Identification of Ion-Pair Structures in Solution by Vibrational Stark Effects
John Hack1,2, David C Grills1, John R Miller1
1Chemistry Department, Brookhaven National Laboratory , Upton, New York 11973-5000, United States.
The Journal of Physical Chemistry. B
|January 26, 2016
Summary
This study identifies solvent-separated ion pairs (SSIPs) in low-polarity solvents using infrared spectroscopy. Researchers distinguished between free ions (FI), contact ion pairs (CIP), and SSIPs, advancing understanding of ion pairing in chemistry.
Area of Science:
- Physical Chemistry
- Spectroscopy
- Materials Science
Background:
- Ion pairing influences diverse fields like batteries and organic photovoltaics.
- Distinguishing between free ions (FI), contact ion pairs (CIP), and solvent-separated ion pairs (SSIPs) in solution is challenging, particularly for organic radicals and nonmetal electrolytes.
Purpose of the Study:
- To provide evidence for SSIP formation in low-polarity solvents.
- To achieve distinct measurements of CIP, SSIP, and FI.
- To utilize the vibrational Stark effect (VSE) for identifying ion-pair structures.
Main Methods:
- Time-resolved infrared (IR) detection following pulse radiolysis.
- Analysis of the ν(C≡N) infrared band of a nitrile-substituted fluorene radical anion.
- Electronic structure calculations and solvent dependence studies.
Main Results:
- Unambiguous assignment of the FI peak using time-resolved IR.
- Observation of two distinct red-shifted peaks assigned to CIP and SSIP in the presence of nonmetal electrolytes.
- Demonstration of VSE-induced blue-shifts in intramolecular ion pairs, supporting IR shift origins.
Conclusions:
- SSIPs exist in low-polarity solvents.
- The vibrational Stark effect (VSE) is a reliable tool for identifying ion-pair structures, including SSIPs.
- This work enhances the understanding of ion-pair behavior in solution chemistry.
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