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Deep Eutectic Solvents

1. Physicochemical and Structural Investigation of L- Threonine/Glycerol-based Deep Eutectic Solvent Using Experimental and Molecular Modelling Approaches. ( New Journal of Chemistry )

One of the most popular topics in sustainable chemistry is the creation of new eco-friendly solvents. Deep eutectic solvents (DESs) have been established as accessible and affordable substitutes for ionic liquids. Herein, we studied the formation of a novel amino acid-based DES (AADES) considering L-threonine as a hydrogen bond acceptor and glycerol as a hydrogen bond donor. Owing to their complexity, a comprehensive understanding of DESs requires combined efforts that integrate experimental observations with computational approaches. The validation of the synthesized DES was initially performed through FTIR spectroscopy, where significant changes in bond-bending vibrations indicated strong non-covalent bond formation. In differential scanning calorimetry (DSC) analysis, the 1 : 3 Thr/Gly deep eutectic system displayed phase transitions marked by a pronounced peak at −22 °C, which was lower than the melting points of threonine (Thr) and glycerol (Gly). 1H-NMR studies also revealed hydrogen bonding intermolecular interactions between glycerol and threonine. Deshielded chemical shifts of proton signals of both glycerol and threonine are due to the local changes in electron density induced by the closeness of electronegative oxygens via the inductive effect, which supports the formation of the conformer (3 : 1 glycerol/threonine DES). Molecular dynamics (MD) simulation was employed to acquire a comprehensive understanding of how these solvents form and function at both the molecular and macroscopic levels. RDF and CDF analyses revealed the non-bonding sites (C–O⋯H and CO⋯H), interaction intensity (2–4.7 Å), and predominant angles (135–180 and 0–30 degrees) governing the process by which hydrogen bonds originate in the DES. SDF uncovered the conformations of the liquid DES and highlighted its variability, particularly in terms of clusters. The bulk properties of DESs are of paramount significance because they are intricately linked to their diverse applications. Transport property values were obtained through specialized MD simulations, providing crucial insights into the behavior of the systems. Non-equilibrium molecular dynamics (NEMD) simulations were performed to assess the rheological properties of viscosity at three distinct temperatures (298 K, 313 K, and 328 K). The obtained viscosity, surface tension, and self-diffusion coefficient values appear practical and fall within a reasonable range when compared to those of other well-known DESs.

2. Elucidating the Structure, Dynamics, and Interaction of a Choline Chloride and Citric Acid-Based Eutectic System by Spectroscopic and Molecular Modeling Investigations ( ACS omega)

Eutectic solvent systems are versatile solvents that have found widespread use in numerous applications. Traditional solvents are homogeneous, having only one component, and their chemistry is relatively simple, with some exceptions. On the other hand, deep eutectic solvents (DESs) comprise binary components, generally a donor and an acceptor in hydrogen bonding with varying ratios. The interaction chemistry among the donor and acceptor involved in hydrogen bonding in DESs is complicated. Although numerous research is focused on the synthesis and application of DESs, few studies are reported to elucidate the complex structure and dynamic and interaction behavior of DESs. In this study, we employed calorimetry, vibrational spectroscopy techniques including FTIR and Raman, and nuclear magnetic resonance to derive insight into the structural feature and noncovalent contact of choline chloride (ChCl) and citric acid (CA) while they formed DESs. The 1:1 ChCl/CA eutectic system showed phase transitions and melting peaks with the most pronounced peak at 156.22 °C, suggesting the DESs melting at a lower temperature than the melting temperatures of ChCl and CA. In addition to IR and Raman findings, 1H NMR investigations demonstrate hydrogen bonding intermolecular interactions between ChCl and CA, supporting the formation of 1:1 ChCl/CA DESs based on the deshielded chemical shifts of the proton for Ch. The interaction of the chloride anion with the methyl protons (H4) and methylene protons (H3) of ChCl as well as the strong hydrogen bonding interactions between the hydroxyl hydrogen (H1) of ChCl with one of CA’s carbonyl oxygens both supported the formation of conformer E. In addition, molecular dynamics followed by the density functional theory (DFT) was employed to visualize the structure and interaction of DESs using the ωB97XD theory and 6-311++G (d,p) basis set. Both experimental and theoretical IR, Raman, and structural analyses provided evidence of the formation of DESs by possessing hydrogen bonds. These multifaceted experimental and computational investigations provide details of structural and intermolecular interactions of ChCl/CA DESs.

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