Ab initio MP2, MCSCF and MR-SDCI study on the structure of O₄ and comparison with the hypervalent CO₃ and SO₃ species

Ab initio second-order Møller-Plesset (MP2), complete active space multiconfigurational SCF (CASSCF), multireference second-order perturbation theory (CASPT2) and multireference configuration interaction (MR-SDCI) calculations have been performed on the structure of O₄ with D_3h symmetry. The results have been compared with the hypervalent CO₃ and SO₃ molecules. It was found that the three molecules present two stable structures with symmetry C_2v and D_3h respectively. While most of the methods predict the former to be the most stable structure for CO₃ - although CASPT2 and Davidson's correct MR-SDCI gave the opposite result - all methods predict the D_3h structure as the global minimum for O₄ and SO₃. At the theoretical level employed here O₄ is stable towards decomposition into O₃(¹A₁) and O(³P) by more than 40 kcal mol^-1, for which reason it is suggested that it should be observable among the products of the reaction of oxygen atoms with ozone molecules.