Ab initio study of the O4 molecule

SCF-CI calculations were done on tetratomic oxygen complexes at various geometries. The results point to the existence of a metastable covalent molecule O₄ completely different from the van der Waals structure (O₂)₂ detected experimentally. At its equilibrium geometry, the O₄ molecule is a quasi-square (r(OO) ≈ 1.4 Å), slightly twisted out of plane, corresponding to the symmetry group D_2d. The activation energy of the reaction O₄(¹A_g) → 20₂(X ³Σ^?_g) is found to be ≈ 15 kcal/mole, that of the inverse reaction, ≈ 75 kcal/mole.