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 Ca2- 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 Ca2- with the fully occupied π orbital in the O2- is stronger than that with the semioccupied π orbital, controlling the dynamics.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry