High-Level VSCF/VCI Calculations Decode the Vibrational Spectrum of the Aqueous Proton.

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• The hydrated excess proton (H+) is fundamental in aqueous chemistry, forming diverse structures with water molecules.
• The infrared (IR) spectrum of the aqueous proton exhibits a broad
• proton continuum
• (1000–3000 cm^-1), challenging to interpret due to spectral breadth and vibrational anharmonicity.

Purpose of the Study:

• To accurately reproduce and interpret the linear IR spectrum of the aqueous proton.
• To elucidate the relationship between spectral features and the underlying molecular structures and vibrational dynamics of the hydrated proton.

Main Methods:

• Utilized reactive molecular dynamics to generate protonated water hexamer clusters (H⁺(H₂O)₆).
• Employed a high-level local monomer quantum method with a many-body potential energy surface to calculate IR spectra.
• Incorporated anharmonic effects and quantum mechanical treatment for the proton, validating against gas-phase spectra.

Main Results:

• Successfully reproduced the aqueous proton's linear IR spectrum by focusing on H⁺(H₂O)₂ core structures within larger clusters.
• Anharmonic calculations significantly improved agreement with experimental spectra compared to the double-harmonic approximation.
• The proton stretching mode strongly correlates with the proton-oxygen distance (⟨R_OH⟩), with spectral regions mapping to specific configurations and vibrational couplings.

Conclusions:

• The developed computational approach accurately models the complex IR spectrum of the aqueous proton.
• The study provides detailed insights into how different spectral frequencies correspond to specific proton hydration structures and vibrational dynamics.
• This work advances the understanding of proton transport and reactivity in aqueous systems.