Hydrogen bond dynamics in aqueous NaBr solutions.

Area of Science:

• Physical Chemistry
• Chemical Physics
• Spectroscopy

Background:

• Water's hydrogen bond network is crucial for its unique properties.
• Understanding how solutes affect water structure is vital in chemistry and biology.
• Sodium bromide (NaBr) is a common electrolyte with significant effects on aqueous solutions.

Purpose of the Study:

• To investigate the influence of NaBr concentration on hydrogen bond dynamics in water.
• To quantify the effects of NaBr on water's structural and orientational relaxation.
• To analyze vibrational population relaxation in aqueous NaBr solutions.

Main Methods:

• Ultrafast 2D IR vibrational echo spectroscopy.
• Polarization-selective IR pump-probe experiments.
• Measuring spectral diffusion of the OD stretching mode of HOD in H2O.

Main Results:

• Increasing NaBr concentration (up to ~6 M) slows global hydrogen bond structural rearrangement, increasing the time constant from 1.7 to 4.8 ps.
• Orientational relaxation also slows with increasing NaBr concentration, with the slowest component increasing from 2.6 ps (pure water) to 6.7 ps (~6 M NaBr).
• Vibrational population relaxation of the OD stretch significantly slows down as NaBr concentration increases.

Conclusions:

• NaBr concentration directly impacts water's hydrogen bond dynamics, slowing both structural rearrangement and orientational relaxation.
• The observed slowing suggests increased structural order or restricted motion in water at high NaBr concentrations.
• These findings provide quantitative insights into solute-solvent interactions in concentrated electrolyte solutions.