Systematic ab initio calculations on the energetics and stability of covalent O4

Ab initio calculations with highly correlated methods together with extensive basis sets have been used to obtain the most accurate heat of formation and stability with respect to dissociation (into molecular oxygen) for the chemically bound tetraoxygen molecule. Our calculations show that the heat of formation is significantly smaller and that the barrier to dissociation is larger than previously assumed. In particular, we have shown that the previous theoretical estimate for the heat of formation of tetraoxygen was in error by a significant amount (18%-24%) owing to lack of accuracy in the theoretical method then used. Our best estimates places that value in the range 93-95 kcal/mol and this should be taken into consideration when discussing the possible relevance of tetraoxygen in a variety of experiments, as well as in the fundamental atmospheric chemical processes where oxygen species participate.