Eigen versus Zundel complexes in HCl-water mixtures

The hydration behavior of hydrogen ions in water remains an unresolved topic in physical chemistry. While Eigen and Zundel complexes are two proposed models for proton solvation, the exact distribution of these species in solution is still debated. Prior research has shown that protons in water form hydrogen bonds with surrounding molecules, but the structural details remain unclear. This uncertainty has driven continued investigation into the nature of proton hydration. Neutron diffraction has been used to study proton environments in aqueous systems, but interpretation of the data is complex. The distinction between Eigen and Zundel complexes is challenging due to overlapping structural features. No prior work has resolved the ambiguity in how hydration networks influence proton behavior. This gap motivated a reanalysis of existing data using new simulation assumptions. The goal is to better understand the hydration structure of H+ in HCl-water solutions.

This study aimed to reevaluate the hydration structure of hydrogen ions in a 1:9 HCl-water solution using a novel approach to Monte Carlo simulations. The researchers sought to determine whether Eigen or Zundel complexes dominate in this system. They tested three different initial assumptions about proton hydration in their simulations. Each model allowed for different proton species: bare H+, H3O+, or H5O2+. The purpose was to assess how each model fits the neutron diffraction data. The study also aimed to explore the role of hydrogen ion-chloride ion interactions. The researchers wanted to clarify the ambiguity in distinguishing between the two proton hydration models. Their approach was designed to refine the interpretation of experimental hydration patterns.

The researchers used Monte Carlo simulations to model the hydration of H+ ions in a 1:9 HCl-water solution. They tested three different starting hypotheses for proton hydration. One model assumed bare H+ ions, another assumed all H3O+ ions, and the third assumed all H5O2+ ions. Each simulation included water and chloride ions in the system. The simulations were compared to existing neutron diffraction data. The team evaluated how well each model matched the experimental results. They analyzed hydrogen bond lengths and the number of hydrogen bonds formed by each proton species. The simulations allowed for dynamic interactions between water and proton species.

All three simulation models provided a satisfactory fit to the neutron diffraction data. The simulation with bare H+ ions showed that each H+ formed one strong and short hydrogen bond with water. On average, 75% of these H+ ions also formed a second, slightly longer hydrogen bond. This result could be interpreted as a high prevalence of asymmetric Zundel complexes. Alternatively, it could suggest the formation of distorted H3O+ ions. These distorted ions may form two or three hydrogen bonds, leading to Eigen complex-like structures. The study found no clear separation between Eigen and Zundel complexes in the data. The continuous hydrogen bond network between water and protons made structural distinctions difficult. The simulations also revealed the influence of hydrogen ion-chloride ion contacts in the solution.

The new analysis supports the idea that the solution contains a mixture of hydration structures, rather than a clear dominance of one complex type. The researchers found no definitive evidence to favor Eigen over Zundel complexes. The data suggest that both models may represent overlapping hydration states. The continuous hydrogen bond network in the solution complicates the distinction between the two models. The study highlights the challenges in interpreting proton hydration structures from experimental data. The results are not inconsistent with the previous conclusion that Eigen complexes are prevalent. The simulations show that hydration structures can vary dynamically in the solution. The role of chloride ion interactions remains an important factor in the hydration process.