From Lithium and Sodium Superoxides to Singlet-Oxygen - Insights into the Mechanism of Dissociation Using SHARC-MD

The formation of highly reactive singlet oxygen from alkaline superoxides presents an important reactivity of this component class. Investigations of the reaction paths such as disproportionation of LiO₂ and NaO₂ have been presented. Furthermore, the dissociation of these superoxide systems have been discussed as an alternative reaction channel that also allows the formation of singlet oxygen. 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 triplet oxygen ³O₂ and singlet oxygen 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 points 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.