HCl hydrates as model systems for protonated water

Area of Science:

• Computational chemistry
• Spectroscopy
• Materials science

Background:

• Ab initio molecular dynamics simulations investigate vibrational dynamics and spectra of crystal HCl hydrates.
• Distinct protonated water forms, Eigen ion H3O+(H2O)3 and Zundel ion H2O...H+...OH2, are identified in different hydrate compositions.
• These hydrates provide a unique environment to study protonated water species in a semi-rigid matrix.

Discussion:

• BLYP/DZVP-level calculations accurately reproduce experimental spectra via Fourier transform of the system dipole.
• Broad proton bands (800-1000 cm-1) indicate diverse solvation environments influenced by crystal vibrations.
• Analysis of aqueous HCl solutions suggests a continuous distribution of proton structural properties, lacking a clear criterion to separate Eigen-like and Zundel-like forms.

Key Insights:

• Crystal HCl hydrates host distinct protonated water structures, offering insights into proton dynamics.
• Simulations validate experimental spectra, highlighting the role of solvation and crystal vibrations.
• The study challenges the existence of distinct criteria for differentiating Eigen-like and Zundel-like protonated water forms in aqueous solutions.

Outlook:

• Further investigation into dipole derivatives and their relation to IR intensities can enhance understanding of H-bonding and solvation effects.
• Exploring the influence of Wannier centers on electron density representation can refine the analysis of spectral intensities.
• This research paves the way for more accurate modeling of proton behavior in condensed phases.