DFT Study of the Energetic and Noncovalent Interactions between

Article pubs.acs.org/JPCB

DFT Study of the Energetic and Noncovalent Interactions between Imidazolium Ionic Liquids and Hydrofluoric Acid Marco V. Velarde,† Marco Gallo,*,† P. A. Alonso,† A. D. Miranda,‡ and J. M. Dominguez‡ †

Facultad de Ciencias Quı ́micas, Universidad Autónoma de San Luis Potosı ́ (UASLP), Av. Manuel Nava No. 6, Zona Universitaria San Luis Potosı ́, San Luis Potosı ́ 78210, México ‡ Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas Norte 152, C.P. 07730 Distrito Federal, México S Supporting Information *

ABSTRACT: In this work, we evaluated the energetic interactions between imidazolium ionic liquids (ILs) and hydrofluoric acid, as well as the cation−anion interactions in ILs. We used DFT calculations that include dispersion corrections employing the PBE and M06 functionals. We tested 22 ILs, including [C4MIM][PF6], [C4MIM][NTf2], and [C4MIM][CH3COO], obtaining interaction energies in the range of −27 to −13 kcal/mol with the PBE functional. The NCI (noncovalent interaction) index developed by Yang and collaborators (J. Am. Chem. Soc. 2010, 132, 6498−6506; J. Chem. Theory Comput. 2011, 7, 625−632) also was used for mapping the key noncovalent interactions (hydrogen bonds, van der Waals, and steric repulsions) between the anions and cations of ILs and also for interactions of ILs with hydrofluoric acid (HF). The results obtained show that the anions have a stronger effect with respect to cations in their capacity for interacting with hydrofluoric acid, and the strongest interaction energies occur in systems where the key noncovalent interactions are mainly hydrogen bonds. The [C4MIM][PF6], [C4MIM][NTf2], and [C4MIM][BF4] ionic liquids displayed the weakest cation−anion interactions.

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INTRODUCTION ILs (ILs) have been the subject of scientific interest due to their unique properties, such as having melting points under 100 °C, low vapor pressures, good thermal stabilities, and favorable solvating behaviors for a wide range of polar and nonpolar compounds.1,2 ILs often are labeled as green solvents due to their low volatility behavior.1,2 The properties of ILs can be fine-tuned for specific applications by changing their ionic components.3 Common ions present in ILs are bis((trifluoromethyl)sulfonyl)imide (NTf2), hexafluorophosphate (PF6), tetrafluoroborate (BF4), carboxylates, pyridine, and imidazolium. These ILs have different side chains, each combination resulting in ILs with different thermodynamic, catalytic, and transport properties.3 As the number of combinations between the ions forming ILs increases, experimental testing becomes unfeasible and impractical. On the basis of these limitations, computational studies are important for understanding key energetic interactions and, in general, the behavior of ILs in solution. The group of Professor Maginn et al.1,4 was among the first in carrying molecular simulations in ILs using an all-atom force field. They studied the [C4MIM][PF6] IL; from the radial distribution functions, they noticed the existence of a longrange order in the liquid due to the electrostatic interactions. They noticed that the anion orients close to the C2 carbon of the imidazolium ring. Chaban et al.5 considered ILs the holy grail in material science because of their increasing range of applications. © XXXX American Chemical Society

Hydrogen bonds are known to be of great importance in IL interactions and were the subject of recent studies.1,6−8 In conjunction with noncovalent interactions, they are crucial for understanding the behavior of ILs as solvents. Szefczyk et al.9 studied the components of the intermolecular interaction energy in the [BMIM][PF6] IL using the Hybrid Variation-Perturbation Theory. This study was performed on configurations obtained from molecular dynamics simulations. From the calculated components of the interaction energy, the electrostatics was considered as an important energetic factor. The extraction of a polar compound from a nonpolar solution is a specific engineering problem, as in the case of hydrofluoric acid in oils in the petroleum industry. Alkylation is a common process used in oil refineries to produce high-octane gasoline from isoparaffin olefins.10,11 This process uses sulfuric or hydrofluoric acid as the catalyst, leaving traces of sulfur or fluorine in the final product.10,11 Residual concentrations of hydrofluoric acid are enough to cause severe corrosion in pipe lines, storage tanks, containers, and valves.10−12 In addition, hydrofluoric acid (HF) may cause health problems and contamination postcombustion.10,11 These problems have motivated the search of HF scavenger agents. Because of their strong ionic interaction with polar compounds, the nature and quantification of the molecular interactions between ILs Received: January 8, 2015 Revised: March 21, 2015

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DOI: 10.1021/acs.jpcb.5b00229 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

between solutes and ILs. These authors also indicated that in CO2 and SO2, the interactions between the gas and anion are larger than gas−cation interactions. Maggin et al.20 measured the solubility of gases benzene, carbon dioxide, nitrous oxide, ethylene, ethane, oxygen, and carbon monoxide in 1-n-butyl-3-methylimidazolium tetrafluoroborate and 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. These authors observed that benzene, carbon dioxide, and nitrous oxide presented strong interactions with the ILs and also large solubilities. These authors noticed a large CO2 absorption capacity in ILs with the bis(trifluoromethylsulfonyl)imide anion. The CO2 absorption capacity was independent of the cation used (imidazolium, pyrrolidinium, or tetraalkylammonium). Recently Maginn et al.,21 using molecular simulations, studied the solubility and dynamics of water in ILs that react with CO2. The cation tetrabutylphosphonium was combined with different aprotic heterocyclic anions. They noticed the existence of correlations between thermodynamic properties and the strength of interaction between water and anions. The calculated Henry’s Law constant revealed that water has a higher solubility in [P4444][2-CNpyr-CO2] than in [P4444][2CNpyr] because of the stronger interaction between water and the [2-CNpyr-CO2] anion. Recently, Chen et al.22 published a review of the gas solubility in ILs. This review included solubility information on the following gases such as CO2, SO2, CO, N2, O2, H2, H2S, N2O, CH4, Ar, Kr, Xe, hydrofluorocarbons, NH3, and H2O (water). They mentioned that solubility information for mixed gases in ILs is very rare. This information is necessary in developing chemical processes because the gas solubility can change dramatically due to the presence of other gases. Chen et al.22 mentioned that even with the limited solubility data reported today, ILs present high solubility for certain individual gases and are capable of dissolving certain gas mixtures. They suggested that in the future, ILs would replace volatile organic solvents without the need to introduce significant changes in the flow diagram and equipment in the chemical process industry. Jalili et al.23 obtained experimentally the gaseous solubilities of carbon dioxide, hydrogen sulfide, and their binary mixture in the IL 1-octyl-3-methylimidazolium bis(trifluoromethyl)sulfonylimide ([C8mim][Tf2N]). They noticed that the solubility of H2S is around two times higher than that of CO2. They also performed quantum calculations using DFT/ B3LYP with 6-311+G* and 6- 311++G(2d,2p) basis sets. These calculations were carried under vacuum conditions. They noticed that the energetic interaction of H2S with the anion and cation of the IL is higher than that with CO2, leading to a conclusion that the high solubility of H2S in [Cnmim][Tf2N] ILs is due to stronger interaction of H2S with the [Tf2N] anion in comparison to CO2. It has been noted by Costa Gomes et al.24 that theoretical studies of interaction energies between a solute and a solvent in vacuum only provide limited information concerning the interactions in the condensed phase because vacuum studies do not take into consideration the solvent−solvent and solute− solute interactions. To our knowledge, the best strategy to determine the gas solubility in the condensed phase is obtained by calculating the free energy of solvation. Free-energy perturbation techniques in conjunction with molecular dynamics simulations can be used to obtain the free energy of solvation.25 After obtaining the free

and HF need to be elucidated (energetic contribution to solubility). Prasad et al.13 performed ab initio calculations to study the energetic interactions between ILs and SO2, CO2, and N2, indicating that the interactions of the gases with the anion were stronger than those with the cation in polar gases. They also stated that molecular interactions are important to determine the gas solubility, but other factors such as the free volume need to be considered in the quantification of gas solubility. Costa Gomes et al.14 in conjunction with experimental measurements performed molecular simulations to study the solubility of fluorinated gases in trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide IL and to elucidate the solvation behavior at the atomistic level. Other works on this topic have been reported. For example, Pomelli et al. 15 showed the potential of ILs in the desulfurization of oils, finding a similar ordering between the experimental solubilites and the calculated interaction energies of the H2S−anion complexes. These complexes did not include the cation of the ILs. Szefczyk et al.9 indicated that the study of interactions between isolated ions is limited because of the absence of dynamic and entropic effects of the liquid phase. However, for some ILs, these authors mentioned the possibility of relating experimental thermodynamic properties with molecular properties obtained from ab initio calculations. Damas et al.16 studied the interactions between gases (CO2, SO2, and H2S) with the anion and cation from ILs and also the interactions between a cation and anion. Their calculation were based on isolated ions using two different functionals, the B3LYP/6-311+G** level of theory and the M06-2X functional that contains dispersion effects. They observed that the cation− anion interaction for the C1mim−anion series followed this trend: [C1mim][Tf2N] < [C1mim][PF6] < [C1mim][BF4], with a binding energy above −300 kJ/mol. They indicated that high CO2 solubility in [C4MIM][Tf2N] is caused by the weakening of the cation−anion interaction. This weakening increases the free volume in the ILs, providing more space to absorb gases (entropic contribution to solubility). However, Costa et al.17 observed in their molecular dynamics simulations in [C2mim][Tf2N] that the free volume plays a secondary role in the absorption of CO2 in the IL. They placed stronger importance on thermodynamic variables than the use of free volume. Damas et al.16 emphasized in their calculations that the energetic contribution to the solubility cannot be neglected; they showed that anion−gas interactions are larger than cation−gas ones in the case of CO2, SO2, and H2S gases. Ghobadi et al.18 studied the solubility of sulfur dioxide (SO2) and carbon dioxide (CO2) in imidazolium ILs using NPT Monte Carlo simulations. They calculated the gas solubility using the Widom particle insertion method. These authors proposed a gas solubility index in the IL defined as the quotient of the solute−IL interaction over the cation−anion interaction energy density. Strong solute−IL interactions result in higher solubility (high numerator). If the cation−anion interaction in the ions is weak (small denominator in the solubility index), the gas solubility increases. Shi et al.19 studied the solubility of SO2, CO2, N2, O2, and some of their mixtures in the IL 1-hexyl-3-methylimidazolium bis(trifluoromethyl)sulfonylimide ([hmim][Tf2 N]) using Monte Carlo simulations. Their results indicated higher van der Waals interaction with respect to electrostatic interaction B


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The DFT geometries calculated using the M0635 functional were carried in the Gaussian 09W software36 using the 6311+G(d,p) basis set, starting from the PBE-TS optimized structures and employing the counterpoise method37 for the Gaussian basis sets’ superposition error. Wave functions were obtained by using the Gaussian 09W software36 in the ground state using DFT calculations with the M06 functional and employing the 6-311+G(d,p) basis set. Results from the wave function were used as input in the NCIPLOT30 program to map the noncovalent interaction surfaces, and these surfaces were visualized with the VMD software package.38 It is known that the PBE functional has problems in obtaining accurate values for the interaction energy in systems with strong noncovalent interactions caused by dispersion.35 Yang et al.29,30 mentioned that the lack of dispersion effects in some functionals affects only the energy calculation, while the electronic density used in its NCI index is unaffected. To improve the energy values, the Tkatchenko and Scheffler28 correction for dispersion was included in the PBE functional (PBE-TS). Also, the M0635 functional that includes built-in dispersion effects was tested.

energy of solvation, other thermodynamics properties such as the enthalpy of solvation and entropy of solvation can be obtained using thermodynamics calculus.14,26 We emphasize that in the absence of free energy of solvation values, the energy of interaction between a solute and a solvent (energetic component), along with cation−anion interactions (entropic component16), can be used with limitations as reference parameters to infer the solubility order for a given solute in different solvents. In this work, we first calculated the interaction energy between the complete ILs (anion and cation) with H2S using DFT with the PBE27 functional that includes the Tkatchenko and Scheffler28 corrections for dispersion (PBE-TS) and DNP+ (double numerical basis sets that include polarization and diffusion functions). We observed that the interaction energies obtained for these systems followed a trend similar to the experimental solubilities,15 as shown in Table 1. Table 1. Solubilities and Interaction Energies between ILs and H2S

1-butyl-3-methylimidazolium chlorine 1-butyl-3-methylimidazolium tetrafluoroborate 1-butyl-3-methylimidazolium bis((trifluoromethyl) sulfonyl)imide 1-butyl-3-methylimidazolium hexafluorophosphate a

solubility (mole fraction)a

interaction energy (kcal/mol)

[C4MIM] [Cl] [C4MIM] [BF4] [C4MIM] [NTf2]

0.86 ± 0.03

−14.56

0.79 ± 0.02

−9.12

0.77 ± 0.03

−8.63

[C4MIM] [PF6]

0.72 ± 0.02

−8.61

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RESULTS AND DISCUSSION Twenty-two ILs were evaluated using PBE-TS and M06 functionals in order to determine their energetic interactions with HF. These ILs were studied based on their capabilities to remove contaminant gases.10,11,22,39 Experimental studies showed the capability of ILs in the removal of traces of hydrofluoric acid and organofluorinated compounds from alkylated gasolines.10,11,39 Also Chen et al.22 presented a review of the ILs capabilities to remove gases such as CO2, SO2, CO, N2, O2, H2, H2S, N2O, CH4, Ar, Kr, Xe, hydrofluorocarbons, NH3, and so forth. The energetic interactions obtained are shown in Table 2. The strongest interaction energy with hydrofluoric acid using the PBE-TS functional is found in the ILs having trifluoroacetate and carboxylate anions. In contrast, the [PhCH2MIM] cation in conjunction with the [PF6] and the [C4MIM] cation with the [NTf2] anion have the lowest interaction energy with hydrofluoric acid when using the PBE-TS functional. As observed, the different side chains in the IL cations have a smaller effect in the interaction energy. The systems tested were composed of only one IL molecule and one hydrofluoric acid molecule. The largest differences in the interaction energies between the M06 and PBE-TS functionals occur in the [C4MIM][CF3COO] and [C4MIM][C3F7COO] ILs, with differences of 8.84 and 8.37 kcal/mol, respectively. The energetic interactions between the cation and anions in the ILs using the PBE-TS with DNP+ basis sets are shown in Table 3. The weakest cation−anion interactions (entropic contributions16) are present in the [C4MIM] cation along with the [PF6], [BF4], and [NTf2] anions. If the entropic contribution16 dominates the gas solubility, these three ILs would be excellent solvent candidates for HF removal. On the other hand, if the energetic contribution dominates the solubility, the [C4MIM] cation with the [CH3COO] and [CF3COO] anions would be the best solvent candidates for HF removal. The results from the optimized geometries indicate that the hydrofluoric acid tends to be positioned between the cation and anion. The hydrogen in the acid follows the anion, while the

Conditions: 25 °C and 1400 kPa.15

The interaction energy between ILs and hydrofluoric acid was obtained from DFT calculations. The NCI index29,30 was used to gain insight on the type of IL interactions with hydrofluoric acid. The magnitude of the energetic interaction between two molecules is found as the difference in energy of the combined and isolated molecules and is used as a reference parameter for the strength of the interaction between an IL and the hydrofluoric acid. The cation−anion interactions also were calculated in order to estimate the entropic contribution16 to the gas solubility in ILs. The NCI index29,30 used to map the noncovalent interactions in a molecular system is based on the electron density (ρ) and its reduced gradient (s). In order to differentiate between repulsive and attractive interactions, the NCI index uses the sign of the second eigenvalue λ of the electron density Hessian (sin(λ2)ρ).

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COMPUTATIONAL METHODOLOGY Geometry optimizations, including the calculation of vibrational frequencies, were performed using the DMOL331,32 module included in the Accelrys Material Studio 6.1 software package.33 The DFT calculations included GGA and all-electron core treatment with the PBE-TS27,28 functional that includes the Tkatchenko and Scheffler28 corrections for dispersion. DNP+ numerical bases similar to the 6-311+G(d,p) basis set were employed. These numerical basis sets have a smaller superposition error than Gaussian basis sets.34 Around 10 different starting geometries were used to find the global minimum for each system, C


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The Journal of Physical Chemistry B Table 2. Interaction Energies (kcal/mol) between ILs and HF 1-octyl-2,3-dimethylimidazolium acetate 1-octyl-3-methylimidazolium acetate 1-butyl-3-methylimidazolium butanoate 1-butyl-3-methylimidazolium acetate 1-butyl-3-methylimidazolium trifluoroacetate 1-vinyl-3-butylimidazolium trifluoroacetate 1-butyl-3-methylimidazolium benzoate 1-butyl-3-methylimidazolium heptafluorobutanoate 1-propenyl-3-butylimidazolium trifluoroacetate 1-butyl-3-methylimidazolium tetrafluoroborate 4-methyl-N-butylpyridinium tetrafluoroborate 1-benzyl-3-methylimidazolium hexafluorophosphate 1-octyl-3-methylimidazolium tetrafluoroborate 1-propenyl-3-butylimidazolium bis((trifluoromethyl)sulfonyl) imide 1-butyl-3-methylimidazolium hexafluorophosphate 1-benzyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl) imide 1-butyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl) imide 1-vinyl-3-butylimidazolium bis((trifluoromethyl)sulfonyl) imide 1-vinyl-3-butylimidazolium hexafluorophosphate

[ODMIM] [CH3COO] [C8MIM] [CH3COO] [C4MIM] [C3H7COO] [C4MIM] [CH3COO] [C4MIM] [CF3COO] [VBIM] [CF3COO] [C4MIM][ba]

PBE-TS

M06

−26.29

−21.49

−26.38

−20.17

−25.03

−19.96

−26.07

−19.13

−27.66

−18.81

−23.88

−21.26

−26.09

−20.10

[C4MIM] [C3F7COO] [PropBIM] [CF3COO] [C4MIM][BF4]

−22.32

−13.94

−21.74

−20.11

−16.12

−12.70

[C4M-py][BF4]

−18.41

−15.11

[PhCH2MIM] [PF6] [C8MIM][BF4]

−12.99

−7.62

−15.81

−12.00

[PropBIM] [NTf2]

−17.93

−12.11

[C4MIM][PF6]

−14.57

−10.64

[PhCH2MIM] [NTf2]

−14.81

−9.57

[C4MIM] [NTf2]

−13.35

−11.05

[VBIM][NTf2]

−16.30

−12.10

[VBIM][PF6]

−14.42

−10.75

Table 3. Interaction Energies (kcal/mol) between Cations and Anions in ILs with a ±1 Charge PBE-TS [ODMIM][CH3COO] [C8MIM][CH3COO] [C4MIM][C3H7COO] [C4MIM][CH3COO] [C4MIM][CF3COO] [VBIM][CF3COO] [C4MIM][ba] [C4MIM][C3F7COO] [PropBIM][CF3COO] [C4MIM][BF4] [C4M-py][BF4] [PhCH2MIM][PF6] [C8MIM][BF4] [PropBIM][NTf2] [C4MIM][PF6] [PhCH2MIM][NTf2] [C4MIM][NTf2] [VBIM][NTf2] [VBIM][PF6]

−137.06 −136.85 −118.10 −116.15 −109.94 −124.71 −116.18 −108.25 −129.56 −93.78 −103.72 −101.90 −114.01 −103.70 −82.77 −105.60 −90.19 −104.15 −99.96

The strong hydrogen bond shown in Figure 1a loses strength (as shown in Figure 1b) to favor the appearance of new hydrogen bonds. Figure 2a for IL [C4MIM][BF4] (medium to weak interaction energy with HF) shows that most of the interactions between the cation and anion are van der Waals surfaces. Adding the HF acid breaks the van der Waals surface and creates two hydrogen bonds, one hydrogen bond (electronic density of 0.048 au) between the anion and the hydrogen in the hydrofluoric acid and a second low electronic density (0.023 au) hydrogen bond between the fluor in the anion and the hydrogen from the imidazolium ring in the cation. In Figure 3a for IL [PhCH2MIM][NTf2] (very weak interaction energy with HF), we observed a van der Waals surface larger than the one shown in Figure 2a between the anion and the cation. The main interaction in this IL is a large van der Waals surface shown in Figure 3a that becomes considerably smaller (as shown in Figure 3b) in order to fit the HF into the structure. Adding the HF also breaks the van der Waals surface and creates a hydrogen bond between the oxygen and the hydrogen in the acid. In addition, a new hydrogen bond appears between the nitrogen in the anion and the hydrogen in the imidazolium ring cation. In Figure 1, we observed very small van der Waals surfaces, compared to the larger van der Waals surfaces present in Figures 2 and 3. Evaluating by electronic density, we found that the hydrogen bond with a 0.07 au density value in the carboxylate anion shown in Figure 1b is stronger than the hydrogen bond in the [BF4] anion shown in Figure 2b with an electron density value of 0.048 au. It also is stronger than the hydrogen bonds in the [PF6] and [NTf2] anions with an electron density value of around 0.04 au. This strong hydrogen bond, taken with the other hydrogen bonds that appeared when introducing HF, creates an energetic advantage for the carboxylate anion in comparison to the other anionsa higher interaction energy. Table 4 shows the difference in length for the hydrogen bond formed between the hydrogen from the HF acid and the IL anion, using two different functionals. The greatest difference in hydrogen bond length (0.173 Å) between the two functionals is

fluor in the acid interacts with parts of the imidazolium ring with a strong preference for the hydrogen atoms in the imidazolium ring and hydrogen atoms from the side chains close to the imidazolium ring. Rai and Maginn40 also identified this zone in the imidazolium ring as possible interaction sites. Comparison of the geometrical structures and the noncovalent interaction surfaces for ILs, with and without the HF acid obtained with the NCI index, is displayed in Figures 1−3 for three different interaction energies in this order: strong, medium-weak, and very weak. These figures indicate the appearance of hydrogen bonds between the acid and the anion in the ILs. In Figure 1a for IL [C 4 MIM][C 3 H 7 COO] (strong interaction energy with HF), we observed a strong hydrogen bond between the hydrogen in the imidazolium ring and the oxygen in the anion. Furthermore, there are hydrogen bonds present between the hydrogen atoms in the side chains and the oxygen atoms in the anion. Introducing the HF into the system causes the appearance of new hydrogen bonds between the hydrogen in the acid and the oxygen in the anion and between the fluor in the acid and the hydrogen from the methyl group in the cation. D


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Figure 2. Optimized structures and NCI surfaces for (a) [C4MIM][BF4] and (b) [C4MIM][BF4] + HF. The surfaces are displayed on a blue−green−red scale with respect to the values of sin(λ2)ρ, ranging from −0.08 to 0.08 au. The strongest attractions related to hydrogen bonds are displayed in blue, weak interactions such as van der Waals forces are displayed in light green, and steric repulsions are shown in red. The gradient isosurfaces with s = 0.5 au carbon atoms are displayed in light blue, nitrogen atoms are in dark blue, oxygen atoms are in red, fluor atoms are in pink, hydrogen atoms are in white, and the boron atom is in gray.

Figure 1. Optimized structures and NCI surfaces for (a) [C4MIM][C3H7COO] and (b) [C4MIM][C3H7COO] + HF. The surfaces are displayed on a blue−green−red scale with respect to the values of sin(λ2)ρ, ranging from −0.1 to 0.10 au. The strongest attractions related to hydrogen bonds are displayed in blue; weak interactions such as van der Waals are displayed in light green color, and steric repulsions are displayed in red. The gradient isosurfaces with s = 0.5 au carbon atoms are displayed in light blue, nitrogen atoms are in dark blue, oxygen atoms are in red, fluor atoms are in pink, and hydrogen atoms are in white.

The interaction energy for [C4MIM][CH3COO] obtained with the M06 functional is −19.13 kcal/mol; using the same anion with the [C4M-py] cation, the interaction energy is −23.99 kcal/mol. The interaction energies differ from 3 to 4 kcal/mol when changing the cation from imidazolium to pyridinium in the systems presented in Table 6, compared to differences in energies as high as 8−9 kcal/mol when changing some of the anions.

observed in the [ODMIM][CH3COO] IL, while the difference in the interaction energy is 4.80 kcal/mol. The geometries for the ILs systems optimized with both functionals are in general very similar. A path to full comparison of all bond lengths and angle measurements for ILs [C 8 MMIM][CH 3 COO], [PhCH2MIM][NTf2], and [C4MIM][PF6] can be found in the Supporting Information of this paper. Table 5 shows the difference in length between the hydrogen in the HF acid and the closest atom in the IL cation. As observed, these distances are larger than the HF distance with the IL anion. The solute is closer to the anion than the cation in the ILs studied. In order to evaluate the effect of the cation compared to the anion in the energetic interactions with HF, we used cations based on pyridinium in conjunction with some of the anions listed in Table 2. The interactions energies are presented in Table 6.

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CONCLUSIONS The anion in the ILs seems to be a key factor in the energetic interactions of HF with the IL as promising solvent candidates for extraction of hydrofluoric acid. This leaves the cation free to be chosen, considering other process conditions (e.g., viscosity, extraction cycles, IL cost). This is in agreement with other studies that found anions as important factors in the IL interactions with small molecules.7,13,16,19−21,41 Chaban et al.5 indicated the existence of studies that showed an insignificant solubility increase of the CO2 and SO2 gases with the alkyl E


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Table 5. Comparison in the Cation−HF Bond Length (Å) between Two Functionals [ODMIM][CH3COO] [C8MIM][CH3COO] [C4MIM][C3H7COO] [C4MIM][CH3COO] [C4MIM][CF3COO] [VBIM][CF3COO] [C4MIM][ba] [C4MIM][C3F7COO] [PropBIM][CF3COO] [C4MIM][BF4] [C4M-py][BF4] [PhCH2MIM][PF6] [C8MIM][BF4] [PropBIM][NTf2] [C4MIM][PF6] [PhCH2MIM][NTf2] [C4MIM][NTf2] [VBIM][NTf2] [VBIM][PF6]

Figure 3. Optimized structures and NCI surfaces for (a) [PhCH2MIM][NTf2] and (b) [PhCH2MIM][NTf2] + HF. The surfaces are displayed on a blue−green−red scale with respect to the values of sin(λ2)ρ, ranging from −0.08 to 0.08 au. The strongest attractions related to hydrogen bonds are displayed in blue, weak interactions such as van der Waals are displayed in light green, and steric repulsions are shown in red. The gradient isosurfaces with s = 0.5 au carbon atoms are displayed in light blue, nitrogen atoms are in dark blue, oxygen atoms are in red, fluor atoms are in pink, hydrogen atoms are in white, and sulfur atoms are displayed in yellow.

PBE-TS

M06

1.289 1.401 1.410 1.410 1.424 1.489 1.391 1.495 1.422 1.524 1.514 1.579 1.465 1.554 1.607 1.630 1.556 1.610 1.614

1.462 1.479 1.505 1.495 1.480 1.455 1.424 1.584 1.497 1.616 1.568 1.617 1.557 1.662 1.638 1.689 1.615 1.680 1.635

M06

2.326 2.077 2.047 2.078 2.508 2.176 2.843 2.228 2.814 2.945 2.907 2.185 2.111 2.010 3.119 2.681 2.017 3.027 2.984

2.501 2.106 2.098 2.137 2.261 2.604 2.690 2.293 2.048 2.892 2.981 2.370 2.051 2.236 2.959 2.949 2.076 2.295 2.856

Table 6. Interaction Energies (kcal/mol) between Pyridinium-Based ILs and HF M06 4-methyl-N-butylpyridinium acetate 4-methyl-N-butylpyridinium chlorine 4-methyl-N-butylpyridinium tetrafluoroborate 4-methyl-N-butylpyridinium bis((trifluoromethyl)sulfonyl)imide

Table 4. Comparison in the Anion−HF Hydrogen Bond Length (Å) between Two Functionals [ODMIM][CH3COO] [C8MIM][CH3COO] [C4MIM][C3H7COO] [C4MIM][CH3COO] [C4MIM][CF3COO] [VBIM][CF3COO] [C4MIM][ba] [C4MIM][C3F7COO] [PropBIM][CF3COO] [C4MIM][BF4] [C4M-py][BF4] [PhCH2MIM][PF6] [C8MIM][BF4] [PropBIM][NTf2] [C4MIM][PF6] [PhCH2MIM][NTf2] [C4MIM][NTf2] [VBIM][NTf2] [VBIM][PF6]

PBE-TS

[C4M-py] [CH3COO] [C4M-py][Cl] [C4M-py][BF4] [C4M-py] [NTf2]

−23.99 −19.30 −15.11 −14.35

ILs, in the event that the energetic interaction dominates the solubility. On the other hand, if the entropic contribution16 dominates the solubility, the preferred solvents would be the [C4MIM] cation with the [PF6], [BF4], and [NTf2] anions due to the presence of the weakest cation−anion interactions. The cation−anion interaction strength16 is responsible for generating voids in the IL structure, increasing the gas solubility. It is important to emphasize that the calculations in this work using only one IL molecule are limited because they do not take into consideration the effect of the whole IL medium (condensed phase). Dong et al.8 found that five ionic pairs are necessary to reproduce the whole IR spectra for [Emim][BF4] and [Emim][PF6] ILs in the condensed phase. The effect of the IL−IL interactions in the liquid phase and their influence in the interaction energy with HF and entropic effects are important factors that need to be studied in order to obtain the free energy of solvation and the solubility of solutes in ILs.

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ASSOCIATED CONTENT

S Supporting Information *

Comparisons of bond lengths, angles, and dihedrals for ILs [C8MMIM][CH3COO], [PhCH2MIM][NTf2], and [C4MIM][PF6]. This material is available free of charge via the Internet at http://pubs.acs.org.

chain length in the following ILs: [C4C1IM][BF4], [C6C1IM][BF4], [C8C1IM][BF4], [C2C1IM][TFSI], [C4C1IM][TFSI], [C 6 C 1 IM][TFSI], [C 8 C 1 IM][TFSI], [C 10 C 1 IM][TFSI], [C2C1IM][PF6], [C4C1IM][PF6], and [C6C1IM][PF6] . Several and strong hydrogen bonds are also the key in finding the best solvent for the extraction of hydrofluoric acid in

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: 52-444-826-2300. Fax: 52-444-826-2300. F


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The Journal of Physical Chemistry B Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We are grateful to Prof. Weitao Yang for providing the NCIPLOT software used in this work. M.V.V. acknowledges a graduate fellowship from CONACYT. M.G. acknowledges the financial support from the CONACYT-SENER 130363 and the CONACyT-CB-180071 grants.

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DFT Study of the Energetic and Noncovalent Interactions between

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