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Solvation of Dimethyl Succinate in a Sodium Hydroxide Aqueous Solution. A Computational Study Xiuquan Sun,† Tsun-mei Chang,‡ Yang Cao,§ Satomi Niwayama,§ William L. Hase,§ and Liem X. Dang*,† Chemical and Materials Sciences DiVision, Pacific Northwest National Laboratory, Richland, Washington 99352, Department of Chemistry, UniVersity of WisconsinsParkside, Kenosha, Wisconsin 531412, and Department of Chemistry and Biochemistry, Texas Tech UniVersity, Lubbock, Texas 79409-1061 ReceiVed: March 3, 2009
Molecular dynamics simulations were carried out to study dimethyl succinate/water/NaOH solutions. The potential of mean force method was used to determine the transport mechanism of a dimethyl succinate (a diester) molecule across the aqueous/vapor interface. The computed number density profiles show a strong propensity for the diester molecules to congregate at the interface, with the solubility of the diester increasing with increasing NaOH concentration. It is observed that the major contribution to the interfacial solvation free-energy minimum is from electrostatic interactions. Even at higher NaOH concentrations, the increasing electrostatic interaction between the diester and ions is not large enough to favor the solvation of diester in bulk solutions. The calculated solvation free energies are found to be -2.6 to -3.5 kcal/mol in variant concentrations of NaOH aqueous solutions. These values are in qualitative agreement with the corresponding experimental measurements. The computed surface potential indicates that the contribution of diester molecules to the total surface potential is about 25%, with the major contribution from interfacial water molecules. I. Introduction Ester hydrolysis is an essential reaction and routinely carried out via diverse organic synthetic methods. However, it has been difficult to monohydrolyze symmetric diesters, although such monohydrolysis may be accomplished involving enzymes.1-5 Therefore, a limited number of examples of nonenzymatic selective monohydrolysis have been reported.6-10 However, selective monohydrolysis of one of the two identical ester groups in symmetric diesters is challenging, and therefore, the isolation and purification of the products requires substantial efforts. Recently, Niwayama et al. developed a highly efficient and practical method for the monohydrolysis of symmetric diesters.11-14 The selective monohydrolysis was found to occur in an aqueous KOH or NaOH medium, more efficiently consisting of a small amount of a polar aprotic cosolvent slightly miscible with water such as THF or acetonitrile.12-14 Though conjectures have been made regarding the reaction mechanism, there is not an atomic-level understanding of the origin of this selective monohydrolysis. The experiments show that diesters possessing two carboalkoxy groups in “cis” or “germinal” orientation undergo particularly efficient monohydrolysis, and a steric effect in the atomic-level mechanism has been suggested.11 Also of interest, and not resolved, is the role of the solvent. Several theoretical and experimental studies have been performed to understand the uptake of the symmetric diester dimethyl succinate in water.15,16 However, a thorough understanding of the behavior of dimethyl succinate in water is far from complete. Molecular dynamics simulations provide a powerful approach to examine the miscibility of dimethyl * To whom correspondence should be addressed. E-mail: liem.dang@ pnl.gov. † Pacific Northwest National Laboratory. ‡ University of WisconsinsParkside. § Texas Tech University.
succinate in water. This technique has been successfully applied to study many interfacial systems for decades, including ion and molecule solvation, transport, and reactions at interfaces,17-19 by providing structural and dynamics information at an atomic and molecular scale. In the work reported here, molecular dynamics simulations were performed to investigate the manner in which dimethyl succinate molecules are distributed in an aqueous NaOH medium. THF is not included in the solution since selective monohydrolysis also occurs in its absence.14 An atomic-level understanding of the selective monohydrolysis of the diesters requires understanding of how they are distributed and solvated in water. Our hypothesis for this high selectivity is that the hydrophobic diester molecules preferentially congregate at the water interface, where the monohydrolysis then occurs. The goal of this research is to investigate such suggestions and begin the establishment of an atomic-level mechanism for the selective monohydrolysis of diesters. II. Models and Methods The Dang-Chang (DC97) water model was used to represent the water interactions as well as sodium and hydroxide ions,20 and a detailed explanation of its parameters may be found in earlier work.20 Atomistic-scale models with atom-based polarizabilities were utilized to model the diester.21 The force field parameters for the diester were taken from a generalized Amber and OPLS-AA force field.22,23 The potential parameters used for water, ions, and diester in our simulation are summarized in Table 1. The systems were studied at a constant temperature of 298 K using a weak-coupling algorithm.24 The water and diester molecules were kept rigid using the SHAKE algorithm.25 Particle mesh 3D periodic Ewald summation was applied to calculate the electrostatic interactions.26 The LJ interactions were truncated at 11 Å with tail corrections. In the free-energy calculation, one diester molecule was presented in each simulation at several NaOH concentrations. For the simulations’
10.1021/jp901950g CCC: $40.75 2009 American Chemical Society Published on Web 04/09/2009
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TABLE 1: The Potential Parameters for Water, Ions, and Diester Molecule Used in This Study molecule H2 O Na+ OHdiester
atom type
σ (Å)
ε (kcal/mol)
q (e)
R (Å3)
O H M Na O H methoxy C R-alkoxy H alkoxy O carbonyl C carbonyl O alkyl C alkyl H
3.221 0.000 0.000 2.350 3.519 0.000 3.400 2.471 3.000 3.400 2.960 3.400 2.650
0.1825 0.0000 0.0000 0.1300 0.1825 0.0000 0.1094 0.0157 0.1700 0.0860 0.2100 0.1094 0.0157
0.000 0.519 -1.038 1.000 -1.200 0.200 0.160 0.030 -0.330 0.510 -0.430 -0.120 0.060
0.000 0.000 1.444 0.240 2.300 0.000 0.878 0.135 0.465 0.616 0.434 0.878 0.135
surface potential and number density profiles, 1000 DC97 water molecules were used to solvate 6 diesters with a range of NaOH molecules. The concentrations of NaOH were 0, 0.5, and 1.0 M, corresponding to the number of Na+ and OH- ion pairs 0, 9, and 18, respectively. The dimensions of the simulated sample were approximately 26 × 26 × 150 Å3, thereby forming two liquid/vapor interfaces with the two free volumes of 50 Å on both sides of the z axis. All simulations were equilibrated for 500 ps, and the analyses were performed during the subsequent 1 ns run. A constrained mean force technique was applied to examine the transport free energies of a single solute molecule across the interface. The free energy difference between two states is given by
∆F(zs) ) F(zs) - F0 ) -
∫z
zs
0
〈fz(zs′)〉dzs′
(1)
F0 is the free energy at a reference point, which we take to be the gas phase. The 〈fz(z)〉 is the average force exerted on the solute molecule along the z axis. For this free-energy calculation, one solute molecule is present in each simulation. Surface potentials are calculated by an atomic approach. For our polarizable potential model, the total electric potential difference across the interface results from partial charges and an induced dipole distribution. The contribution from partial charges is defined as27
∆φq(z) ) φq(z) - φq(z0) ) -
∫zz Ez(z')dz' 0
(2)
where z0 denotes a reference point in the charge-free (gas) region and Ez(z) is the electric field derived from point charges as a function of z across the gas-liquid interface. The induced dipole contribution is given by
∆φµ(z) )
1 ε0
∫zz 〈Fµ(z')〉dz' 0
(3)
Here, Fµ (z) is the z component of the induced dipole moment density profile. III. Results and Discussion A. Ion Solvation Structures. Figure 1a,b shows the hydration structures of Na+ and hydroxide ions based on their radial distribution functions. For Na+, the first oxygen shell peaks at around 2.4 Å, and the first hydrogen shell is centered at 3 Å. These results can be compared to the experimental hydration structures of Na+ ions using neutron diffractions techniques.28 Even at different NaOH concentration, the simulated hydration
Figure 1. (a) gNaOw(r) and gNaHw(r) for 0.5 M NaOH aqueous solution at 298 K. (b) gOHOw(r) and gOHHw(r) for 0.5 M NaOH aqueous solution at 298 K.
structures of the Na+ ion excellently reproduce the experimental results since the positions of the hydration shells are known to be insensitive to the base concentration. However, because the concentration of the base (1 NaOH/100 water) in our simulation is much lower than the lowest experimental concentration (1 NaOH/12 water), the intensities of the RDF peaks are different from the neutron diffraction results. The simulated intensity increases with decreasing NaOH concentration, consistent with the experimental trend. The hydration shell of hydroxide in our simulation is shown in Figure 1b. Small differences are observed between the simulation and the neutron diffraction experiment; that is, the first oxygen shell is at 2.7 instead of 2.3 Å in neutron diffraction, and the corresponding first hydrogen shell is at about 1.7 instead of 1.4 Å in neutron diffraction. This shortcoming is likely due to the deficiency in our classical OH- model; there is no proton sharing allowed between OH- and the nearestneighbor water molecules. B. Potential of Mean Force (PMF). Shown in Figure 2 are the PMF profiles for transporting a diester molecule across the aqueous/vapor interface at several NaOH concentrations. The vertical dashed line is the Gibbs dividing surface (GDS) of water. A free-energy minimum is observed around the GDS for all three systems. The calculated solvation free energies are -2.6, -2.7, and -3.5 kcal/mol for pure water, 0.5 M NaOH, and 1.0 M NaOH solutions, respectively, and the estimated uncertainty is (0.3 kcal/mol in our simulations. The values are in qualitative agreement with the corresponding experimentally measured value of -6.67 kcal/mol.16 One interesting observation is that the difference between the minimum free energy at the interface and the relative free energy in the gas phase is almost
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Figure 2. PMF of one dimethyl succinate molecule crossing the aqueous/vapor interfaces of pure water and 0.5 and 1.0 M NaOH solutions with the vapor phase as the reference point.
Figure 3. The density profiles for species of the solution/vapor interface containing 6 dimethyl succinate, 9 NaOH, and 1000 water molecules (1.0 M NaOH) at 298 K. GDS denotes the Gibbs dividing surface.
identical (∼6.8 kcal/mol) for all three solutions. It is argued that because sodium and hydroxide ions are depleted from the water/vapor interface,29,30 the local environments at the interfaces for all three systems will be very similar (only water present at the interfaces). As a consequence, the contribution to the free energy only comes from the water molecules solvating the diester. This explanation is supported by experimental measurements of the uptake of dimethyl succinate using the droplet train technique.16 In the measurements, the uptake coefficient, γ, of dimethyl succinate on an aqueous surface was found to be independent of the aqueous-phase composition (NaOH aqueous solution in the experiments). The situation is different for bulk solutions. When the concentration of NaOH is low, the free energy is about the same as that of pure water. At a higher concentration (1.0 M NaOH), the solvation free energy is lowered by about 0.8 kcal/mol. In principle, the solvation free energy of a Lennard-Jones solute in the solvent is a result of a balance between creating a cavity and charging up the electrostatic interactions by the solvent environment. For neutral species like the water and diester molecules, the major contribution to the solvation free energy is from the electrostatic interactions. This is a direct consequence of the more favorable electrostatic interactions between diester and water molecules in the liquid phase than that of the gas phase. When the concentration of NaOH is low, the total solvation free energy is close to the free energy of solvating the diester in pure water. With increasing NaOH concentration, electrostatic contributions (Coulombic and polarization) to the solvation free energy increase, further stabilizing the solvation of the diester in water. Although the solubility of the diester in aqueous NaOH solution increases with NaOH concentration, the free-energy minimum is still at the aqueous/vapor interface, which leads to a limited aqueous solubility of the diester. The free energy of solvating the diester in water can also be calculated using the self-consistent reaction-field method.15 In this calculation, the dielectric constant is used to compute the electrostatic contribution, and the van der Waals contribution is evaluated by a linear relationship with the atomic surface. The calculated result is about -6.0 kcal/mol, which is twice that of our simulation value of -2.6 kcal/mol. In our opinion, the comparison may not be appropriate because the two approaches are not equivalent. C. Density Profiles and Surface Potentials. Figure 3 shows the density profiles of the species along the normal to the 1.0 M NaOH solution/vapor interface as obtained from molecular
dynamics simulations. There are two interfaces in each of our simulations due to our simulation setup. The results shown are the average of the two half-boxes. The diesters are only present at the interface, as expected. The same behavior was also observed in pure water and 0.5 M NaOH aqueous solutions (data not shown). This observation is consistent with the free-energy prediction that the energy minimum is at the interface despite the increased solubility of the diester with increasing NaOH concentration. The depletion of sodium and hydroxide ions at the interface is found in our simulations, and a small increase in the ion densities (especially for Na+) is detected about 7 Å away from the interface. A snapshot taken from the simulation of the diesters in 1.0 M NaOH solution is given in Figure 4. All diester molecules are clearly shown to be present at the interfaces, and from which, the ions are depleted. These observations are consistent with the conclusions drawn from free-energy calculations. Surface tensions are also calculated for the three systems from our simulation. The computed values are 60.4, 62.6, and 65.0 dyn/cm for pure water, 0.5 M NaOH, and 1.0 M NaOH solutions, respectively. These results are in agreement with the experimental trend that the simple base will increase the surface tension of water.31 Figure 5a shows the electric field along the normal direction of the interface calculated for the system with 6 diesters solvated in 1000 water molecules at 298 K. The corresponding surface potentials for the two interfaces are given in Figure 5b. From the density profiles, it is evident that the diester molecules are only present at the interface, from which the ions are repelled. Thus, it is expected that the dominant contribution to the electric field and surface potential, across the interface, will come from water and the diester. The calculated total electric field and surface potential display similar behavior to that in pure water.20,32 The electric field shows a maximum at the interface when approached from the vapor phase toward the bulk solution and a diminishing electric field above the interface. For the pure water system, the electric field at the interface mainly arises from an orientational ordering of water molecules near the surface.33 The total electric field at the interface of our diester/ water system is estimated to be 0.16 ( 0.01 V/Å, which agrees with the value in pure water of 0.16 V/Å. The diester contribution to the electric field is shown in Figure 5a, which is about 25% of the total electric field. The major contribution still comes from water molecules at the interface exhibiting strong orientational ordering as in pure water. The calculated surface potential is about 0.55 ( 0.01 V for the diester in pure
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Figure 5. (a) The total and dimethyl succinate electric field of the solution containing 6 dimethyl succinate and 1000 water molecules at 298 K. (b) The total and dimethyl succinate surface potential of the same system as in (a).
Figure 4. A snapshot of the system described in Figure 3.
water at 298 K. One interesting feature from the simulations is the asymmetric shape of the surface potentials across the two interfaces. By inspecting the component of the surface potential from the diester molecules (also shown in Figure 5b), it is clear that the asymmetry of the total surface potential is a result of the asymmetry of the diester surface potential. This is a finite system size effect resulting in an uneven distribution of the diester molecules at the two interfaces. For example, the number density ratio of diesters at the two interfaces is 1:2 in this case. As a consequence, the symmetry of the diester surface potential is broken. IV. Conclusion The free energy of solvating a diester in various NaOH aqueous solution concentrations was calculated using the PMF technique. The results reveal that the solubility of the diester increases with increasing NaOH concentration. However, the minimum free energy for the diester solvation is always found at the interface regardless of the NaOH concentration for the system size investigated in our simulations. The major contribution to the solvation free energy is from electrostatic interactions. The increasing electrostatic interaction between the diester and ions at higher NaOH concentration is not large enough to favor diester solvation in the bulk solution. Because the Na+ and OHions are repelled from the interface, the local environments are the same at interfaces of aqueous solutions with different NaOH
concentrations. Therefore, the free energy for the uptake of the diester at the aqueous/vapor interface, from the vapor phase, is the same for the three systems that we studied. Our free-energy calculation supports experimental measurements that the uptake coefficient of dimethyl succinate on an aqueous surface is independent of the aqueous-phase composition.16 The surface tension increases with concentration of the NaOH aqueous solution. This finding is consistent with the conclusions of previous studies. According to our surface potential calculations, the contribution of the diester molecules to the total surface potential is about 25%, with the major contribution from the interfacial water molecules. The total surface potential of the diester aqueous solution is close to that of pure water. This indicates a strong orientational preference for the interfacial water molecules at the interface even in the presence of diester molecules. The insights of the structural and energetic information of organic molecules at interfaces are important to understand chemical reactions that occur at interfaces. This work focuses on systems at aqueous/vapor interfaces. Further investigations of liquid/liquid interfaces will be helpful to understand these types of kinetic processes. Acknowledgment. This work was performed at Pacific Northwest National Laboratory under the auspices of the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy (DOE). Battelle operates Pacific Northwest National Laboratory the DOE. The computer resources are provided by the Division of Chemical and Materials
Solvation of Dimethyl Succinate in Sodium Hydroxide Sciences. The contributions to the research from Texas Tech University are based on work supported by the National Science Foundation under Grant No. CHE-061532, the Robert A. Welch Foundation under Grant No. D-0005, and National Science Foundation-CAREER (Grant No. CHE-0443265). References and Notes (1) Ohno, M.; Otsuka, M. Org. React. 1989, 37, 1. (2) Sano, S.; Ushirogochi, H.; Morimoto, K.; Tamai, S.; Nagao, Y. Chem. Commun. 1996, 1775. (3) Schoffers, E.; Golebiowski, A.; Johnson, C. R. Tetrahedron 1996, 52, 3769. (4) Yu, M. S.; Lantos, I.; Peng, Z. Q.; Yu, J.; Cacchio, T. Tetrahedron Lett. 2000, 41, 5647. (5) Lane, J. W.; Halcomb, R. L. J. Org. Chem. 2002, 68, 1348. (6) Corey, E. J. J. Am. Chem. Soc. 1952, 74, 5897. (7) Vecchi, A.; Melone, G. J. Org. Chem. 1959, 24, 109. (8) Strube, R. E. Org. Synth. 1963, IV, 417. (9) Grakauskas, V.; Guest, A. M. J. Org. Chem. 1978, 43, 3485. (10) De Kimpe, N.; Boeykens, M.; Tehrani, K. A. J. Org. Chem. 1994, 59, 8215. (11) Niwayama, S. J. Org. Chem. 2000, 65, 5834. (12) Niwayama, S. J. Synth. Org. Chem. Jpn. 2008, 66, 983. (13) Niwayama, S.; Cho, H. J.; Lin, C. L. Tetrahedron Lett. 2008, 49, 4434. (14) Niwayama, S.; Wang, H.; Hiraga, Y.; Clayton, J. C. Tetrahedron Lett. 2007, 48, 8508. (15) Aleman, C.; Puiggali, J. Chem. Phys. 1997, 222, 9.
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