Triplet versus singlet chemiexcitation mechanism in dioxetanone: a CASSCF/CASPT2 study

Chemiluminescence is a fundamental process of chemistry consisting in the conversion of chemical energy stored in chemical bonds into light. It is used by nature and by man-made technology, being especially relevant in chemical analysis. The understanding of the phenomenon strongly relies in the study of peroxide models such as 1,2-dioxetanones. In the present contribution, the singlet S2 and the triplet T2 potential energy surfaces of the unimolecular decomposition of 1,2-dioxetanone have been mapped along the O–O and C–C bond coordinates on the grounds of the multiconfigurational CASPT2//CASSCF approach. Results confirm the energy degeneracy between T2, T1, S1, and S0 at the TS region, whereas S2 is unambiguously predicted at higher energies. Triplet-state population is also supported by the spin–orbit couplings between the singlet and triplet states partaking in the process. In particular, the first-principle calculations show that decomposition along the T2 state is a competitive process, having a small (~3 kcal/mol) energy barrier from the ground-state TS structure. The present findings can explain the higher quantum yield of triplet-state population with respect to the excited singlet states recorded experimentally for the unimolecular decomposition of 1,2-dioxetanone models.