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

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.

Mechanism of Heterogenization of Dirhodium Catalysts: Insights from DFT Calculations.

Dirhodium(II) complexes such as [Rh2(TFA)4] bound to a functionalized mesoporous SBA-15 carrier material have proven to be valuable candidates for heterogeneous catalysis in the field of pharmaceutical synthesis. However, the mechanistic steps of immobilization by linker molecules containing carboxyl or amine functionalities remain the subject of discussion. Here we present a theoretical study of possible mechanistic binding pathways for the [Rh2(TFA)4] complex through model representations of synthetically investigated linkers, namely n-butylamine and n-butyric acid. Experimentally proposed intermediates of the immobilization process are investigated and analyzed by density functional theory calculations to gain insights into structural properties and the influence of solvation. An evaluation of the thermodynamic data for all identified intermediates allowed distinguishing between two possible reaction pathways that are characterized by a first axial complexation of either n-butyric acid or n-butylamine. In agreement with results from NMR spectroscopy, singly or doubly n-butylamine-fixated complexes were found to present possible immobilization products. Initial binding through a carboxy-functionalized linker is proposed as the most favorable reaction pathway for the formation of the mixed linker pattern [Rh2(TFA)3]·(n-butylamine)·(n-butyrate). The linkers n-butyric acid and n-butyrate, respectively, are found to exhibit an unaltered binding affinity to the dirhodium complex despite their protonation states, indicating invariance to the acidic environment unlike an immobilization by n-butylamine. These results present a theoretical framework for the rationalization of observed product distributions while also providing inspiration and guidance for the preparation of functionalized heterogeneous SBA-15/dirhodium catalyst systems.