Issue 4, 2024

Mechanism of ionic dissociation of HCl in the smallest water clusters

Abstract

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)n−1 (HCl–H2O cluster), i.e., HCl(H2O)n−1 + 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−1 (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.

Graphical abstract: Mechanism of ionic dissociation of HCl in the smallest water clusters

Supplementary files

Article information

Article type
Paper
Submitted
24 Nov 2023
Accepted
03 Jan 2024
First published
04 Jan 2024

Phys. Chem. Chem. Phys., 2024,26, 3623-3631

Mechanism of ionic dissociation of HCl in the smallest water clusters

H. Tachikawa, Phys. Chem. Chem. Phys., 2024, 26, 3623 DOI: 10.1039/D3CP05715A

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