Theoretical Calculation of the Thermodynamic Properties of 20 Amino

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

Theoretical Calculation of the Thermodynamic Properties of 20 Amino Acid Ionic Liquids Bingyu Nie, Ruifang Li, Yang Wu, Xing Yuan, and Wei Zhang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b06813 • Publication Date (Web): 25 Oct 2018 Downloaded from http://pubs.acs.org on October 27, 2018

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The Journal of Physical Chemistry

Theoretical Calculation of the Thermodynamic Properties of 20 Amino Acid Ionic Liquids †

Bingyu Nie†a, Ruifang Li a, Yang Wua*, Xing Yuanb, and Wei Zhangb* a

b

College of Chemistry, Key Laboratory of Green Synthesis and Preparative

Chemistry of Advanced Materials, Liaoning University, Shenyang 110036, China Abstract: The thermodynamic properties of gas-phase amino acid ionic liquids (AAILs) containing 20 amino acids ([AA]−) and 1-ethyl-3-methylimidazolium ([Emim]+) are studied using a combination of the ab initio method, molecular dynamics simulations, Born–Haber (BH) cycle analysis, and isodesmic reactions. The M06-2X/TZVP method is used to explore the structure and dissociation enthalpies of [Emim][AA] by considering dispersion interaction, and the MP2/Aug-cc-pVTZ method is used to correct these enthalpies. The vaporization enthalpies of all 20 AAILs are calculated by molecular dynamics simulations, and the gas-phase formation enthalpies (ΔfH) of the 20 [AA]− anions and [Emim]+ cation are calculated by the DFT/M06-2X method and isodesmic reaction approaches. To obtain the ΔfH of the AAILs, interconnections in the corresponding BH cycles are evaluated. Systematic study of the 20 [Emim][AA] ion pairs provides some initial factors contributing to the thermodynamic properties of AAILs: including length of the alkyl chain, interatomic electronic effects, steric repulsion from the cyclic group, and H-bonds formed by functional groups. Generally speaking, the results of this work provide insights into the structure-property relationships of not only ionic liquids, but 1 †

These authors contributed equally to this work. ACS Paragon Plus Environment

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also any ionic or molecular substance. 1. Introduction Room temperature ionic liquids (ILs) have recently received considerable attention because of their unique physicochemical properties. Most ILs are chemically stable, noncombustible, and poorly volatile.1-3 A wide liquid temperature range is useful for versatile solvation applications, and the ability to efficiently dissolve both polar and nonpolar compounds fosters development of IL-based reaction media and separation setups.4 The advantages of ILs lie in the tailorability of the anion or cation， which can cause changes in properties and functions of the resulting compound.5 For example, introduction of ester groups to the alkyl side chain of imidazolium cations can improve the biodegradability of the resulting IL.6 Amino acids have been used as precursors to develop reusable materials based on modifiable ILs and reduce their preparation cost.7 In 2003, Bao et al. first obtained ILs from natural amino acids and paved the way for development of amino acid ILs (AAILs).8 In 2005, Ohno et al. used 1-ethyl-3-methyl imidazole ([Emim]+) as a cation and 20 natural amino acids as anions to synthesize AAILs.9 Kou et al. reported a new generation of cations, that were directly derived from α-amino acids and their ester salts, and prepared novel natural ILs [AA]X (X = Cl−, NO3−, BF4−, PF6−, ...) and [AAE]Y (Y = NO3−, BF4−, PF6−, Tf2N−, ...).7, 10 The most exciting features of these AAILs are that they are biodegradable，nontoxic, and inexpensive. Egorova et al. found that the toxicity of ILs with amino acid anions ([AA]–) is lower than that of ILs with BF4−, PF6−, and HSO4− anions.11 Wu et al. investigated AAILs as electrolytes in energy-storage devices and 2

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found that AAIL-based capacitors can work at higher temperatures compared with normal IL-based capacitors with similar capacitance.12 The physicochemical properties of AAILs can easily be adjusted for a wide range of tasks. [Emim][Pro] can be used as a green additive in the separation of acetonitrile-and-water system.13 [AA]– ions with a carboxyl group and a primary amine group can improve the CO2 absorption capacity of ILs.14 Natural amino acids, such as L-proline are outstanding ligands used in organocatalysis and organometallic catalysis. Chen et al., developed task-specific ILs (TSILs) through ionic pair coupling of the imidazolium cation of the modified polystyrene support with L-proline.15 AAILs are, therefore, ideal candidate platforms for other TSILs or functional ILs.10, 16 Many AAIL systems have been studied over the years via experimentation and theoretical researches. Differential scanning calorimetric measurements can be used to measure the glass-transition temperature (Tg) of AAILs.17 Yang et al. used the standard addition method to obtain the experimental density and surface tension data of [Emim][Gly].18 Norman et al. investigated the properties of AAILs through neutron diffraction and found that addition of the OH group to serinate ILs leads to stronger bonding interactions.19 Sistla et al. used Fourier transform infrared spectroscopy to AAILs and found that the primary amine (–NH2) is converted into a secondary amine (–NH) after reacting with CO2 to form a carbamic acid group or carbamate group.14 Despite these interesting findings, however, the design and application of AAILs based on structural data and thermodynamic properties remain limited due to the lack of experimental values that can be used directly.20 3



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Theoretical calculations not only provide visualized structural information but also enable the prediction of microscopic and macroscopical properties.21 Good agreement exists between theoretical and experimental results because of the development of computational and theoretical studies on the conformational equilibrium of ions in the nanosegregated polar and nonpolar domains.20, 22 In addition, given that the physicochemical properties of AAILs can be tuned by modifying the functional group, systematic theoretical studies on the relationship between molecular structure and physicochemical properties can promote the exploitation of AAILs.21, 23 For example, Mohajeri et al. investigated the relationship between Tg and the interaction energy of AAILs via density functional theory ( DFT) method.24 In more recent work, several thermochemical properties of ILs, such as their dissociation enthalpy (ΔdissH), vaporization enthalpy (ΔvapH), fusion enthalpy, solvation enthalpy, and lattice enthalpy (ΔlattH) of ILs were computed by the G3MP2 method.25 Zhu et al. computed the ΔfH of glycine-based sulfate/bisulfate AAILs using the Born–Haber (BH) cycle.3 Similarly, the ΔlattH and ΔfH of [AA]X and [AAE]X AAILs were calculated via the BH cycle based on the MP2/6-311++G** method.20 Ebrahimi et al. predicted the thermophysical properties of some AAILs, as well as the influence of [AA]− tuning, cation length, and temperature on these properties, through the B3LYP/6-311++G** method.26 Besides the ab initio method, classical molecular dynamics (MD) simulation is a proven powerful tool for predicting the thermodynamic properties of ILs through microscopic knowledge.6, 27-28 Liu et al. calculated the volume expansivity and heat capacity of [Emim][Gly] by MD 4


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simulation and obtained good agreement between the theoretical results and experimental data.29 In our previous study, some thermochemical properties, such as ΔvapH, were calculated for [Emim][Gly] by MD simulation.30 To study the potential application of AAILs in various fields of in the chemical industry, the physiochemical properties of these ILs must be analyzed.31 Previous research has studied the viscosity, density, heat capacity, and melting point of AAILs.24, 29, 32 However, systematic study on thermochemical properties, especially the ΔfH of AAILs remain lacking. Therefore, prediction of certain key physiochemical properties by theoretical calculation is of considerable interest. In this article, we report the computational thermodynamics properties of 20 AAILs. We used the DFT and MP2 methods, MD simulations, BH cycles and isodesmic reactions for our study. The structures of the AAILs and their ΔdissH were explored by DFT/M06-2X, which is widely used because it can account for dispersion, unlike DFT/B3LYP. The ΔdissH obtained were corrected by MP2/Aug-cc-pVTZ. The ΔvapH of 20 AAILs were obtained by MD simulations, and the forces and potential energies required for MD integration were obtained directly from Amber-type parameters and restraint electrostatic potential (RESP) charges. Isodesmic reactions are especially powerful tools to estimate the ΔfH of 20 neutral amnio acids, their corresponding anions, and [Emim]+. Finally, based on BH thermocycles, the ΔfH of the 20 AAILs were calculated. The relationship between molecular structure and thermodynamic properties was clearly and systematically explored.

2. Computational Details 5



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2.1 MD Simulation A classical MD simulation program (Tinker 6.333) was used for the simulations of the 20 AAILs, and systematic study of ΔvapH was conducted through MD simulations. A cubic box with periodic boundary conditions applied in three dimensions was used to obtain the bulk behavior of AAILs. The van der Waals terms are described by the Lennard-Jones 12-6 potentials. Intramolecular terms related to bonds and angles are taken from AMBER-type force fields developed by Cornell’s group.34 The RESP charges35 were fitted to determine the atomic charges of the [Emim][AA] via the RED III scheme.36 During the simulation intramolecular 1-4 interactions were considered by the 1.0 scale factor. The Ewald summation method was used for the treatment of long-range electrostatic forces, in which a cutoff of half-length of the simulated box was used for the real part of electrostatic interactions. The complete set of force field parameters is given in the Supporting Information (Table S1 and Figure S1). The initial system geometries were generated by randomly inserting 126 pairs of anions and cations of AAILs into the simulation cell and then allowing the system to relax. This step was followed by equilibration of the system for 2 ns within the NPT ensemble. Berendsen thermostat and barostat were used to control the temperature and pressure at 298 K and 0.1 MPa, respectively.37 After equilibration, the total intermolecular energies and densities were monitored until a steady state was achieved. Then, the trajectory of the next 1 ns was performed in the NPT ensemble. Finally, considering the specific viscosity of the ILs, the production stage was continued for 3 ns under the NVT ensemble. Based on the MD simulation 6


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results, the ΔvapH values of the 20 AAILs were calculated. During the production stage, configurations of the simulation box were recorded every 0.1 ps for further structural analysis. The calculated site-site radial distribution functions for all 20 [Emim][AA] are presented in the Supporting Information. (Figure S2). 2.2 Ab Initio Method Using the Gaussian 09 package,38 the gas-phase ion pairs and the corresponding monomers of 20 [Emim][AA] were explored. The B3LYP method is used extensively in studies on thermodynamic properties, and it has been previously shown to be a useful tool for evaluating ILs.39 However it has not been explicitly developed to treat dispersion interactions. M06-2X, a hybrid functional identical to B3LYP but augmented with a basis set superposition errors and dispersion correction, was developed to treat transition states and weak interactions.40 The DFT/M06-2X functional highlights the importance of activation energies and dispersion interactions; it is one of the best functionals for a combining research on main-group thermochemistry, kinetics, and noncovalent interactions.41 Thus, the structures and ΔdissH of 20 AAILs were optimized by the M06-2X/TZVP method, and compared with the corresponding B3LYP results.32 The ΔdissH were calculated without considering deformation energies between ion pairs and the isolated monomers, because the geometries of the monomers in complexes differ slightly from their optimal geometries when isolated. To obtain accurate energies, we also used the MP2/Aug-cc-pVTZ method to correct the ΔdissH. 2.3 Born-Haber (BH) Cycle 7



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The BH cycle established by Hess's law links the phase transitions of process. This method can be used to predict different thermodynamic properties. For example, ΔfH, which cannot be obtained directly from experiments,25 can be obtained only by empirical methods in many studies.42 In this report, density functional data, MD simulation results, and BH cycles can be combined to determine the ΔfH of (

[Emim][AA] (Figure 1). In this figure,

(

(

, and

(

are the gas-phase ΔfH of the [Emim]+ cation, gas-phase ΔfH of the [AA]− anion,

(

(

b

(

(

(

(

(

(

(

(

(

Figure 1. Combination of the Born-Haber cycle and thermodynamic properties of AAILs. and the

and

( (

(

are cation and anion in the gas-phase, respectively,

is the ion pair in the gas-phase.

(

,

(

,

(

and

(

are

o t t ble ele ent . ΔdissH ΔvapH n ΔfH are the dissociation, vaporization and

formation enthalpies, respectively.

ΔdissH of AAIL, and ΔvapH of AAIL, respectively. Combining these properties with the BH cycle, the ΔfH of AAIL,

(

can be presented as: (

(

(1)

The ΔvapH of the 20 AAILs were predicted by MD simulation, and their ΔdissH were predicted by DFT/M06. The ΔfH of anions and cations are difficult to obtain experimentally. Thus, isodesmic reactions are introduced to predict the ΔfH of cation and anions. In this case, four of the quantities in Equation (1) are determined, and then 8


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the ΔfH of AAILs can be calculated systematically.

3. Results and Discussion 3.1 Geometric Structures As suggested by Dupont, pure ILs based on the imidazolium cation are H-bonded polymeric supramolecules. Such a structure is generally true for both the solid and liquid phases and apparently maintained to a great extent even in the gas-phase.43 Many researchers have explored the hydrogen bonds formed between cation and cation, anion and anion, cation and anion in detail by experimental and theoretical studies.44 In the case of imidazolium-based ILs, the cation-cation and anion-anion interactions can be excluded because of the strongly interacting between [Emim]+ and anions.45 For [Emim][AA], the oxygen atoms of amino acid anions can interact with the hydrogen of the imidazolium ring and the hydrogen of the methyl or methylene group, which means five regions of S1, S2, S3, S4, and S5 are available for the formation of intermolecular H-bonds,32 as shown in Scheme S1 (Supporting Information). From a previous study, among the three hydrogens of the imidazolium ring in region S1, C2-H is well known to be the most acidic and presents the strongest interaction with amino acid anions.19 Therefore, we only optimized these isomers in region S1 for 20 [Emim][AA] by M06-2X/TZVP. The optimized structures are shown in Figure 2; in the figure, the oxygen atoms of [AA]− form two stable H-bonds with the imidazole ring. Figure 2 also lists the H-bond lengths obtained by M06-2X/TZVP, and the corresponding results calculated by DFT/B3LYP for comparison. For all calculated 20 ion pairs of [Emim][AA], the strong H bond (r = 1.725-2.206 Å), is to 9



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the most acidic C2-H, and a weaker interaction is with the C6-H (r = 2.089-2.248 Å). The H-bonded interaction between [Emim]+ and [AA] result in nearly linear hydrogen bonds, and the ranges of C2-H···O and C6-H···O angles are 142.62157.27 and 113.01160.17, respectively, which are in accordance with the typical H-bond. For typical H-bonds, the length is less than the corresponding atomic van der Waals non-bonded radius and the angle is usually linear. 46 (The specific bond and angle data can be found in the Table S2 of Supporting Information) While the shortest H-bond between the carbonyl oxygen and C2-H is 1.725 Å (in [Emim][Gly]), another H-bonds between the carbonyl oxygen and C6-H, at 2.089 Å (in [Emim][Ala]), can also be considered fairly short. Considering the H-bonding interactions of [Emim][AA] in region S1, the H-bond lengths of C2-H···O are shorter than those of C6-H···O, which is in agreement with previous results.32 Thus, the C2-H···O interaction is dominant in [Emim][AA] because of the higher acidity of C2-H. The length of H-bond is also affected by the alkyl chain length of the anion. Calculations by Rao et al. on AAILs have shown that the H-bond lengths increase steadily while the corresponding binding energies decrease with increasing anion alkyl chain length.47 Our calculations also indicated similar results. Taking [Emim][Ala] and [Emim][Val] as examples, the H-bonds of C2-H···O in these ILs are 1.738, and 1.806 Å, respectively. For comparison, the corresponding lengths of C2-H···O determined by B3LYP/6-311+G(d,p) are between 1.653 and 1.766 Å. Thus, the H-bonds optimized by the M06-2X/TZVP method are longer than those optimized by the B3LYP/6-311+G(d,p) method. 10


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Figure 2. Geometric structures of the ion pairs [Emim][AA] optimized by the M06-2X/TZVP and B3LYP/6-311+G (d,p) methods with bond lengths in Å.

3.2 Dissociation Enthalpy The ΔdissH is the energy needed to disrupt an ion pair gas-phase to form infinitely separated ions

(

and

( (

in the

.25 In general, mass

spectrometry experiments can be used to qualitatively determine ΔdissH.48 However, measurement is difficult because of the instability of the ILs in the gas-phase; thus, theoretical calculation methods are used to obtain ΔdissH.39 Several authors have shown that the calculated ΔdissH are highly dependent on the method and basis set used. Although DFT/B3LYP is very popular, it is unable to treat dispersive interactions, especially for IL ion pairs. Certain hybrid or dispersion-corrected functionals, such as M06-2X41, 49 and BLYP-D50 at the basis set limit, have been proven to provide good accordance with MP2 values. Thus, in the present paper we use the M06-2X/TZVP method to explore the ΔdissH of 20 [Emim][AA]; and the 12


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single-point energies determined by the MP2/Aug-cc-pVTZ method are also provided. The following equation is used to calculate the ΔdissH of AAILs at 298 K. (2) Table 1 shows the ΔdissH obtained by the M06-2X/TZVP and MP2/Aug-cc-pVTZ methods; for comparison, we also listed the corresponding B3LYP/6-311+G(d,p) results. The calculation method selected exerts an obvious influence on the results. The ΔdissH calculated by the hybrid M06-2X/TZVP method ranged from 371.51 kJ·mol−1 to 418.17 kJ·mol−1, in agreement with the corresponding results determined by MP2/Aug-cc-pVTZ, and are generally larger than the MP2 values by less than 2.47%. Obviously, the B3LYP method underestimates these dissociation energies. Differences in the values calculated by the M06-2X/TZVP and B3LYP/6-311+G(d,p) methods ranged from 14.56 kJ·mol−1 to 26.59 kJ·mol−1. The calculated results also indicate that the functional groups of the amino acid anions influence the ΔdissH of the AAILs to a certain extent. The phenyl group of [Emim][Phe], for example, induces a ΔdissH of 399.82 kJ·mol−1, which is smaller than the ΔdissH obtained from similar ILs with only alkyl side chains, such as [Emim][Ala] (418.17 kJ·mol−1). This difference could be due to the hyperconjugation effect between the benzene ring and the carbonyl group in the [AA]−, which delocalizes negative charges more extensively over the anion and reduces interaction energies. The intramolecular H-bonds formed between the functional group and carbonyl O atoms may also decrease the proton-accepting ability of carbonyl O atoms and reduce the ΔdissH of the related ILs. The hydroxyl H atom of [emim][Thr], for example, forms an intramolecular H-bond 13



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with one of the carbonyl O atoms (distance of approximately 1.975 Å), and compared with other AAILs, this intramolecular H-bond produces a lower ΔdissH of 388.41 kJ·mol−1 when calculated by the M06-2X/TZVP method. [Emim][Asn] and [Emim][Gln], which feature amide groups, yielded fairly low ΔdissH of 371.51 and 380.12 kJ·mol−1, respectively, because of the influence of intramolecular H-bonds between the amide H and carbonyl O. Ludwig et al. explored the dissociation energies of

imidazolium-based

ILs,

-351.94,

-370.15,

and

-384.41

kJ·mol−1

for

[1,3-m-im][NTf2], [1,2-m-im][NTf2], and [1-m-im][NTf2], respectively.51 Moreover, this result indicated that the different position of methyl functional group would modify the H-bond capabilities and lead to the dissociation energetic differences. In general, the ΔdissH of AAILs are relatively comparable to conventional ILs. For example, the ΔdissH of [Emim][BF4] and [Emim][PF6] were calculated by the M06L/6-311+G method to be 370.0 and 351.2, respectively.52 Table 1. Dissociation enthalpies (kJ·mol−1) of [Emim][AA] at the M06-2X/TZVP, MP2/Aug-cc-pVTZ and B3LYP/6-311+G(d,p) levels AAILs

M06-2X/TZVP

MP2/Aug-cc-pVTZ

B3LYP/6-311+G(d,p)

[Emim][Gly]

417.95

411.87

396.80

[Emim][Ala]

418.17

410.44

394.39

[Emim][Val]

410.05

409.96

388.15

[Emim][Leu]

410.26

404.36

387.36

[Emim][Ile]

402.64

404.91

387.48

[Emim][Ser]

398.01

401.99

383.45

[Emim][Thr]

388.41

386.63

368.95

[Emim][Pro]

412.52

407.87

391.13

[Emim][Asp]

399.40

423.35

380.60

[Emim][Asn]

371.51

372.12

350.95 14


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[Emim][Glu]

397.59

420.45

375.30

[Emim][Gln]

380.12

378.51

349.62

[Emim][Lys]

406.74

406.51

387.40

[Emim][Arg]

394.54

391.34

373.34

[Emim][Cys]

413.94

403.97

387.35

[Emim][Met]

400.96

396.99

378.91

[Emim][His]

404.69

400.21

379.75

[Emim][Trp]

408.12

402.31

385.47

[Emim][Phe]

399.82

398.84

380.22

[Emim][Tyr]

396.95

397.55

377.58

3.3 Vaporization Enthalpy The enthalpy of vaporization, ΔvapH, is the energy required to transform a given quantity of a liquid to gas at a given pressure. For ILs, the ion pair remains in the gas phase, which means interactions with only a single counter-ion are maintained by each ion. The ΔvapH can be determined by different experimental methods, such as thermogravimetric analysis,53-54 line of sight mass spectrometry55 and quartz crystal microbalance.56 Some approaches to the modeling of ΔvapH of ILs have been established,39, 47, 57-58 for example, MD simulation, ab initio cluster approach, and Kabo’s method. In the present paper, we explore the ΔvapH for all 20 [Emim][AA] on

nte

(3)

ILs by MD simulations, where Uionpair is the intermolecular energy between the cation and anion in the gas phase, which can be calculated by the M06-2X/TZVP method. The internal energy in the liquid state Uinter is also calculated by MD simulation, R is the gas constant, and T is the temperature. The calculated ΔvapH of [Emim][AA] are shown in Table 2; here, the available 15



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experimental ΔvapH are also listed. The calculated ΔvapH values are in good agreement with the experimental results. The experimental ΔvapH of [Emim][Gly], for example, is 139.7 kJ·mol−1, and the MD simulation result is 152.81 kJ·mol−1. The small deviation may be due to the factor that the vapH calculations by MD simulation should use the gas phase ion pairs which are very different in case of [AA]  showing strong competition between enthalpy and entropy.

58

These will be systematically

explored in the following paper. Obviously, ΔvapH is considerably influenced by the functional group of [AA]−. The ΔvapH of [Emim][AA], which features with amine, is considerably higher than that of other AAILs due to the formation of H-bonds. The ΔvapH of [Emim][Asn], at 205.51 kJ·mol−1, may be ascribed to the contribution of intramolecular H-bonds formed between the amine group and the carbanyl group (‒CONH2···O=CO‒). The amino group not only forms intramolecular H-bonds but also forms intermolecular H-bond with anions or cations in other ionic pairs. This intermolecular H-bond can influence ΔvapH. Compared with those of [Emim][Leu] (147.55 kJ·mol−1) and [Emim][Ile] (149.74 kJ·mol−1), which only have four carbon side chains on their anions, the ΔvapH of [Emim][Lys] is higher at 174.55 kJ·mol−1. Among the AAILs evaluated, [Emim][Arg

howe

the h he t ΔvapH, 215.69

kJ·mol−1, likely because of the amine group and two imino groups in [Arg]−. As shown in Table 2, [Emim][Cys] presents the smallest ΔvapH among the AAILs evaluated (120.11 kJ·mol−1) , likely because the mercapto group on the weakens H-bond interaction.59-60 Besides, the short alkyl chain of cysteine anion also have effect on the small value.61 The ΔvapH of the AAILs show marked influences by cyclic 16


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groups. For example, the ΔvapH of [Emim][Phe] is obviously small at 124.56 kJ·mol−1. By comparison, AAILs with an amino group on their cyclic groups show considerably higher ΔvapH because of formation of intermolecular H-bonds. For examples the ΔvapH of [Emim][His] and [Emim][Trp] (160.79 and 181.68 kJ·mol−1) are higher than those of AAILs with a phenyl group only. Other studies have found that the alkyl chain length of the imidazole cation

n

e t ΔvapH, which increases with increasing alkyl

chain length.62 In our studies, the ΔvapH of [Emim][Ala] calculated by MD simulation is 133.99 kJ·mol−1; the ΔvapH of [Bmim][Ala]63 is higher (162.3 kJ·mol−1) in the experiment. The ΔvapH of pure [Emim][AA] is similar to those of typical ILs. For example, the experimental ΔvapH of [Emim][BF4], [Emim][NTf2], and [Emim][PF6] are 157, 134 and 151 kJ·mol−1, respectively.25 Obviously, the ΔvapH of AAILs are relatively higher than those of ordinary molecular solvents because of strong electrostatic ionic interactions in the former. Table 2. Uionpair, Uinter, and ΔvapH calculated by MD methoda (kJ·mol−1) AAILs

Uionpair

Uinter

[Emim][Gly]

–383.39

[Emim][Ala]

vapH

MD

Expt

–533.72

152.81

139.7b

–413.42

–544.93

133.99

159.9b

[Emim][Val]

–371.00

–526.67

158.15

130.8c

[Emim][Leu]

–383.70

–528.77

147.55

[Emim][Ile]

–380.31

–527.57

149.74

[Emim][Ser]

–411.73

–553.27

144.02

[Emim][Thr]

–379.26

–555.64

178.86

[Emim][pro]

–386.23

–537.23

153.48

[Emim][Asp]

–387.93

–577.87

192.42

[Emim][Asn]

–384.41

–587.44

205.51 17



a

[Emim][Glu]

–380.28

–582.76

204.96

[Emim][Gln]

–374.00

–578.99

207.47

[Emim][Lys]

–377.27

–549.34

174.55

[Emim][Arg]

–367.75

–580.96

215.69

[Emim][Cys]

–402.28

–519.91

120.11

[Emim][Met]

–377.16

–529.77

155.09

[Emim][His]

–429.41

–587.72

160.79

[Emim][Trp]

–382.31

–561.51

181.68

[Emim][Phe]

–415.95

–538.03

124.56

[Emim][Tyr]

–397.70

–562.39

167.17

ionpair

U

244.1d

is the intermolecular energies between the cation and anion in the gas phase,

Uinter is the internal energies in the liquid state, b

Page 18 of 31

vapH

is the enthalpy of vaporization.

Reference,63 cReference,62 dReference.64

3.4 Formation Enthalpy 3.4.1 ΔfH of [Emim]+ and [AA]− The ΔfH of [Emim]+ and [AA]− are calculated through isodesmic reactions. An isodesmic reaction is a reaction in which numbers of the same bond type are conserved in the reactants and products.65 It can be used to correct systematic errors resulting from spin contamination and bond environments66-67 and increase the accuracy of calculated ΔfH in the gas phase.68 In this study, the ΔfH of 20 [AA]− anions and the [Emim]+ cation are obtained by isodesmic reactions (Scheme 1) and calculated by M06-2X/TZVP. To verify the accuracy of isodesmic reactions, the ΔfH of neutral amino acid molecules are first calculated. The ΔfH of neutral glycine can be estimated based on Reaction (1) in Scheme 1 and on the standard ΔfH of CH3COOH, CH3NH2 and CH4 which can be obtained from CRC Handbook of Chemistry and Physics.69 The ΔfH for neutral amino acid molecules are listed in Table 3. Compared with the experimental values of the seven neutral amino acid molecules Gly, Ala, Val, 18


Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Leu, Pro, Met, and Phe, the ΔfH of are −392.1, −465.9, −455.1, −486.8, −366.2, −413.5, and −312.9 kJ·mol−1, respectively. The corresponding theoretical ΔfH calculated by the M06-2X/TZVP method are in very close agreement with the experiment values, as the maximum difference is less than 8.57%. Dorofeeva et al. proved that theoretical values calculated by isodesmic reactions are more reliable than experimental data.70 NH2CH2COOH+CH4 NH2CH2COOH

N

N

N

N

N

N

CH3NH2+CH3COOH

(1)

NH2CH2COO-+H+

(2)

+3CH4+4NH3

3NH2CH3+NH2C2H5+CH2=CH2+CH2 =NH+CH3NH3+ (3)

+3CH4+4NH3

4NH2CH3+NH2C2H5+CH2=CH2+CH2 =NH2+

(4)

+3CH4+4NH3

4NH2CH3+NH3+C2H5+CH2=CH2+CH2=NH

(5)

Scheme 1. Isodesmic reactions of [Emim]+ and [Gly]–

The neutral amino acid can be decomposed into the corresponding anion and equal moles of proton; as an example, Reaction (2) of Scheme 1 presents the constructed isodesmic reactions of [Gly]–. The other corresponding reactions of 20 [AA]– can be seen in the Supporting Information ( Scheme S2 ). The ΔfH of 20 [AA]– anions are listed in Table 3. The calculated ΔfH of [Gly]− is −493.70 kJ·mol−1, interestingly, the ΔfH of [Glu]− is −936.34 kJ·mol−1. Such results are obviously due to the different functional groups in [AA]−. For [Glu]−, the contributions of the carboxyl group and intramolecular H-bonding to the ΔfH are very large. [Trp]− has the largest ΔfH among the AAILs studied, −350.20 kJ·mol−1, and the large volume of the amino 19



Page 20 of 31

acid anion may contribute to this observation ([Trp]− has the largest size among the 20 amino acid anions this work).71 Considering the dispersion of positive charges in the imidazole ring of [Emim]+, three isodesmic reactions for [Emim]+ are built to calculate ΔfH [Scheme 1, Reactions (3)–(5)]. The detailed calculation processes are given in the Supporting Information [Reactions (S1)–(S18)], and our calculated ΔfH of [Emim]+ is 659.19 kJ·mol−1. A similar result of 603.044 kJ·mol−1 for [Emim]+ was obtained from volume-based thermodynamics estimates by Klapötke et al.72 Therefore, the ΔfH calculated by isodesmic reactions are in good agreement with available experimental and theoretical values. 3.4.2 ΔfH of [Emim][AA] Based on the ΔdissH, ΔvapH,

(

(

and

(

(

values, the ΔfH

of 20 AAILs are explored through BH cycles, and the calculated values are listed in Table 3. As shown in the calculated results, the ΔfH of AAILs calculated by the M06-2X/TZVP method are generally more negative than those calculated by the MP2/Aug-cc-pVTZ method. For example, the ΔfH of [Emim][Gly] are –405.27 and –399.19 kJ·mol−1 when calculated by the M06-2X/TZVP and MP2/Aug-cc-pVTZ methods, respectively. As illustrated in Table 3, structure-property relationships are very important for AAILs. Many researchers have suggested that for more complex compounds, besides the total number of carbon atoms, ΔfH depends on the type, length, and degree of branching in the molecule, as well as the presence of non-organic atoms such as oxygen and heavy atoms.71 Thus, in our project, we considered that the ΔfH of 20 AAILs calculated by the corresponding BH cycles are 20


Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


related to not only ΔvapH, ΔdissH, ΔfH(cation), and ΔfH(anion) but also to the structure of [Emim]+ and especially [AA]−. The ΔfH of AAILs are affected by anion alkyl chain length. As the length of the anion alkyl chain in the ILs increases, the ΔfH of [Emim][Gly],

[Emim][Ala],

[Emim][Val],

and

[Emim][Leu]

obtained

by

M06-2X/TZVP gradually decrease to −405.27, −421.45, −492.79, and −495.23 kJ·mol−1, respectively. These results may be due to the electron-donating effect of the methyl group and van der Waals interactions between the alkyl chain.48 Some specific AAILs such as [Emim][Lys] and [Emim][Arg], wh h how ΔfH of −478.81 and −447.55 kJ·mol−1, respectively, are very interesting. The long alkyl chains of these AAILs allow for a large number of rotational degrees of freedom,73 which could increase ΔfH. Functional groups, such as the hydroxyl group (–OH), can influence the ΔfH significantly due to strong interatomic electronic effects.74-76 Thus, [Emim][AA] with hydroxyl groups have obviously lower ΔfH than AAILs with alkyl group. For example, compared with [Emim][Ala], [Emim][Ser] has more hydroxyl groups and a relatively lower ΔfH, −599.71 kJ·mol−1. A similar effect can be observed for [Emim][Tyr] and [Emim][Phe] with ΔfH of −504.25 and −301.08 kJ·mol−1, respectively. In [Emim][Thr], the hydroxyl group replaces the methyl group in [Emim][Val], which would cause intramolecular H-bonds to form between –OH and the O=CO‒ carbonyl group. Besides [Emim][AA], neutral amino acids with hydroxyl groups may also show increase ΔfH (Table 3). Mercapto group (–SH) also exert some influence on the ΔfH of the AAILs. The lower electronegativity of the mercapto group can weaken the 21



Page 22 of 31

stability of the amino acid anion, which would produce a relatively larger ΔfH. [Emim][Cys], which features a mercapto group, has a relatively higher ΔfH of −368.21 kJ·mol−1; [Emim][Ser] , which features a highly electronegative group (–OH) h

ΔfH of –599.71 kJ·mol−1. Because the –SH group in the side chain has

comparatively poorer H-bonding characteristics than other groups, the interactions between each ion are weaker.59 Thus, the ΔfH of [Emim][Cys] is clearly higher than those of most AAILs considered in this work. The ΔfH of AAILs with phenyl or pentacyclic groups are considerably higher than those with alkyl side chains, likely due to higher steric repulsion from the cyclic group. Steric effects limit free rotation between nearby substituent groups, and the corresponding stability of AAILs decreases as a result of this limitation.24, [Emim][Pro], for example, h

71, 77

ΔfH (−350.47 kJ·mol−1) that is higher than those of

most AAILs with open-chain groups. The ΔfH of [Emim][Phe], which features a phenyl group is higher than that of [Emim][Ala]. Dong et al. found that the cyclic structure of proline and phenylalanine may increase the ΔfH of their derivatives.20 Similarly, the ΔfH of [Emim][Tyr] is also considerably higher than that [Emim][Ser]. Because of the two-cycle structure of [Trp]−, [Emim][Trp]

e ent the l

e t ΔfH

(−280.81 kJ·mol−1) among the AAILs studied in this work. As demonstrated in Table 3, [Emim][Glu] has the lowest ΔfH (−879.70 kJ·mol−1) among the AAILs studied; this observation is attributed to [Glu]− containing more carboxyl groups than other AAILs. As a π-acceptor, the carboxyl group stabilizes the radical, which leads to an increase n

‒ (

bon o e .74 Such a result is in 22


Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


agreement with the available values. Albahri et al. calculated ΔfH using the structural group contribution method and suggested that carboxyl groups offer significant contributions to ΔfH, and the value is approximately −410.20 kJ·mol−1.71 The ΔfH of [Emim][Asp] is only slightly higher than that of [Emim][Glu], which features a similar functional group, becaue of the shorter alkyl chain of [Asp]−. AAILs with anions containing acylamino groups in [AA]− have lower ΔfH. For example, the ΔfH of [Emim][Asn] and [Emim][Gln] are −682.55 kJ·mol−1 and −710.63 kJ·mol−1, respectively. The strong H-bond between the amino group and carbonyl group ‒NH2···O=C(NH2 ‒ induce stability in the structures of [Asn] and [Gln], thus lowering their energies. Table 3. Formation enthalpies (kJ·mol−1) of AA, [AA]– and [Emim][AA] M06-2X/TZVP AAILs [Emim][Gly] [Emim][Ala] [Emim][Val]

[AA]–

[Emim][AA]

[Emim][AA]

a

–493.70

–405.27

–399.19

a

–528.48

–421.45

–413.72

a

–583.78

–492.79

–492.70

a

–596.61

–495.23

–489.33

AA –399.54 –425.96 –477.36

MP2/Aug-cc-pVTZ

–392.1 –465.9 –455.1

[Emim][Leu]

–492.44

–486.8

[Emim][Ile]

–497.72

–604.56

–497.75

–500.02

[Emim][Ser]

–585.14

–716.87

–599.71

–603.69

[Emim][Thr]

–629.64

–766.79

–674.87

–673.09

[Emim][Pro]

–339.75

–443.66

–350.47

–345.82

[Emim][Asp]

–789.00

–919.10

–851.73

–875.68

[Emim][Asn]

–596.19

–764.72

–682.55

–683.16

[Emim][Glu]

–807.84

–936.34

–879.70

–902.56

[Emim][Gln]

–619.20

–782.23

–710.63

–709.02

[Emim][Lys]

–447.88

–556.71

–478.81

–478.58

[Emim][Arg]

–366.28

–496.51

–447.55

–444.35

[Emim][Cys]

–374.94

–493.35

–368.21

–358.24

–366.2a

23



a

[Emim][Met]

–417.94

[Emim][His]

–413.5(expt)a

Page 24 of 31

–538.87

–435.73

–431.76

–259.70

–376.87

–283.16

–278.68

[Emim][Trp]

–230.45

–350.20

–280.81

–275.00

[Emim][Phe]

–319.83

–435.89

–301.08

–300.10

[Emim][Tyr]

–479.16

–599.32

–504.25

–504.85

–312.9(expt)a

Experimental values from the CRC Handbook of Chemistry and Physics.

4. Conclusions Systematic study of the gas-phase ion pairs of AAILs, i.e., [Emim][AA], provides new insights into the structure, ΔdissH, ΔvapH, ΔfH, and structure-property relationships of these AAILs. Calculations were performed via ab initio method, MD simulation, BH cycle, and isodesmic reactions. The [Emim][AA] ILs were formed by coupling the imidazolium cation [Emim]+ with 20 natural amino acid anions, [AA]−. All of the ion pairs of [Emim][AA] can form strong H-bond interactions, which are dominated by the side-chain structure and the functional group of the amino acid anions. The calculations indicate that the presence of a phenyl group produces a decrease n ΔdissH, while functional groups induce more extensive delocalization of negative charges over the anion, consequently reducing the corresponding interaction energies. The ΔvapH obtained by MD are consistent with the reported experimental observations. The larger ΔvapH of [Emim][AA] with amino group on [AA]− indicate that H-bond interactions obviously contribute to the ΔvapH obtained. Combining the BH cycles, quantum chemical calculations, MD simulations, and isodesmic reaction results, the ΔfH of 20 [Emim][AA] are predicted. Strong interatomic electronic effects and H-bonds caused by the hydroxyl, carboxyl, and acylamino groups lead to lower ΔfH. Furthermore, the 24


Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


low electronegativity of mercapto group and high steric repulsion from cyclic groups increase the ΔfH of [Emim][AA]. The structure-property relationship analysis of 20 kinds of [Emim][AA] ILs provides some initial factors contributing to the future design of novel functionalized AAILs. The cation-anion electrostatic attractions were observed to affect the melting point, and the van der Waals repulsions of the alkyl chains can be applied to estimate the cohesively energy and transformation property, intermolecular - interactions partially resulted in the nanostructured organization, and 3D H network of H bonds could describe the solubility especially for dissolving biomaterials. And those factors are interplayed to influence the properties of ionic liquid systems, which are interesting and challenging subjects for further studies. Acknowledgements: We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (No. 21373104) and Program for Liaoning Excellent Talents in University, China (No. LJQ2015042). Supporting Information: Table S1 lists the detailed force field parameters for [Emim][AA]. H-bonded lengths and angles for [Emim][AA] optimized by M06-2X/TZVP are listed in Table S2. The charges of the atoms calculated by the RESP method are shown in Figure S1. The site-site radial distribution functions gH-O(r) are ploted for [Emim][AA] in Figure S2. The structural formula for [Emim]+ and [AA]– can be seen in Scheme S1. The isodesmic reactions of 20 [AA]– can be seen in Scheme S2. Detailed isodesmic reactions used to calculate the ΔfH of [Emim]+ and [Gly]– are also shown. This material is available free of charge through the Internet at http://pubs.acs.org. 25



Page 26 of 31

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Theoretical Calculation of the Thermodynamic Properties of 20 Amino

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