Theoretical and experimental study of choline chloride-carboxylic acid deep eutectic solvents and their hydrogen bonds

In the field of green chemistry, deep eutectic solvents (DES) have seen extensive use and research in last two decades. Two carboxylic acid-choline chloride DES were studied using experimental Fourier Transform Infrared Spectroscopy (FTIR) and computational Density Functional Theory (DFT). Geometry optimization, vibrational analysis, and assignment of vibrational frequencies were performed using two DFT methods: B3LYP and BLYP. The resemblance to the experimental FTIR vibrations with calculated vibrations established that both the methods can be used to simulate experimental DES. Vibrations that would be impossible to identify and characterize using experimental methods were identified using DFT simulations. Optimized structures as well as experimental spectra were analyzed to prove the existence of hydrogen bonds and to study their lengths and changes when DES are synthesized from components. Atoms in Molecules theory was applied to study hydrogen bonding in these DES. The chlorine ion in choline chloride was found to form a strong hydrogen bond with acetic acid and formic acid in their DES formed with choline chloride. The choline ion was also found to act as a hydrogen bond donor in forming a strong hydrogen bond with acetic acid and formic acid.