed electron paramagnetic resonance studies were performed on the Superoxide ion, O₂-, encaged in the nanoporous crystal 12CaO'7Al₂O₃ (C12A7). Hyperfine splitting due to ¹⁷O in O₂- was observed in C12A7 heated in 40% ¹⁷O-enriched O₂ gas at 550 °C, providing solid evidence that encaged O₂derives from gaseous O₂ in the atmosphere via reactions with free oxygen ions in the cage. Angular variations of g values, ¹⁷O hyperfine splitting in the single crystal at 20 K, and ²⁷Al electron spin-echo envelope modulation pattern for the powder at 4 K clarified that an O₂- is encaged in a subnanometer-sized cage of C12A7 and adsorbed on a Ca^2- ion forming a part of the cage wall. Further, two oxygen atoms in the O₂ion are equivalent in the cage, indicating that the O₂- ion takes a "side-on" configuration. Then, the O-O bond is perpendicular to the 2-fold rotation axis (C₂ \\ (100)) across the center of the cage and directed to two oxygen ions forming the cage wall. Variation in the g values with temperature indicates that the O₂- may be regarded as solidlike below ∼20 K and above this temperature the rocking motion of the O-O bond about the C₂ axis is activated with an increase in temperature, but the anisotropy in the motion remains even at 400 K. This result may be explained by a view that Coulombic interaction of the Ca^2- with the fully occupied π orbital in the O₂^- is stronger than that with the semioccupied π orbital, controlling the dynamics.