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Sequential proton transfer through water bridges in acid-base reactions
Omar F Mohammed1, Dina Pines, Jens Dreyer
1Max Born Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max Born Strasse 2A, D-12489 Berlin, Germany.
Science (New York, N.Y.)
|October 8, 2005
Summary
Proton transfer in water occurs via a sequential hopping mechanism. A transient hydrated proton intermediate forms, bridging the acid and base through water molecules.
Area of Science:
- Physical Chemistry
- Chemical Dynamics
- Spectroscopy
Background:
- Proton transfer is fundamental to many chemical and biological processes.
- Understanding the real-time dynamics of proton transfer in aqueous solutions is crucial.
- Previous studies have proposed various mechanisms for proton exchange between acids and bases.
Purpose of the Study:
- To investigate the real-time mechanism of proton transfer between aqueous Brønsted acids and bases.
- To identify transient intermediates involved in the proton exchange process.
- To elucidate the role of water molecules in mediating proton transfer.
Main Methods:
- Utilized ultrafast infrared spectroscopy to monitor proton transfer in real time.
- Employed optically triggered photoacid proton donors (ROH) to initiate the reaction.
- Analyzed transient absorption spectra to identify intermediate species and reaction pathways.
Main Results:
- Observed a transient marker band indicating the formation of a hydrated proton intermediate (H3O+) before base protonation.
- Evidence suggests a sequential, von Grotthuss-type, proton-hopping mechanism through water bridges.
- The intermediate hydronium cation (H3O+) appears stabilized in the Eigen configuration within the ionic complex (RO-...H3O+...B-).
Conclusions:
- Proton exchange between aqueous acids and bases proceeds via a stepwise proton hopping mechanism mediated by water.
- A transient hydrated proton (H3O+) is a key intermediate in this process.
- The findings provide direct spectroscopic evidence for the von Grotthuss mechanism in aqueous proton transfer.
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