AB-INITIO MP2, MCSCF AND MR-SDCI STUDY ON THE STRUCTURE OF O-4 AND COMPARISON WITH THE HYPERVALENT CO3 AND SO3 SPECIES

Ab initio second-order Moller-Plesset (MP2), complete active space mul ticonfigurational SCF (CASSCF), multireference second-order perturbati on theory (CASPT2) and multireference configuration interaction (MR-SD CI) calculations have been performed on the structure of O-4 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 stab le 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 re sult - all methods predict the D-3h structure as the global minimum fo r O-4 and SO3. At the theoretical level employed here O-4 is Stable to wards decomposition into O-3((1)A(1)) and O(P-3) 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 molecul es.