The Thermal Decomposition of 1,2-Dioxetane Revisited

The ground state (S0) and lowest energy triplet state (T1) energy surfaces of the parent dioxetane have been extensively explored using various CASSCF active spaces with MP2 corrections in several basis sets. In particular, the singlet/triplet surface crossing regions have been examined and the spin - orbit coupling and energetics computed. The computed energy barrier for the ring-opening of dioxetane is 16 kcal mol-1, which is lower than the experimentally observed threshold (22 kcal mol-1) for unsubstituted dioxetane decomposition. However, the surface topology is in good agreement with the experimental observations. The barrier for O - O cleavage on the ground state surface is found to lie at nearly the same energy as the transition structure for C - C biradical cleavage on the triplet energy surface. More significantly, the computational results indicate that the singlet and triplet surfaces do not cross along the minimum energy path (MEP) between the ground state O - O cleavage transition state and the singlet biradical. as previously thought. Instead, the S0 → 3(3π) surface crossing is prompted by a motion orthogonal to the reaction coordinate, which has components along both the OC - CO torsional and O - C - C asymmetric bending vibrational modes. In particular, we find evidence for a singlet/triplet crossing "line" that spans the ground state O - O cleavage valley and lies a few kcal mol-1 higher in energy. The computed spin - orbit coupling between the ground state and triplet 3(3π) surfaces is large (ca. 60 cm-1) throughout this crossing region. Therefore it is suggested that facile intersystem crossing (ISC) from the ground state to the triplet surface can occur anywhere along the MEP. ISC leads to production of a .OCH2 - CH2O. triplet biradical that can either fragment to form triplet products or undergo ISC back to the ground state surface. The existence of a triplet/singlet crossing region located very close to the computed triplet biradical, suggests that this species is metastable with a short (picosecond) lifetime.