Structural analysis of natural deep eutectic solvents. Theoretical and experimental study

A theoretical and experimental study was performed on natural deep eutectic solvents (NADES) formed by lactic acid-glucose (LGH), citric acid-fructose (CFH) and citric acid-glucose (CGH). The presence of nuclear Overhauser effect (NOE) in the proton nuclear magnetic resonance (H-1 NMR) spectra of the NADES was studied. The spatial proximity between the NADES components was experimentally confirmed by the detection of multiple NOE effects in the dilutions analyzed. LGH showed the best outcome by partially maintaining its supramolecular structure throughout the dilutions. In order to rationalize the intermolecular interactions generated among the components, a theoretical study was performed using a density functional theory (DFT) computational method. A simplified dimeric model of the NADES was selected in order to achieve a rapid screening of the system searching for interactions between their constituents. In agreement with the experimental evidence, the calculations allowed to confirm the spatial proximity, by finding at least two hydrogen bonds between the components of every NADES.