A stability analysis of choline chloride: urea deep eutectic solvent using density functional theory

Abstract

Deep eutectic solvents (DES) are eutectic mixtures of hydrogen bond acceptors and hydrogen bond donors. The stability of DES depends on the nature of interatomic interactions during their structural formation. This study focuses on the structural analysis of choline chloride (ChCl)-urea DES system (molar ratio 1:2) at the atomic level using density functional theory.Both gauche- and trans-conformers of choline were considered for geometric optimization with chloride anion at various orientations. The most stable structure of each conformer was further considered with urea to optimize the DES structure. Energy Decomposition Analysis (EDA) study showed that the binding energy (ΔEbind) for gauche-DES was more negative (−82.59 kJ/mol) than trans-DES (−7.60 kJ/mol). This indicated lower bond strength in trans-DES, hence higher stability in gauche-DES. It also agrees with calculation by dispersion energy, the Bader’s atoms-in-molecules theory for electrostatic interactions, the frontier molecular orbital analysis and Non-Covalent Interactions (NCI) analysis.