<scp>l</scp>-Proline–Carboxylic Acid-Based Hydrophobic Deep Eutectic Solvents: Thermophysical Properties and DFT Structural Insights

Hydrophobic deep eutectic solvents (HDESs) are emerging as potential substitutes for traditional organic solvents. Their distinctive features, namely, low aqueous solubility and tunable structures, make them suitable for diverse uses. The present work describes the synthesis and spectroscopic (¹H NMR, ¹³C NMR, and FTIR) characterization of l-proline-carboxylic acid-based HDESs. Their thermophysical behavior in terms of parameters such as density, dielectric constant, conductivity, and refractive index has been examined. Density functional theory calculations with an emphasis on geometry optimization, HOMO-LUMO analysis, quantum theory of atoms in molecules (QTAIM), and analysis of the noncovalent interactions (NCI) were undertaken to gain structural insights, e.g., molecular stability, electronic properties, and intermolecular interactions. Among the three l-proline-based HDESs, l-proline:HexA shows the highest stability, demonstrated by its maximum stabilization energy value of -24.453 kcal mol^-1. The findings suggest that l-proline-based HDESs demonstrate stable intermolecular interactions, favorable electronic properties, and advantageous thermophysical characteristics, making them appealing for a variety of potential applications.