Metastable Shared Proton Complexes in Aqueous Sulfuric Acid Solutions

The formation of metastable, shared proton contact ion pairs (CIPs) between anions in aqueous sulfuric acid (H₂SO₄) solutions is investigated at a concentration of around 0.5 mol/L (pH = 0.3). The CIP can form a relatively long-lived Zundel-like complex, where two anions share a single proton, forming an unusual and strong hydrogen bonding configuration between the two anions. Utilizing <i>ab initio</i> molecular dynamics (AIMD) simulations combined with free energy sampling, the free energy profile and the dissociation barrier of the anion pairing are estimated. Additionally, quantum mechanics (QM) calculations are adopted to reveal the stabilizing interactions in the complex. Through calculated radial distribution functions (RDFs), the solvation structures of anions are further resolved. These findings underscore the important role of induction interactions and solvation effects in stabilizing the CIPs. The insights presented in this work enhance our understanding of aqueous acidic solutions and highlight that such solutions may exhibit interesting behavior beyond simple hydronium cation dissociation from its conjugate base.