Amino acid ionic liquids based on imidazolium-hydroxyl functionalized cation: New insight from molecular dynamics simulations

Accepted Manuscript Amino acid ionic liquids based on imidazolium-hydroxyl functionalized cation: New insight from molecular dynamics simulations

Mostafa Fakhraee PII: DOI: Reference:

S0167-7322(18)33381-6 https://doi.org/10.1016/j.molliq.2019.01.109 MOLLIQ 10340

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

2 July 2018 26 December 2018 21 January 2019

Please cite this article as: M. Fakhraee, Amino acid ionic liquids based on imidazoliumhydroxyl functionalized cation: New insight from molecular dynamics simulations, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.01.109

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ACCEPTED MANUSCRIPT

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Amino Acid Ionic Liquids Based on ImidazoliumHydroxyl Functionalized cation: New Insight from Molecular Dynamics Simulations

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Mostafa Fakhraee* Department of Chemistry, Sharif University of Technology, Tehran, 11365-11155, Iran

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(*Corresponding author e-mail: [email protected])

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ACCEPTED MANUSCRIPT Abstract Various thermodynamic and structural properties of amino acid ionic liquids (AAILs), comprising 1-(2Hydroxyethyl)-3-methyl imidazolium ([C2OHmim]+) cation mixed with Glycinate [Gly], Serinate [Ser], Alaninate [Ala], and Prolinate [Pro] AA anions are explored using molecular dynamic (MD) simulations

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and quantum theory of atoms in molecules (QTAIM) analysis. In general, the simulated thermodynamic

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results are in good agreement with the reported experimental data. Structural dependence of vdW- and

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electrostatic energies of AAILs is [Pro] > [Ala] > [Ser] > [Gly] and [Gly] > [Ala] > [Pro] > [Ser], respectively. The similar trend of electrostatic energies is found for their interaction energies. Molecular

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level information on organization of ions are attained by computing site-site cation-cation, cation-anion, and anion-anion radial- and spatial distribution functions (RDF and SDF). According to the cation-cation

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RDFs and SDFs results, the weakest cation-cation interactions belongs to [C2OHmim][Gly] IL and strongest of corresponding interactions is attributed to AAILs composed of [Pro] anions. On the contrary,

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the reverse trend of cation-cation RDFs and SDFs is observed for cation-anion interactions. These results

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are also confirmed by reduced density gradients (RDG). Site-site anion-anion RDFs between acidic hydrogen and interactive site of AA anions (carrying high negative charge) become sharper and more

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intensive in the following order: [Ser] < [Gly] < [Ala] < [Pro]. This trend arises from more association of

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the smaller anions such as [Gly] and [Ala] with their cations, which leads to weakening cation-cation and anion-anion interactions. Even though, [Ser] has more polar group than [Ala], more intramolecular

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hydrogen bonds in [Ser] anion are believed to be responsible for aforementioned trends. These results are in excellent consistency with the calculated thermodynamic properties, RDG, and interaction energies. Amazingly, intramolecular hydrogen bond between O atom of –OH group and H atoms of imidazoliumring can be clearly understood from the average structures of cations. Keywords: amino acid ionic liquids, thermodynamic and structural properties, molecular dynamics simulation, quantum theory of atoms in molecules analysis, intramolecular hydrogen bond

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ACCEPTED MANUSCRIPT 1. INTRODUCTION Ionic liquids (ILs), known as ambient temperature molten salts, having melting point below 100 o

C.1 They are being widely applied in many application areas, and have attracted most attention

in theoretical and experimental studies, due to their extraordinary properties.1-4 One of the most

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valuable researches on ILs is to relate physicochemical properties of these compounds to their

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structures and composition of the ions.4-8 This allows designing tailor-made ILs regarding to

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specific applications.3 Indeed, a microscopic knowledge on properties of ILs is needed for following such approach. With the aid of molecular dynamics (MD) simulation as a predictive

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tool, precious information about structure and microscopic properties of ILs can be obtained.5-8

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Regarding to the reported studies, many ILs seem to be toxic to aquatic organisms and human

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cells.2,9-11 Therefore, the toxicity of these materials remains one of the main obstacles, limiting their commercial applications.9,10 Nowadays, synthesizing ILs with the least toxicity is

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intended.11 Toward this aim, embedded oxygen in the form of ester, ether, and hydroxyl

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functional groups into the side chain of imidazolium cation decreased the toxicity.9-11 For this reason, hydroxyl-functionalized ILs have been synthesized and meticulously investigated.11-17

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Imidazolium-based ILs with hydroxyl functionality have applications in many diverse areas such

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as capturing greenhouse gases,18-26 stabilizer solvents for the synthesis of nanostructure material and metal oxide powders,27-30 suitable solvents for increasing the selectivity of Diels-Alder reactions,28-32 improving the enzyme activity of ILs.27 Additionally, amino acid ionic liquids (AAILs) are a new class of ILs that indicate low toxicity due to their biological nature.2,4,11 Therefore, combination of amino acid anions with low toxic cations results in biocompatible, low toxic, and biodegradable ILs.2,4,11 Recently, Ghanem et al.11 synthesized and characterized four new ILs based on 1-(2-hydroxyethyl-3-methylimidazolium) cation ([C2OHmim]) with

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ACCEPTED MANUSCRIPT glycinate [Gly], serinate [Ser], alaninate [Ala], and prolinate [Pro] anions. They measured their thermophysical properties such as density, viscosity, surface tension, heat capacity, molecular volume, standard molar entropy, and lattice energy.11 Besides, their results showed that that these new AAILs can be considered as eco-friendly compounds.11

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Keeping the above facts in mind, the present study states the results of MD simulations and

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QTAIM analysis on hydroxyl-functionalized ILs based on the [C2OHmim] cation partnered with four AA anions, including [Gly], [Ala], [Ser], and [Pro]. Ball and stick models of chosen ILs

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alongside their atomic labels are depicted in Figure S1 of the Supporting Information. Target ILs in this study are purposefully selected, so that the effects of hydroxyl functional group (-OH) and

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different combination of anions will be monitored. Eventually, the achieved results of these ILs

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would be compared with the experimental results of analogous and homologous imidazolium-

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based ILs. 2. COMPUTATIONAL METHOD

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2.1. Molecular dynamics simulations

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The AMBER33 and OPLS-AA34 force field parameters were utilized for simulation of anions and cation, respectively. The details of applied force fields are provided in the Supporting

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Information. In order to generate starting structure of each system, isolated ion pairs were optimized at B3LYP/6-311++G(d,p) level of theory with the Gaussian 03 suite of programs.35 Afterwards, the optimized structures were randomly replicated 216 times into a large simulation box. The MD package DL_POLY 2.18 36 was used for simulating target systems under periodic boundary condition. These configurations were randomized by annealing simulations at desired temperature for 1 ns, followed by long-time simulations (5 ns) to achieve equilibrium. Finally, 4 ns production runs were performed to estimate thermodynamic and structural properties within 4

ACCEPTED MANUSCRIPT temperature range of 293.15-373.15 K. The equations of motion were integrated using the Verlet leapfrog algorithm37 using a time step of 1 fs. Isothermal-isobaric ensemble (NPT, P=1 atm) simulations were conducted using Nosé-Hoover thermostat and barostat,38 with relaxation times of 0.1 and 0.5 ps, respectively. Short-range interactions were considered up to 16 Å and

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electrostatic interactions were taken into account using the Ewald summation method with a

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precision of 1 × 10-6.39

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2.2 Ab initio calculations

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The first insight into the nature of cation-anion interactions were obtained by optimizing the structures of isolated ion pairs using Gaussian 03 package.35 Afterwards, Electronic, binding, and

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interaction energies of the most stable conformers were calculated at B3LYP/6-311++G(d,p)

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theoretical level. Subsequently, the achieved optimized structures from the ab initio calculations were utilized for computing the reduced density gradient (RDG) using QTAIM method at the

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same level of theory. Eventually, the ab initio results were used for interpreting the MD findings.

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3. RESULTS AND DISCUSSION 3.1. Thermodynamic properties

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3.1.1. Volumetric properties

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To evaluate utilized force field and conducted simulation, several thermodynamic properties such as density, isobaric heat capacity, surface tension, molecular volume, standard entropy, and lattice energy are estimated and compared with experimental data reported by Ghanem et al. 11 in temperature range of 293.15-373.15 K. Fig. 1 shows the simulated and experimental densities (ρ) versus temperature. The corresponding numerical data of the simulated density and molar volume are given in Table S1 and Table S2 of the Supporting Information, respectively. Simulated density is observed to follow the trend: 5

ACCEPTED MANUSCRIPT [C2OHmim][Gly] > [C2OHmim][Ser] > [C2OHmim][Pro] > [C2OHmim][Ala]. Larger steric hindrance of [Pro] anion is probably compensated by its higher molecular mass. In general, the simulated densities are in good agreement with the experimental data.11 The obtained densities are clearly lower than those for ILs composed [C2OHmim] cation paired with typical inorganic

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anions.20 It may be related to higher steric hindrance of larger AA anions, which leads to a

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remarkable reduction in the packing efficiency. On the contrary, the simulated density in this

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work are higher than those of analogous AAILs containing non-functionalized cation

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([C2mim][AA]),2,4,40 due to the presence of –OH functional group.

The density values as a function of temperature can be used to determine the thermal expansion

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coefficient (αP) at atmospheric pressure using Eq. 1, as follows:41

(1)

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1    P       T  P

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where ρ is the density. The simulated and experimental values of αP are summarized in Table S3

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of the Supporting Information. As it can be seen, both of αP data slightly decreased with increasing temperature. The higher values of αP can be found for [C2OHmim][Pro]. This may be

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related to the reduction in the packing efficiency, owing to the considerable steric hindrance of

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its voluminous anion.20

The molecular volume (V), standard molar entropy (S0), and lattice energy (Upot) of AAILs were calculated using Eqs. 2-4.11,41,42 Lattice energy is an important thermodynamic quantity to evaluate the stability of an ionic material, which can be defined as the energy required to remove the ions from their locations in the crystal structure to infinite separation.

V

V M  m NA  N

(2)

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ACCEPTED MANUSCRIPT  V  S 0 (298)  1246.5  3   29.5  nm 

(3) 1/ 3

     U pot  2 I  1/ 3     1981.2   M  V 

 103.8

(4)

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where M is molar mass, Vm implies to the molar volume, NA denotes to the Avogadro’s constant,

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V is the molecular volume, and ρ is the density.11,41 Glasser established that there is linear

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relationship between standard entropies-molecular volumes of ionic solids and organic liquids.42 According to the fact that an IL possesses large organic cations, Glasser applied average values

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of the linear correlation constants to achieve the absolute entropies at ambient conditions (Eq.

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3).42 In eq. 4, α and β are fitted coefficients and I is the ionic strength factor, which are equal to 117.3 kJ mol−1, 51.9 kJ mol−1 and 1, respectively for the case of an IL of formula MX salts with

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charge ratio (1:1).42 The computed results of these thermodynamic properties at 298.15 K are

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summarized in Table 1. The molecular volume increases in the following trend: [C2OHmim][Gly] < [C2OHmim][Ala] < [C2OHmim][Ser] < [C2OHmim][Pro]. A similar trend

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of molecular volumes is exactly observed for the standard molar entropies. Furthermore, the

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reverse order of molecular volume can be seen for the lattice energies. Totally, molecular volumes and standard molar entropy of the selected hydroxyl functionalized AAILs in the

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current work are lower than those reported for non-functionalized AAILs.40 which is a sign of better organization in the existence –OH functional group.11 This is also confirmed by the higher values of the lattice energies of hydroxyl functionalized AAILs. It may be caused by formation of the stronger hydrogen bonds network due to the presence of hydroxyl group. The predicted results of V, S0, and Upot are in good consistency with the experimental data.11 3.1.2. Enthalpy of vaporization and surface tension

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ACCEPTED MANUSCRIPT To calculate enthalpy of vaporization ( H vap ), long-time gas-phase simulations (40 ns) were m performed on a single ion pair of chosen ILs in NVT ensemble. The enthalpy of vaporization can be defined, as follows:41,43,44 H mvap (T )  U m,gas -U m,liquid  RT

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(5)

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where Um,gas and Um,liquid imply to the molar configuration energy of gas and liquid phase,

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respectively. The obtained values of H vap are indicted in Fig. 2 and their numerical values are m

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presented in Table S4 of the Supporting Information. The computed results disclosed a

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descending trend of H vap with increasing the temperature. The enthalpy of vaporization varies m in the following order: [C2OHmim][Ser] < [C2OHmim][Ala] < [C2OHmim][Gly] <

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[C2OHmim][Pro].

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The surface tension (σ) was computed over a temperature range of 293.15 K. to 353.15 K using the empirical equation of Zaitsau et al.,45 as follows: vap H m  A ( Vm2 / 3 N 1A/ 3 )  B

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(6)

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The first term of Eq. 6 is related to the dispersive interactions in which A = 0.01121, Vm is the molar volume, and NA denotes the Avogadro’s number. Also, non-dispersive forces such as

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dipole-dipole, and electrostatic are considered in Eq. 6 with B constant, which is equal to 2.4 kJ.mol-1.41,45 Fig. 3 compares the predicted and experimental data of the surface tension. Numerical values of surface tension are collected in Table S5. The maximum value of surface tension is ascribed to AAILs composed of [Gly] anion, followed by [Ala] anion and the lowest of the corresponding property is attributed to AAILs containing [Pro] and [Ser] anions, respectively. This trend is in excellent agreement with the reported experimental data.11

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ACCEPTED MANUSCRIPT 3.1.3. Isobaric heat capacity The isobaric heat capacity (CP) for each system was estimated from procedure described by Cadena et al.,46 as follows: total potential energy (U) splits into intermolecular (UNB) and intramolecular (UINT ) terms. Therefore, total enthalpy splits into ideal gas- and residual terms,

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H=Hid + Hres, where Hid = UINT + K + NkBT and Hres = UNB + PV – NkBT. Then, isobaric heat

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isobaric heat capacity can be expressed by means of Eq. 7: 1,5,41,46

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capacity is defined as CP (T,P) = CPid (T) + CPres (T,P).5,41,46 The residual contribution of the

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NB    H res       U    P V    Nk B C Pres (T , P)    T     T  P   P  T  P

(7)

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To obtain the ideal gas contribution to the isobaric heat capacity, thermochemistry calculations in

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the frequency analysis were performed on the isolated cation and anion, separately at B3LYP/6-

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311++G(d,p) level of theory (Table S6 of the Supporting Information). 5,41,46 The simulated and experimental values of CP were compared in Fig. 4 and their numerical values are summarized in

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Table S7 of the Supporting Information. An ascending trend of CP can be observed with increasing temperature, because of the activation of more vibrational degrees of freedom.41 Over

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the whole temperature range, the isobaric heat capacity follows the trend [C2OHmim][Ala] >

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[C2OHmim][Pro] > [C2OHmim][Ser] > [C2OHmim][Gly].11 Although nearly large deviations at some target temperatures between simulated and experimental results of CP are viewed, it seems to be reasonable.5

3.1.4. Refractive index The refractive index (nD) determines how much light is bent, when entering a material. The Lorentz−Lorenz relationship was applied to estimate nD, as:41

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ACCEPTED MANUSCRIPT 2  nD  1  N  V  A    n 2  2  m 3 0  D 

(8)

where Vm is the molar volume, ε0 denotes to the vacuum permittivity, and α is the mean polarizability. In order to calculate nD, the mean polarizability was computed using ab initio

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calculations at the B3LYP/6-311++G(d,p) level.41 Refractive index decreased with increasing

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temperature (Table 2). The high packing efficiency in AAIL composed of [Gly] anion, the more light is refracted. Electron cloud for the bulkier anion [Pro] is easily distorted by an external

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electric field of neighbor molecules. As a result, [C2OHmim][Pro] has higher values of nD than [C2OHmim][Ser] and [C2OHmim][Ala]. Eventually, the more bending light in [C2OHmim][Ala]

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can be observed than [C2OHmim][Ala], due to the higher density values of [C2OHmim][Ala].

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3.2. Structural properties

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3.2.1. VdW, electrostatic, and interaction energy The predicted results of vdW (EvdW), electrostatic (Eelect), and interaction energies (Eint) of studied

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AAILs are collected in Table 3. Electrostatic energies are dominant interactions in the bulk phase

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of these AAILs. Structural dependence of vdW energies is as follows: [Pro] > [Ala] > [Ser] > [Gly]. Albeit, [Ser] anion possess an oxygen atom more than [Ala] anion, smaller vdW energies

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is unexpectedly observed for [Ser] anion over the whole target temperature. Highest electrostatic interaction is attributed to the ILs composed of [Gly], followed by [Ala], and the weakest cationanion association belongs to AAILs consisting of [Pro] and [Ser] anions, respectively. The interaction energies (Eint) can be calculated using Eq. 9 41

Eint  EvdW  Eelect

(9)

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ACCEPTED MANUSCRIPT The same trend of electrostatic energies is found for interaction energies. Lower steric hindrance of [Gly] anion and consequently interactions at the closer distances are believed to be responsible for their higher interaction energies. This trend is comparable with the reported data for nonfunctionalized imidazolium-based ILs coupled with AA anions.2

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Moreover, electronic (ΔEele), binding (ΔEbin), and interaction energies (ΔEint) of the most stable conformer of selected AAILs were determined at B3LYP/6-311++G(d,p) level of theory using

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the following formulas:47

(10)

Ebin  E( B3LYP) IL  E( B3LYP) cat  E( B3LYP) ani   (ZPVE )

(11)

Eint  E( B3LYP) IL  E ( B3LYP) cat  E( B3LYP) ani   (ZPVE )  (TE)

(12)

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Eele  E( B3LYP) IL  E( B3LYP) cat  E( B3LYP) ani 

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where E(B3LYP) denotes to the electronic energy, ZPVE implies to the zero-point vibrational energy, and TE is the thermal energy.47 It is worthy of note that the basis set superposition errors

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(BSSE) were removed by use of the counterpoise method (CP).48 Exactly the same trends of MD

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results can be observed for the ab initio energies. It can be observed from Table 4, the strongest cation-anion interactions is ascribed to AAILs consist of [Gly], followed by and [Ala] and [Pro],

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and the weakest of the matching interactions belong to [C2OHmim][Ser]. Totally, the observed trend for interaction energies is in good consistency with the lattice energy (Upot) and surface tension (σ) findings. 3.2.2. Non-covalent interactions QTAIM analysis by computing RDG was used to get insight into the weak non-covalent interactions of optimized AAILs structures, which is defined using Eq. 13:49,50

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ACCEPTED MANUSCRIPT RDG 



1 2(3 )

2 1/ 3

(13)

 4/3

Fig. 5 indicates the most stable configurations of chosen AAILs accompanied with the RDG isosurfaces. It is worth nothing that vdW interactions indicates in green color, blue color denotes

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to the regions with strong attraction, while the regions with strong repulsive are shown in red

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color.49-51 The RDG isovalue is 0.7 au and Sign (λ2)*ρ values have been mapped onto the RDG

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isosurfaces in the region from -0.03 to +0.03 au.49-52 As it can be seen from Fig. 5, O atoms of anions significantly oriented toward H2 site and their strong interactions are clearly distinguished

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by blue color. Besides, N site of [Gly] and [Ala] anions is strongly interacted with H9 site of

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cation. Due to the presence of OH functional group in [Ser] anion, a competition between HN and OH sites to interact with cation is seen. This brings about weakening H9-N and O-HN interactions.

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As an outcome, the weakest cation-anion interactions can be observed for [C2OHmim][Ser].

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Interestingly, vdW interactions are dominant in this AAIL. Precisely, weakening cation-anion interactions in AAILs composed of [Ser] anion is related to the existence of OH functional

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group. Also, O atoms of [Pro] anion have strong interactions with H2 and H9 simultaneously,

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then, the weakest N-H9 interactions can be viewed for [C2OHmim][Pro]. Such orientation may be related to the high steric hindrance of [Pro] anion. Intramolecular hydrogen bond between O

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and HN atoms of anions is the main feature of RDG results. This intramolecular hydrogen bond in [Ser] anion would be deteriorated owing to N-HO intramolecular interaction. The unexpected trends of vdW-, electrostatic-, and interaction energies for AAILs containing [Ser] and [Ala] are vindicated by the RDG findings. 3.2.3. Cation-cation interactions Radial distribution function (RDF) provides microstructure information into the nature of interactions as well as the arrangement of ions, which can be expressed as Eq. 14:52,53 12

ACCEPTED MANUSCRIPT Ni N j    V g ij (r )    r  r i (t )  r j (t )  N i N j i 1 j i 1 



    t

(14)

where the bracket denotes to ensemble average on the distance between atoms of types i and j. Due to the presence of –OH functional group, site-site cation-cation interactions should be

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emphasized. Fig. 6 indicates site-site cation-cation RDFs at 373.15 K accompanied with the

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average structures of cations. Intramolecular hydrogen bond between O9-H2 and O9-H5 can be

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understood from the average structure of cation. Neighbor cations interact with reference cations

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mainly via H5, followed by H2. Even though, H4 and H5 sites have nearly the similar position, formation of O9-H5 intramolecular hydrogen bond is appears to be responsible for this

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observation. As it can be seen from Fig. S2, the other Hn sites of a cation have lower contribution to cation-cation interactions. The intensity of O9-Hn RDFs decreases in the following trend: [Pro]

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> [Ser] ≈ [Ala] > [Gly] (Fig. 6).

Further information periphery cation-cation interactions are achieved from visualizing the spatial

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distribution functions (SDFs) of H2, H5, and H9 around O9 site. The SDFs were calculated by means of trajectory analyzer and visualizer (TRAVIS)53 from 4 ns simulations at 373.15 K. As

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can be seen from Fig. 7, O9 site mainly reside around H5, followed by H2 and H9, respectively.

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The thickness of these isosurfaces becomes bulkier with increasing the size of anions, suggesting stronger interionic interactions among cations composed of bulkier anions. This is comparable with the cation-cation RDFs results. 3.2.4. Cation-anion interactions Additional information of ions interactions can be gained by computing cation-anion RDFs. It should be emphasized that the strong sites of anions, carrying high negative charge, were chosen based on ESP (Electrostatic Potential Map) results in Fig. S3. According to the site-site cation13

ACCEPTED MANUSCRIPT anion RDFs in Figs. 8 and 9, the stronger cation-anion association is attributed to ILs containing [Gly] and [Ala] anions. The weaker ones belong to ILs composed of [Ser] and [Pro] anions, which is due to the more involvement of their cations in cation-cation interactions (Figs. 6 and 7). Actually, the strongest cation-anion interactions mostly occurred through N and O atoms of

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reference anions with H9 site of surrounding cations. To be more precise, the first peak intensity

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of O-Hn and N-Hn sites increases in the following order: H5 < H4 ≤ H2 < H9. This trend is in a

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good agreement with RDG findings. The N-H9 sites RDFs begin at smaller distances ~ 1.5 Å (~ 1.75) for [C2OHmim][Pro]) in comparison with the other N-Hn RDFs (~ 2.25 Å). Nearly the

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similar trend can be found for O-Hn RDFs. In general, O-Hn RDFs have higher intensity than

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N-Hn RDFs. The weaker O-Hn and N-Hn RDFs are shown in Figs. S4 and S5 of the Supporting Information. Furthermore, RDFs between O9 site of cation and HN site of anion is depicted in

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Fig. S5. These results are in comparable with the RDG findings.

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Additionally, [Ser] anion has OH site which thought to be interactive with acidic hydrogen of

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cation. Fig. 10 represents the cation-anion OH-Hn RDFs for [C2OHmim][Ser] IL. The maximum intensity of OH-H9 RDF occurs at smaller distance (~1.8 Å) than those for the other

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corresponding sites (~2.5 Å). Weaker OH-Hn RDFs are demonstrated in Fig. S6. Owing to the

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existence of OH functional group in this anion, the weakest cation-anion interactions is attributed to [C2OHmim][Ser], which is previously verified by the interaction energies and RDG results. In general, stronger interactions are happened along OH-Hn sites than N-Hn for [C2OHmim][Ser] IL. Bearing in mind the cation-anion RDFs results, the SDFs between N-H9 and O-H9 sites are depicted in Fig. 11. In this presentation, anions are reference and the probabilities of the presence of neighbor cations are observed. A competitive behavior with a remarkable propensity for O atom to interact with H9 in comparison with N-H9 is found, which is figured out by the thickness 14

ACCEPTED MANUSCRIPT and orientation of O-H9 isosurfaces. Furthermore, O-H9 isosurfaces are placed at the inner shell of interactions area, while for another one is located at outer shell. 3.2.5. Anion-anion interactions Owing to the presence of acidic hydrogen in anions such as HN in all target anions as well as HO

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site in [Ser], anion-anion interactions are considered by calculating O-HN, N-HN RDFs in Fig. 12. The most outstanding feature of these RDFs is the strong correlation between O atoms of

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references anions and HN sites of neighbor anions, which becomes stronger in the following

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order: [Ser] < [Gly] < [Ala] < [Pro]. The maximum intensity of aforementioned RDFs is located at around 1.6 Å and moves toward 2 Å for IL composed of [Ser] anion. Taking into account the

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average structures of anions in Fig. 12, this strong O-HN correlation comes from the

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intramolecular hydrogen bond. This finding is previously confirmed by the RDG results. The OHN SDFs are also shown in Fig. 12. The direct O-HN interactions are mostly seen for [Pro] anion,

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followed by [Ala], whereas this correlation becomes weaker in [Gly] anion, which can be easily

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realized by declining the thickness of isosurfaces. The lowest intensity of O-HN RDFs is observed for [Ser] at further distances. This is due to the presence of OH group as a second polar

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group in this anion. With a closer look to the average structure of [Ser] anion, a simultaneous

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competition for hydrogen-bond-formation between O…HN and HO…N sites can be observed. The finding is in good consistency with SDF isosurface of O-HN in [Ser] anion in Fig. 12. Although, [C2OHmim][Ser] IL has almost similar cation-cation RDFs to [C2OHmim][Ala] and weaker cation-anion interactions than [C2OHmim][Ala], this is compensated with anion-anion interactions. Regarding to the existence of –OH functional group in [Ser] anion, more RDFs are required to have better evaluation of anion-anion interactions in this anion. As it can be simply viewed from Fig. 13, the highest RDF peak intensity is ascribed to N-HO interaction that is 15

ACCEPTED MANUSCRIPT positioned at nearly 1.5 Å. Even though, the SDF isosurface represents strong intermolecular interactions between N and HO sites, the strong intramolecular hydrogen bond (as previously proved by the average structure of [Ser] anion in Fig. 12) is believed to be responsible for intensive RDF peak of N-HO sites. This result is also verified by optimizing structures of anions

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at B3LYP/6-311++G(d,p) level of theory in Fig. S7. Likewise, OH-HN and O-HO RDFs contain

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two consecutive and intense peaks. Precisely, RDF of OH-HN is composed of two successive

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peaks, where the first one is located at 2 Å and the second one is occurred at approximately 3.3 Å. The similar RDF pattern is viewed for O-HO sites with lower intensity than OH-HN RDF at

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farther distances.

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In addition, N-HN is another path should be considered in anion-anion interactions. According to

M

Fig. 12 (inner panel), a broad peaks can be found for N-HN RDFs. Regardless of [Ser] anion, RDFs reach to their maximum based on the size of anions. It is worthy of note that N-HN RDF

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for [C2OHmim][Ser] starts at shorter distance than the other AAILs and includes three

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successive peaks, placing at almost 2 Å, 2.5 Å, and 9 Å, respectively. As it was already viewed for O-HN RDFs (Fig. 12, outer panel), the intensity of N-HN RDFs becomes broader with

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decreasing the size of anions, as: [Pro] > [Ala] > [Gly].

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Summing up, strongly associated cation-anion for AAILs composed of [Gly] and [Ala] can be understood, which leads to less correlation of cation-cation and anion-anion interactions for these AAILs. Cation-anion interactions for weakly coordinating anion, including [Pro] anion are substituted by strong cation-cation and anion-anion interactions. Eventually, interactive sites of [Ser] anion are mostly occupied by intramolecular hydrogen bond, therefore, the weakest cationanion and anion-anion interactions belong to AAIL containing this anion. Given the fact that cation-anion interaction have the main contribution to the interaction energies, the minimum 16

ACCEPTED MANUSCRIPT values of interaction energies is viewed for [C2OHmim][Ser]. These findings are in good agreement with the predicted values of surface tension and lattice energy. 4. CONCLUSIONS

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Molecular dynamics simulations and QTAIM analysis were performed to calculate

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thermodynamic and structural properties of amino acid ionic liquids (AAILs), containing 1-(2-

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Hydroxyethyl)-3-methyl imidazolium ([C2OHmim]+) cation paired with Glycinate [Gly], Serinate [Ser], Alaninate [Ala], and Prolinate [Pro] AA anions. Several thermodynamic

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properties such as density, molar volume, thermal expansion coefficient, isobaric heat capacity,

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surface tension, molecular volume, standard entropy, lattice energy, and refractive index were determined. The simulated thermodynamic properties were in good agreement with the reported

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experimental data. VdW- and electrostatic energies increased in the following order: [Pro] >

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[Ala] > [Ser] > [Gly] and [Gly] > [Ala] > [Pro] > [Ser], respectively. The same trend of electrostatic energies can be observed for the interaction energies. These orders were also

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verified by the RDG results. The RDG results indicated the strong intermolecular O…H2,

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N…H9, and O…HN hydrogen bonds for all selected AAILs. Site-site cation-cation, cation-anion, and anion-anion RDFs and SDFs were evaluated to obtain structural information from

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microscopic point of view. Cation-cation RDFs and SDFs findings revealed that the weakest interionic interactions is ascribed to [C2OHmim][Gly], while the strongest of matching interactions is attributed to [C2OHmim][Pro] AAIL. Cation-anion RDFs and SDFs demonstrated the opposite trend of above-mentioned interactions, which is in an excellent consistency with the RDG findings. Intramolecular hydrogen bonds between O atom of –OH functional group and H atoms of imidazolium-ring can be understood from cation-cation interactions. Interestingly, the intensity of anion-anion RDFs varies in the following order: [Ser] < [Gly] < [Ala] < [Pro]. This 17

ACCEPTED MANUSCRIPT was rationalized by more involvement of the smaller anions such as [Gly] and [Ala] with their cations. Therefore, weaker association of cation-cation and anion-anion was observed for these AAILs. Although, [Ser] anion has more polar group than [Ala], the weakest cation-anion and anion-anion interactions were found for AAIL composed of this anion. According to the average

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structures of anions, more intramolecular hydrogen bonds network in [Ser] anion are suggested

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AN

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computed values of surface tension and lattice energy.

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to be responsible for observed conflict. Totally, structural results are in good agreement with the

18

ACCEPTED MANUSCRIPT SUPPORTING INFORMATION 1. Force field function and parameters, 2. Ball and stick model of the chosen AAILs, 3. Numerical values of the thermodynamic properties such as density (Table S1), molar volume (Table S2), thermal expansion coefficient (Table S3), molar enthalpy of vaporization (Table S4), surface tension (Table S5), ideal

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contribution to the isobaric heat capacities (Table S6), isobaric heat capacities (Table S7),

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3. Structural properties.

AUTHOR INFORMATION

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Corresponding Author

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* E-mail: [email protected].

ACKNOWLEDGMENTS

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The author is grateful to the High Performance Computing Centre (HPCC) of Sharif University

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CE

PT

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of Technology for generously providing the computing facilities.

19

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ACCEPTED MANUSCRIPT Table 1. Simulated and experimental molecular volume (V), standard molar entropy (S0), and lattice energy (Upot) at 298.15 K. V (nm3)

S0 (J mol−1 K−1) Sim. Exp.

UPOT (kJ·mol−1) Sim. Exp.

ILs

Sim.

Exp.

[Gly]

0.2471

0.2443

337.5

334

477.7

[Ala]

0.2949

0.2915

397.1

393

456.3

458

[Ser]

0.2960

0.2935

398.4

395

455.8

457

[Pro]

0.3283

0.3244

438.7

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Quantity

434

443.9

445

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ACCEPTED MANUSCRIPT Table 2. Simulated refractive index (nD) data accompanied with their uncertainties at the rage of 293-373 K. nD [C2OHmim][Ser] Sim. 1.5370±0.0052 1.5329±0.0051 1.5290±0.0056 1.5269±0.0060 1.5235±0.0057 1.5218±0.0067 1.5185±0.0073 1.5166±0.0066 1.5141±0.0067

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[C2OHmim][Ala] Sim. 1.5231±0.0038 1.5216±0.0038 1.5193±0.0040 1.5160±0.0040 1.5120±0.0044 1.5069±0.0043 1.5048±0.0046 1.5016±0.0055 1.5001±0.0058

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[C2OHmim][Gly] Sim. 1.5791±0.0054 1.5758±0.0056 1.5731±0.0059 1.5704±0.0062 1.5637±0.0064 1.5609±0.0068 1.5556±0.0077 1.5545±0.0071 1.5455±0.0074

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ILs T/K 293 303 313 323 333 343 353 363 373

29

[C2OHmim][Pro] Sim. 1.5434±0.0037 1.5423±0.0038 1.5386±0.0043 1.5339±0.0041 1.5296±0.0043 1.5229±0.0045 1.5223±0.0050 1.5189±0.0050 1.5165±0.0052

ACCEPTED MANUSCRIPT Table 3. Simulated values of vdW- (EvdW), electrostatic- (Eelect), and interaction energies (Eint) of AAILs as a function of temperature. vdW / kJ.mol-1 [C2OHmim][Gly] Sim.

293 303 313 323 333 343 353 363 373

-48.13 -47.39 -46.89 -46.61 -46.02 -45.94 -45.16 -44.66 -42.48

[C2OHmim][Ala] Sim.

[C2OHmim][Ser] Sim.

-55.15 -54.60 -54.08 -53.56 -53.22 -52.51 -51.65 -51.38 -50.88

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-59.69 -59.16 -58.78 -58.00 -57.11 -55.97 -54.64 -54.31 -54.09

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ILs T/K

[C2OHmim][Pro] Sim.

-84.61 -84.22 -83.85 -82.68 -82.02 -81.25 -79.66 -78.80 -77.96

ILs T/K

[C2OHmim][Gly] Sim.

293 303 313 323 333 343 353 363 373

-1353.29 -1351.76 -1347.31 -1344.53 -1342.55 -1340.98 -1339.49 -1336.16 -1332.36

[C2OHmim][Ala] Sim.

[C2OHmim][Ser] Sim.

293 303 313 323 333 343 353 363 373

-1401.42 -1399.15 -1394.20 -1391.14 -1388.57 -1386.92 -1384.65 -1380.82 -1374.84

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-328.94 -326.90 -323.06 -320.38 -318.16 -315.74 -312.20 -310.44 -307.28

[C2OHmim][Pro] Sim.

-813.47 -810.92 -809.72 -808.01 -805.88 -803.11 -800.37 -798.20 -797.27

Eint / kJ.mol-1

[C2OHmim][Ala] Sim.

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[C2OHmim][Gly] Sim.

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M

AN

-1086.37 -1085.08 -1082.12 -1081.28 -1079.66 -1074.28 -1067.91 -1066.26 -1066.05

ILs T/K

AC

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Ecoul / kJ.mol-1

-1146.06 -1144.24 -1140.90 -1139.28 -1136.77 -1130.25 -1122.55 -1120.57 -1120.14

[C2OHmim][Ser] Sim.

-384.09 -381.50 -377.14 -373.94 -371.38 -368.25 -363.85 -361.82 -358.16

30

[C2OHmim][Pro] Sim.

-898.08 -895.14 -893.57 -890.69 -887.90 -884.36 -880.03 -877.00 -875.23

ACCEPTED MANUSCRIPT Table 4. Electronic energies (Eelec), binding energies (Ebin), and interaction energies (Eint) of isolated ion pairs obtained at the B3LYP/6-311++g(d,p) level. E / kJ mol-1 [C2OHmim][Ala]

[C2OHmim][Ser]

[C2OHmim][Pro]

ΔEele

-431.74

-431.27

-401.12

-430.97

ΔEbin

-422.88

-422.55

-394.91

-421.32

ΔEint

-422.02

-421.83

-392.57

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T

[C2OHmim][Gly]

31

-419.62

ACCEPTED MANUSCRIPT

Figure Captions Figure 1. (Online color) Predicted values of the density (ρ) of explored AAILs at temperature range of 293.15-373.15 K. The fitted line is related to the simulated data.

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vap Figure 2. (Online color) Simulated values of the molar enthalpy of vaporization ( H m ) at the

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temperature range of 293.15-353.15 K.

Figure 3. (Online color) Simulated values of the surface tension (σ) at the temperature range of

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293.15-353.15 K. The fitted line is related to the simulated data.

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Figure 4. (Online color) Computed isobaric heat capacity (CP) within temperature range of

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293.15-353.15 K. The fitted line is related to the simulated data. Figure 5. (Online color) Reduced density gradient (RDG) isosurface for AAILs achieved at the

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B3LYP/6-311++g(d,p) level of theory. The RDG isovalue is 0.7 and Sign(λ2)*ρ values have been

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and red (repulsion).

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mapped onto the isosurfaces ranging from -0.03 to +0.03 au in blue (attraction), green (vdW),

Figure 6. (Online color) O9-Hn=2,4,5,9 RDFs of cation-cation organization at 373.15 K alongside

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the average structure of cations. Figure 7. (Online color) O9-Hn=2,5,9 SDFs isosurfaces of cation-cation interactions at 373.15 K. Isosurfaces are drawn 7 times the average density. Figure 8. (Online color) Estimated N-Hn=2,4,5,9 RDFs of cation-anion organization for all selected AAILs at 373.15 K. Figure 9. (Online color) calculated O-Hn=2,4,5,9 RDFs of cation-anion organization for all selected AAILs at 373.15 K. 32

ACCEPTED MANUSCRIPT Figure 10. (Online color) Computed OH-Hn=2,4,5,9 RDFs of cation-anion organization for AAIL composed of [Ser] anion at 373.15 K. Figure 11. (Online color) O-H9 and N-H9 SDFs isosurfaces of cation-anion interactions for all selected AAILs at 373.15 K. Isosurfaces are drawn 12 times the average density.

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Figure 12. (Online color) The average structures of AA anions at 373.15 K accompanied with

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the hydrogen bonds which are shown by dotted line and distances are in Å. (Outer panel) O-HN

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RDFs and SDFs of anion-anion organization for all selected AAIL at 373.15 K. Isosurfaces are drawn 5 times the average density. (Inner panel) N-HN RDFs of anion-anion arrangement for all

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selected AAIL at 373.15 K.

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Figure 13. (Online color) (Inner panel) N-HO and OH-HN, SDFs of anion-anion arrangement for [C2OHmim][Ser] at 373.15 K. Isosurfaces are drawn 5 times the average density. (Outer panel)

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anion-anion N-HO, OH-HN, OH-HO, and O-HO RDFs for [C2OHmim][Ser] at 373.15 K.

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Fig. 1

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Fig. 11

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Graphical Table of Contents

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Highlights

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Several thermodynamic and structural properties of selected ILs were investigated Combined MD-ab initio calculations were used to calculate Cp and refractive index The stronger cation-anion interaction is associated with the weaker cation-cation interaction Intramolecular O…H hydrogen bonds can be found from the average structures of cations

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