Proton transfer in concentrated aqueous hydroxide visualized using ultrafast infrared spectroscopy

Area of Science:

• Physical Chemistry
• Spectroscopy
• Chemical Dynamics

Background:

• The solvation structure and proton transfer mechanism of hydroxide ions in aqueous solution are not fully understood.
• Understanding these processes is crucial for explaining rapid ion diffusion.

Purpose of the Study:

• To elucidate the solvation structure and proton transfer mechanism of the hydroxide ion (OH⁻).
• To investigate the dynamics of proton transfer in aqueous solutions using advanced spectroscopic techniques.

Main Methods:

• Femtosecond infrared spectroscopy, including pump-probe, photon echo peak shift, and two-dimensional infrared (2D IR) spectroscopy.
• Measurements were performed on dilute HOD in NaOD/D₂O solutions at varying deuteroxide concentrations.

Main Results:

• A spectral feature decaying on a ~110 fs timescale was assigned to proton sharing between HOD and OD⁻.
• 2D IR spectra revealed population exchange between OH⁻ and HOD over picosecond timescales, indicating deuteron transfer.

Conclusions:

• The study provides insights into the transient configurations and proton transfer dynamics involving hydroxide ions.
• A spectral model was developed to explain experimental data, yielding a lower bound of 3 ps for deuteron transfer kinetics.