Theoretical Study of Acetic Acid Association Based on Hydrogen

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Theoretical Study of Acetic Acid Association Based on Hydrogen Bonding Mechanism Minhua Zhang, Lihang Chen, Huaming Yang, and Jing Ma J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 26 May 2017 Downloaded from http://pubs.acs.org on May 30, 2017

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The Journal of Physical Chemistry

Theoretical Study of Acetic Acid Association Based on Hydrogen Bonding Mechanism

Minhua Zhang1, 2, Lihang Chen1, 2, Huaming Yang1, 2, Jing Ma1, 2, * 1

Key Laboratory for Green Chemical Technology of Ministry of Education, R&D Center for Petrochemical Technology, Tianjin University, Tianjin 300072, China 2 Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University, Tianjin 300072, China *Corresponding author phone: (86)02227406119; fax: (86)02227406119 email: [email protected]

Abstract: Combining molecular dynamics (MD) and quantum chemistry (QM) simulation, the association mechanisms of acetic acid (AA) systems were examined. DFT methods were proposed to compare the hydrogen-bonding energies of variable acetic acid dimers, and definitely provided the stable dimers configuration. Geometry parameters of dimers were also obtained by QM calculations, which were taken as the characteristic criteria for further MD analysis. Proportion of different acetic acid dimers in gas phase was obtained by Radial distribution function (RDF) analysis, and cyclic dimer with two O1-Ho hydrogen bonds was demonstrated as the most stable structure. While in the more complex liquid phase, the linear chain form was proved to be the most stable one. Furthermore, in the acetic acid-crotonaldehyde solution, the association configuration of acetic acid changed from the linear chain form to the cyclic dimer structure as the acetic acid concentration decreased gradually. This result would be significant for the chemical separation process of acetic acid-crotonaldehyde solutions. 1. Introduction Vapor-liquid equilibrium (VLE) properties of fluids play an important role in chemical engineering field. However, the VLE properties of acetic acid behave severe non-ideality due to the intermolecular association. 1 Hence, it is significant to study the hydrogen bonding interactions between these molecules in a variety of phases. The strength of the hydrogen bond lies between weak chemical bonds and strong physical intermolecular interactions. 2-4 There is a complex association within acetic acid as it has two kinds of hydrogen bond donors (hydroxyl hydrogen and methylic hydrogen atoms) and acceptors (carbonyl oxygen and hydroxyl oxygen atoms). 5 This molecule, as the prototypical associating system, has been studied experimentally and theoretically to reveal the hydrogen bonding mechanism. Various associated configurations were found in acetic acid gas, liquid, and in acetic acid solutions of different concentrations. In the gas phase, there is now a consensus that the predominant structure is the 1

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cyclic dimer. The stability of this structure can be explained by the formation of two equivalent O-H--O=C hydrogen bonds between the two molecules. 6-9 Karle 10 and Desrisne 11 investigated the molecular structure of acetic acid by electron-diffraction method, and found that the acetic acid dimer was a planar ring structure formed by O-H---O=C hydrogen bonds. The cyclic structure is found to be more stable than the others. 12-13 Recently, with the development of the computational chemistry, the properties of the structures have been investigated from the other views. Chocholousǒvá 14 proposed six kinds of dimer structures by ab initio and DFT calculations. Yang 15 utilized semi-empirical methods of quantum chemistry, finding eight kinds of acetic acid dimer structures, and demonstrated that the cyclic dimer with double O-H---O=C hydrogen bonds was the most stable, consistent with the results of Chocholousǒvá. While a variety of dimer structures have been found, their distributions under different conditions are still unknown. The study of the distribution of the dimers will be of great interests for chemical engineering applications. Liquid acetic acid molecules are also affected by hydrogen bonding interactions. But the structure in the liquid phase is more complex than that in the gas. Both experimental and theoretical methods have been used to reveal the acetic acid structure in liquid phase. 5, 16-23 Based on the neutron diffraction experiment, Bertagnolli 24-25 considered that the main associated configuration in the liquid phase was cyclic dimer structure. They found that the hydrogen bond length was constant as the substance passed from the crystalline to the liquid state. Compared to the chain structure present in the crystalline state, the cyclic dimer structure better described the structure of liquid acetic acid. Furthermore, Nielson 26 assigned three distinctive peaks in the Raman spectrum to the three fundamental modes of the cyclic dimer by means of depolarization ratios, isotope effects, and the calculations with empirical force fields. 27-30 Flakus 31 and Benmalti 32 also confirmed this observation. However, researchers have recently found that the chain-like formation was the main structure present in liquid acetic acid. Nakabayashi 16 found that the melting of the crystalline acetic acid did not affect its peak position in the low-frequency Raman spectrum by Raman spectra and ab initio MO calculations, indicating that the chain clusters like the crystal network fragment were the predominant structures in liquid acetic acid. The same conclusion was obtained by Nasr 17-18, Takamuku 33 and Briggs 23. Since the hydrogen bonds in liquid acetic acid are more complex, the main association structure has not been confirmed. Hence, exploring the association of acetic acid in the liquid phase is still a great challenge. As a special associating system, the associating form will also vary with the concentration of acetic acid in the solution. For instance, in high concentrations of water solution, the existence of water molecules would destroy the former oligomers and form new ones 22, 30. In the production of vinyl acetate by acetic acid and acetylene, the product also contains a mixture of acetic acid and crotonaldehyde. However, whether the presence of crotonaldehyde affects the association behavior in liquid acetic acid has never been investigated so far, which will guide the separation of acetic acid-crotonaldehyde solution. 2


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In our investigation, four kinds of acetic acid dimer structures in the gas phase were selected as subjects by DFT methods. The geometry parameters and stability of different dimer structures have also been compared in the quantum chemistry calculations. The characteristic distance between carbonyl carbon atoms is chosen as criteria for subsequent research. Molecular dynamics (MD) simulations were carried on the acetic acid systems in the gas and liquid phases, as well as the acetic acid-crotonaldehyde system. Based on the characteristic distance of different dimers, the proportion of each dimer was obtained for the first time by the analysis of the radial distribution function (RDF). The stable structure of acetic acid in the liquid phase and the influence of acetic acid concentration on the acetic acid-crotonaldehyde solution were also investigated. It is worth mentioning that crotonaldehyde is for the first time chosen as the solvent to study the influence on acetic acid association, which is meaningful for the separating operations in the chemical engineering industry. 2. Methods 2.1 Quantum chemical calculations The structures of acetic acid dimers in the gas phase and acetic acid-crotonaldehyde clusters were optimized at the DFT/B3LYP/6-31G** level based on the previous study.14 The hydrogen-bonding energies of dimers were also obtained by Grimme’s functional B97D including dispersion corrections. Final hydrogen-bonding energies of acetic acid dimers were corrected for the basis set superposition error (BSSE)34 and the deformation energies of monomers. All of the quantum chemical calculations in this paper were performed employing Gaussian 09 packages.35 The structure and atom labels of an acetic acid molecule are shown in Fig. 1.

Fig. 1 Structure and atom labels of acetic acid molecule 2.2 Molecular dynamics simulations In this study, MD simulations were carried out utilizing Discover module in Materials Studio software, and COMPASS force field was used as the atomic force field 36, which has been developed by combining results of ab initio calculations and an empirical fitting of condensed-phase properties. 512 molecules were contained in pure vapor, liquid acetic acid phase and the acetic acid-crotonaldehyde solutions. Cubic simulation cells were constructed by Amorphous Cell module according to the experiment densities. The cutoff distance for L-J interactions was set as half of the cell length, and the long-range intermolecular forces were evaluated using Ewald method in periodic boundary conditions. Simulations were performed in the NPT ensemble with a time step of 0.5 fs. The temperature were controlled by means of the 3



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Nose thermostat. The simulations included initial equilibration runs of 400 ps, followed by simulation runs of 400 ps to collect statistically meaningful properties. 2.3 Radial distribution functions (RDF) The RDF is an important function to characterize the structure of the fluid and amorphous solids, describing the distribution of groups. In the thermodynamics theory, if the accurate RDF could be gained under certain conditions, all of the thermodynamics quantities would be deduced from it. The RDF is the main information that connects the structure and attributes of the system. The association interactions of acetic acid molecules in this study could be analyzed by the position and value of the maxima in the intermolecular carbonyl oxygen-hydroxyl hydrogen and carbonyl carbon-carbonyl carbon RDFs. The position of the maxima explains the hydrogen-bonding distance and the peak value could express the relative strength of hydrogen bond. The proportion of different dimers was obtained by the ratio of the g(r) at the characteristic distance between the carbonyl carbon atoms of the different dimers. 3. Results and discussion 3.1 Quantum chemical calculations on acetic acid association structure. Based on the formation of hydrogen bond as criterion, we have found variety of dimeric configurations in our previous study. Four relatively stable configurations were selected for our study, as is shown in Fig. 2. The hydrogen-bonding energies of the structures are summarized in Table 1. As is presented, the hydrogen-bonding energies calculated by B97D has little difference from the results calculated by B3LYP due to the dispersion correction to the energies. However, the results between the two functional and basis sets are relatively close, and present the same relations between the four types of dimers. That is, the most stable dimer is the structure with the cyclic arrangement containing two carbonyl oxygen (O1)–hydroxyl hydrogen (Ho) hydrogen bonds. The hydrogen-bonding energies of dimer B and dimer C have little difference, meaning the carbonyl oxygen (O1)–methyl hydrogen (Hc) hydrogen bond and the hydroxyl oxygen (O2)–hydroxyl hydrogen (Ho) hydrogen bond have similar strength. Based on the comparation, it can be easily explained that the dimer D structure with two O1-Hc hydrogen bonds has the least hydrogen-bonding energies.

Fig. 2 Four main acetic acid dimer structures in gas phase 4


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Table 1 Values of Interaction Energies (ΔE) and the Interaction Energies with BSSE in the Acetic acid Dimersa B3LYP/6-31G** B97D/6-31G** B97D/6-311G** Structure ΔE ΔE(BSSE) ΔE ΔE(BSSE) ΔE ΔE(BSSE) Dimer A -24.35 -20.14 -24.92 -20.91 -23.46 -19.81 Dimer B -11.06 -7.92 -11.43 -8.49 -10.10 -7.85 Dimer C -11.96 -8.93 -12.16 -9.36 -11.43 -8.95 Dimer D -4.78 -2.59 -3.41 -2.31 -5.28 -3.60 a. Energy units are kcal/mol. Table 2 Intermolecular atom distances in the four dimer structures Structure O1 - Ho O2 – Ho O1 – Hc C1 – C1 O1 – O2 Dimer A 1.636 --3.820 2.643 Dimer B 1.855 1.925 -4.284 2.700 Dimer C 1.790 -2.257 4.204 2.768 Dimer D --2.375 4.662 5.518 a. Distance units are Å in the Table; b. “--” means no such hydrogen bond in the structures. The geometry parameters are summarized in the Table 2. It can be seen that in the most stable dimer structure (dimer A), the distance of intermolecular O1-Ho atoms is 1.636 Å, less than this in dimer B and C structures. This can also explain why the stabilization of dimer A is higher than the others—the hydrogen bond lengths are shorter in more stable dimer structures. For instance, the distances between O1 and Ho atoms in the dimer A, B and C are separately 1.636 Å, 1.855 Å, and 1.790 Å, same with the stability order. The distance between O1 and Hc atoms in dimer C is 2.257 Å, shorter than this in dimer D (2.375 Å). Even so, the distinctions of hydrogen bond length in variable dimer structures are still little (From 1.636 Å to 1.855 Å for O1–Ho, 2.257 Å to 2.375 Å for O1-Hc). It is difficult to determine the dimer configuration by this distance. However, the distance between the intermolecular carbonyl carbon (C1) atoms can be used as the characteristic distance for the disparate dimer structures. As is present in the Table 2, the distances between the intermolecular C1 atoms in the conformers are separately 3.820 Å, 4.284 Å, 4.204 Å, and 4.662 Å, behaving the same trend with the hydrogen-bonding energies. In addition, the differences between the disparate conformers are obvious enough to distinguish each dimer structure. As for the distance between O1 and O2 atoms, dimer A, B and C present similar value (2.643 Å, 2.703 Å, and 2.768 Å), because all of them contain the O1–Ho hydrogen bonds. The distance in the dimer D is 5.518 Å, longer than in the others, for the reason that there are two O1–Hc hydrogen bonds in the dimer D structure. 3.2 Acetic acid association in the gas phase 3.2.1 Influence of temperature In this work, MD simulations were employed on the pure vapor acetic acid molecules at the temperature of 423.15-583.15 K and the pressure of 100 kPa. The RDFs of intermolecular O1-Ho and C1-C1 atoms were utilized to investigate the 5



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conformer structure in the gas phase. Fig. 3 presents the RDFs of intermolecular O1-Ho and C1-C1 atoms at 100 kPa. O1-Ho

C1-C1

(a)

(b)

Fig. 3 RDFs between acetic acid molecules vs temperature at 100kPa: (a). O1-Ho; (b). C1-C1 In Fig. 3(a), all of the O1-Ho RDFs exhibit a distinct peak at around 1.78 Å, indicating the location as a typical hydrogen bond distance. The value has a little difference with the quantum chemistry calculation results, in which the typical hydrogen bond distance is 1.64 Å. This may be caused by the molecular vibration that we did not consider in the ground state quantum chemistry calculations. The effect of the temperature on molecular thermal motion could also weaken the hydrogen bond strength. The peak heights gradually decrease as the temperature increases from 423.15 K to 503.15 K, showing the hydrogen bonds are destroyed with the increase of temperatures. When temperatures are higher than 503.15 K, heights of distinct peaks exhibit no variation, implying that the conformer distributions in the gas phase are stable. Fig. 3(b) presents the intermolecular C1–C1 RDFs at 100 kPa. The characteristic peak is exhibited at around 3.80 Å, same with the quantum chemistry results. According to the definition of RDF and the method in our former investigation, 37 proportion of each dimer could be obtained based on the C1–C1 distances. For instance, the characteristic distance of dimer A is 3.80 Å, and the RDF value can be obtained from the picture, so the proportion equals to the value of dimer A divide by the total value of the all dimers. The results are shown in Table 3. From Table 3, dimer A has also proved to be the most stable dimer, whose average proportion is about 35.90%. Dimer C is the next, with the average proportion of 24.56%. The proportions of dimer B and D reflect a different trend depending on the temperature change. Furthermore, when it is lower than 503.15K, proportions of the four dimers change regularly with the temperature increasing: proportions of dimer A and D decrease, while dimer B and C are increasing. When it is higher than 503.15K, the effect of temperature on the association is small, and the change of the proportion of each dimer is small and irregular.

6


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Table 3 Proportion (%) of different acetic acid dimers at various temperatures. Dimer proportions Temperature A B C D 423.15K

39.47

14.51

24.34

21.68

463.15K

38.02

17.07

25.39

19.52

503.15K

35.96

21.68

26.23

16.13

543.15K

32.61

21.74

23.91

21.73

583.15K

33.46

20.83

22.93

22.78

3.2.2 Influence of pressure The effect of pressure on the association of acetic acid is great. To reveal the effect of pressure, MD simulations were employed at 463.15 K and the pressure of 50 kPa, 100 kPa, 200 kPa and 500 kPa respectively. The intermolecular RDFs of O1-Ho and C1-C1 atoms between acetic acid molecules were investigated to obtain the regularity of association. Fig. 4 shows the RDFs between different atoms with the change of pressure. The characteristic peaks appear separately at 1.78 Å and 3.80 Å, same with the isothermal conditions, which can be testified by the other investigations. 9 Simultaneously, as the pressure increases from 50 kPa to 100 kPa, the heights of the RDF peaks decrease rapidly, meaning that the association of the molecules is weakened. When the pressure is higher than 100 kPa, the RDF peaks show little change with the variation of the pressure. Unlike the isobar-variable-temperature process, the isothermal-variable-pressure process is more complicated in the gas phase. According to the definition of radial distribution function, g(r) presents the ratio of the local density to the average density, where the change of density indicates the variation of the pressure. The fluctuation of the pressure would influence both of the parameters. Due to the different sensitivity of the local density and average density to pressure, decreasing of the peak heights can be explained as that the average density increases more rapidly than the local density with the increase of the pressure. O1-Ho

C1-C1

(a)

(b)

Fig. 4 O1-Ho RDFs between acetic acid molecules vs pressure at 463.15K: (a). O1-Ho; (b). C1-C1 7



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According to Fig. 4(b) and corresponding criteria, distributions of acetic acid dimers are presented in Table 4. As is shown in Table 4, dimer A has the greatest proportion in the whole pressure range. When the pressure is lower than 100 kPa, the proportion of the four dimers is not stable. However, the proportion of the four dimers shows no variation when the pressure is higher than 100 kPa. Table 4 Proportion (%) of different acetic acid dimers at various pressures Dimer proportions Pressure A B C D 50 kPa

42.72

12.89

20.83

23.56

100 kPa

38.02

17.07

25.39

19.51

200 kPa

37.63

17.44

25.89

20.12

500 kPa

39.12

16.81

24.79

19.28

3.3 Acetic acid association in the liquid phase 3.3.1 The associating properties of the acetic acid liquid The association properties of acetic acid molecules in the liquid phase were rarely studied since the association of liquid molecules is more complex than gas. MD simulation was employed to investigate the intramolecular and intermolecular properties of acetic acid at 298.15 K and 100 kPa.

Fig. 5 RDFs between acetic acid molecules at T=298.15 K and P=100 kPa Fig. 5 presents the intramolecular and intermolecular RDFs of acetic acid molecules. The intramolecular RDF is defined as the RDF between atoms in the same molecule, and the intermolecular RDF is between atoms in different molecules. The positions of the peaks in the intramolecular RDF represent the distance among the atoms respectively, that is, the bond lengths in the molecule. Simulation length of 8


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each bond is compared with the experiment results proposed by Imbert, in Table 5.

21

presented

Table 5 Experiment and simulation results of bond length in liquid acetic acid. Bond ra , Å rb ,Å21 O2-Ho

0.970

0.950

O2-C1

1.350

1.342

C1-O1

1.210

1.219

C1-C2

1.510

1.495

C2-Hc

1.090

1.099

a. Bond length obtained by molecular dynamics simulations; b. Bond length measured by neutron diffraction with empirical potential structure refinement (EPSR) simulation. Table 5 shows that configuration parameters obtained by MD simulation have little variation with experiments. Bond between O2 and Ho atoms has the highest deviation of 0.02 Å, probably since that the formation of hydrogen bond reduce the bond length. The dash line in Fig. 5 presents the intermolecular RDF between acetic acid molecules. Due to the complexity in the liquid acetic acid molecules, it presents no regularity. For the purpose of investigating special hydrogen bonding interactions in liquid phase, intermolecular RDFs of Ho and O1 atoms with other atoms were proposed in Fig. 6.

(a)

(b)

Fig. 6 Intermolecular RDFs at T=298.15 K and p=100 kPa: (a). Ho-other atoms; (b). O1–other atoms Obviously, the first peak between Ho and O1 atoms, at 1.76 Å, demonstrates the formation of the hydrogen bond in the liquid phase. From Fig. 6(b), it can be seen that the Ho atom is the nearest atom to the O1 atom, followed by Oh, O1, C1, C2 and Hc atoms in sequence, same with the results obtained in experiment, reflecting the strong associating interaction and the short-range order-long-range disorder phenomenon in the liquid acetic acid. 9



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(a)

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(b)

Fig. 7 RDFs between intermolecular C1s, intramolecular O1 and Ho at T=298.15K and p=100kPa (a) and the structure of liquid acetic acid (b) The intramolecular O1-Ho RDF is presented in Fig. 7(a), showing two obvious peaks at the distance of 2.4 Å and 3.0 Å, representing the cis and trans isomers of acetic acid molecules separately. The formation of the acetic acid molecules in the system can be analyzed by the intermolecular C1-C1 RDF. The first peak appears at the distance of 4.5 Å, meaning the distance of the C1 atoms in the main associated configuration. And the second weak peak appears at the distance of 8.8 Å. Based on the hydrogen bonding formation in Fig. 5, the main associated configuration in the liquid phase of acetic acid proves to be the linear chain form, developed from the acetic acid crystal, which is in agreement with the experiment results, 16, 21, 23 as is shown in Fig. 7(b). 3.3.2 Influence of temperature For further investigation on the effect of temperature, MD simulations were employed at 100 kPa and various temperatures to investigate intermolecular RDFs of O1-Ho and C1-C1 atoms.

O1-Ho

C1-C1

(b)

(a)

Fig. 8 RDFs between acetic acid molecules vs. temperature at 100 kPa: (a). O1-Ho; (b). C1-C1 As is shown in Fig. 8(a), the peak appears at 1.76 Å between O1 and Ho atoms, 10


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indicating that the distance is hydrogen bond length. As the temperature rises from 303.15 K to 383.15 K, the decrease of the peak values is unsharp compared with acetic acid vapor. Molecular spacing in liquid phase is much shorter than in vapor phase, hence the effect of temperature is weak. As is shown in Fig. 8(b), the characteristic distance between intermolecular C1 atoms is 4.50 Å, and another strong peak is obtained at around 9.00 Å. The distances correspond to the distances of C1 atoms between acetic acid molecules, proving the linear chain form in the acetic acid system. 3.3.3 Influence of pressure In this work, we chose the pressure in the range of 10~500 kPa to investigate the intermolecular RDFs of O1-Ho and C1-C1 atoms at 303.15K. As is presented in Fig. 9, the first peaks are presented at 1.76 Å and 4.50 Å between intermolecular O1-Ho and C1-C1 atoms separately, in consistent with the isobar-variable-temperature process. Meanwhile, the peak values showed no variation with the pressure increasing, indicating that the pressure has no influence on the associating reaction in the liquid acetic acid molecules.

C1-C1

O1-Ho

(b)

(a)

Fig. 9 RDFs between acetic acid molecules vs temperature at 303.15 K: (a). O1-Ho; (b). C1-C1 3.4 Acetic acid association in binary system 3.4.1 High concentration of acetic acid At 303.15 K and 100 kPa, MD simulations were conducted at acetic acid mole fraction of 99%, 90%, 80% and 60%. The intermolecular RDFs of O1-Ho atoms and C1-C1 atoms at different concentrations were investigated and presented in Fig. 10.

11



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O1-Ho

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C1-C1

(a)

(b)

Fig. 10 RDFs between acetic acid molecules vs concentration of acetic acid at 303.15 K, 100 kPa: (a). O1-Ho; (b). C1-C1

The characteristic distances of intermolecular O1-Ho and C1-C1 atoms are 1.76 Å and 4.50 Å, same as them in the liquid phase. While acetic acid concentration decrease from 99% to 60%, the O1-Ho RDF peak values are enhancing gradually, indicating that the O1-Ho hydrogen bonds gradually increase. According to Fig. 10(b), the first peak of intermolecular C1-C1 RDF increases inconspicuously with the acetic acid concentration decreasing from 99% to 80%. While the acetic acid concentration reduced from 80% to 60%, the position of the first peak shifted toward left. However, the shape of the graph is same as it in the liquid phase, revealing the main form in crotonaldehyde solution is still chain-like form. The quantum chemistry results give the same conclusion. Fig. 11 presents the optimized structure of a four molecule acetic acid (AA) chain combining with one crotonaldehyde molecule (CA). The optimized result shows a planar structure similar with the chain. It seems that the crotonaldehyde molecule can hardly break the chain. Hence, in high acetic acid concentration solution, the association of acetic acid molecules is similar with the liquid phase.

Fig. 11 Optimized structure of acetic acid -crotonaldehyde mixture at high acetic acid concentration 3.4.2 Low concentration of acetic acid

12


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C1-C1

O1-Ho

(a)

(b)

Fig. 12 RDFs between acetic acid molecules vs concentration of acetic acid at 303.15 K, 100 kPa: (a). C1-C1; (b). O1-Ho The association results is more fluctuanting in the low acetic acid concentration system. As is presented in Fig. 12(a), with the decrease of acetic acid concentration from 39% to 10%, the peak value at r=4.50 Å reduces gradually in the C1-C1 intermolecular RDF figure, and the peak at r=3.80 Å rises simultaneously. The latter distance is exactly the distance of carbonyl carbon atoms in the cyclic dimer of double O1-Ho hydrogen bond. Fig. 13 gives the optimized structure of the acetic acid (AA) chain combining with six crotonaldehyde molecules (CA). It is obvious that the planar chain structure was broken due to the interaction of crotonaldehyde. Therefore, when the acetic acid concentration is low, the cyclic dimer is increasing to be the main structure in the solution. The associated configuration of acetic acid proved to be transitioned from liquid phase to gas phase with the reduction of acetic acid concentration.

Fig. 13 Optimized structure of acetic acid -crotonaldehyde mixture at low acetic acid concentration

13



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4. Conclusions Acetic acid associated configurations under different conditions were investigated by combining quantum chemistry and MD methods. Quantum chemistry results illustrate that dimer A structure with two O1–Ho hydrogen bond is the most stable dimer structure. The distance between intermolecular C1 atoms is chosen as the characteristic distance to distinguish each dimer structure. Associating interactions in pure acetic acid gas and liquid phase at variable temperature and pressure, and in acetic acid-crotonaldehyde solution at different concentration have been simulated using MD method. Through the RDF analysis, distributions of various dimers at different conditions were obtained. For pure acetic acid molecules in the gas phase, the association proportion decreases with the increase of temperature and pressure. When the temperature is higher than 503.15 K or the pressure is larger than 100 kPa, the proportion of the associating molecules achieves equilibrium. Through the RDF investigation in acetic acid gas, dimer A proved to be the most stable structure with the proportion of 32%-40%. The distribution varies regularly change with temperature and pressure. With regards to the liquid phase, linear chain form is found to be the main configuration in the liquid phase. The acetic acid molecules are connected by O1-Ho and O1-Hc hydrogen bond. Associating molecules varied little with temperature and pressure. For acetic acid-crotonaldehyde solution at 303.15 K and 100 kPa, with acetic acid concentration decreasing, the main association structure is transformed from linear chain form into cyclic dimer. Quantum chemistry results also give the same results that the chain structure will be broken when the crotonaldehyde molecules are increasing. And the transformation mainly happens at the acetic acid concentration of 80%. Acknowledgement The authors are grateful for the financial support from the National Natural Science Foundation of China (21406156). We also thank Professor Lichang Wang for her help with the Gaussian 09 package.

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Theoretical Study of Acetic Acid Association Based on Hydrogen

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