Reactivity and electronic states of $O4$ along minimum energy paths

Ab initio calculations, of the ground and low-lying electronic states of $O4$ along the minimum energy paths (MEP) for the reactions $O2(X 3Σg−)+O2(X 3Σg−)→O4(X 1A1)$ and $O2(X 3Σg−)+O2(X 3Σg−)→O3(X 1A1)+O(3P)$ have been performed. Our $CASSCF(16,12)+CASMP2$ calculations using the $5s4p3d2 f$ ANO basis set provide a solid basis to establish the stability of the $O4$ chemically bound molecule. Surface crossings between singlet and triplet states have been found and further characterized by evaluating their spin–orbit coupling matrix elements. Our calculations show that for the ozone formation reaction there should be spin–orbit coupling transitions allowing vibrational to electronic energy transfer around the saddle point region. The formation of vibrationally excited $O2(a 1Δg,v)$ is predicted to contribute to the dark channel reported in experiments.