Acidity scales of deep eutectic solvents based on IR and NMR

Area of Science:

• Green Chemistry
• Supramolecular Chemistry
• Analytical Chemistry

Background:

• Acidic deep eutectic solvents (ADESs) are versatile but require accurate acidity characterization for performance prediction.
• Existing methods for measuring ADES acidity are limited, hindering their broader application and optimization.
• Understanding both Lewis and Brønsted acidities is crucial for tailoring ADES properties.

Purpose of the Study:

• To develop novel, solvent-free methods for quantifying Lewis and Brønsted acidities in ADESs.
• To establish reliable probes for precise acidity measurements.
• To provide a molecular-level understanding of acidity in ADESs.

Main Methods:

• Utilized acetonitrile as an infrared (IR) probe to measure Lewis acidity by monitoring the C≡N stretching frequency (ν(C≡N)).
• Employed trimethylphosphine oxide (TMPO) as a phosphorus-31 nuclear magnetic resonance (NMR) probe for quantifying both Lewis and Brønsted acidities.
• Established a linear correlation between the IR shift of ν(C≡N) and the effective charge density of metal cations.

Main Results:

• Developed two efficient, solvent-free methods for identifying and characterizing Lewis and Brønsted acidities in ADESs.
• The IR approach accurately measures Lewis acidity, while the NMR approach precisely quantifies both Lewis and Brønsted acidities.
• A strong linear relationship was found between the IR spectral shift and metal cation charge density, enhancing Lewis acidity understanding.

Conclusions:

• The study presents two practical and accurate methods for ADES acidity determination.
• These methods offer a molecular basis for optimizing ADES performance in diverse applications.
• The findings contribute to the advancement of analytical techniques in green solvent research.