Ionic effects on the proton transfer mechanism in aqueous solutions

Area of Science:

• Physical Chemistry
• Chemical Physics
• Solution Chemistry

Background:

• Proton dissociation (PD) and proton transfer (PT) reactions are fundamental in aqueous solutions.
• The influence of ions on the kinetics of these processes is significant but not fully understood at a molecular level.
• A detailed molecular mechanism explaining the ionic effect on PD and PT rates is lacking.

Purpose of the Study:

• To investigate the excited-state proton transfer (ESPT) reaction of a photoacid in aqueous metal chloride solutions.
• To elucidate the molecular-level mechanism by which ions affect ESPT rates.
• To correlate cation properties with changes in activation energy and local H-bond structure.

Main Methods:

• Utilized time-resolved fluorescence spectroscopy to study the ESPT dynamics.
• Employed quantum chemical calculations to model the reaction mechanism and energetics.
• Investigated solutions containing a photoacid and various aqueous metal chlorides.

Main Results:

• The activation energy (E_a) for the photoacid's ESPT reaction increased with increasing cation charge density (ρ_cat).
• Local hydrogen bond (H-bond) structure within the ionic hydration shell correlated with E_a and ρ_cat.
• Increased cation charge density destabilized the delocalized positive charge in the transition state, raising E_a.

Conclusions:

• The study provides a molecular-level understanding of the ionic effect on proton transfer reactions.
• Cation charge density is a key factor modulating the activation energy of ESPT.
• The findings elucidate the mechanism of ionic influence on proton dissociation and transfer in aqueous solutions.