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An infrared nuNH scale for weakly basic anions. Implications for single-molecule acidity and superacidity.
Evgenii S Stoyanov1, Kee-Chan Kim, Christopher A Reed
1Department of Chemistry, University of California, Riverside, CA 92521, USA.
Tri-n-octylammonium salts reveal anion basicity and acid strength. N-H stretching frequencies correlate with acidity, offering insights into molecular-level acid properties and potential for discovering new superacids.
Area of Science:
- Physical Chemistry
- Spectroscopy
- Acid-Base Chemistry
Background:
- Tri-n-octylammonium salts with weakly basic anions were studied.
- Understanding the relationship between anion basicity and acid strength is crucial.
- Previous studies often focused on bulk properties, limiting molecular-level insights.
Purpose of the Study:
- To investigate the N-H stretching frequencies of tri-n-octylammonium salts.
- To correlate these frequencies with the basicity of various anions.
- To establish a molecular-level indicator of acid strength in condensed phases.
Main Methods:
- Infrared (IR) spectroscopy was used to measure N-H stretching frequencies.
- Salts were studied in different solvents (carbon tetrachloride, 1,2-dichloroethane) and physical states (crystalline, oil).
- Analysis focused on the relationship between frequency shifts and anion properties.
Main Results:
- N-H stretching frequencies decreased in the order: fluoroanions > carboranes > oxyanions.
- This trend reflects the relative basicities of the anions and, by inference, the acidity of their conjugate acids.
- Ion pair formation (contact vs. solvent-separated) was observed depending on solvent polarity.
Conclusions:
- N-H stretching frequencies provide a useful qualitative indicator of individual molecule acidity.
- The data offer a condensed-phase analogy to gas-phase acidity.
- Anion chemical stability is key for high acidity, guiding the search for stronger acids.
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