Dimers of oxygen molecules in the pores of Ca₆A zeolite

Abstract

Dimers of O₂ molecules are formed in the pores of Ca₆A zeolite at temperatures lower than 100 K. The O₄ molecule is diamagnetic and located on the Ca²⁺ ions. No isotope exchange takes place between ¹⁶O₂ and ¹⁸O₂via O₄ molecules, suggesting that O₄ is either a linear, chemically bound molecule or a van der Waals molecule. The heat of formation of adsorbed O₄ is only 2.1 kJ mol^-1 from adsorbed O₂ molecules, and is estimated to be 26 kJ mole^-1 from gaseous O₂. Semi-quantitative studies show that Ca²⁺ markedly weakens the van der Waals force between O₂ molecules. These results suggest that the O₄ molecule can exist as a linear molecule on Ca²⁺ in Ca₆A zeolite, and is a van der Waals species in free space.