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

How Meaningful is the Halogen Bonding in 1-Ethyl-3Methyl Imidazolium Based Ionic Liquids for CO Capture? 2

Lucas Lodeiro, Renato Contreras, and Rodrigo Ormazabal-Toledo J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b04990 • Publication Date (Web): 23 Jul 2018 Downloaded from http://pubs.acs.org on July 23, 2018

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The Journal of Physical Chemistry

How Meaningful is the Halogen Bonding in 1-Ethyl3-Methyl Imidazolium Based Ionic Liquids for CO2 Capture? Lucas Lodeiro,a Renato Contreras,a Rodrigo Ormazábal-Toledo*b a

Departamento de Química, Facultad de Ciencias, Universidad de Chile, Las Palmeras 3425,

Casilla 653, Santiago de Chile. bCentro Integrativo de Biología y Química Aplicada (CIBQA), Universidad Bernardo O Higgins, Santiago 8370854, Chile

ACS Paragon Plus Environment

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ABSTRACT. We report on several parameters that can be used to describe the 1-ethyl-3-methyl-4,5(X2)imidazolium cations (where X=H, Br and I) within the Canongia-Lopez and Padua Force Field (CL&P) framework. Geometrical parameters like intramolecular distances and Radial Distribution Functions are close to the experimental structure. Density values obtained with our force field are within the expected ones from CL&P calculations in related systems. This information is used to simulate through molecular dynamics the solubilization of CO2 by these ILs. For pure ILs, the addition of halides in position 4 and 5 promotes an enhanced hydrogen bond interaction at position 2 with the oxygen atoms in the anion. It is found that CO2 should be in the interstices of the anion-cation 3D network with longer distances than those found in other reports at ab-initio levels, suggesting that halogen bond, if present, may be not the driving force interaction in these systems. Therefore, it seems that CO2 interacts linearly via an oxygen atom with the cation and with the anion through a π-stacking or hydrogen bonded fashions. Solvation enthalpies compare well with the experimentally data, thereby suggesting that halogenated ILs dissolves more efficiently CO2 than C2C1Im+ derivatives. This result suggests that halogenated ILs can be considered as reliable candidates for CO2 capture.


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INTRODUCTION Human activity is, nowadays, the most important source of contaminants. Particularly, control of greenhouse gases emission coming from excessive industrialization of the human activity, is one of the main goals within public policies. For instance, in order to control part of this scenario, a key step for CO2 recovering and reconversion is the trapping of CO2 prior to its recycling.

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Physical and chemical capture of CO2 has become a field of increasing activity.

However, the poor reactivity of CO2 difficult its interaction towards several species used as solvent, nucleophiles, etc. Commonly, CO2 must be activated by using electrochemical over potentials5,

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or expensive catalysts.7 Nevertheless, considering the pioneer work of Dupont,8

followed by part of us,9 the use of ionic liquids may be considered as a safely and green route to activate CO2 for subsequent use in organic synthesis.10-12 In this sense, ionic liquids (ILs) have attracted great interest in different fields of chemistry during the last three decades.13-15 For instance, their dual role as reaction media and potential catalysts in several reactions at room temperature was a starting point for different works in other fields.13-18 ILs are stable salts in the liquid phase near room temperature due to their low symmetry. They are normally formed by a high organic cation (imidazolium, pyridinium and others) and an organic or inorganic anion (tetrafluoroborate, hexafluorophosphate, acetate, etc). Its high combinatorial flexibility provides a significant advantage for “designing” different ILs for a specific task, allowing adjusting its properties for same process.8, 11, 19, 20 Nevertheless, this combinatorial flexibility is at the same time a high disadvantage because it is almost impossible to perform a complete analysis of all possible combinations. At this point, theory may contribute to understand the main interactions of ionic liquids that may be responsible for their properties. Different studies based on the use of one or two ionic


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pairs have been made to understand specific interactions in ILs.7, 9, 21-24 However, omitting the bulk properties of ILs one cannot obtain results that may be directly contrasted with experiments. This difficulty may be avoided by modelling ILs explicitly in a less complex level by using molecular dynamics. Molecular dynamics of ILs have been used to understand the main interactions and distribution in the IL+CO2 systems as well as IL+SO2 systems to understand the selectivity of this gases towards different ILs.25-32 Commonly, in ILs some chemical properties have been attributed to the anion (a so-called anion effect). For instance, ILs based on the [NTf2]– (bistrifluoromethylsulfonylimide anion) displayed an enhanced ability to dissolve CO2 compared to other anions having a marginal effect any modification in the alkyl chain on the cation.31 Moreover, the [C2C1Im][NTf2] IL displays the less viscosity and the less ability to dissolve water within a wide range of ILs, which makes it a good choice for CO2 recovering.20 Imidazolium based ILs are probably the most studied family in the past decade and it is well-known its ability to donate hydrogen bonds through the acidic site in carbon 2. This acidity may be modulated by adding some pattern substitution over the imidazolium ring or the side chains. In the last years, different authors synthetized different [C2C1Im] ILs with the [NTf2] counterion presenting different patterns in the imidazolium ring.3335

For instance, imidazolium rings were obtained by substitution at position 4 and 5 by bromine

and iodine. A complete physicochemical characterization of these new materials was done, providing evidence of halogen bonding within its crystal structure.36 Halogen bonding (XB) is an interesting non-covalent interaction introduced theoretically several years ago to explain some experimental evidence. For instance, XB is a specific interaction between halogen atoms acting as electrophilic centers towards nucleophilic centers.37, 38

This interaction is based on the existence of an anisotropic distribution in the map of


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electrostatic potential of the halogen atom. This anisotropy generates a positive charged region, called σ-hole, which may interact electrophilically with centers with enhanced negative charge in a donor-acceptor way. It has been found that in charged systems the main contributions to the XB are electrostatic.38, 39 Nowadays, XB is a non-covalent interaction widely studied in crystal engineering, organometallic networks, polymers, catalysis, pharmacology and other fields.37, 38, 40-44

Several authors have studied XB in ionic liquids through ab-initio calculations to stablish the role of this interaction in the potential CO2 capture. In those studies, some cations were used as halogen source. Particularly, the halogenated version of the [C1C1Im]+ cation at positions 2,4,5 of the imidazolium ring showed an enhanced interaction towards a nucleophilic center (between 4 and 5 times stronger) respect to similar neutral systems.39 This novel work allows thinking XB as a design tool for task-specific ionic liquids for CO2 capture. Other authors reported on abinitio studies in halogenated ILs with the [C3C1Im]+ cation brominated in its propyl side-chain, as well as molecular dynamics studies about ILs based on fluoroalkylmethylimidazolium cation on its side-chains to study the interaction with CO2 and other gases. Results obtained do not reveal clearly the importance (if any) of XB in an effective CO2 capture.39, 45, 46 Nevertheless, the use of molecular dynamics for studying the role of halogen bonding in ionic liquids is yet scarce; moreover, the study of CO2 capture and reactivity in these materials is still an open problem. For this reason, this work is devoted to analyzing a family of ILs based on the NTf2 anion together

with

[C2C1Im]+

cation

derivatives.

Particularly,

the

1-ethyl-3-methyl-4,5-

(X2)imidazolium cation (where X=H, Br and I) where used to unravel the role of XB in the interaction towards CO2 and their competition with hydrogen bonds in these materials. To do


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this, an update of the CL&P force field was implemented for a proper description of halogenated cations.

COMPUTATIONAL DETAILS. Force Field Parameters. The CL&P force field developed by Canongia-Lopes and Padua, was chosen for a proper description of Ionic Liquids.25 However, this force field has not been implemented a proper description for halogenated imidazolium rings. The lacking parameters were obtained by scanning over the optimized geometry at the HF/6-31G(d)-PP//MP2-cc-pVTZ-PP level of theory (PP is LanL2DZ pseudopotential used for iodine). The equilibrium bond/angle and oscillator constants were obtained by fitting the energy profile to a harmonic oscillator. A detailed description of the procedures and numerical values of fitting to complete CL&P force field is given as Supporting Information. Molecular Dynamics Simulations. All trajectories were obtained with GROMACS 5.1.4 software.47 In all cases, a simulation box containing a fixed number of particles was obtained with Packmol,48 that was minimized with the steepest descent algorithm in GROMACS. After this, a short simulation in a canonical ensemble (NVT) was carried out during 1 ns with a timestep of 2 fs. Finally, a simulation in the isothermal-isobaric ensemble (NPT) was obtained during 40 ns with a timestep of 2 fs. The last 30 ns were used as production run to obtain statistical information as density and radial or spatial distribution function using Travis program.49 In all cases, charges were refitted with CHelpG method as implemented in Gaussian 09 at the MP2 level together with the cc-pVTZ for C, H, N and Br atoms and the LanL2DZ pseudopotential for I atom.50 In a first step, a simulation of pure


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ILs was obtained to compare computational results with crystallographic data to assess the reliability of the force field. In a second step, a simulation box containing ILs and CO2 in a 0.038 molar fraction was simulated.

This molar fraction corresponds to the mean value of

experimentally obtained CO2 concentration in these ILs.31 RESULTS AND DISCUSSION The set of ILs differs only in its cation, specifically in the 2,4 and 5 substituted derivatives. For this reason, for an easy description of these systems, we introduce the following nomenclature: H, B and I for Hydrogen, Bromine and Iodine, respectively. Then, the HHH, HBB and HII acronyms correspond to the [1-ethyl-3-methyl imidazolium][NTf2], [1-ethyl-3-methyl-4,5dibromoimidazolium][NTf2]

and

[1-ethyl-3-methyl-4,5-diiodoimidazolium][NTf2],

ILs

respectively. Additionally, the suffix -CO2 was introduced to denote those systems with CO2 added. 1. CL&P parameters for halogenated ionic liquids. Parameters corresponding to the halogen positions in the cation were obtained, as well as the parameters for CO2 (see Supporting Information for more details). Particularly, X-CW bonds and X-CW-NA and X-CW-CW angles were refitted. Additionally, the CW-CW bond was also obtained considering that this bond is central in the extra perturbation promoted by the voluminous halogen atoms. For CO2, the C-O bond and the O-C-O angle parameters were also obtained. Figure 1 shows the atom labelling for the halogenated cation and Table 1 and Table 2 a show a list of bonded and non-bonded parameters used in this work.


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Figure 1. Atom labelling from CL&P force field for atoms in the 1-ethyl-3-methyl-4,5dihaloimidazolium cation. Parameters were obtained from energy profiles for each bond and angle (see Figure S1 in Supporting Information). Values obtained were compared with experimental bond lengths and angles reported elsewhere.36,

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As shown, values obtained from our ab-initio calculations

compare well with the experimental ones, as shown in Table 1.

Table 1. Bonded parameters for CO2 and 4,5-X2-emim+ cation Molecule

Bond

r0,experimental (Å)

r0 (Å)

kr (kJ mol-1 Å-2)

CO2

C-O

1.16251

1.170

10704

HBB cation

Br-CW

1.85136

1.844

2240

CW-CW

1.34536

1.395

4700

I-CW

2.04836

2.011

2011

CW-CW

1.36436

1.389

4044

Molecule

Angle

θ0,experimental (degrees)

θ0 (degrees)

kθ (kJ mol-1 rad-2)

CO2

O-C-O

180.051

180.0

454.0

HBB cation

Br-CW-NA

123.036

124.7

852.9

Br-CW-CW

129.636

129.6

788.3

HII cation


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HII cation

I-CW-NA

124.536

123.4

889.9

I-CW-CW

128.636

130.3

837.5

Table 2: Non-bonded parameters for CO2 and 4,5-X2-emim+ cation. Molecule

Atom

σ (Å)

ε (kJ mol-1)

CO2

O

3.050

0.65731

C

2.800

0.22231

HBB cation

Br

3.470

1.96852

HII cation

I

3.750

2.51252

Atomic charges were recalculated for each molecule since the principle of partial charge transferability, is only true within similar molecules.25 Changing from hydrogen to bromine or iodine in the imidazolium ring, substantially changes the electronic density of the system and their atomic charges (see Section 2 in Supporting Information). Additionally, Figure 2 shows a comparison of the map of electrostatic potential for the cations HHH (Figure 2a), HBB (Figure 2b) and HII (Figure 2c). The electropositive zone in the opposite side of the CW-X bond was found for HBB and HII. This zone is more positive for HII than HBB as previously reported by Politzer.37 On the other hand, for HHH the 4,5 region of imidazolium shows a completely positive zone, being the most positive zone the C2 substituent of the molecule. Note also, the change of electron density distribution by changing from HHH to HBB or HII. This response may be traced to the change in electronegativity passing from hydrogen to the halides.


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Figure 2. Map of electrostatic potential for cations (a) HHH, (b) HBB and (c) HII.

2. Validation of Force Field for pure ILs. Simulation of pure HHH, HBB and HII were conducted at 25°C and 90°C to obtain densities that were compared with experimental reports (See Table S1 in Supporting Information). Moreover, radial distribution functions (RDF) were obtained to define intermolecular distances that could be consistent with the crystallographic data. To analyze geometric patterns in the 3D network of HHH, HBB and HII, Figure 3 shows atom-to-atom radial distribution function of cation (atoms 2, 4 and 5) respect to anion (atoms F and O) for HHH, HBB and HII simulated at 25°C.


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Figure 3. Atom-to-atom Radial distribution function of selected atoms in (a) HHH, (b) HBB and (c) HII cations respect to F or O in anion. Solid blue H (position 2 in imidazolium)-F (in NTf2); dashed blue X4,5-F (NTf2); solid red H (position 2 in imidazolium)-O (in NTf2); dashed red X4,5O (NTf2). Positions 4 and 5 in imidazolium ring present similar responses since their RDF are quite similar for different pairs of atoms in the cation-anion interaction for the tree cations. However, position 4 shows higher peaks than position 5. This situation may be traced to the fact that position 5 is closer to the ethyl side-chain promoting a more intense steric effect, hindering the anion approach. For hydrogen atom at position 2 (H2) similar responses may be observed for the three ILs considered, showing a strong interaction with oxygen atoms in NTf2 near 2.5 Å (Figure 3a). Experimentally, H-O distance was found to be 2.5 Å which could be considered as a weak hydrogen bond.36 On the other hand, the interaction with fluorine atoms in NTf2 were found at longer distances (near 5.4 Å), showing a low-intensity shoulder at 2.6 Å. This result reveals that the chemical environment of H2 in imidazolium remains unaltered by substituent in position 4 and 5. For the HHH system H atoms at positions 4 and 5 have peaks at similar distances than H2, but their intensity is clearly different, as shown in Figure 3a. Interaction with oxygen atoms have less intensity, meanwhile with F atoms of NTf2, intensity is more important. This response may be traced to a major presence of oxygen atoms than fluorine atoms near H2 and, on the other


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hand, the vicinity of H4,5 have more presence of fluorine atoms. This observation is in line with values of CHelpG charges obtained for HHH cation and NTf2 anion, because H2 atom is slightly more positive than H4,5 and O is slightly more negative than F in NTf2. For HBB and HII, distributions are quite similar, however positions 4 and 5 have slight differences (Figure 3b and 3c). For instance, distance and peak for HII (Figure 3c) are greater than HBB (Figure 3b) in the correlation towards F and O in NTf2. In HBB peak between Br-O and Br-F atoms appears at 3.3 Å and 3.4 Å, respectively and for HII both peaks appear at 3.3 Å (experimentally, the Br-O, Br-F, I-O and I-F distances are 3.0 Å, 3.3 Å, 3.1 Å and 4.3 Å respectively). This result suggests that interactions obtained by molecular dynamics with the CL&P with Br and I data are pretty like those experimentally found from crystallographic data.36 Comparatively, distances between 4,5-halogen positions and O or F atoms are close to those obtained by ab-initio calculations by other authors.38, 39, 45, 53 An analysis of solvation shells is presented in Figure 4, where RDF of the geometric center of cation and anion and interanion were obtained for the production run at 25°C.

Figure 4. Radial distribution function between geometric centers of cation and anion for (a) HHH, (b) HBB and (c) HII. For the three ILs there exists a clear ionic structures of alternated solvation shells, typically observed in other ILs. This result supports the idea that these materials are not ionic pairs, but


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liquids formed by ions where their interactions are mainly based on coulombic interactions. Considering this result, the study of ionic liquids interactions and their physicochemical responses should be made using a 3D network more than a single ion pair. Most probable distance for the cation-anion are 4.9, 4.8 and 4.8 Å and for anion-anion are 8.3, 8.6 and 8.6 Å for HHH (Figure 4a), HBB (Figure 4b) and HII (Figure 4c), respectively. For HBB and HII, the first peak is thinner and long than in HHH. This result suggests that for HHH IL the cation-anion interaction could be present in several ways and for HBB and HII just some distributions are possible. Nevertheless, the size of bromine and iodine could be also important to justify results in Figure 4. To gain more insights about the degree of aggregation of these materials, spatial distribution function (SDF) was obtained and is presented on Figure 5.

Figure 5. Spatial Distribution Function for cations (a) HHH, (b) HBB and (c) HII. For cations color code is: sky blue for fluorine and red for oxygen in NTf2, respectively. For cations in Figure 5, SDF shows that the interaction with oxygen is established at H2 position (red zones), being less important in HHH (Figure 5a) than in HBB (Figure 5b) or HII (Figure 5c). Nitrogen atom (not shown) displays a tiny interaction near the imidazolium cation. This interaction may be attributed to a displacement promoted by F or O interaction. Fluorine atom interacts with the imidazolium ring and the ethyl side chain. For HBB and HII there exists


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an interaction near halogens and between them that is absent in HHH. In all cases oxygen atoms have more important interactions with hydrogen at position H2, as suggested in the RDF (see Figure 3a-c). This result is natural considering that position H2 is the most positive zone of cation and oxygen and nitrogen present the most negative zone in anion. However, nitrogen is sterically hindered to interact with imidazolium, so its interaction will be less important. Fluorine atoms do not present a characteristic interaction with any site of the cation: they are delocalized around imidazolium. Nevertheless, only F atoms interacts directly with cations, but not in the σhole zone, thereby revealing that XB may be not so important in these systems. For anions in the same system (see Supporting Information), there exists a difference between HHH and HBB or HII. Substituent at position 4 is located near fluorines and substituent at position 5 have no a preferred position. This response may be attributed to the ethyl side chain that prevents the approach between position 5 and the anion. For position 4, this situation is avoided considering a more facile approach to the anion. For all cases, hydrogen in position 2 is highly localized near oxygen atoms, and as discussed previously, competes for fluorine atoms with positions 4 and 5.

3. ILs in interaction with CO2. To test the usefulness of HBB and HII to catch CO2, a system containing CO2:IL at a 0.038 molar fraction was studied. Working temperature was 25°C for HHH and HBB. For HII working temperature was 90°C because at this temperature this IL is liquid. Calculated densities are equally than those obtained for the pure systems: 1.58 g/ml, 1.99 g/ml and 2.17 g/ml for HHHCO2, HBBCO2 and HIICO2, respectively. This is a well-known result in ILs where, in contrast to molecular solvents, density varies slight and smoothly with the addition of a gas. In ILs, interactions are mainly coulombic and they are not affected by adding little quantities of a


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neutral gas.31 Additionally, anion/CO2 and cation/CO2 SDFs were obtained (see Figure 6) to analyze the distribution of oxygen atoms in CO2 surrounding the anion or cation.

Figure 6. Probability (80%) to find oxygen atoms in CO2 surrounding (a) HHH, (b) HBB at 25°C and (c) HII cations at 90°C. For the systems HHHCO2 and HBBCO2 (Figure 6a and Figure 6b), those zones with more probability to find CO2 correspond to those zones with low probabilities to find anions as displayed previously in Figure 5a-b. Same response was obtained for the case of anion surrounded by oxygen. These results reveal that CO2 do not competes in the interactions towards the cation or anion, but it only fills those vacant sites in the IL network. HII was omitted in the following discussion (Figure 6c), since as stated before, the working temperature at 90°C is not an appropriate choice for comparison with HHH and HBB. Nevertheless, as could be visualized in Figure 6c, CO2 is in similar zones than HBB, but less specific. That is, due to the high kinetic energy of CO2 at 90°C their localization is not so restricted as in HBB, but the interactions presented are roughly the same. Taken this into account, any conclusion should be extensive to the HII system. For the anion the situation is similar since CO2 fills the vacant sites near the oxygen atoms in NTf2, probably through the coordination with the central C atom. This response


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is more important when HBB is the cation of the system. Respect to cation, one may see that CO2 interacts with substituents in positions 4 and 5 of the imidazolium ring. In position 2 the interaction is less important, presumably due to the presence of anions surrounding it. In HBB the interaction towards oxygen diminishes respect to HHH, suggesting a poor interaction between O in CO2 and Br in HBB. This result shows that the approximation of CO2 to HBB will be no longer possible through halogen bonding. Even though our calculations do not include explicitly the σ-hole as proposed by Jorgensen for different halobenzenes, charges over bromine and iodine are much more positive than hydrogen in HHH. This result reveals an important feature in ILs, some interactions could be important in principle, however one must analyze the complete 3D network and not only a portion that satisfy the hypothesis. To explain better this point, we calculate the energies of 4 different complexes commonly formed in the cation and CO2 association as suggested in other reports at the M06-2x/6-31+G(d,p)//MP2/AUG-cc-pVTZ level of theory (see Figure 7).

Figure 7. Possible conformers of CO2 and HBB. (a) π-stacking interaction, (b) hydrogen bond interaction, (c) halogen bond interaction and (d) halogen bridged interaction. Distances are in Angstroms and energies in kcal/mol.


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As displayed in Figure 7, relative energies for each conformer are different. The most stable conformer is a π-stacking interaction between O in CO2 and the aromatic system in HBB (Figure 7a). Then, a hydrogen bond interaction with more than 2 kcal/mol may be established (Figure 7b). Next, halogen bonding interaction presents almost 4 kcal/mol more than the π-stacking interaction (Figure 7c). Finally, the interaction between CO2 and both bromine atoms in HBB have 5.9 kcal/mol (Figure 7d). Considering these results, the interaction between CO2 and HBB should be mainly promoted by the aromatic system in the imidazolium ring and as hydrogen bonds with the acidic H2 site. This result is key for it, since it reveals the same trend observed in Figure 6a-b, where SDF for HHH and HBB are mainly located over the cation and near the H2 site; which are the most stable conformers according to ab-initio calculations presented in Figure 7a. RDF in Figure 8 between oxygen in CO2 and several atoms in cation and anion was obtained to give more insights about how CO2 interacts with cation.


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Figure 8. Radial distribution function between O (black) and C (red) atoms in CO2 and atoms in position 2 (solid line) and 4 (dashed line) in cation or oxygens in anion.

To compare the geometric feature observed in Figure 7, RDF was obtained for the O in CO2 respect to position 2 and 4 in cations HHH and HBB (Figure 8). HII was omitted, since as stated before, the working temperature at 90°C is not an appropriate choice for comparison with HHH and HBB, however, any conclusion should be extensive to this system. As displayed in Figure 5, CO2 interacts with cation through positions 2 and 4 (or 5). RDF shows that these pairs present a peak at 260 pm (2.6 Å) that could be related to a π-stacking or hydrogen bond interactions. Considering that ab-initio calculations predict that these interactions present lower energies, halogen bond through σ-hole could be assessed as an irrelevant interaction in these cation-anion


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pairs. Other calculations predict similar distances for halogen bonding,38,

39

nevertheless, the

probability to find CO2 near bromine or iodine may be discarded because anions occupies these sites around the cation. Hydrogen bond interactions are preponderant respect to halogen bond, keeping it as a possible interaction but with low intensity. Another interesting feature observed in Figure 8 is that distributions of carbon and oxygen atoms in CO2 are mutually related: it seems that peaks associated with carbon atoms is followed by the peaks of oxygen atoms and vice versa. In the case of anion, peak for carbon atom is followed at 0.2 Å by a peak of oxygen atom. No other relevant peaks are observed for these atoms. This response implies that three atoms in CO2 are very close to the anion. For HHH and HBB cations, peaks of oxygen atom are followed by a peak of carbon atom at 1.0 Å and another peak of oxygen atom at 1.0 Å. This separation is close to the O-C bond (1.17 Å), suggesting that CO2 interacts with cation through only one oxygen atom in a similar fashion as displayed in Figure 7a-c. This result is relevant considering that RDF and SDF shows the same response observed in the ab-initio calculations: those conformers with lower energy are the same structures found in the molecular dynamics simulation. On the other hand, the anion could interact in a different way, that is with C atom directly attached to anion and both oxygen near it. Finally, solvation enthalpies were calculated for the three systems. Values obtained are ∆Hsolv = -12.5, -25.9 and -18.2 kJ/mol for HHH (at 25°C), HBB (at 25°C) and HII (at 90°C). Reported enthalpy for HHH is -14.2 ± 1.6 kJ/mol, revealing that our model based on the CL&P force field well correlates with experimental values.31 In all cases, values obtained reveals that CO2 is stabilized after dissolution in the different ILs, being this trend more pronounced for HBB and HII. However, enthalpy it is not a correct descriptor for solubility. In this case Gibbs Free Energy should be obtained, but this is out of the scope of the article; nonetheless, with some


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approximation these values could be useful. For instance, we have the solubilization of a gas in a liquid, so the entropy change should be negative. Moreover, considering that CO2 does not affect RDF and SDF of cation-anion interactions, suggests that for HHH, HBB and HII entropic change should be similar since it depends on the solubilization of the gas and not on the nature of the ILs. With these arguments enthalpy could be used as a raw descriptor of solubility founding that CO2 is easily solubilized in HBB then in HII and finally in HHH. This trend may be attributed to different electrostatic responses obtained by the different reactivity patterns imposed. This order reinforces that halogen bond may be not so meaningful to dissolve CO2 in this family of ILs. Nevertheless, is important to stress that halogenation is relevant to enhance the CO2 solvation by perturbing the electron density of the imidazolium ring, allowing to establish stronger interaction with the π-system or stronger hydrogen bonds.

CONCLUSIONS In this work we obtained several parameters to describe the 1-ethyl-3-methyl-4,5(X2)imidazolium cations (where X=H, Br and I) within the CL&P framework. Geometrical parameters as intramolecular distances and RDF are close to the experimental structure; moreover, densities obtained with our force field are within the expected values from CL&P calculations in related systems. The reliability of the proposed modification of the CL&P force field was first validated for the IL-H2O system, wich are well known experimentally to be hydrofobic IL's. The complete description of this validation step is described in detail in supporting information. This information was useful to simulate through molecular dynamics the solubilization of CO2 by these ILs. For pure ILs, the addition of halides in position 4 and 5 promotes a more intense interaction of H atom at position 2 with oxygen atoms in anion


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compared with cation HHH. On the other hand, the interactions with bromine and iodine are similar than those obtained by crystallographic methods but do not reveal the characteristic directionality observed in the halogen bonded systems, suggesting that in liquid phase hydrogen bonds are more probable to find. Addition of CO2 reveals common features than those observed in experimental works. For instance, CO2 should be in the interstices of the anion-cation 3D network with longer distances than those found in other reports at ab-initio levels, suggesting that halogen bond if present may be not the preponderant interaction in these systems. Moreover, with the results obtained here, the interaction of CO2 towards cations and anions are well related to ab-initio calculation showing that this gas interacts linearly through one oxygen atom with cation and through its three atoms with anion in a π-stacking or hydrogen bonded fashions. Finally, solvation enthalpies are well related with experimentally obtained ones and suggest that halogenated ILs dissolves more efficiently CO2 than HHH for its capture.

ASSOCIATED CONTENT Supporting Information. Detailed description of force field development. CHelpG atomic charges for HHH, HBB and HII cations, NTf2 anion and CO2. A detailed description of Molecular dynamics of HHH, HBB and HII with water. Simulation boxes and their volumes and densities. Determination of production dynamics simulation time. Complete reference 50. AUTHOR INFORMATION Corresponding Author *[email protected] ACKNOWLEDGMENT


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The authors thanks FONDECYT of Chile grants N° 11160061 and N° 11160780

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