Mechanism of ionic dissociation of HCl in the smallest water clusters

The dissociation of strong acids into water is a fundamental process in chemistry and biology. Determining the minimum number of water molecules that can result in an ionic dissociation of hydrochloric acid (HCl → H⁺ + Cl^-) remains a challenging subject. In this study, the reactions of H₂O with HCl(H₂O) (HCl-H₂O cluster), i.e., HCl(H₂O) + H₂O (n = 3-7), were investigated by using the direct ab initio molecular dynamics (AIMD) method. Direct AIMD calculations were performed to set the collision energy of H₂O to zero for all trajectories. For n = 3, no reaction occurred. In contrast, HCl dissociated to H⁺ + Cl^- at n = 4, forming a contact ion pair (cIP) and solvent-separated ion pair (ssIP) as products. The reactions were expressed as HCl(H₂O)₃ + H₂O → H₃O⁺(H₂O)₂Cl^- (ssIP), and HCl(H₂O)₃ + H₂O → H₃O⁺(Cl^-)(H₂O)₂ (cIP). The ion pair (IP) products were dependent on the collision site of H₂O relative to HCl(H₂O)₃. For n = 5-7, both IPs were formed through the reaction between H₂O and HCl(H₂O) (n = 5-7). The reaction between HCl and (H₂O)₄ (HCl + (H₂O)₄ → HCl(H₂O)₄) was non-reactive in IP formation. The reaction mechanism was discussed based on the theoretical results.