From lithium and sodium superoxides to singlet-oxygen - insights into the mechanism of dissociation using SHARC-MD.

ABSTRACT

The parasitic formation of singlet oxygen in aprotic alkaline/air batteries presents a challenge for the technical development of these systems. Avoidance strategies and investigation of reaction paths such as disproportionation of LiO2 and NaO2 have been presented. Furthermore, the dissociation of these superoxide systems have been discussed be as an alternative reaction channel. Here, we present a fundamental study of the electronic nature and dissociation behaviour of the alkali superoxides. The molecular systems were calculated at the CASSCF/CASPT2-level of theory. We determined the minimum energy crossing points along the dissociation required to form 3O2 and 1O2. Building on these results, a surface-hopping AIMD-simulation was performed employing the SHARC program package to follow the electronic transitions along the minimum energy crossing pooints during the dissociation. The feasibility of populating the electronic state corresponding to the formation of singlet oxygen during dissociation was demonstrated. For LiO2, 6.85% of the trajectories were found to terminate under formation of 1O2, whereas for NaO2 only 1.68% of the trajectories ended up in 1O2 formation. This represents an inverse trend to that reported in the literature. This observation suggests that the dissociation is a viable, monomolecular reaction path to 1O2 that complements the disproportionation pathway.