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 [Sn2Cl5]- and [Zn2Cl5]- 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.