Ion Pair Supramolecular Structure Identified by ATR-FTIR Spectroscopy and Simulations in Explicit Solvent*

Area of Science:

• Physical Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Ion pairing in aqueous solutions influences chemical and physical properties.
• Understanding the structure of alkali-metal carboxylate ion pairs is crucial for various chemical processes.

Purpose of the Study:

• To investigate the supramolecular structure of carboxylate alkali-metal ion pairs in aqueous solutions.
• To determine the appearance concentration thresholds of contact ion pairs.
• To elucidate the spectroscopic sensitivity of the carboxylate group and the roles of cations and water molecules.

Main Methods:

• Attenuated Total Reflectance Fourier-Transform Infrared (ATR-FTIR) spectroscopy.
• Molecular simulations in explicit solvent and frequency calculations.
• Quantum chemistry calculations including conformational explorations, optimizations, and electron density analyses.

Main Results:

• ATR-FTIR spectra showed cation-specific behaviors across a 0.25-4.0 M concentration range.
• Contact ion pair appearance concentration thresholds were measured between 1.9 and 2.6 M, varying with the cation.
• Identified main supramolecular structures contributing to experimental spectra through advanced computational methods.

Conclusions:

• The study reveals cation-specific mechanisms governing ion pairing in aqueous solutions.
• Spectroscopic sensitivity of the carboxylate group is influenced by cation and water interactions.
• Combining advanced experimental and theoretical techniques is essential for accurate ion pairing descriptions.