Investigation of Alcohol Conformer Distribution and Hydrogen Bonding in (2,2′-Difurylmethane + n-propanol or n-butanol) Binary Mixtures Using Molecular Dynamics Simulations
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Molecular dynamics simulations of 2,2′-difurylmethane (DFM)/n-propanol and DFM/n-butanol binary liquid mixture have been performed using the optimized potentials for liquids simulations all-atom. The density and excess molar volume were computed with DFM mole fraction ranging from 0 to 1. There is excellent agreement between the calculated and experimental density in the entire DFM composition range. Excess molar volume is negative and positive in the lower and higher mole fraction of DFM respectively, which is in accord with the experimental data (Mokate and Ddamba in J Solut Chem 35:1493–1503, 2006. https://doi.org/10.1007/s10953-006-9080-7). The conformer distributions for n-propanol and n-butanol in neat system and in the presence of DFM were similarly studied. It was found that in both cases there is a gradual increase in the gauche conformation population as DFM mole fraction is increased from 0 to 1. Furthermore, the correlation between the dihedral angles of n-propanol and n-butanol with the oxygen of DFM-acidic hydrogen (ODFM–Halc) radial distributions are investigated. The correlation is positive for trans conformation and negative for gauche conformation, which is attributed to the structural ease of hydrogen bond donation by both alcohols. Finally, the spatial distribution of DFM and n-butanol around a DFM molecule are examined. It is revealed that both molecules orient uniquely around the DFM molecule, which modifies the intermolecular interactions.
KeywordsMolecular dynamics Hydrogen bonding Conformer Excess molar volume Radial distribution functions
2,2′-difurylmethane (DFM) is an organic molecule consisting of two furan rings separated by a methylene bridge via one of the alpha carbons. It is a colorless liquid at room temperature. DFM is widely used as a flavorant and odorant in the food industry . Many industrial chemicals contain small amounts of DFM as a side product. For example, polyfurfuryl alcohol polymer (PFA) resin has been found to contain many chemicals one of which is DFM . A review of the uses and applications of furylarylmethanes in general can be found in Ref. . Among those, include applications as monomers and linkers in the synthesis of industrially important polymers and resins. DFM and its derivatives have been used in synthesis of calixarenes which are useful as sensors for ion selective chromatography . Furylarylmethanes are favored as starting raw materials for the production of polymers because they can be made from furfuryl alcohol which can be obtained from agricultural waste . DFM can also be made furfuryl alcohol thus it can be considered a greener solvent. Since DFM is a polar aprotic solvent, it can be used to modify the polarity of alcohols. Motivated by this, Mokate and Ddamba probed the volumetric properties of binary mixtures of DFM with methanol in a temperature range of 288.15–308.15 K . They later studied the volumetric properties of DFM with aliphatic alcohols up to n-hexanol under the same conditions [1, 6]. In terms of excess molar volumes at 298 K, lower chain alcohols (methanol and ethanol) show volume contraction at almost all compositions, while long chain alcohols (pentanol and hexanol) largely show volume expansion at all compositions. Propanol and butanol show both volume contraction (at low DFM composition) and volume expansion (at high DFM composition) . Few binary mixtures exhibit both volume contraction and expansion [7, 8]. The existence of such behavior shows the presence of both negative and positive volume contributing factors and dominance of one over the other depending on the mixing ratios. Negative contributing factors include dipole–dipole interactions between the different molecules and geometrical interstitial accommodation into each other’s cavities whereas positive contributions include repulsions within the system and unfavorable molecular packing . In the case of DFM/n-alcohol mixtures, it has also been suggested that the observed changes in excess molar volumes arise due to changes in the proportion of different conformers of the alcohol molecules as the mole fraction of DFM increases . This can easily be investigated using molecular dynamics.
In our previous paper we used molecular dynamics and ab initio studies to investigate details of the DFM/n-propanol binary mixture . Our results showed evidence of hydrogen bonding between the acidic proton in n-propanol and the DFM oxygen atoms. The effects of mixing DFM and n-propanol on each of the two molecules were investigated mainly using radial distributions functions. The results showed that no significant changes in the local structure of both molecules occur upon mixing at different compositions, indicating that the solvents do not interact strongly. However we did not study the distribution of n-propanol conformers in that system. In this paper, the DFM/n-butanol binary mixture is studied. Due to an extra CH2 group, n-butanol has more conformational isomers than n-propanol. For both alcohols, the effect of DFM concentration on the distribution of conformers is investigated.
For both DFM/n-propanol and DFM/n-butanol binary liquid mixtures, molecular dynamics simulations were performed using the DLPOLY 4.08 package . The optimized potentials for liquid simulation All Atoms (OPLS-AA) force field was used to describe the systems [11, 12]. Initial configurations were generated using Packmol . Analysis of the trajectories was performed using TRAVIS . The dynamics were performed in the Isothermal isobaric ensemble at 298.15 K and 1.0 atm using the Nose–Hoover thermostat and barostat, whose relaxation times were set to 0.1 ps and 0.5 ps respectively. The leapfrog-verlet algorithm was used for integration of equations of motion with a time step of 1.0 fs. The cut-off distance was set to 12 Å. The smooth particle mesh ewald summation was used for treating long range interactions. For each mixture, a total of 1000 molecules were placed in a cubic box. The system was then equilibrated for 1 ns and production dynamics were collected over 3 ns using a time step of 1.0 fs. This time was considered enough to capture the full dynamics given that a 15 ns long run (5 ns equilibration and 10 ns production run) using 900 DFM/100 n-butanol mixture gave essentially the same results.
3 Results and Discussions
3.1 Conformer Distribution in Pure Alcohols and in Mixtures
n-propanol has five possible conformers due to the two torsional angles, C–C–C–O and C–C–O–H. On the other hand, n-butanol has one more torsional angle, C–C–C–C, which leads to more unique conformers. Thus, it has a total of fourteen conformers. The common practice for naming the conformers is to use two letters (for n-propanol) or three letters (for n-butanol) notation to denote the two or three dihedral angles which can be either gauche (G or G′ g or g′) or trans (T or t). Lower case letters represent the C–C–O–H torsion and the primed letters are for negative dihedral angles. For example, the Tg’ conformer of n-propanol has dihedral angles of 180° for the C–C–C–O torsion, − 60° (or 300°) for C–C–O–H torsion. The GGt conformer of n-butanol has dihedral angles of 60°, 60° and 180° for the C–C–C–C, C–C–C–O and C–C–O–H torsions respectively. Visuals of the five n-propanol conformers and fourteen n-butanol conformers can be found in Ref.  supporting information. The relative amounts of the conformers in equilibrium systems were calculated by integrating the appropriate dihedral distribution functions. The integrations were from 0° to ~ 120° for the G/g configurations, ~ 120° to ~ 240° for the T/t configurations and ~ 240° to 360° for the g’ configurations. From the C–C–C-O dihedral angle distributions of pure n-propanol the trans configurations have a slight dominance (34.5%) over the gauche configurations which together contribute 65.5%. In the C–C–O–H dihedral angle g and g’ configurations each contribute 31% for a total of 62% and the trans configurations contribute the remaining 38%. This is comparable to recent Raman spectroscopy studies by Chen and co-workers which showed that the gauche configurations in C–C–O–H contribute 67% . The same authors also obtained 63% using molecular dynamics calculations. Thus, in totality the most predominant conformer is Tt. Recent high level ab initio calculations suggest the Gt conformer is the global minimum followed by the Tt conformer lying 0.07 kcal/mol above it .
In the case of n-butanol gas phase ab initio studies using HF/6-31G  and M08-HX/6-31 + G(2df, 2p)  place the TTt conformer 0.17 kcal/mol and 0.32 kcal/mol respectively above the TGt which is the most stable. The latter predicts two other conformers of lower energy lie below the TTt. However, from our calculations the TTt conformer dominates at all concentrations of DFM.
3.2 Hydrogen Bonding in Alcohols
3.3 Analysis of the First Solvation Shell
Molecular dynamics simulations using OPLS-AA force field was used to investigate the thermodynamics and structural properties of DFM/n-butanol binary mixture. The calculated results satisfactorily reproduce the experimental density. In case of the excess molar volume there is a reasonable discrepancy with the experimental data. Nevertheless, the calculated excess molar volume trend is consistent with experimentally calculated results. Excess molar volumes exhibit negative deviation for lower DFM mole fractions and positive deviation for higher mole fractions.
For structural investigations, we have calculated the conformer distributions, hydrogen bond interactions and first solvation shell populations for DFM/n-propanol and DFM/n-butanol mixtures. The results indicate that there is a small increase in gauche conformation for both alcohols as the DFM mole fraction is increased from 0 to 1. This suggests that gauche conformers are more able to incorporate into the DFM local network at higher DFM mole fractions. Correlations between alcohol conformer and the alcohol/DFM hydrogen bonding show that there is positive correlation for trans conformations and negative values for gauche conformations. This is not surprising since trans conformers have structurally less hindered with respect to approaching the DFM oxygen atoms for hydrogen bonding. The excess mole fractions of DFM/n-propanol indicate that the first coordination shell has a huge impact on the overall observed bulk properties. Finally, the spatial distribution of DFM and n-butanol around DFM molecule shows n-butanol molecules are mostly located above and below DFM furan ring, while DFM molecules are mostly on the sides. This arrangement maximizes the interaction energy between DFM and n-butanol and minimizes repulsion between DFM–DFM furan ring.
The Authors would like to thank the Botswana International university of Science and Technology for supporting the research and scholarship for OWK. We also would like to thank University of Botswana CSHPC for use of their computational resources.
Compliance with Ethical Standards
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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