Sulfuric acid (SA) is a critical compound in both atmospheric chemistry and various industrial processes. Its aqueous solutions, particularly in small water clusters, play a significant role in atmospheric reactions, while concentrated solutions are widely used as electrolytes in electrochemical applications, such as the oxygen evolution reaction. While molecular dynamics (MD) simulations offer insights into various critical systems at the atomic level, they are limited by their inability to capture bond breaking and forming, such as proton transfer events, due to fixed bonding topologies. Ab initio molecular dynamics (AIMD) simulations, coupled with density functional theory (DFT), provide a more accurate depiction of interactions, since electronic structure of the system is explicitly considered, and the ability to simulate processes that involve bond formation and breaking. Previous studies have largely focused on structure and dynamics of single anion, overlooking the interactions between themselves. In this study, we employ AIMD simulations with umbrella sampling to explore the dissociation dynamics of sulfate/bisulfate anions in SA aqueous solutions. By analyzing the trajectories and conducting structural analysis, we find that a proton is consistently shared between the two sulfate/bisulfate anions in the contact ion pair (CIP) structures, forming a Zundel-like complex, either as (SO₄^2-) -- H⁺ -- (SO₄^2-) or (SO₄^2-) -- H⁺ -- (HSO₄^-). This configuration, together with the solvation effect of water molecules, leads to the formation of a stable CIP, which is only 0.5 kcal/mol less stable than the solvent-separated ion pair (SSIP) according to the free energy profile. The dissociation of this CIP requires overcoming a free energy barrier of 0.7 kcal/mol. We further use structural and hydrogen bond analysis to gain deeper insights into the dissociation process. Our findings provide a deeper understanding of the dynamic behavior of sulfate and bisulfate anions in aqueous environments, shedding light on their fundamental role in various chemical processes.

Sijia Chen

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