Microscopic Mechanism of Proton Transfer in Pure Water under Ambient Conditions

J Huo and JH Chen and P Liu and BK Hong and J Zhang and H Dong and SH Li, JOURNAL OF CHEMICAL THEORY AND COMPUTATION, 19, 4243-4254 (2023).

DOI: 10.1021/acs.jctc.3c00244

Watermolecules and the associated proton transfer (PT) are prevalentin chemical and biological systems and have been a hot research topic.Spectroscopic characterization and ab initio molecular dynamics (AIMD)simulations have previously revealed insights into acidic and basicliquids. Presumably, the situation in the acidic/basic solution isnot necessarily the same as in pure water; in addition, the autoionizationconstant for water is only 10(-14) under ambient conditions,making the study of PT in pure water challenging. To overcome thisissue, we modeled periodic water box systems containing 1000 moleculesfor tens of nanoseconds based on a neural network potential (NNP)with quantum mechanical accuracy. The NNP was generated by traininga dataset containing the energies and atomic forces of 17 075configurations of periodic water box systems, and these data pointswere calculated at the MP2 level that considers electron correlationeffects. We found that the size of the system and the duration ofthe simulation have a significant impact on the convergence of theresults. With these factors considered, our simulations showed thathydronium (H3O+) and hydroxide (OH-) ions in water have distinct hydration structures, thermodynamicand kinetic properties, e.g., the longer-lasting and more stable hydratedstructure of OH- ions than that of H3O+, as well as a significantly higher free energy barrierfor the OH --associated PT than that of H3O+, leading the two to exhibit completely different PTbehaviors. Given these characteristics, we further found that PT viaOH(-) ions tends not to occur multiple times or betweenmany molecules. In contrast, PT via H3O+ cansynergistically occur among multiple molecules and prefers to adopta cyclic pattern among three water molecules, while it occurs mostlyin a chain pattern when more water molecules are involved. Therefore,our studies provide a detailed and solid microscopic explanation forthe PT process in pure water.

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