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pH Jumps in a Protic Ionic Liquid Proceed by Vehicular Proton Transport
Sourav Maiti1, Sunayana Mitra2, Clinton A Johnson2
1Central Laser Facility, RCaH, STFC-Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, United Kingdom.
The Journal of Physical Chemistry Letters
|August 23, 2022
Summary
Excess protons in ethylammonium formate (EAF) primarily transfer via hydrogen bonds within picoseconds. Proton diffusion is limited, suggesting vehicular transport in this protic ionic liquid (PIL).
Area of Science:
- Physical Chemistry
- Spectroscopy
- Ionic Liquids
Background:
- Protic ionic liquids (PILs) are promising media for chemical reactions.
- Understanding proton dynamics in PILs is crucial for their application.
- Ethylammonium formate (EAF) is a representative PIL for studying proton transfer.
Purpose of the Study:
- To investigate the dynamics of excess protons in EAF over a wide timescale (femtoseconds to microseconds).
- To elucidate the mechanisms of proton transfer and transport following photoexcitation.
- To provide a comprehensive understanding of the photoprotolytic cycle in PILs.
Main Methods:
- Visible pump mid-infrared probe spectroscopy was employed.
- Photoexcitation of 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) initiated proton transfer.
- Analysis of transient-infrared spectra captured proton dynamics.
Main Results:
- Proton transfer to EAF occurred predominantly (∼70%) via a preformed hydrogen-bonded complex within picoseconds.
- Protons diffused only a few solvent shells before recombination, indicating limited long-range transport.
- A kinetic isotope effect near unity (kH/kD ≈ 1) suggested vehicular proton transfer.
Conclusions:
- The study reveals the complete photoprotolytic cycle of excess protons in EAF.
- Proton transport in this PIL is characterized by short-range diffusion and vehicular mechanisms.
- Findings offer critical insights into proton dynamics within ionic liquid systems.
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