Conformation of Ethylene Glycol in the Liquid State - ACS Publications

Article pubs.acs.org/JPCB

Conformation of Ethylene Glycol in the Liquid State: Intra- versus Intermolecular Interactions Aman Jindal and Sukumaran Vasudevan* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: Ethylene glycol is a typical rotor molecule with the three dihedral angles that allow for a number of possible conformers. The geometry of the molecule in the liquid state brings into sharp focus the competition between intra- and intermolecular interactions in deciding conformation. Here, we report a conformational analysis of ethylene glycol in the liquid state from ab initio molecular dynamics simulations. Our results highlight the importance of intermolecular hydrogen bonding over intramolecular interactions in the liquid, with the central OCCO linkage adopting both gauche and trans geometries in contrast to the gas phase, wherein only the gauche has been reported. The influence of intermolecular interactions on the conformation of the terminal CCOH moieties is even more striking, with certain regions of conformational space, wherein the ethylene glycol molecule cannot participate with its full complement of intermolecular hydrogen bonds, excluded. The results are in agreement with Raman and NMR spectroscopic studies of liquid ethylene glycol, but at the same time they are able to provide new insights into how intermolecular interactions favor certain conformations while excluding others.

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INTRODUCTION Ethylene glycol (HOCH2CH2OH) or 1,2-ethanediol is a typical rotor molecule with three dihedral angles, which can, in principle, give rise to 33 conformers. Of these, only 10 are unique for the isolated molecule, with the all-trans conformer of C2h symmetry and the remaining nine of lower symmetry and multiple degeneracies.1 The structure of the ethylene glycol molecule in the gas phase has been extensively studied by multiple techniques that include electron diffraction as well as microwave and infrared spectroscopic methods.2−4 It has also been the subject of a number of quantum chemical calculations at varying levels of theory.5−7 Both experiment and theory concur that the most stable conformer is one, wherein the central OCCO linkage adopts a gauche (G) arrangement, whereas the two CCOH moieties adopt trans (t) and gauche (g′) respectively, so that the overall conformation may be represented as tGg′; a similar conformation is adopted by ethylene glycol in the crystalline state.1−9 The next higher energy conformation is gGg′ (g and g′ refer to gauche conformers obtained from clockwise and counterclockwise rotations about the C−O bond).9 Much of the current debate on the gas-phase structure of ethylene glycol has focused on assessing the relative importance of intramolecular hydrogen bonding, involving the two vicinal hydroxyl groups, versus hyperconjugation, or the gauche effect, in establishing the conformation of the central OCCO linkage.7,9−11 The jury, however, is still out, and the final word is yet to be written. Ethylene glycol is an important molecule widely used, along with water, in antifreeze formulations as well as a heat-transfer agent. Establishing the conformation and nature of interactions © 2017 American Chemical Society

in the neat liquid is a prerequisite to a molecular understanding of how the hydrogen-bonding network in mixtures are modified leading to a depression of the freezing point and elevation of the boiling point, allowing it to function as an antifreeze as well as a heat-transfer agent. The conformation of ethylene glycol in the liquid is a more complex problem than that in the gas phase because of the competition between intra- and inter-molecular interactions in the condensed phase. The ethylene glycol molecule can, in principle, participate in four intermolecular Hbonds because each molecule has two proton donor hydroxyl groups and the two oxygen atoms that can act as proton acceptors.12 In addition, there is always the possibility of intramolecular H-bonding. Experimental studies of the liquid have been limited to determining radial distribution functions (RDFs) either from X-ray or neutron scattering measurements.13,14 Information from NMR studies too is available. These studies have generally focused on ethylene glycol in solutions but as part of the investigation reported data for the neat liquid.15−17 The 13C satellites present in the proton NMR have been used to establish that there is a gauche preference of up to 86% in liquid ethylene glycol, but this conclusion has been contested in subsequent reports.16 A very early Raman study too had hinted that unlike in the crystalline state, in the liquid a fraction of the molecules were in the trans geometry.18 Raman spectra studies of liquid ethylene glycol, at high Received: March 26, 2017 Revised: May 9, 2017 Published: May 11, 2017 5595

DOI: 10.1021/acs.jpcb.7b02853 J. Phys. Chem. B 2017, 121, 5595−5600

Article

The Journal of Physical Chemistry B

Figure 1. (a) Snapshot of the postequilibration simulation cell for liquid ethylene glycol. (b) The calculated RDF. For comparison, the experimental RDF from neutron scattering experiments14 is also shown. Calculated partial RDFs (c) g(rHH), (d) g(rXH), and (e) g(rXX), where H refers to the hydroxyl protons, whereas X refers to all other atoms of the ethylene glycol molecule. For comparison, the experimentally determined partial RDFs from neutron-scattering experiments38 are also shown.

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SIMULATION METHODOLOGY Ab initio MD simulations were performed using the CPMD v4.1 program on a Cray XC40 cluster using 480 (20 × 24) processors.27−29 The simulation cell consisted of 100 ethylene glycol molecules confined in a cubic box of side 20.998 Å with periodic boundary conditions. The dimensions of the cell ensured that the density was identical to the experimental density of liquid ethylene glycol at 25 °C, 1.11 g cm−3. The simulation began with a classical MD run using parameters of the universal force field to model the ethylene glycol molecules.30 For the initial set up of the simulation cell, the most stable conformer of the ethylene glycol molecule in the gas phase, the tGg′ geometry (Figure S1), was chosen. The final structure obtained from the classical MD simulations was then used as the initial configuration for the ab initio MD simulations. In the ab initio MD simulations, all 100 ethylene glycol molecules were considered explicitly. In the CPMD simulations, the electronic structure of the ethylene glycol molecules was computed by density functional theory using the Becke, Lee, Yang, and Parr (BLYP) gradientcorrected exchange-correlation functional with dispersive interactions accounted for using the empirical correction of Grimme.31−33 Core electrons were treated using the normconserving atomic pseudopotentials of Troullier−Martins, whereas valence electrons were represented in a plane wave basis set truncated at an extended energy of 70 Ry.34 The fictitious electron mass parameter was equal to 400 au. Wave

pressures, have also indicated that for a fraction of the molecules, the central OCCO has a trans conformation.19 The neat liquid presents inherent problems for detailed spectroscopic studies, and it is therefore not surprising that liquid ethylene glycol has been the subject of a number of classical molecular dynamics (MD) simulations using different force fields. Although most force fields are able to reproduce the gross features of the experimental RDF, the results for the crucial dihedral angle distribution are rather disconcerting, as the observed distribution of the intramolecular OCCO and HOCC angles depends critically on the choice of the force field used in the simulation.12,20−24 For example, the OPLS-AA force field, widely used in the simulation of liquids, favors a trans conformation for the central OCCO torsion, whereas the modified OPLS-AA-SEI favors the gauche.21,24 The fraction of molecules in the liquid where the central dihedral OCCO angle adopts a trans conformation can vary anywhere from 5 to 95% depending on the force field used.12 These results are surprising considering that the ethylene glycol molecule has been widely used as a template for the development of force fields for more complex systems.25,26 In these circumstances, it is natural to turn to ab initio MD for a more realistic assessment of the conformation of ethylene glycol in the liquid state. Here, we report a conformational analysis of the ethylene glycol molecule in the liquid state from ab initio MD simulations. 5596


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Figure 2. (a) Distribution of the three dihedral angles, OCCO (φ), HOCC (θ), and CCOH (θ*) for the ethylene glycol molecules in the ensemble (the value in the gas-phase is indicated by the star and that in the crystal by a square).8 (b) Distribution by dihedral angles θ and θ* over the last 1 ps of the simulation. The dark-blue shaded regions have the lowest population. Typical structures in these forbidden regions are indicated. These are structures with dihedral angle values (0, 0*), (0, 150*), and (−150, 150*).

functions were optimized until a convergence value of 1 × 10−8 au was reached. Prior to the simulations the choice of functional was validated by carrying out quantum chemical calculations (Gaussian 09) of an ethylene glycol molecule using the dispersion corrected BLYP/6-311++G** method and comparing the results with those obtained using the higher level CCSD(T)/aug-cc-pVTZ method (Supporting Information, S1).35 It was found that the dihedral angles for the optimized geometry (Figure S1) obtained using the BLYP/6311++G** are comparable to that obtained from calculations using CCSD(T)/aug-cc-pVTZ (Table S1). We have also compared the relative energies of different conformers with the reported CCSD(T)/cc-pVDZ//MP2/cc-pVDZ values.1 The dispersion corrected BLYP/6-311++G** method is able to predict the order of the higher energy conformers (Table S2). It is also well documented that dispersion corrected DFT is able to provide an accurate (comparable to CCSD(T)) description of intermolecular interactions and hydrogen bonding.36,37 The CPMD simulations were performed for a total of 6.0 ps. The time step applied during the simulations was 4 au (1 au = 0.024 fs), with temperature maintained at 300 K using a Nosé− Hoover thermostat. The simulations were initiated using an NVT (T = 300 K) ensemble that was subsequently switched to an NVE ensemble postequilibration. We used the trans−gauche ratio of the central OCCO linkage of the ethylene glycol molecule as a measure of equilibration, a constant value signifying that equilibrium was attained (Figure S2). This typically occurred after 2.5 ps from the start of these simulations. All of the results reported here are from an analysis of trajectories obtained from the NVE ensemble.

the two results are in good agreement. We have also calculated the partial RDFs, g(rHH), g(rXH), and g(rXX), where H refers to the hydroxyl hydrogens of the hydroxyl groups and X refers to all other atoms of ethylene glycol, including the methyl hydrogens. These are shown in Figure 1c−e along with the experimental partial RDFs obtained from neutron-scattering experiments.38 The agreement is reasonable, with the calculated RDFs able to reproduce most features of the experiment. It may be noted that most classical force fields are unable to reproduce all of the features of the partial RDF, with studies concluding that more realistic models are required.12 The fact that the RDFs obtained from ab initio MD simulations (Figure 1b−e) are comparable to the experimental data gives us confidence that the simulations are truly representative of the liquid state of ethylene glycol. The conformation of ethylene glycol is defined by the three dihedral angles, the central OCCO (φ) and the two terminal HOCC (θ and θ*) dihedral angles (we use the asterisk to distinguish the two, otherwise equivalent, HOCC dihedrals). We have analyzed the conformation of ethylene glycol molecules in our simulation of the liquid by determining the values of these three angles for all 100 molecules in the ensemble. This is shown in Figure 2a, wherein each point represents the (φ, θ, θ*) values for a molecule in the ensemble. Also shown in Figure 2a is the projection of the dihedral angle values in the (φ, θ), (φ, θ*), and (θ, θ*) planes. The reported dihedral (φ, θ, θ*) values of the ethylene glycol molecule in the gas and the crystalline phases are also indicated. We first consider the distribution in the values of the central OCCO dihedral angle (φ). It may be seen that they fall in two regions (the shaded regions in Figure 2a); the majority of the molecules lie in the region 60 ± 20° that we formally define as gauche, whereas the rest lie between 180 ± 30°, designated as trans. It may be recalled that the simulations were initiated with the central OCCO dihedral for all molecules in the ensemble adopting the gauche conformation with φ ≈ 60°, the value obtained from quantum chemical calculations of the isolated molecule. At equilibrium, the population of the trans conformation is ∼20% (we have, in fact, used the trans− gauche ratio as a measure to judge whether our simulations

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RESULTS AND DISCUSSION A snapshot of the postequilibrium simulation cell is shown in Figure 1a. RDFs are important criteria to describe the average structure of any liquid. To ensure that the results of our simulations are truly representative of the liquid state of ethylene glycol, the RDF was computed and compared to the RDF reported from neutron scattering experiments on liquid ethylene glycol (Figure 1b).14 As may be seen from Figure 1b, 5597


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Figure 3. Distribution of the intra- and inter-molecular rOH···O distances and ∠O−H···O angles.

Figure 4. Calculated partial RDFs: (a) g(rOH···O) and (b) g(rO···O). The positions of the first minima are indicated. (c) Distribution of hydrogen bonds per ethylene glycol molecule in the ensemble. The geometric criteria rOH···O ≤ 2.4 Å, rO···O < 3.5 Å angle ∠O−H···O ≥ 110° were used to define the H-bond.

0*), and (±150, ±150*). Typical conformations of the ethylene glycol molecule in these regions are shown in the structures in Figure 2b. On the basis of their geometries, it is easy to understand why these forbidden regions in the θ−θ* map exist. The ethylene glycol molecule can, as stated earlier, participate in four intermolecular H-bonds, as there are two proton donor hydroxyl groups and two oxygen atoms that can act as proton acceptors. An examination of the structures of the ethylene glycol molecule in the forbidden regions shows that these geometries do not permit the ethylene glycol molecule from attaining its full complement of four inter-molecular Hbonds. They could still, however, possess intra-molecular Hbonds except for those molecules, wherein the central OCCO linkage is trans. The importance of intermolecular interactions in determining the conformation of ethylene glycol in the liquid state is also implied from the distribution of the rOH···O distances and ∠OH···O angles (Figure 3). As can be clearly seen in Figure 3, intermolecular ∠O−H···O angles are close to 180° and OH···O

have reached equilibrium; see Figure S2). The results are in broad agreement with those of the earlier NMR and Raman spectroscopic studies that had suggested that for a fraction of molecules in the liquid, the central OCCO dihedral of ethylene glycol adopts the trans conformation, in contrast to the gas and crystalline phases, where only the gauche conformer exists. The dihedral angle distribution of the ethylene glycol molecules (Figure 2a) indicates that the two terminal dihedral angles (θ and θ*) explore a much wider region of conformational space as compared with the central OCCO (φ) dihedral. We confirmed that the values of the two dihedrals are not correlated and neither are changes in their values concerted (Supporting Information S3). A closer examination, however, reveals an interesting pattern. In Figure 2b, the values of the two dihedrals, θ and θ*, for each of the 100 molecules have been plotted for multiple frames spaced 0.01 ps apart for the last 1 ps of the simulation. The figure clearly shows that there are “forbidden” regions in the θ−θ* map. These are regions centered around the θ−θ* values (0, 0*), (0, ±150*), (±150, 5598



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distances, much shorter than the intramolecular OH···O distances, indicating that geometries that favor intermolecular OH···O interactions dominate over those that favor only intramolecular interactions. These results clearly highlight the importance of intermolecular interactions in deciding the “allowed” conformation of ethylene glycol in the liquid state. It should be noted that many of the structures in the forbidden regions have geometries that are quite close to the structure of ethylene glycol in the gas phase, with comparable intramolecular rOH···O and rO···O distances as well as values of the ∠O−H···O angle. These structures, however, have a limited ability to participate with a full complement of four intermolecular hydrogen bonds and, hence the absence of these structures in the liquid. We have also estimated the average number of H-bonds per ethylene glycol molecule in the liquid state by defining a simple distance and angle criteria for the existence of an H-bond. We define two “cutoff” distances, rO···O and rOH···O, as an upper bound along with the value of the ∠O−H···O angle as the lower bound for the formation of an H-bond between donor and acceptor oxygen atoms. The cutoff values for rO···O and rOH···O were obtained from the distance values of the first minima in the computed partial RDFs, g(rO···O) and g(rOH···O) (Figure 4a,b).23 The values are 2.4 and 3.5 Å, respectively. We used a lower bound of 110° for the ∠O−H···O angle based on the IUPAC recommendations.22,39,40 Using these geometrical values, the distribution of H-bonds per ethylene glycol molecule in the liquid was estimated (Figure 4c). The average number of H-bond per ethylene glycol molecule from the ab initio MD simulations is 3.8. These results highlight the importance of intermolecular hydrogen bonding in the liquid state of ethylene glycol.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b02853. (S1) Validation of the choice of the DFT functional; (S2) evolution of the trans/gauche ratio for the central OCCO linkage as a function of simulation time; (S3) correlation coefficient, ⟨Δθ, Δθ*⟩, for fluctuations in the terminal CCOH dihedral angle pair (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91-80-2293-2661. Fax: +91-80-2360-1552/0683. ORCID

Sukumaran Vasudevan: 0000-0002-5059-6098 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Supercomputer Education and Research Center (SERC) at the Indian Institute of Science, Bangalore, India for providing the CRAY XC40 computational facility. S.V. thanks the Department of Science and Technology, Govt. of India, for the J. C. Bose national fellowship. The authors thank Professor E.A. Arunan for suggesting the problem as well as useful discussions.

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REFERENCES

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CONCLUSIONS In summary, we have carried out a conformational analysis of ethylene glycol in the liquid state using ab initio MD simulations. The use of ab initio methods is necessitated by the fact that the results of classical MD simulations depend critically on the choice of the force field. Our results highlight the importance of intermolecular hydrogen bonding in deciding the conformation that the ethylene glycol molecule adopts in the liquid state. We observe that in the liquid, in contrast with the gas phase, the central OCCO linkage adopts both gauche and trans geometries, with an equilibrium trans fraction of ∼20%. The results indicate that intermolecular interactions have a greater influence as compared with intramolecular interactions irrespective of whether the origin of the latter is the gauche effect or hydrogen bonding. The influence of intermolecular interactions on the conformation of the terminal CCOH moieties is even more striking. The CCOH dihedral angle adopts a wide range of values, and specific conformations like trans or gauche are not easily delineated. What is clearly discernable, however, is that there are certain regions of conformational space of the CCOH dihedrals that are excluded. These forbidden regions are those wherein the geometry restricts the ethylene glycol molecule from participating with a full complement of four intermolecular hydrogen bonds. We believe that the significance of these results in establishing the conformation is the first step toward understanding the structure of liquid ethylene glycol, and subsequently on how it functions as an antifreeze in water−ethylene glycol mixtures. 5599


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Conformation of Ethylene Glycol in the Liquid State - ACS Publications

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