Quantum Chemical Insight into the Interactions and Thermodynamics

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Quantum Chemical Insight into the Interactions and Thermodynamics Present in Choline Chloride Based Deep Eutectic Solvents Durgesh Vinod Wagle, Carol A Deakyne, and Gary A. Baker J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b04750 • Publication Date (Web): 07 Jun 2016 Downloaded from http://pubs.acs.org on June 10, 2016

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The Journal of Physical Chemistry

Quantum Chemical Insight into the Interactions and Thermodynamics Present in Choline Chloride Based Deep Eutectic Solvents Durgesh V. Wagle, Carol A. Deakyne, and Gary A. Baker* Department of Chemistry, University of Missouri-Columbia, 125 Chemistry Building, Columbia, Missouri 65211, USA 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 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 correlate 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.

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Introduction The emergence of ionic liquids (ILs) in the past decade or two has revolutionized the field of chemistry. The strong ionic nature of ILs arising from discrete bulky organic cations and weakly coordinating anions not only results in low melting temperatures (< 100 °C) but also leads to favorable attributes such as high thermal stability, excellent chemical stability, nonflammability, low vapor pressure, and good recyclability.1 Unfortunately, recent developments have demonstrated considerable toxicity and poor biodegradability associated with many ILs.2-5 Moreover, commercial scale applications of ILs are limited by the high cost of synthesis, thus making it imperative to develop an environmentally friendly class of solvents from renewable and non-toxic sources.6 More recently, organic-based deep eutectic solvents (DESs) have emerged as cost-effective and bio-friendly alternatives to ILs, while retaining their characteristics such as non-flammability, low vapor pressure, and good recyclability.7-10 Abbott et al. first reported that mixing a high melting quaternary ammonium salt, such as choline chloride (302 °C), with a high melting hydrogen-bond donor (HBD), such as urea (133 °C), in an appropriate molar ratio results in a eutectic mixture that is a liquid at room temperature. The melting point of the eutectic mixture is governed by the amount of HBD in the mixture, with a reported minimum of 12 °C achieved in the choline chloride/urea-based DES containing 67 mol% urea.8 Several DES mixtures have emerged since, but choline chloride is a prominently featured quaternary ammonium salt in DESs. This salt is economically produced in large quantities and can be used with a variety of HDBs such as amides, carboxylic acids, glycols and phenols, thus offering a wide range of options.10 The flexibility in DES composition and “the sustainable” nature of DES systems have broadened their applications into nanochemistry,11 electrochemistry,12,13


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catalysis,14 polymer synthesis,15 gas separation,16,17 generation of mesoporous carbons,18,19 and biomass treatment.20 With the rise in popularity and potential applications of DESs, a fundamental molecularlevel understanding of their structure and dynamic nature takes on great importance. Recent years have witnessed a significant amount of experimental research on DESs;21,22 however, computational studies remain in their infancy. Recent molecular dynamics (MD) simulations by Sun et al. on DESs involving choline chloride and urea in different ratios revealed a disruption of the long-range ordering of choline chloride by urea, long hydrogen-bonding lifetimes of urea with chloride anion, and significant moderation in urea–urea and choline–chloride interaction energies, which resulted in a significant melting point depression for the DES mixture.23 Similarly, Perkins et al. made observations using MD simulations and infrared spectroscopy that suggested the presence of strong trans-type hydrogen-bonding interactions between the Cl– and urea to maximize the number of hydrogen bonds, leading to the low-melting nature of this DES.24 Our previous calculations on choline chloride/glycerol revealed stronger binding of glycerol with chloride anion as compared to that with choline cation, leading to higher translational diffusion of choline cation in the DES.25 Using DFT calculations and atoms in molecules (AIM) analysis, Garcia et al. pointed out that there is a linear relationship between the melting temperatures (Tf) and the change in the electron density at the cage critical points of the hydrogen-bonding networks in cholinium-based DESs.26 Rimsza et al. performed quantum chemical calculations on Cu and CuO in reline to map out the possibility of proton hopping between anionic urea species in the DES.27 Zhang et al. studied the mechanism of the electrochemical deposition process of magnesium chloride hexahydrate in a choline chloride/magnesium chloride hexahydrate-based DES using a combination of experiments and 3 ACS Paragon Plus Environment


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quantum chemical calculations.28 Although computational research on the properties of DESs is beginning to emerge, most of the work has been focused exclusively on eutectic mixtures based on the choline chloride/urea system. Herein, we build upon the work reported by Garcia et al. to further our understanding of DES systems by studying them through infrared spectroscopy and quantum mechanical (QM) approach.29 Specifically, we probe into the intermolecular interactions between choline chloride and three different types of hydrogen-bond donors (urea (U), ethylene glycol (EG) and malonic acid (MA)) using electronic structure calculations. We focus on the molecular geometries, charge delocalization effects, and thermochemistry of these systems, thus offering a deeper insight into the eutectic nature of DESs than previously available. In terms of nomenclature in this article, we will use the following parenthetical abbreviations for the three DES systems under investigation: 1:2 choline chloride/urea (reline), 1:2 choline chloride/EG (ethaline), and 1:1 choline chloride/MA (maloline), wherein the molar ratios of the DES components are denoted (Figure 1). Experimental methods Choline chloride (99 %), urea (99.5 %), ethylene glycol (99 %) and malonic acid (99 %) were bought for Sigma-Aldrich. The three DES mixtures were synthesized according to previously reported procedures.8,10,30 Infrared measurement on the DES mixtures was done using the NEXUS 670 FT-IR instrument from GMI Inc. Computational Methods The geometry optimization of the DES mixtures was carried out using various starting orientations of the HBD molecules (in-plane and out-of-plane) around choline chloride, with the 4 ACS Paragon Plus Environment

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HBDs placed at the ammonium and/or hydroxyl end. Initial optimizations carried out at the HF/6-311G(d,p) level of theory were followed by reoptimization at the higher M06-2X/631++G(d,p) level. All calculations were done for a singlet ground state with no symmetry restrictions. Absence of imaginary frequencies for the optimized structures ensured minima. Mulliken population and electrostatic potential charge analyses were performed at the higher level of theory. The basis set superposition error (BSSE) was evaluated using the Boys and Bernadi counterpoise (CP) procedure.31 The thermochemical characteristics of the DESs were tested for their consistency and reproducibility by performing single-point energy calculations on the above optimized geometries with 1) the M06-2X method using the aug-cc-pVDZ basis set and 2) the MP2 method (to assess electron correlation effects) using the 6-31++G(d,p) and augcc-pVDZ basis sets. All calculations reported here were carried out using the Gaussian 09 C.01 program package.32 Scaling factor of 0.986 was used for the correction of vibrational frequencies calculated at M06-2X/6-31++G(d,p) level.33 Charge decomposition analysis was performed using the AOMix program.34,35 The dispersion correction in the M06-2X calculations was taken into account for the thermochemistry of DES formation using the method developed by Grimme.36 Thermochemical values for the choline chloride (ChCl), HBDs and DES mixtures were obtained using the following equations: ΔXChCl = XChCl – XCh – XCl

(1)

ΔXHBD = X(HBD)2 – 2XHBD

(2)

ΔXDES = XDES – Xcholine chloride – nX(HBD)

(3)

Here, X is the BSSE-corrected interaction energy (E), enthalpy (H) or free energy (G) where n = 1 (maloline) or 2 (reline and ethaline). 5 ACS Paragon Plus Environment


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Results and Discussion Optimized Geometry A preliminary study of the effect of method and basis set on the geometries of individual components as well as DES systems confirmed the reproducibility and reliability of the results under consideration. The M06-2X functional is known to provide accurate insights into hydrogen bonding, charge transfer, dipole interactions, and non-covalent interactions including dispersion interactions.33,37 Interactions within HBD systems (i.e., between two urea molecules, two EG molecules and two MA molecules) and between choline and chloride were investigated. The minimization of the urea–urea system resulted in a “double two-center” structure, a hydrogenbonded, eight-membered ring arrangement involving two N–H···O interactions (Figure 2A), the most frequently found orientation in urea clusters.38, 39 The N···O (2.991 Å) and H···O (1.992 Å) distances exhibit good agreement with those from the previously reported QM calculations on urea clusters.39 Optimization of the geometry of the EG dimer (Figure 2B) yielded a cyclic cooperative hydrogen-bonding pattern with a H···O bond distance of 1.944 Å, which is consistent with the bond length reported by Kumar.40 For the MA pair, the O···O distance of 2.614 Å in the O–H···O interaction (Figure 2C) is in excellent accord with the distance of 2.630 Å reported for MA crystals.41 Also, the ∠O–H···O bond angle of 177.9° in MA is within the range of 175–178° observed for the MA crystals.41 In choline chloride, the computed C–N, C–O, and C–C distances were within 0.01 Å of the bond lengths found in the crystal structure reported by Hjortas et al.42 With the successful geometrical predictions, in silico formation of clusters is representative of the DESs was pursued.


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Information on the structure and orientation of choline chloride and HBDs in the DESs is critical for understanding the eutectic nature, as the way molecules pack/interact determines their physical properties such as melting point, viscosity, and thermal stability. To this effect, we have collected and analyzed the IR spectra and compared it to the calculated vibrational spectra of the DESs using the M06-2X theory at 6-31++G(d,p) level. Table 1 provides a list of vibrational modes associated with the interactions between various components within the DES systems. It should be noted that for the experimental spectra, despite the tiny amounts of samples that were employed to obtain a spectrum, the quantity used contains a much larger number of individual molecules compared to the few molecules studied in the calculations. Since the number of molecules varies within the two methods, they form hydrogen-bonds to a slightly different extent. Therefore, when an IR spectrum is acquired, the IR absorption bands will occur at varying frequencies for each of these individual bonds, resulting in peak shift and broadening, as it is an average of all these slightly different absorptions,43 which is otherwise not observed in the computed spectra. The features of the calculated spectra for the optimized structures of reline, ethaline, and maloline correlated well with the features of their corresponding experimental vibrational spectra (Figure 2) producing correlation coefficients (r2) of 0.996, 0.995, and 0.989 (Figure 3) respectively, for the three DESs studied. These results highlight the reliability of the computational method employed here for the optimization of the DES systems and provide a platform to assign the individual interactions in the calculated vibrational spectra with the peaks observed within the experimental vibrational spectra. The DES systems exhibit cooperative hydrogen-bonding networks wherein the HBD molecules interact with the cation as well as the anion to form a complete loop (cycle). In reline, urea interacts with Cl– via two N–H···Cl– interactions with H···Cl– distances of 2.283 Å and 2.302 7 ACS Paragon Plus Environment


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Å, thus forming a six-membered ring-like structure (Figure 3A). A relatively weak urea–urea interaction is retained after formation of the DES via two cis-N–H···O=C interactions with H···O distances of 2.032 Å (Figure 3A), as compared to 1.992 Å in the HBD dimer (Figure 2 A). The calculated IR vibrational frequency for the associated N–H stretch increases from 3407 cm–1 in the urea dimer to 3576 cm–1 in reline, again indicative of hydrogen bond weakening. Interestingly, a strong in-plane interaction between the C=O on urea and three C–H bonds on the methyl groups of the choline cation is observed, with distances of 2.256 Å, 2.416 Å, and 3.166 Å, an indication of C–H···lone pair interactions. Similarly to reline, ethaline exhibits three C– H···O interactions between the oxygen on EG and the methyl protons on the choline cation, with H···O interaction distances of 2.146–2.440 Å. These distances are shorter than those for the H···Cl– interactions (2.271–2.474 Å) between EG and Cl–. The Cl– forms the centerpiece in the ethaline complex by interacting with five hydroxyl groups, one from the choline cation and four from the two EG molecules (Figure 4B). Moreover, the EG acts as a bidentate HBD, leading to two spirally connected seven-membered rings sharing the Cl–, thus enhancing the cooperative hydrogen-bonding ability in the DES and leading to higher stability. This arrangement causes the hydrogen bonds observed in the HBD dimer (Figure 4B) to be nearly completely disrupted in the DES (Figure 4B and 5B). In maloline, unlike reline and ethaline, the DES mixture is based on a 1:1 stoichiometry of choline chloride and malonic acid. The MA interacts strongly with the choline cation and Cl–. The hydroxyl group of the carboxylic acid functionality on MA interacts via a H···Cl– (1.923 Å) interaction, whereas the C=O end interacts with the methyl protons on choline via C–H···π interactions with H···O distances ranging from 2.178 Å to 2.363 Å (Figure 4C). The overall structural characteristics found in this work for the three DES systems agree well with those reported previously.26


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A pseudo bond order (BO) analysis was used to evaluate the strength of the interactions of the Cl– with choline and the HBDs. The analysis revealed bond orders of 0.449, 0.227, and 0.377 between the carbon atoms in the choline backbone and the Cl– in reline, ethaline, and maloline, respectively. Furthermore, choline interacts with Cl– through multiple sites with a few strong (hydrogen-bonding) and multiple weak (electrostatic) interactions. The sum of the BOs of these individual choline–Cl– interactions is presented in Table 2 for each DES system. Interestingly, this analysis indicates that the choline–Cl– interaction in reline strengthens, whereas in ethaline and maloline it weakens significantly. This behavior is consistent with the change in the charge on the Cl– before and after formation of a DES and in the choline–Cl– interaction distances (Figures 4D and 5). Moreover, for the DES systems under consideration, there is a linear relationship between the total bond order associated with the interaction between choline and Cl– and the melting temperatures of the DES systems. This indicates that the melting temperature depends upon the ability of the HBD molecules to fine tune the choline–Cl– interaction (Figure 6). The HBDs also interact with Cl– through multiple sites involving –NH2 and –OH protons with bond orders ranging between 0.120–0.151, which are significantly lower than those for the choline–Cl– interaction (with an exception in maloline: 0.377). Overall, the optimized geometries and bond orders of the DESs indicate that mixing of choline chloride and HBDs results in: 1) moderation/modification of the interactions present in the parent components and 2) formation of multiple weak non–covalent interactions, such as C–H···lone pair and C– H···π, leading to formation of the DESs. In the correlation plot of Figure 6, we used our experimentally-determined melting point for reline. Although the literature recurrently reports a freezing temperature of 285.5 K for reline, in our experience this DES invariably solidifies upon short-term ambient storage. Indeed, we 9 ACS Paragon Plus Environment


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find that rigorously-dried reline exists as a liquid but eventually turns into a waxy white solid after standing for several days at room temperature (see Figure S1, SI). We infer from this behavior that the liquid reline observed by researchers is actually in a supercooled state. In line with this, while initially a stable and homogeneous fluid, reline eventually freezes to form a waxy, white solid which is then observed to melt at 305 ± 2 K using a capillary-based melting point apparatus. Charge transfer In a recent review on DESs, Carriazo et al. proposed that the charge delocalization between the anion and HBDs that occurs via hydrogen-bonding interactions is responsible for the depression in the freezing point of the DES mixtures.15 To address this proposed charge delocalization, in this study we have employed the charge decomposition analysis (CDA) developed by Frenking and co-workers to elucidate the direction and extent of charge transfer (CT) between the various constituents of the DESs. In this analysis, the CT interaction between two fragments is divided into three terms: 1) electron donation from fragment A to B, 2) electron donation from fragment B to A, and 3) reorganization of electron density due to electron– electron repulsion between the fragments. The CDA analysis of the DES systems suggests a significant amount of charge transfer from choline chloride to the HBDs in the DES mixture. The total amount of charge transfer from choline chloride to the HBDs increases proceeding from reline to ethaline to maloline as 0.122 e–, 0.154 e–, and 0.174 e–, respectively. The CT was further investigated using the CHELPG data. It was observed that the net positive charge on the cation increases and the net negative charge on the anion decreases along with the net increase in the negative charge on the HBDs – a clear indication of CT occurring from the choline cation as well as the chloride anion to the HBDs through hydrogen-bonding and electrostatic interactions 10 ACS Paragon Plus Environment

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(Table 3). To decipher the nature of the CT process between the various components of the DESs, we looked at the density of states (DOS) to investigate the orbital contributions from the various components. Figure 7 provides a comparison of the DOS for choline chloride in the absence and presence of the three DES systems. It is observed that the DOS feature of the choline cation is increasingly perturbed from reline to ethaline to maloline, a trend that is in good agreement with the CHELPG charge changes (Table 3) and CDA analysis discussed above. Moreover, the DOS features of choline in ethaline and maloline are considerably altered compared to reline and indicate charge donations from orbitals that are much lower in energy compared to that of the HOMO. The DOS feature of Cl– in the DESs (–6.8 to –9.0 eV) is reduced in intensity compared to that of the free choline chloride with no significant change in shape, an indication that Cl– plays a smaller role compared to choline in the CT process. Moreover, the negative shift in the Fermi energy levels of the DOS features associated with the choline chloride in the DESs (Figure 7, inset) further confirms that the CT occurs from choline chloride to the HBDs.44 Thermochemistry In this study of DESs, of particular interest is the change in the thermodynamics associated with the formation of DESs. To test for the consistency and reproducibility of the results, the thermochemical analysis for choline chloride, dimers of HBDs, reline, ethaline, and maloline was done at the M06-2X/6-31++G(d,p), M06-2X/aug-cc-pVDZ, MP2/aug-cc-pVDZ and MP2/6-31++G(d,p) levels of theory on the geometries optimized at the M06-2X/631++G(d,p) level (Table 4). Irrespective of the basis set, the interaction energies calculated with the M06-2X method are within 2 kcal/mol of the value calculated with the MP2 method. In addition, the calculated M06-2X/6-31++G(d,p) interaction energy of 16.31 kcal/mol for the 11 ACS Paragon Plus Environment


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urea–urea interaction is in reasonable agreement with the interaction energy of 15.57 kcal/mol obtained at the lower B3LYP/6-31G(d,p) level used by Yilgör et al.39 The negative sign of the interaction energies and enthalpies for the various DESs (Table 5) points to favorable interactions between the HBDs and choline chloride. Analysis of the entropic component of the thermochemistry was done to gain more insight into the Δrxn2S of the DES systems, wherein we looked at the translational, rotational, and vibrational entropy contributions. This decomposition of the entropic contributions to DES formation revealed that ΔSrot and ΔStrans do not contribute significantly to the differences in the Δrxn2S values for the three DES systems. That ΔSrot and ΔStrans for maloline are about half the magnitude of those for reline and ethaline is consistent with the stoichiometry of the reaction. The differences in the ΔSvib values, however, do correlate directly with the differences in the Δrxn2S values (Tables 5 and 6). This behavior is a manifestation of the differences in the strength of the intermolecular interactions that lead to the 3D cage-like structures of the DESs. Overall, however, the entropic contributions to the ∆rx2G values, although significant, are outweighed by the enthalpic contributions, making DES formation spontaneous in the gas phase. Conclusions In this study, we have shown that the DESs considered exhibit cooperative hydrogenbonding networks wherein the strong interactions (HBD–choline, HBD–chloride, and choline– chloride) form a complete loop (cycle). The choline–chloride interaction is largely retained after formation of the DESs, whereas the HBD–HBD interactions are significantly disrupted. Both cation as well as anion exhibit strong binding with the HBD molecule. In the case of reline and ethaline, the HBD interacts with Cl– as a bidentate ligand to form six- and seven-membered ring12 ACS Paragon Plus Environment

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like structures, respectively. The choline cation interacts with the HBDs via methyl protons forming C–H···lone pair and/or C–H···π type (reline, maloline) interactions. The charge decomposition and DOS analyses clearly indicate that CT occurs from the choline chloride to the HBDs through hydrogen-bonding and electrostatic interactions. The changes in the CHELPG atomic charges further suggest that the CT from choline to the HBD is larger than that from Cl– to the HBD. Indeed, one effect of the HBD tuning of the choline-Cl– interactions on the physical properties of the DESs is clarified by the direct correlation between the melting points of the DESs and the sum of the bond orders associated with the choline-Cl– interactions in the DESs. The enthalpic contribution dominates the entropic contribution to the free energy of formation of the DESs, making the formation process spontaneous in the gas phase.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx. Image sequence illustrating the supercooled nature of reline.



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Figures and Tables Table 1. Comparison between experimental vibrational frequencies and those calculated at the M06-2X/6-31++G(d,p) level.a

Reline

Ethaline

Maloline

Calc. ν (cm–1) 3683 3611 3509 3377 3049 1738 1619 1495 1427 1124

Bond

Stretch

Interaction

Interacting species

N–H O–H N–H N–H C–H C=O N–H C-H C–H C–O

asym asym sym sym sym sym asym scissors asym asym

N–H···Cl O–H···Cl N–H···O=C N–H···Cl

urea···Cl– choline··· Cl– urea···urea urea··· Cl– choline urea···urea urea choline choline choline

3674 3609 3137 3046 3001 1487 1416 1370 1094

O–H O-H C–H C–H C–H C-H C–H C–H, C–O C–H, C–O

asym asym asym asym asym scissors sym wagging wagging

O–H···Cl O–H···Cl –N(Me)3, –CH2– –CH2– –CH2–

O–H···Cl O–H···Cl

EG··· Cl– choline··· Cl– choline EG EG choline choline choline, EG, Cl– EG··· Cl–

3754 3594 3171 3050 2748 1860 1766 1446 1300

O–H O–H C–H C–H O–H C=O C=O C–H C–H

asym asym asym sym asym sym sym scissors wagging

–COOH O–H···Cl –N(Me)3 –CH2– –COOH –COOH –COOH –N(Me)3, –CH2– –CH2–

MA choline··· Cl– choline choline, MA MA MA MA choline MA

C=O···H–N


Exp. ν (cm–1) 3323 3198 3024 1672 1612 1476 1445 1169 3332 3025 2940 2870 1485 1412 1315 1204 3308 2940

2568 1738 1724 1480 1418


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1249 C–H wagging –CH2– MA 1384 1225 C–H wagging –CH2– MA 1347 1147 C–H wagging –CH2– MA 1201 a A scaling factor of 0.986 was used for the vibrational frequencies calculated at M06-2X/631++G(d,p).

Table 2. Sum of the bond orders (Σ BOs) for the interactions between the components of the DES mixtures.a Cho···Cl 0.937 0.196 0.648 0.876

HBD···Cl HBD···Cho Reline 0.300 0.288 Ethaline 0.428 0.511 Maloline 0.479 0.058 Choline Chloride – – a Σ BO = Σ BObonding interactions – Σ BOanti-bonding interactions

Table 3. CHELPG charges on the choline, chloride and dimers of HBDs (urea, ethylene glycol and malonic acid) before and after formation of DES mixtures. Pure components Choline Chloride Urea Ethylene glycol Malonic acid

Reline 0.825 –0.781 –0.044 – –

0.780 –0.780 0 0 0

DES mixtures Ethaline Maloline 0.857 0.832 –0.689 –0.723 – – –0.168 – – –0.109


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Table 4. Interaction energies (Δrxn1E, kcal/mol) for the choline chloride and HBD dimer complexes before formation of DESs.

6-31++G(d,p) aug-cc-PVDZ

Urea M06-2X MP2 –16.31 –14.08 –14.45 –14.71

HBD dimer Ethylene glycol M06-2X MP2 –22.39 –20.79 –20.42 –20.53

Malonic acid M06-2X MP2 –17.32 –16.24 –16.56 –16.85

Choline chloride M06-2X MP2 –100.22 –99.62 –101.38 –101.48

Table 5. The change in the energies, free energies, enthalpies and entropies associated with formation of the DES systems.a

Reline Ethaline Maloline

∆rxn2E (kcal/mol) –43.19 –43.96 –22.25

Δ rxn2G (kcal/mol) Δ rxn2H (kcal/mol) –23.13 –44.68 –17.36 –41.08 –18.31 –29.71 a M06-2X/6-31++G(d,p) data in kcal/mol

Δ rxn2S (kcal/mol . K) –0.072 –0.079 0.009

Table 6. The change in the translational (∆Strans), rotational (∆Srot), and vibrational (∆Svib) entropies associated with formation of the DES systems.

Reline Ethaline Maloline

∆Strans (kcal/mol . K) –0.075 –0.075 –0.037

∆Srot (kcal/mol . K) –0.044 –0.044 –0.021


∆Svib (kcal/mol . K) 0.046 0.039 0.067


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Figure 1. Scheme depicting the three DESs studied in this work. Reline, ethaline, and maloline represent urea, glycol, and carboxylic acid-based DES systems, respectively.


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Figure 2. A comparison between the experimental and calculated vibrational spectra at M062X/6-311++G(d,p) method for the DES systems. The calculated spectrum qualitatively reproduced the features in the experimental spectrum.



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Figure 3. Correlation between experimental and calculated vibrational spectra indicating the reliability of the M06-2X/6-311++G(d,p) method used for the DES systems.

Figure 4. Optimized geometries of the HBD dimers at the M06-2X/6-311++G(d,p) level of theory depicting H···B (B = O and Cl) interatomic distances. A) urea, B) ethylene glycol, C) malonic acid and D) choline choride.


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Figure 5. Optimized geometries of the DESs at the M06-2X/6-311++G(d,p) level of theory depicting interatomic distances. A) reline, B) ethaline, and C) maloline. The interatomic distances in red denote choline···Cl–, in green denote HBD···Cl–, in blue denote choline···HBD, and in black denote HBD···HBD interactions.

Figure 6. The total choline–chloride bond orders versus melting temperatures of the DES mixtures.



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Figure 7. Comparison between density of states (DOS) features for the choline chloride constituents (a) before and (b–d) after formation of a DES. The choline is represented by the green profiles and chloride by the blue profiles. To the right are shown expanded regions of interest taken from the DOS plots to the left. The vertical dashed red line is a reference line to aid in visual comparison. 24 ACS Paragon Plus Environment

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