Biomimetic hydride transfer catalysts are a promising route to efficiently convert CO2 into more useful products, but a lack of understanding about their atomic-scale reaction mechanisms slows their development.
One such reaction is using borohydride (BH4) for a hydride transfer to CO2. We obtained reactant and product states of the reaction from high-temperature Born-Oppenheimer molecular dynamics (BOMD) of this reaction. Two chain-of-state methods were used to identify reaction pathways: NEB and single-ended GSM. First, we used the high-temperature BOMD structures to obtain a 0 K NEB pathway of the reaction. Second, we used a global optimization code, ABCluster, to identify initial structures for a single-ended growing string method calculation. These two methods provide potential reaction pathways for BH4 reduction of CO2 at 0 K. To examine solvent effects at different levels of theory, we replaced specific portions of the explicit solvent molecules with different dielectric continuum (SMD, PCM, COSMO, CANDLE, etc.) or hybrid (COSMO-RS, ESM-RISM) solvent models. Furthermore, we investigated the role of counter ions in reaction energetics and solvent models.
Since chain-of-state methods provide pathways at 0 K we also used umbrella sampling with BOMD to obtain a potential of mean force (PMF) of the reaction. This PMF simulation can provide even more physical insight into the reaction with dynamic (thermal) contributions at a significant computational cost. We found that the pathways are generally similar, but not always. The PMF energies are not always consistent with chain-of-state methods using continuum or hybrid solvent models. This points towards needing guidelines of when PMF simulations are necessary or when it can be confidently ignored.
Alex Maldonado, Mitch Groenenboom
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