Unimolecular Decomposition Mechanism of 1,2-Dioxetanedione: Concerted or Biradical? That is the Question!

Abstract

Determination of the ground- and excited-state unimolecular decomposition mechanisms of 1,2-dioxetanedione gives a level of insight into bimolecular decomposition reactions of this kind for which some experimental results are reported. Although a few studies have put some effort to describe a biradical mechanism of this decomposition, there is still no systematic study that proves an existence of a biradical character. In the present study, state-of-the-art high-level multistate multiconfigurational reference second-order perturbation theory calculations are performed to describe the reaction mechanism of 1,2-dioxetanedione in detail. The calculations indicate that the decomposition of this four-membered ring peroxide containing two carbonyl carbon atoms occurs in concerted but not simultaneous fashion, so-called "merged", contrary to the case of unimolecular 1,2-dioxetane and 1,2-dioxetanone decompositions where biradical reaction pathways have been calculated. At the TS of the ground-state surface, the system enters an entropic trapping region, where four singlet and four triplet manifolds are degenerated, which can lead to the formation of triplet and singlet excited biradical species. However, these excited species have to overcome a second activation barrier for C-C bond cleavage for excited product formation, whereas the ground-state energy surface possesses only one TS. Thus our calculations indicate that the unimolecular decomposition of 1,2-dioxetanedione should not lead to efficient excited-state formation, in agreement with the lack of direct emission from the peroxyoxalate reaction.

Mechanistic insights into the efficient intramolecular chemiexcitation of dioxetanones from TD‐DFT and multireference calculations

“…By its turn, for OH, the energy gap is of 4.0‐10.0 kcal mol −1 , between IRC of 0.20 and 9.94 amu 1/2 bohr. It should be noted that previous studies indicate that multireference calculations should predict even smaller gaps between S 0 and S 1 . This was ascribed to the importance of multireference correlation in these systems, which implies that S 0 and S 1 become degenerated in the biradical region.…”

“…An active space of 10‐in‐8 was chosen for the different dioxetanones (Supporting Information Figures S5 and S6). The choice of active space was made based on previous studies and correspond to OO and CC σ bonding and σ* antibonding orbitals, CO π bonding and π* antibonding orbitals, and oxygen lone pairs orbitals . The MS‐CASPT2 eliminate the method was used to compute the dynamical electron correlation .…”

“…It should be noted that previous studies indicate that multireference calculations should predict even smaller gaps between S 0 and S 1 . [22][23][24][34][35][36][37][53][54][55] This was ascribed to the importance of multireference correlation in these systems, which implies that S 0 and S 1 become degenerated in the biradical region. The energetic error present in this region may result from spin contamination in the reference state introduced by a BS technology.…”

“…The choice of active space was made based on previous studies and correspond to O O and C C σ bonding and σ* antibonding orbitals, C O π bonding and π* antibonding orbitals, and oxygen lone pairs orbitals. [23,[53][54][55] The MS-CASPT2 eliminate the method was used to compute the dynamical electron correlation. [71] Six roots were considered simultaneously.…”

“…ing the biradical region: O C C O dihedral angle larger than55 and O C C angles smaller than 117 . Our results indicate that these conditions are not the same for dioxetanones.…”