What the anion tells us about the structure of tetrabutylammonium salt/phenol-based deep eutectic solvents? A combined infrared spectroscopic and DFT study

Abstract

In this study, a series of new Deep Eutectic Solvents (DESs) composed of phenol as the HBD (Hydrogen Bond Donor) and tetrabutyl ammonium (TBA) salts with different molecular anions as HBA (Hydrogen Bond Acceptor) were prepared and investigated using ATR-IR spectroscopy and DFT calculations in order to understand the solvation mechanism at work in these DESs. In particular, the OH stretching mode of phenol is significantly shifted to lower frequencies in the DESs in comparison with that reported for pure phenol. Similarly, characteristic vibrational modes of the anions in the investigated DESs display significant shifts in frequency in comparison with that measured for the pure TBA salt. As a representative example, the frequency shifts observed for the azide anion as a function of the HBA:HBD molar ratio were fully interpreted by the DFT calculations putting in evidence strong hydrogen bonding interactions that occurs between the anion and the phenol molecules. Interestingly, a minimum of the calculated free energy interaction of −25.9 kcal/mol was found at a 1:3 HBA:HBD molar ratio that is consistent with thermodynamic results reported for the Choline-chloride/Phenol DES for which the lowest freezing temperature was reported at this ratio.