Molecular and Spectroscopic Insights into a Metal Salt-Based Deep Eutectic Solvent: A Combined Quantum Theory of Atoms in Molecules, Noncovalent Interaction, and Density Functional Theory Study

Abstract

Deep eutectic solvents (DESs) based on metal halide salts are highly catalytic, low toxic, reusable, cost-effective, and have higher thermal stability than their analogue ionic liquids (ILs). In this work, we have reported the formation mechanism of metal salt-based DESs at the molecular level along with their charge-transfer analysis and thermodynamics associated with their formation using density functional theory. The DES systems analyzed in the present work were choline chloride and tin(II)chloride (DES1) and choline chloride and zinc(II)chloride (DES2), both in a molar ratio of 1:2, respectively. An excellent correlation is obtained between the theoretically calculated IR spectra of the DES systems and the previously reported experimental findings for the formation of the complex systems. The DESs were found to be stable systems due to traditional hydrogen bonding and electrostatic interactions resulting in the ionic species [Sn 2 Cl 5 ] − and [Zn 2 Cl 5 ] − and are elucidated with the help of electronic structure calculations. CHELPG partial charge analysis and natural bond orbital analysis suggest a charge transfer from Cl − (chloride) to Ch + (choline) and metal salts in the DES structures. The atom-in-molecules and noncovalent interaction (NCI) analysis suggest a strong electrostatic interaction within the DES2 system as compared to DES1. Higher stability and reactivity are observed in the DES2 system based on the frontier molecular orbital analysis. Our analysis offers important insights into the formation mechanism of these economic IL analogues.

Synthesis of biofuel precursors from benzaldehyde and cyclopentanone via aldehyde–ketone condensation in a deep eutectic solvent system

“…The TE, DE and ZPVE of DES ChCl/Fa (−1024.41 Ha, −51.44 kcal mol −1 and 145.48 kcal mol −1 , respectively) were observed to be significantly higher than its HBA (ChCl) and HBD (Fa), which might be attributed to the classic hydrogen bonds (–N–H⋯Cl) formed between ChCl and Fa resulting in the DES being a stable system with high catalytic capability. 38 Compared to DES ChCl/Fa, the catalytic system ChCl/Fa–SnCl 4 was found to be a more robust structure with TE, DE and ZPVE of −9048.69 Ha, −56.73 kcal mol −1 and 149.23 kcal mol −1 , respectively due to the feasible interaction structures among the ChCl, Fa and SnCl 4 , which were primarily formed by coordination bonds, electrostatic interactions and H-bonds. 38,39 A charge transfer was conducted from chloride ion (Cl − ) to choline ion (Ch + ) in DES and tin ion (Sn 4+ ) in SnCl 4 .…”

“…38 Compared to DES ChCl/Fa, the catalytic system ChCl/Fa–SnCl 4 was found to be a more robust structure with TE, DE and ZPVE of −9048.69 Ha, −56.73 kcal mol −1 and 149.23 kcal mol −1 , respectively due to the feasible interaction structures among the ChCl, Fa and SnCl 4 , which were primarily formed by coordination bonds, electrostatic interactions and H-bonds. 38,39 A charge transfer was conducted from chloride ion (Cl − ) to choline ion (Ch + ) in DES and tin ion (Sn 4+ ) in SnCl 4 . 38 The coupling Cl − in ChCl and H + in Fa was capable of forming a strong H-bond (H⋯Cl) similar to hydrogen chloride.…”

Ionic-Liquid-Assisted Capture of Volatile Low-Carbon Alcohols in Printing Plant Exhaust Gas

“…The type and strength of interactions between different molecular segments can be visually analyzed through quantum chemical calculation. − In this work, electrostatic potential (ESP) and independent gradient model (IGM) methods were used to get insight into capturing low-carbon alcohols by [HEMIM]­[PF 6 ] at the molecular level.…”

Solvent Regeneration Methods for Combined Dearomatization, Desulfurization, and Denitrogenation of Fuels Using Deep Eutectic Solvents

“…DESs have been actively applied as a promising alternative to volatile organic compounds and ionic liquids. − DESs are attractive for several reasons, including their inexpsensive to moderate cost of synthesis, the absence of purifying steps required before use, their low flammability, and their biodegradability. ,− Since their discovery, the application of DESs in various fields has shown the bright future of DESs as a promising solvent. − Such an example could be found in fuel purification processes, where DESs led their effectiveness in improving the quality of fuels. − One of the most advanced applications of DESs in fuel purification is the simultaneous removal of different aromatics. Choline chloride was used as an HBA in the DESs developed by Larriba et al to eliminate BTEX ( i.e.…”

“…DESs have been actively applied as a promising alternative to volatile organic compounds and ionic liquids. 9 − 12 DESs are attractive for several reasons, including their inexpsensive to moderate cost of synthesis, the absence of purifying steps required before use, their low flammability, and their biodegradability. 1 , 13 − 15 Since their discovery, the application of DESs in various fields has shown the bright future of DESs as a promising solvent.…”