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Demystifying the Diffuse Vibrational Spectrum of Aqueous Protons Through Cold Cluster Spectroscopy
Helen J Zeng1, Mark A Johnson1
1Sterling Chemistry Laboratory, Yale University, New Haven, Connecticut 06520, USA;
Annual Review of Physical Chemistry
|March 1, 2021
Summary
The hydrated proton, or H+(aq), is complex. Gas-phase studies of H+(aq) cluster ions reveal its chemical nature and the Grotthuss mechanism for charge transport in water.
Area of Science:
- Physical Chemistry
- Chemical Physics
- Spectroscopy
Background:
- The hydrated proton, H+(aq), is a fundamental species in aqueous chemistry.
- Its complex nature is often oversimplified in routine pH measurements.
- Understanding H+(aq) is crucial for various chemical and biological processes.
Purpose of the Study:
- To review gas-phase studies of H+(aq) cluster ions over 30 years.
- To elucidate the chemical nature of H+(aq) in water.
- To reveal molecular-level insights into the Grotthuss mechanism.
Main Methods:
- Isolation of H+(aq)⋅(H2O)n cluster ions in the gas phase.
- Cryogenic cooling in radiofrequency ion traps.
- Two-color, infrared-infrared (IR-IR) double-resonance spectroscopy.
Main Results:
- Established a clear picture of how local hydrogen-bond topology influences spectral signatures.
- Provided insights into the Grotthuss relay mechanism for proton translocation.
- Demonstrated the complexity of the hydrated proton species.
Conclusions:
- Gas-phase cluster ion studies significantly advance our understanding of H+(aq).
- Local hydrogen-bond structure dictates the excess proton's spectral characteristics.
- Future studies can now focus on ultrafast relaxation dynamics of H+(aq).
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