Switching between Proton Vacancy and Excess Proton Transfer Pathways in the Reaction between 7-Hydroxyquinoline and...

Area of Science:

• Physical Chemistry
• Photochemistry
• Chemical Kinetics

Background:

• Bifunctional photoacids possess both acidic and basic sites, enabling the study of proton transfer reactions.
• Proton transfer in protic solvents occurs via protolysis and hydrolysis, influenced by acid-base strength.
• 7-hydroxyquinoline (7HQ) is a bifunctional photoacid with a known hydroxide/methoxide transport mechanism in deuterated methanol (CD3OD).

Purpose of the Study:

• To investigate how the addition of a formate base affects the reaction pathways of the bifunctional photoacid 7-hydroxyquinoline (7HQ).
• To elucidate the role of formate in modulating proton transfer dynamics in CD3OD solution.

Main Methods:

• Time-resolved infrared (IR) spectroscopy was used to monitor the IR-active marker modes of photoexcited 7HQ and formic acid (HCOOD).
• Classical molecular dynamics (MD) simulations were employed to complement spectroscopic data.
• Experiments were conducted with varying concentrations of formate in CD3OD.

Main Results:

• The presence of formate significantly alters the proton transfer pathways of 7HQ.
• Spectroscopic and simulation data reveal distinct contributions from 'tight' and 'loose' 7HQ-formate reaction pairs.
• Formate acts as a proton relay, facilitating proton transfer from the 7HQ hydroxyl group via a methanol/formate bridge to the quinoline nitrogen.

Conclusions:

• Formate addition provides a new mechanism to control and study proton transfer in bifunctional photoacids.
• The orientation of the 7HQ hydroxyl group influences the effectiveness of the formate-mediated proton relay.
• This study deepens the understanding of solvent and additive effects on ultrafast proton transfer dynamics.