Modeling Bicarbonate Formation in an Alkaline Solution with Multi-Level Quantum Mechanics/Molecular Dynamics Simulations

Understanding carbonate speciation and how it may be modulated is essential for the advancement of carbon dioxide (CO₂) capture and storage technologies, which often rely on the transformation of CO₂ into carbonate, e.g. via the formation of carbonate minerals. To date, few atomic-level, quantum-mechanics-based simulations have been carried out to characterize how carbonic acid (H₂CO₃) and bicarbonate (HCO−3HCO3−) form in aqueous solution, and how pH affects this process. Recently, Martirez and Carter utilized rare-event sampling density functional theory molecular dynamics simulations in combination with multi-level embedded correlated wavefunction theory, thus accounting for both solvent dynamics and electron correlation accurately, to elucidate the mechanism of H₂CO₃ formation in neutral solution (J. Am. Chem. Soc., 145, 12561, 2023). Here, we perform a complementary simulation using the same method to map out the energetics of HCO−3HCO3− formation from dissolved CO₂ in basic solution. We find that, as in H₂CO₃ formation, including water dynamics is important to obtain an accurate prediction of the energetics for the aforementioned reaction. Furthermore, only with MD did we identify the correct pathway for the reaction, in which water – not hydroxide – acts as the initial nucleophile and only at the transition state does it lose a proton.