Molecular Dynamics Simulations of Amide Functionalized Imidazolium

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Molecular Dynamics Simulations of Amide Functionalized Imidazolium Bis(trifluoromethanesulfonyl)imide Dicationic Ionic Liquids Hassan Khakan, and Saeid Yeganegi J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b03917 • Publication Date (Web): 14 Jul 2017 Downloaded from http://pubs.acs.org on July 14, 2017

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The Journal of Physical Chemistry

Molecular Dynamics Simulations of Amide Functionalized Imidazolium Bis(trifluoromethanesulfonyl)imide Dicationic Ionic Liquids Hassan Khakan, Saeid Yeganegi* Department of Physical Chemistry, University of Mazandaran Babolsar, IRAN Corresponding author- E-mail: [email protected] Office phone: +98-1135342380 Fax: +98-1135302350

ACS Paragon Plus Environment

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Abstract In the present study the structure and dynamics of three dicationic ionic liquids (DILs) with a functional amide group in the imidazolium ring with bis(trifluoromethanesulfonyl)imide, [TFSI]anion has been studied by molecular dynamics simulations. Densities, radius distribution functions (RDFs), combined distribution functions (CDFs), spatial distribution functions (SDFs), mean-square displacements (MSD), and self-diffusivities for the ions have been calculated from MD simulations. The calculated densities for [C4(amim)2][TFSI]2 at different temperatures agreed well with the experimental values. The calculated RDFs and CDFs show that anions are well organized around the amide group and imidazolium rings and the favorite sites of interaction of the [TFSI]− ion are the hydrogen atoms of amide group and hydrogen atoms of imidazolium ring of the cation. The calculated MSDs indicated that the diffusion coefficients of the studied DILs are one order of magnitude smaller than that of dicationic ionic liquids with a comparable molar mass.


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1. Introduction Ionic liquids (ILs) are a group of organic salts that attracted much attention from the scientific community in recent years.

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These organic salts have unusually low melting temperatures and

negligible vapor pressure.2 The possibility of designing application-specific ionic liquids by combining different cations and anions, is an attractive feature to both academic institutes and industrial societies.3–12 Actually, ILs have been the subject of considerable studies on tailoring them as the reaction or extraction media in industry where they are usually known as green or designer solvents.

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The usage and applications of ionic liquids in chemistry and biochemistry

have traversed many areas.2,14–29 Geminal dicationic ionic liquids (DIL) are an interesting family of ILs generally composed of two identical imidazolium or pyrrolidinium cations linked to each other by an alkyl chain which is paired with two singly charged anions.30–32 Their properties can tuned up by utilizing different linkage chains and adopting various kinds of functional groups in the cation. The understanding of physicochemical properties of DILs in terms of molecular parameters is an interesting research topic. Due to the large number of possible combinations of cations and anions moieties to form a DIL, a larger variability in the physical properties of DILs than the MILs can be expected. Where it is possible to “tuned”, controlled or altered their properties to a specific task by changes in their structure.31,33,34 DILs have been investigated in various applications such as solvents for high- temperature organic reactions,35 chromatography,36 electrolytes,37 and extracting35. Using of DILs in industrial process and academic researches rely on the understanding of their thermophysical properties in terms of molecular parameters. Unlike MILs, most of these thermophysical properties are depended to the structural changes30,38 rather than temperature changes. The effects of these structural variations on the physicochemical and solvation properties of this class


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of ionic liquids are examined experimentally31,39–46 and theoretically.32,40,44,46–49 In 2009, Pitawala et al.39 studied the conductivity and thermal properties of new dicationic ionic liquids based on the bis(trifluoromethanesulfonyl)imide [TFSI]- anion, the cation contains two imidazolium rings and connected by either a pentane or a decane hydrocarbon chain and different side groups by dielectric spectroscopy and differential scanning calorimetry. Their results show that the length of the alkyl chain on the cation has no, or weak, influence on the glass transition temperature Tg, whereas the presence of rigid aromatic side groups strongly increases Tg. In 2011 Shirota et al.45 compared the liquid density, shear viscosity, thermal properties, and surface tension of the imidazolium-based dicationic and monocationic ILs with four anions ([NTf2]-, [NPf2]-, [BF4]-, and [NO3]-). Their results showed that density; glass transition temperature and melting point, surface tension and shear viscosity of DILs are higher than that of mono cationic ILs (MILs) with a comparable mass. In 2012, Yeganegi et al.48 studied the effects of anion type and alkyl chain length on the density and microscopic structure of geminal dicationic ionic liquids by MD simulations. Their results indicate that, unlike those of MILs, the anions and cations of DILs distribute homogeneously in liquid phase and anions have a major role in the transfer of electric current. In 2016 Fareghi et al.44 synthesized two new dicationic ionic liquids (DILs) with diol functional groups on their linkages. They studied the molecular structures by elemental analysis, FT-IR, 1H and

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CNMR spectroscopies and some

thermophysical properties including density, thermal behavior and heat capacity. They also performed DFT quantum chemical calculations to identify the hydrogen bonding between anions and cations. In 2015, Bhargava et al.50 have studied the liquid structure of aqueous solutions of several different imidazolium-based DILs by using molecular dynamics simulations. Recently, Moosavi et al.51 using experimental techniques have measured the effects of temperature and


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alkyl chain length on the density and surface tension of the DILs. They showed that the densities and the surface tensions of the studied DILs decrease with increasing temperature and the alkyl chain length. In 2012, Claros et al.52 synthesized a new amide functionalized dicationic ionic liquid ([C4(amim)2][TFSI]2) and reported the physical properties of this DIL. It is proven that the ILs are good absorbents for CO2 scrubbing proposes.53–55 The observed CO2 adsorption is high for several kinds and especially for amino-functionalized ILs.53–55The knowledge of physical properties is of great importance for process engineering and equipment design.56,57Also understanding the nature of DILs containing functional group from the molecular point of view can help to design task-specific ILs for especial purposes such as CO2 adsorbent process. We used MD simulations to study liquid structure and ion diffusivities of three DILs 1,1′alkan-1,n-di-yl-bis[3-(2-amino-2-oxoethyl)-1H-imidazol-3-ium]

dibis-

(trifluoromethanesulfonyl)imide ([Cn(amim)2][TFSI]2 , n= 2,4, 6). This work is the first MD study on [Cn(amim)2][TFSI]2 ionic liquid. The aim of this study is to investigate the effect of the amide group and the length of the hydrocarbon chain on the liquid structure and ion diffusivities of the [Cn(amim)2][TFSI]2 ionic liquid.

2. Simulation Details In this work, we used LAMMPS simulation package to perform all-atom classical Molecular Dynamics (MD) simulations on a series of [Cn(amim)2][TFSI]2 (where n = 2, 4, 6)) DILs.58 The simulation cell contains 100 cations and 200 anions (6800, 7400 and 8000 atoms for n=2, 4, 6, respectively) where the periodic boundary conditions were employed and the equations of motion were integrated using the Verlet leapfrog scheme. The simulated ILs differed from each other in the length of the hydrocarbon spacer of the cation. The all-atom force field of CanongiaLopes59 with the modification by Yeganegi et al.48 was used for dications [Cn(amim)2]2+(n = 2, 4,


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6). The atomic partial charges of [Cn(amim)2]2+ cations were derived by DFT calculations at B3LYP/6-311++G** level of theory using the ChelpG scheme. The force fields for the anion was also taken from Canongia-Lopes.59 Bonded and non-bonded interactions potential energy is given by the equation,59

= ∑ , − , + ∑ , − , + ∑! , 1 + ∅ + , 1 + 2∅ + 1 ",

+ 3∅ + $, 1 + 4∅ & + ∑ ∑ 4. '

() (* + ,

)*

0)*

+ 4- . / & )*

0)*

1

− & 23 (1) )*

where the total potential energy represents sum of the energy terms involving all the bonds stretching, angles bending, dihedrals torsions as well as non-bonded interactions including Lennard-Jones and electrostatics. The equilibrium bond lengths and angles as well as the corresponding force constants are represented by , , , , , and , respectively. The 6, terms are the Fourier coefficients for describing the dihedral torsions. The partial charge on the ith atom are denoted by 7 , 8 and - are the Lennard-Jones interaction parameters. A cutoff distance of 14 Å was used for the calculation of the non-bonded interactions. Long-range electrostatics were treated by the particle-particle particle-mesh solver (PPPM).60 The initial configurations were generated by Packmol code61 and have equilibrated under isothermal–isobaric (constant NPT) conditions at 1 atm pressure and 328 K temperature. The Nose–Hoover thermostat and barostat with a temperature and pressure damping frequencies of 100 fs and 1000 fs, respectively, was used to maintain the temperature and pressure.62 The density of all studied systems reached the consistent equilibrium value after 2 ns. The VMD63 software has been used for visualization of results. The minimum-energy geometries of isolated cation and anion was determined by performing density functional theory geometry optimizations at B3LYP/6-311++G** level of theory using


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Gaussian03. The vibration analysis was followed to ensure the absence of negative frequencies and verify the existence of a true minimum. The anion and cation charges were set at -1 and +2, respectively. The optimized structures of anion and cations are shown in Figure 1.

Figure 1. Optimized geometries of cations and anion at B3LYP/6-311++G** level of theory. (S, yellow; C, grey; N, dark blue; O, red; F, cyan; H, white).


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The initial configuration of each NVT simulation was taken from the final configuration of a relevant equilibrated NPT simulation. Temperature, total energy and volume were monitored during the equilibration until the corresponding time series were stationary. The mean square displacement MSDs were averaged over a set of at least three independent NVT simulations each with a total 1 ns equilibration followed by a 15 ns production in which the positions of particles were recorded every 500 fs. All subsequent analysis of obtained trajectories was carried out by TRAVIS software developed by Brehm and Kirchner.64

3. Results and Discussion 3.1. Density Liquid phase densities are one of the most copious data for ILs in the literature and they are usually used as a preliminary check for the accuracy of the applied MD method and force field. The size and shape of the ions, cation–anion interactions and molecular packing are the key parameters that affect the liquid density.44 The knowledge of density is important in masstransfer operations like distillation, extraction, absorption, adsorption, metabolic extractive fermentation and also in the performance of biological membranes and many other chemical engineering processes.51 To the best of our knowledge, there is no report on the density of the [Cn(amim)2][TFSI]2 DILs (n=2,6) at different temperatures in the literature. In this work we first calculated the densities of [C4(amim)2][TFSI]2 at several temperatures by NPT simulations and compared it with the experimental results52 in Table 1. The calculated densities of [C4(amim)2][TFSI]2 at all studied temperatures is lower than the experimental values, but the difference becomes smaller as the temperature increases. The absolute percent deviations (%AD) of simulated densities in Table 1 from the experimental values of Claros et al.52 are less than 3% which indicates adequacy of the adopted MD method and force field.


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Table 1. Simulated and Experimental Densities ρ of [C4(amim)2][TFSI]2 ρ (Kg m-3) T(K)

a

This work

Experimentala

%AD

298

1547

1590

-2.70

308

1543

1580

-2.34

318

1540

1572

-2.03

328

1533

1562

-1.86

343

1514

1548

-2.19

From ref 52.

In the next step, the densities of the other studied DILs were calculated at 343 and 328 K and 1 atm and shown in Table 2. It can be seen that the density of [C4(amim)2][TFSI]2 is larger than that of the other studied DILs which indicates a significant packing and a stronger interactions between anions and cations in [C4(amim)2][TFSI]2 . Table 2. Computed Liquid Densities ρ of studied DILs at 328 K, 343 K and 1 atm ρ (Kg m-3) DIL T=328 K

T=343 K

[C2(amim)2][TFSI]2

1454

1448

[C4(amim)2][TFSI]2

1533

1514

[C6(amim)2][TFSI]2

1411

1408

3.2. Radial Distribution Functions The center of mass and partial radial distribution functions g(r) can be used to gain a microscopic view of the liquid structure. Various radial distribution functions (RDFs) were computed from the MD generated trajectories to obtain an insight into the liquid organization of the studied DILs. Figure 2 shows the calculated RDFs of anions [TFSI]- around the geometric center of the imidazolium rings of the cations in the studied DILs. All three RDFs show a peak


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near 6 Å and a broad peak near 13 Å. This suggests that the anions are well organized around the imidazolium rings for all of the studied systems irrespective of the length of alkyl chain.

Figure 2. RDFs of anions around the geometric center of the imidazolium rings for [Cn(amim)2][TFSI]2 (n=2,4,6). Figure 3 shows the center of mass RDFs for cation-anion, anion-anion, and cation-cation interactions. Figure 3(a) exhibit a cation-anion peak at approximately 5 Å for all of the studied DILs. The anion-cation peak for [C6(amim)2][TFSI]2 is broader than the two other systems, which is most likely due to the additional conformational flexibility of the longer alkyl chain for [C6(amim)2]2+ cation.


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Figure 3. Radial distributions functions for the centers of mass (a)anion-cation, (b)anion-anion and (c)cation-cation for [Cn(amim)2][TFSI]2 (n=2,4,6).

The RDFs of anion−anion in Figure 3(b) show a first peak around 7 – 8 Å and a broad second

peak at distances larger than 17 Å, again the distance of 10 Å between the position of the first and the second peaks can be associated with solvation shells for both cations and anions. The RDFs of anion−anion for [C2(amim)2][TFSI]2 and [C4(amim)2][TFSI]2 show a first peak at about 7 Å,

where for [C6(amim)2][TFSI]2 a broad first peak at about 8 Å can be observed. The RDFs of cation-cation in Figure 3(c) show broad peaks around 10 Å.

To elaborate more detailed structure of anions around the imidazolium rings and amide groups we have calculated the PRDFs (Partial Radial Distribution Function) of oxygen, nitrogen and fluorine atoms of the anion and hydrogen atoms (H1, H2, H3) of the cation. The atoms labels in the cations are described in Figure 4 and the calculated PRDFs are shown in Figure 5.


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Figure 4. Atom labeling around the imidazolium ring in [Cn(amim)2]2+ cations used in this work.


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Figure 5. atom-atom RDFs for [Cn(amim)2][TFSI]2 at 328 K and 1 atm. O-H3 (a), O-H1 (b), OH2 (c), N-H3 (d), N-H1 (e), N-H2 (f), F-H3 (g), F-H1 (h), F-H2 (i). The sharp peaks for distances less than 2 Å indicate strong interactions between H3 of the cation and oxygen atoms of the anion. The second peak for H3 at 3.5 Å can be attributed to the correlation with the second shell anions. The peaks at 2.5 Å for H1 and H2 indicate a weak interaction between these hydrogens and the oxygen atoms of the anions. Broad peaks for F-H3, F-H1 and F-H2 indicate a weak interaction between these hydrogen atoms and fluorine atoms of


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the anion. So, the favorite sites of interaction of the oxygen and nitrogen atoms of anion are the H3 atom of the amide group and H1 and H2 atoms of the imidazolium ring respectively. 3.3. Combined Distribution Functions The liquid structure of an IL is a result of the interplay between Columbic, hydrogen bonds (HBs), and dispersion forces. Pervious works have been shown that hydrogen bonding has an important effect on the properties and reaction dynamics of ILs.65–68 According to IUPAC definition,69 for a strong H-bond, the distance between H-atom and the acceptor are less than 2.2 Å and the angle made by the donor, the H-atom, and the acceptor is within the range of 130–180 degree. While, for a weak hydrogen bond, the corresponding distance and angle ranges should be 2.0–3.0 Å and 90–180 degree, respectively.50 To better understand the hydrogen bonding and microstructure of studied DILs, the CDFs (Combined Distribution Function) which include the corresponding RDF and ADF (Angular Distribution Function) of relevant bonds and atoms. A CDF generally provides more information than its constituent radial and angular distribution functions. Figure 6 shows two sets of CDFs composed of the RDF between the H3 and H1 atoms of [Cn(amim)2]2+ (n=2,4 and 6) with the N atom of [TFSI]- as X axis and the ADF between the H3─N3 and H1─C1 vectors in [Cn(amim)2]2+ and the intermolecular H3─N and H1─N vectors between one [Cn(amim)2]2+ and one [TFSI]- as Y axis. In Figure 6 (a-c) one can see that N3─H3─N angle is greater than 135° and the preferred H3-N distance is around 400 pm where a hydrogen bond between H3 atom of cation and N atom of anion can be formed. There is another peak at larger distances around r=600 pm for angles less than 55° which is corresponds to a configuration in which a [TFSI]anion is coordinated to the other two [Cn(amim)2]2+ H3 atom. In Figure 6(d-f) one can see that


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for C1─H1─N angle greater than 100° the most probable H1-N distances is around 500 pm, which indicates a low probability of formation of a hydrogen bond


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Figure 6. CDFs (Combined Distribution Function) showing the hydrogen bond between H3 and H1 atoms of [Cn(amim)2]2+ (n=2,4 and 6) and the N atom of [TFSI]-.

Figure 7 shows the resulted CDFs from combining RDFs between the H1 and H3 atoms of [Cn(amim)2]2+ (n=2,4 and 6) with O atoms of [TFSI]- and an ADF between the H1-C1 and H3N3 vectors in [Cn(amim)2]2+ and the intermolecular H1-O and H3-O vectors between a [Cn(amim)2]2+ cation and a [TFSI]- anion. According to Figure 7(a-c), for N3-H3-O angle greater than 135° the distance between H3 and oxygen atoms is around 200 pm, indicating a strong hydrogen bonding between these atoms. In Figure 7(d-f) there is a possibility for a weak hydrogen bond between H1 and oxygen atoms because the corresponding distance range and angular range are 200–300 pm and 90–180 degree, respectively.


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Figure 7. CDFs (Combined Distribution Function) showing the hydrogen bonding between H3 and H1 atoms of [Cn(amim)2]2+ (n=2,4 and 6) and O atom of [TFSI]-.

3.4. Spatial Distribution Functions (SDFs)


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SDFs show the probability of finding an atom in a certain distance to another atom. The calculated SDFs in Figure 8 show a 3-D organization of the anions around a cation. The SDFs have been calculating from the MD generated trajectories using the TRAVIS software.64 According to Figure 8, the favorite sites for interaction of the TFSI− ion are in order H3 > H1 > H2 hydrogen atoms of the cation where the probabilities of finding TFSI- anions around H3 hydrogen atoms of cations in three dicationic ionic liquids are similar.

Figure 8. Spatial distribution functions (SDFs) of the anions around the H3 (a, b and c), H1 and H2 (d, e and f) hydrogen atoms of cation.

3.5. Diffusion Coefficients The microscopic dynamics of ionic liquids plays a critical role in determining the rheological properties of these compounds. It is worth mentioning that the ionic mass transfer has a crucial effect in chemical reactivity, separations and electrochemical applications of ILs. Self-diffusion coefficient can be obtained from the slope of the long-time limit of the mean square displacement (MSD) using the well-known Einstein relation:


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J

D = lim→! I KLDM 1

(2)

J

Where MSD is defined by

R R KLD = 〈∑N U|Q S − Q 0| 〉 N

(3)

Where R S is the location of the center of mass of ion i at time t. The trajectories were dumped for 15 ns at 500 fs intervals at 328 K after 1 ns equilibration and the self-diffusion coefficients obtained from the slopes of the line fitted to the MSDs in the range 6-11 ns. The cations and anions center of mass mean squared displacements (MSD) for studied DILs calculated from NVT simulations up to 11 ns are shown in Figure 9. The diffusion coefficients calculated as the slope of the MSDs in the asymptotic linear regions (t > 6 ns).

Figure 9. Calculated MSDs of the center of mass cations and anions of three dicationic ionic liquids at 328 K and 1 atm. The calculated diffusion coefficients based on the curves in Figure 9 are presented in Table 3. Our results show that the studied DILs have much smaller self-diffusion coefficients than the


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corresponding mono cationic ionic liquids and dicationic ionic liquids with comparable molar mass. For example, in this work the calculated diffusion coefficients of cation and anion for [C2(amim)2][TFSI]2 is 1.50 × 10−13 m2/s and 2.55 × 10−13 m2/s, respectively (Table 3) which can be compared with that of [bmmim][TFSI] calculated by Kowsari et al.70 at 400 K as 336× 10−13 m2/s and 304× 10−13 m2/s for the cation and anion, respectively. Also, the calculated diffusion coefficients of cation and anion at 450 K by Yeganegi et al.48 for [C9(mim)2][PF6]2 is 10 × 10−13 m2/s and 15 × 10−13 m2/s, respectively. The electrostatic interactions in a DIL is stronger than that of a corresponding MIL with a comparable density because the charge density in a DIL is two times larger than that of the MIL. Simulation work of Tsuzuki et al.71 revealed that the ionic diffusivities for MILs are about 40 times larger when neglecting the electrostatic interactions. The smallness of a DIL ionic diffusivities relative to a MIL with a comparable density can be understood in terms of the larger electrostatic interaction in the DIL.

Table 3. Calculated Diffusion Coefficients of Cations and Anions from the Slope of MSD Plots in Figure 9 for Three Dicationic Ionic Liquids at 328 K and 1 atm DILs

D+ ( 10-13 m2/s) D- ( 10-13 m2/s)

t+

t-

[C2(amim)2][TFSI]2

1.50

2.55

0.227

0.773

[C4(amim)2][TFSI]2

0.912

1.46

0.238

0.762

[C6(amim)2][TFSI]2

1.55

2.35

0.248

0.752

DILs as molten salts can serve as electrolytes in the electrochemical processes and devices.17,18,46,72 The ionic transference number ti is the relative contribution of a given ionic species i in carried total electrical current in the electrolyte. The difference in transference


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numbers arises from the differences in ionic motilities. For a 1:2 electrolyte, MX2, consisting of a cation M2+ and two anions X−, such as the DILs studied in this work, transference numbers can be estimated from the diffusion coefficients of the cation and anion

SW =

XY

XY WXZ

S[ =

XZ

XY WXZ

(4)

The calculated transference numbers are shown in Table 3. According to table 3, the calculated t+ for DILs are less than 0.3. The t− greater than 0.7 indicates that anions carry a substantial part of the electric current in the molten DIL. The larger transference number of the anions than that of the cations caused by the larger diffusion coefficients of the anion in Table 3.

4. Conclusion In this study, thermophysical properties of DILs [Cn(amim)2][TFSI]2 with n= 2, 4, and 6 including liquid phase density, radial and spatial distribution functions and diffusive properties are investigated for the first time using MD simulations. The simulated densities for [C4(amim)2][TFSI]2 agree well with the experimental data. The calculated RDFs show that anions are well organized around the amide group and imidazolium rings. Analysis of partial RDFs and CDFs shows that there are various hydrogen bonds between hydrogen atoms of imidazolium rings and amide group with nitrogen and oxygen atoms of anions where the letter are the strongest. The calculated diffusion coefficients indicate that studied DILs have smaller self-diffusion coefficients than dicationic ionic liquids with a comparable molar mass.

ACKNOWLEDGMENT We acknowledge the University of Mazandaran for support of this work.


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Molecular Dynamics Simulations of Amide Functionalized Imidazolium

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