Ab initio MP2, MCSCF and MR-SDCI study on the structure of O 4 and comparison with the hypervalent CO 3 and SO 3 species

Abstract

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 O 4 with D 3 h symmetry. The results have been compared with the hypervalent CO 3 and SO 3 molecules. It was found that the three molecules present two stable structures with symmetry C 2 v and D 3 h respectively. While most of the methods predict the former to be the most stable structure for CO 3 — although CASPT2 and Davidson's correct MR-SDCI gave the opposite result — all methods predict the D 3 h structure as the global minimum for O 4 and SO 3. At the theoretical level employed here O 4 is stable towards decomposition into O 3( 1A 1) 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.