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Speed limits for acid-base chemistry in aqueous solutions
Mateusz L Donten1, Joost Vandevondele, Peter Hamm
1University of Zurich Institute of Physical Chemistry, Zurich, Switzerland.
Chimia
|May 23, 2012
Summary
Proton transfer reactions, including acid-base recombination, can take microseconds, not milliseconds, near neutral pH. Competing bases create counterintuitive states, like transiently protonated acetate ions despite hydroxyl ions presence.
Area of Science:
- Physical Chemistry
- Chemical Kinetics
Background:
- Proton transfer reactions, such as acid-base recombination, are typically assumed to occur almost instantaneously.
- However, diffusion control and low concentrations near neutral pH can significantly extend these reaction time scales into the microsecond range.
Purpose of the Study:
- To investigate the time scales of proton transfer reactions in a model system with competing bases.
- To illustrate the kinetically controlled behavior and its impact on pH relaxation.
Main Methods:
- Utilized a model acid-base system comprising o-nitrobenzaldehyde (oNBA) as a proton cage.
- Employed acetate ions and hydroxyl ions as competing bases to study pH relaxation dynamics.
Main Results:
- Demonstrated that proton transfer reactions can extend into the microsecond range.
- Observed highly counterintuitive states due to kinetically controlled behavior.
- Showcased transient protonation of acetate ions for hundreds of nanoseconds, even in the presence of hydroxyl ions.
Conclusions:
- Acid-base recombination is not always instantaneous and can be diffusion-controlled, especially at low concentrations.
- Competing bases can lead to unexpected kinetic behaviors and transient intermediate states.
- The study highlights the importance of considering reaction kinetics in understanding pH relaxation processes.
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