Tracking Aqueous Proton Transfer by Two-Dimensional Infrared Spectroscopy and ab Initio Molecular Dynamics...

Area of Science:

• Physical Chemistry
• Chemical Physics
• Spectroscopy

Background:

• Proton transport in water is exceptionally fast due to proton hopping, a phenomenon first described by Grotthuss.
• Despite extensive research, the exact proton hopping time and underlying mechanisms remain challenging to determine experimentally.
• Ultrafast proton transfer dynamics are critical for understanding various chemical and biological processes.

Purpose of the Study:

• To experimentally determine the elusive proton hopping time in aqueous acid solutions.
• To elucidate the molecular mechanisms driving proton hopping dynamics.
• To validate simulation results with experimental observations.

Main Methods:

• Utilized two-dimensional infrared (2D-IR) spectroscopy with methyl thiocyanate as a vibrational probe.
• Performed ab initio molecular dynamics (AIMD) simulations to model proton transfer.
• Analyzed experimental and simulated acid concentration dependencies.

Main Results:

• Successfully extracted chemical exchange rates between hydronium and water ions.
• AIMD simulations confirmed that proton hopping dominates chemical exchange.
• Extrapolated single-step proton hopping time to the dilute limit, consistent with proton mobility and NMR data.
• Identified concerted multi-water molecule hydrogen bond rearrangement as the driver for proton hopping.

Conclusions:

• Provided direct experimental measurement of proton hopping time in dilute acid solutions.
• Confirmed the role of hydrogen bond dynamics in facilitating proton transfer.
• Established a comprehensive understanding of proton transport mechanisms in water.