Quantum Chemical Insight into the Interactions and Thermodynamics Present in Choline Chloride Based Deep Eutectic Solvents

Abstract

We report quantum chemical calculations performed on three popular deep eutectic solvents (DESs) in order to elucidate the molecular interactions, charge transfer interactions, and thermodynamics associated with these systems. The DESs studied comprise 1:2 choline chloride/urea (reline), 1:2 choline chloride/ethylene glycol (ethaline), and 1:1 choline chloride/malonic acid (maloline). The excellent correlation between calculated and experimental vibrational spectra allowed for identification of dominant interactions in the DES systems. The DESs were found to be stabilized by both conventional hydrogen bonds and C-H···O/C-H···π interactions between the components. The hydrogen-bonding network established in the DES is clearly distinct from that which exists within the neat hydrogen-bond donor dimer. Charge decomposition analysis indicates significant charge transfer from choline and chloride to the hydrogen-bond donor with a higher contribution from the cation, and a density of states analysis confirms the direction of the charge transfer. Consequently, the sum of the bond orders of the choline-Cl(-) interactions in the DESs correlates directly with the melting temperatures of the DESs, a correlation that offers insight into the effect of the tuning of the choline-Cl(-) interactions by the hydrogen-bond donors on the physical properties of the DESs. Finally, the differences in the vibrational entropy changes upon DES formation are consistent with the trend in the overall entropy changes upon DES formation.

Charge Spreading in Deep Eutectic Solvents

“…The Hirshfeld‐I ansatz uses an improved partitioning scheme of the electron density compared to the original method proposed by Hirshfeld, which allows to apply this method to ionic systems and produces results strongly correlated to the CHELPG partial charge analysis . As can be seen in Table , the charge of chloride is significantly reduced in all investigated DESs and significantly smaller than in the previous study of Wagle et al., which investigated clusters consisting of one choline chloride ion pair and one or two organic molecules . The deviations between both studies might be attributed to the missing solvation of the gas‐phase calculations in their study.…”

“…Within this Communication, we present the first investigation of DESs by ab initio molecular dynamic simulations, which allows us to study the charge spreading in the liquid state. The charge located at the anion, cation or the organic compound was investigated by static density functional theory calculations for clusters consisting of only one choline chloride ion pair and one or two organic molecules in the gas phase so far . Thus, solvation was neglected in previous studies which leads to a too strong charge localization at the ionic compounds (see discussion below).…”

“…The charge located at the anion,c ation or the organic compound was investigated by static density functional theory calculations for clusters consistingo fo nly one choline chloride ion pair and one or two organic molecules in the gas phase so far. [29,30] Thus,s olvation was neglected in previous studies which leads to at oo strongc hargel ocalization at the ionic compounds (see discussion below). To show that our observations are not limited to as ingle choline chloride DES system, we have investigated three popularD ESs knownf rom the literature: [8,12] 1.…”

Green Separation of Bioactive Natural Products Using Liquefied Mixture of Solids

“…Based on the previous research [29], ethylene glycol interacted into each other by making hydrogen bonding in cyclical pattern and the distance of H-O bond was 1.944 Å, whereas, ChCl in crystalline structure has three bonds consisted of C-N, C-O and C-C, and the distance of each bond is 0.01 Å. ChCl is difficult to convert into liquid at room temperature due to the small distance of ChCl bonds. Based on Wagle et al [29], DES from ChCl and ethylene glycol has two interactions of C-H-O.…”

“…Based on Wagle et al [29], DES from ChCl and ethylene glycol has two interactions of C-H-O. The first one was between oxygen from ethylene glycol and methyl proton from ChCl, and the distance was 2.146-2.440Å.…”