Distinguishing Intra- and Intermolecular Interactions in Liquid 1,2

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B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution

Distinguishing Intra- and Intermolecular Interactions in Liquid 1,2-Ethanediol by H NMR and Ab Initio Molecular Dynamics 1

Ritu Ghanghas, Aman Jindal, and Sukumaran Vasudevan J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b07750 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on October 2, 2018

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The Journal of Physical Chemistry

Distinguishing Intra- and Intermolecular Interactions in Liquid 1,2-Ethanediol by 1H NMR and Ab Initio Molecular Dynamics

Ritu Ghanghas, Aman Jindal and Sukumaran Vasudevan* Department of Inorganic and Physical Chemistry Indian Institute of Science, Bangalore 560012, INDIA

*Author to whom correspondence may be addressed. E-mail: [email protected]. Tel: +91-80-2293-2661. Fax: +91-80-2360-1552/0683;

ACS Paragon Plus Environment

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ABSTRACT

The central OCCO backbone of the 1,2-ethanediol molecule adopts the gauche conformer in the gaseous and crystalline states but exists in conformational equilibrium between gauche and trans in the liquid; an observation that has been attributed to the competition between intra- and intermolecular interactions. Here we show that Nuclear Overhauser Effect (NOE) has the ability to distinguish inter- from intramolecular interactions in liquid 1,2-ethanediol. We do so by exploiting the secondary isotope effect to distinguish the hydroxyl protons of HOCH2CH2OH and the deuterated HOCD2CD2OH in the 1H NMR spectra of mixtures of the two and, in conjunction with ab initio MD simulations show how the interplay between inter- and intramolecular interactions gives rise to the conformational isomers in the liquid state of 1,2-ethanediol.


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INTRODUCTION 1,2-Ethanediol (HOCH2CH2OH) or ethylene glycol, a major component of antifreeze as well as heat-transfer formulations, is one of the simplest bifunctional molecules capable of forming intramolecular hydrogen bonds.1 The two hydroxyl groups and two oxygen atoms present on the molecule can act as proton donors and proton acceptors, respectively, in the formation of hydrogen bonds. In this respect, the 1,2-ethanediol molecule is similar to the water molecule that, too, forms four hydrogen bonds in the liquid state. The similarity, however, ends here as the 1,2ethanediol molecule can participate in both intermolecular as well as intramolecular hydrogen bonding and, in addition, exhibit conformational degrees of freedom. The molecule possess three dihedral angles and can in principle exist in one of 33 = 27 conformations of which 10 are unique.2 The structure of the 1,2-ethanediol molecule in the gas-phase has been extensively investigated using multiple techniques that include electron diffraction as well as microwave and infrared spectroscopy.3–5 These measurements, as well as computational studies, concur that the central OCCO bonds adopts the gauche (G) conformation and that the most stable conformers in the gas phase are the tGg' and gGg' conformers (t and g, in small case, refer to the trans and gauche conformers of the terminal HOCC dihedral angles while that in capital to the central backbone OCCO dihedral).6,7 In the crystalline state, too, the backbone OCCO dihedral adopts the gauche (G) conformation.8 In both the gas and crystalline states of 1,2-ethanediol there is a clear preference for the central OCCO to adopt the gauche conformer. This could be a consequence of either intramolecular hydrogen bonding, involving the two vicinal hydroxyl groups, or hyper-conjugation (the gauche effect) or other bond orbital interactions, or all three; consensus is, however, veering towards the latter two.9,10


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The scenario is, however, very different for 1,2-ethanediol in the liquid state. An early Raman study provided direct evidence that in the liquid state, the 1,2-ethanediol molecule exists as mixture of two rotational isomers distinguishable by the conformation of the central OCCO, either gauche or trans, unlike the solid or gaseous forms where the gauche conformation is adopted exclusively.11 Subsequent studies have used Raman spectroscopy to explore the effect of temperature and pressure on the population of the OCCO trans and gauche conformers.12,13 The

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C

satellites present in the proton NMR have been used to establish that there is a gauche preference of up to 86% in liquid 1,2-ethanediol.14 A recent conformational analysis of liquid 1,2-ethanediol from ab initio molecular dynamics (MD) simulations, too, showed that the central OCCO linkage adopts both trans and gauche geometries, with the relative population of the latter being close to 20%.15 The fact that conformational isomerism of the 1,2-ethanediol molecule is observed only in the liquid state is intimately linked with the existence of three-dimensional network structures in the liquid and reflects the competition between intramolecular and intermolecular interactions in the condensed phase. A more affirmative conclusion would, however, require the ability to distinguish inter- from intramolecular interactions in liquid 1,2-ethanediol, and gauge their relative strengths. Here we have attempted to do so by exploiting the secondary isotope effect16 to distinguish the hydroxyl protons of HOCH2CH2OH (I) and the deuterated HOCD2CD2OH (II) in the 1H NMR spectra of mixtures of the two and, in conjunction with ab initio Car-Parrinello Molecular Dynamics (CPMD)17 simulations show how the interplay between inter- and intramolecular interactions give rise to the conformational isomers in the liquid state of 1,2-ethanediol.


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EXPERIMENTAL The 1H NMR spectra of 1,2-ethanediol (Sigma Aldrich) and the corresponding deuterated HOCD2CD2OH (Cambridge Isotopes) were recorded on a JEOL ECX II NMR spectrometer operating at a proton resonance frequency of 500 MHz Spectra were recorded using a pulse length of 6.25 µs, a spectral width of 3000 Hz, with a relaxation time of 4s and averaged over 8 scans. 1H NOESY were recorded with a mixing time of 1 ms for 16 scans using 1024 data points in both t1 and t2 dimension. The 1D transient NOE measurements of the 1:1 mixture of I and II in CDCl3 were performed by selective inversion of the CH2 resonance at 3.7 ppm and measuring the intensities of the hydroxyl resonances of the mixture at different mixing times with a relaxation delay of 8 s. The 1D NOE experiments used the Double Pulse Field Gradient Spin Echo (DPFGSE) pulse sequence, available as a part of the JEOL pulse program library. SIMULATION METHODOLOGY Ab initio molecular dynamics simulations18 were performed using the, CPMDv4.119 program on a CRAY XC40 machine using 480 (24  20) processors. The CPMD simulations were performed for a cubic simulation cell of side 20.998 Å containing 100 1,2-ethanediol molecules with periodic boundary conditions. The dimensions of cell reproduced the experimental density of 1.12 gcm-3 for liquid 1,2-ethanediol at 25 °C.20 The gradient-corrected exchange correlational DFTBLYP (Becke exchange and Lee-Yang-Parr correlation) functional was used in the present study for electronic structure calculations.21 Dispersive interactions were taken into account by using the Grimme’s empirical correction.22 The choice of the functional had been validated in earlier studies2 by comparing the results obtained from calculations for 1,2-ethanediol using the dispersion corrected BLYP/6-311++G** method with those obtained from higher level CCSD(T)/aug-


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cc-pVTZ (Supporting Information, S1 of Reference 15). After wavefunction optimization, the equations of motion were integrated with a time step of 4 au (0.096 fs) and a fictitious electron mass value of 400 au. The Nose-Hoover thermostat was used to maintain the temperature at 300 K.23 The CPMD simulations were carried out for a total of 10 ps, including a 2.5 ps equilibration run using an NVT ensemble that was subsequently switched to an NVE ensemble for a production run of 7.5 ps. The simulations followed the procedure outlined in Reference 15. Ab initio MD simulations were also performed for 100 molecules at two dilutions of 1,2-ethanediol with chloroform, 25:75 and 50:50; the dimensions of the simulation cell were 20.274 Å and 22.422 Å, respectively. RESULTS AND DISCUSSION The 1H NMR of HOCH2CH2OH (I) and the deuterated HOCD2CD2OH (II) are shown in Figure 1a. The manifestation of the secondary isotope effect may clearly be seen in the position of the hydroxyl protons that appear at 5.43 ppm and 5.04 ppm, respectively. However, in the 1:1 mixture of I and II, rather disappointingly, only a single hydroxyl resonance at 5.38 ppm is observed. The reason for a single resonance in the mixture is that the extended hydrogen bonding in the liquid dominates over the secondary isotope effect in deciding chemical shift values. This was confirmed by recording the spectra of the 1:1 mixture on heating. It is well known that in hydrogen bonded solvents like 1,2-ethanediol the extended hydrogen bonding network is disrupted with increasing temperature leading to a reduction in the degree of hydrogen bonded association of solvent molecules, and consequently the -OH resonances are shifted up-field.24,25 The up-field shift in the position of the -OH resonance of 1,2-ethanediol with temperature has, in fact, been used as an NMR thermometer.26 In the mixture it was observed that with increasing temperature, in addition to the up-field shift of the -OH resonance, the peak broadened


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Figure 1. a) 1H NMR of HOCH2CH2OH (I), deuterated HOCD2CD2OH (II), mixture of (I) and (II), and 1:1 mixture diluted with CDCl3. b) 1H 2D NOESY spectra of the diluted mixture. c) 1D NOE spectrum of the diluted mixture (mole fraction ~ 0.03) at different mixing times on irradiation of the -CH2 peak at 3.7 ppm (indicated with an arrow in the figure). Filled peaks represent the hydroxyl resonances of I (orange) and II (green). Inset shows the NOE build-up curve for OH resonances at 2.82 ppm and 2.78 ppm. (Supporting Information Figure S1). More interestingly at high temperatures (> 100 ºC) the -OH resonance show signs of splitting suggesting that when the hydrogen bonded network is disrupted or weakened, the secondary isotope effect on chemical shifts are observable. However, work-


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ing at such elevated temperatures close to the operational limits of our JEOL 500 MHz NMR spectrometer was considered risky. An alternate way of weakening or disrupting the hydrogenbonded network in liquid 1,2-ethanediol is to dilute it with an ‘inert’ solvent. In solvents capable of forming hydrogen bonds the effect of dilution with an inert solvent is similar to increasing temperature, both lead to a reduction in the average degree of association of the solvent molecules and consequently an up-field shift of the hydroxyl resonance.24,27,28 For example , the OH proton in ethanol, is shifted up-field on dilution with CCl4 , an observation similar to that observed on heating ethanol.29 Here we choose CDCl3, as the inert solvent as earlier studies had indicated that it has no influence on the conformation of 1,2-ethanediol, unlike other common solvents like DMSO.28,30 The 1H spectra of the 1:1 mixture diluted with CDCl3 is shown in Figure 1a. As expected the hydroxyl resonance is upfield shifted; this is a consequence of the weakening of the extended hydrogen bonded network in liquid 1,2-ethanediol on dilution. Care was taken to ensure that at these dilutions the shift in the -OH resonance showed a linear shift with dilution, reminiscent of the observed linear shift with temperature (Supporting Information Figures S2 a,b). It may be observed that on dilution two distinct hydroxyl resonances at 3.27 ppm and 3.23 ppm are seen, corresponding to molecules I and II, respectively. We use the NOE (nuclear Overhauser effect) to distinguish intra- and intermolecular interactions in the diluted mixture of HOCH2CH2OH (I) and the deuterated HOCD2CD2OH (II). The NOE is the cross-relaxation between two nuclear spins, I and S, because of magnetization transfer via dipolar coupling of their magnetic moments and can be either intramolecular or intermolecular in origin; the only constraint being that the spatial separation of spins I and S be within the range of dipolar coupling, typically < 5Å.31,32 The NOE has been widely used to probe local structure in liquids and solutions.33 The 1H 2D NOESY spectra of the diluted mixture (0.12


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mole fraction) is shown in Figure 1b (the cross-peaks with phase opposite to that of the diagonal are shown in red). It may be seen that two cross-peaks, one between the OH and CH2 protons of I and the other between the OH protons of II and the CH2 protons of I, are present. The former would arise from intramolecular NOE relaxation pathways while the latter would be intermolecular in origin as molecule II has no methylene protons. These results may be analyzed more quantitatively from 1D transient NOE experiments on the diluted mixture of I and II by selective inversion of the resonance at 3.7 ppm of

the CH2 methylene protons of I and

measuring the intensities of the two hydroxyl resonances at 2.82 ppm and 2.78 ppm for different mixing times. The spectra at different mixing times and the intensity buildup curve for the hydroxyl resonances as a function of the mixing time are shown in Figure 1c (the intensities were obtained after decomposing the hydroxyl resonances as a sum of two Lorentzians centered at 2.82 and 2.78 ppm). Within the initial rate approximation, the NOE enhancement, ηI{S}, of spin I on inversion or irradiation of spin S, at a mixing time of m is proportional to the cross-relaxation rate, σIS, which −6 in turn depends on their inter-nuclear separation, 𝑟𝐼𝑆 , and is given by the expression

−6 𝜂𝐼 {𝑆} = 𝑘𝜎𝐼𝑆 𝜏𝑚 = 𝑘 ′ 𝑟𝐼𝑆 𝜏𝑚

where k and k' are constants of proportionality.31,34 The initial rate approximation is valid only at short mixing times; at longer times the T1 relaxation of spin I competes with cross relaxation between the I and S spins, resulting in the NOE build up curve deviating from linearity, eventually NOE decaying to zero. The initial linear part of the NOE build-up curve for the resonances at 2.82 ppm and 2.78 ppm have been plotted in the inset of Figure 1c. It may be seen


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that the slopes of the build-up rate for the hydroxyl protons of HOCH2CH2OH (I) and the deuterated HOCD2CD2OH(II) on irradiating the CH2 protons of I are similar, implying that they have similar cross relaxation rates and hence inter-nuclear separations with respect to the methylene protons of HOCH2CH2OH (I). The 1D transient NOE experiments were repeated for other dilutions of the 1:1 mixture of I and II in CDCl3. For all dilutions it was obsereved that the slopes of the build-up rate for the hydroxyl protons were identical (Supporting Information Figures S2 c-f) The 1H NOE of the mixtures, therefore, implies that the intramolecular separation of the OH and CH2 protons and the corresponding shortest intermolecular distances between OH and CH2 protons in liquid 1,2-ethanediol are comparable with similar values. In order to validate the experimental observation, to understand its consequences and whether the NOE measurements on the diluted mixtures are representative of the neat liquid, we turn to ab initio MD (CPMD) simulations. The use of ab initio methods is necessitated by the fact that the results for 1,2-ethanediol obtained from classical molecular dynamics methods depend crucially on the choice of the force-field.35,36 CPMD simulations were performed for the neat liquid as well as for two dilutions of 1,2-ethanediol with CDCl3 , 25:75 and 50:50. The total number of molecules in the simulation cell was 100 for all three simulations. In the CPMD simulations the electronic structure of 1,2-ethanediol and the CDCl3 molecules were computed by density functional theory using the Becke, Lee, Yang and Parr (BLYP) gradient corrected exchangecorrelation functional with dispersive interactions accounted for using empirical correction of Grimme.22,37,38 A snap shot of the cell after 10 ps of simulation is shown in Figure 2a-c for the three concentrations. The 1H NOE NMR measurements had shown that the inter- and intramolecular distances between the methylene protons, H (-CH2), and the protons of the hydroxyl group, H (OH-), in


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Figure 2. Snapshot of the post-equilibration simulation cell for a) 25:75 mixture of 1,2ethanediol and CDCl3, b) 50:50 mixture 1,2-ethanediol and CDCl3 and c) neat 1,2-ethanediol. CDCl3 molecules are shown by Licorice representation. The distribution of intra- and intermolecular distances between the methylene protons, H (-CH2), and the protons of the hydroxyl group, H (OH-) for d) the 25:75 mixture, e) 50:50 mixture and f) 1,2-ethanediol. liquid 1,2-ethanediol are comparable. It is interesting to see whether the MD simulations are able to reproduce these observations and whether it would hold true for all dilutions. In Figures 2d-f, the distribution of the intra- and intermolecular (-CH2) H---H (OH-) proton-proton distances have been plotted. The intramolecular (-CH2) H---H (OH-) distance distribution for the three dilutions is multimodal reflecting the four possible proton-proton distances between the two meth-


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ylene, CH2, protons and the two hydroxyl OH, protons; the intermolecular distribution is bimodal. It may be seen that for all three dilutions the distributions, in the region less than 4.5 Å, are quite similar and that in this region (shaded region of Figures 2d-f) there is a significant overlap between the inter- and intramolecular distance distributions. What is important in the context of the NMR results is that there is a large contribution to the intermolecular (-CH2) H---H (OH-) distribution from proton-proton distances that are less than 4.5 Å; it is for distances less than 5 Å that NOE cross-relaxation, which is dipolar in origin, manifests. The MD simulations clearly indicate that there is a range of intra- and intermolecular (-CH2) H---H (OH-) proton-proton distances that are comparable and fall well within the range of the NOE. The similarities in the distributions in Figures 2d-f clearly suggest that the inferences drawn from the NOE results on the diluted samples, that the inter and intramolecular (-CH2) H---H (OH-) distances are comparable, would also hold true for the neat liquid 1,2-ethanediol. Additional information on the nature of the liquid state of 1,2-ethanediol may be obtained by analyzing the molecules that fall within the shaded region of Figure 2f, these are the molecules for which the intra- and intermolecular (-CH2) H---H (OH-) proton-proton distances are comparable and are responsible for the cross-peaks in the 2D NOESY (Figure 1b). For these molecules we compute the distribution of the intermolecular non-bonded HO--O distances and OH--O bond angles for all pairs of molecules over all trajectories of the simulation. This is shown in Figure 3 along with the distribution of the intramolecular non-bonded HO--O distances and OH--O bond angles. It may be seen there are two regions in the intermolecular distribution that have a significant population of 1,2-ethanediol pairs. The first corresponds to molecular pairs where the intermolecular non-bonded HO--O distance is ~2.9 Å and the OH--O bond angle ~175° and the second to pairs where the intermolecular non-bonded HO--O distances is ~2.9 Å and the OH--O


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Intermolecular

Intramolecular

Counts 1000

OH---O angles (degrees)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 OH---O distances (Angstrom)

OH---O distances (Angstrom)

Figure 3: Distribution of intramolecular (left panel) and intermolecular (right panel) non-bonded HO--O distances and OH--O bond angles for 1,2-ethanediol molecules that lie in the shaded region of Figure 2f. bond angle ~60°. One of the consequences, therefore, of the NMR observation that intra- and intermolecular (-CH2) H---H (OH-) proton-proton distances are comparable, is that for pairs of molecules that satisfy this criterion, the intermolecular non-bonded HO--O distances can be quite short, ~2.9 Å, which is comparable, if not shorter than the intramolecular non-bonded HO--O distance, and additionally are linear. If the non-bonded HO--O distance and its linearity are a measure of the strength of the hydrogen bond39,40 then Figure 3 clearly highlights the importance of intermolecular hydrogen bonding, which is more dominating than intramolecular hydrogen bonding in liquid 1,2-ethanediol.


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CONCLUSIONS In summary, we have shown that NOE can be used to distinguish intra- and intermolecular cross relaxation between the methylene and hydroxyl protons 1,2-ethanediol in the liquid state. We do so by exploiting the secondary isotope effect on chemical shifts to distinguish the hydroxyl protons in a mixture of HOCH2CH2OH and the deuterated HOCD2CD2OH and show from NOE intensities that the intra- and intermolecular distances between the CH2 and OH protons are comparable. Ab initio MD simulations of an ensemble of 100 1,2-ethanediol molecules are able to reproduce the experimental NOE observations. It is also shown from the simulations that for those molecules in the ensemble that satisfy the NOE distance criteria their intermolecular non-bonded HO--O distances are short, ~2.9 Å and O-H---O angle almost linear. The distribution of intra- and intermolecular non-bonded HO--O distances and OH--O bond angles, clearly indicates that the latter interaction dominates. The results suggest that the conformational equilibrium in liquid 1,2-ethanediol between the predominantly (~80%) gauche conformer and the trans conformer arises because of competition between intermolecular hydrogen bonding and intramolecular hyperconjugation that is known to be responsible for the exclusive presence of the gauche conformer in the gas-phase, and not intramolecular hydrogen bonding.


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ASSOCIATED CONTENT *Supporting Information Available (S1) Effect of temperature on the 1H NMR spectra of a 1:1 mixture of HOCH2CH2OH (I) and HOCD2CD2OH (II) 1,2-ethanediol. (S2) Effect of dilution on the 1H NMR and chemical shifts of a 1:1 mixture of HOCH2CH2OH (I) and deuterated HOCD2CD2OH (II), diluted with CDCl3. The transient 1D NOE spectrum of the diluted mixture for different mole fractions, at different mixing times on irradiation of the methylene peak.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: +91-80-2293-2661. Fax: +91-80-2360-1552/0683. ACKNOWLEDGEMENTS The authors acknowledge the support of the JEOL-IISc NMR Collaboration Centre for use of the ECX500II NMR spectrometer and the Supercomputer Education and Research Center (SERC) at the Indian Institute of Science, Bangalore, India for providing the CRAY-XC40 computational facility. The authors thank Dr. Vaishali Arunachalam for discussions and help with experiments. S.V. thanks the Department of Science and Technology, Govt. of India, for the J. C. Bose national fellowship.


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Distinguishing Intra- and Intermolecular Interactions in Liquid 1,2

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