Ab Initio Investigation of the Interionic Interactions in

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Ab Initio Investigation of the Interionic Interactions in Triethylammonium - Based Protic Ionic Liquids: The Role of Anions in the Formation of Ion Pair and Hydrogen Bonded Structure Irina V. Fedorova, and Lyubov P. Safonova J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b10906 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 21, 2018

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The Journal of Physical Chemistry

Ab Initio Investigation of the Interionic Interactions in Triethylammonium - Based Protic Ionic Liquids: the Role of Anions in the Formation of Ion Pair and Hydrogen Bonded Structure Irina V. Fedorova and Lyubov P. Safonova G. A. Krestov Institute of Solution Chemistry of the Russian Academy of Sciences, 1 Akademicheskaya Street, Ivanovo 153045, Russia.

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ABSTRACT. The results of structural analysis and cation - anion interactions of ten ion pairs and their relevance for the physicochemical properties of triethylammonium - based protic ionic liquids are reported. The calculations were mainly performed by dispersion corrected density functional theory method (B3LYP-GD3). It is shown that the dispersion correction is important in the evaluation of the interaction energies of these compounds. The role of anions in the formation of ion pairs and hydrogen bonded structure are analyzed. To obtain a quantitative measure of the strength of H-bonds, the Bader's theory of atoms in molecule (AIM) was applied. The correlations between hydrogen bond lengths, their energies and electron-topological parameters at the H…O bond critical point are presented.


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INTRODUCTION One of the most important tasks of the modern chemistry is to establish the compositionstructure-property relationships for a particular compound. Understanding how the physicochemical properties of compounds are related to their molecular structures promotes the design of new materials that have the desired properties for many industrial and technical applications. From this point of view, the protic ionic liquids (PILs), as a significant subclass of ionic liquids, are peculiar compounds because they are entirely composed of ions. PILs are formed by proton transfer from a Brønsted acid to a Brønsted base. A unique feature of these systems is the presence of the exchangeable proton on the cation. And this can produce hydrogen bonding between the protonated base (cation) and the acid anion and in some cases an H-bonded extended network.1-3 Although electrostatic interaction is dominative between the cation and the anion in these compounds, the hydrogen bonding plays a significant role on the physicochemical properties of PILs, and it is thought the vital interaction to orderly pack the cation and the anion together and determines their structural features.4-9 There are quite a large number of both theoretical and experimental works studying of the hydrogen bonding interactions in these compounds.10-16 Because of the wide variety of cations and anions constituting these PILs, H-bonding for different PILs is very system dependent. For primary alkylammonium-based PILs there is a significant change in hydrogen bonding interactions ranging from short and linear to long and bi/trifurcated and it reflects their macroscopic properties.6 The strong H-bonding interactions in the above-mentioned PILs determine more “solid-like” character of the physical properties, whereas the formation of weaker, bent hydrogen bonds leads to decreasing interionic interactions and the system displays more “liquid-like” properties. Stoimenovski et al.17 investigated protic ionic liquids obtained by proton transfer from acetic acid to a range of amine bases of similar pKa values and came to a conclusion that ionicity and proton transfer large depend on the structure and hydrogen


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bonding ability of the ammonium ions rather than the ΔpKa (the difference between the pKa values of the dissociation constant of the protonated base and acid in water). Numerous studies indicate that the anion nature has a significant effect on the thermal stability of PILs and their decomposition process.18-23 Thermal stability of PILs was shown to increase with the pKa value and the anion and cation sizes. Among tertiary alkylammonium trifluoroacetate PILs those containing less bulky cations have the highest conductivity value.24 And in these cases, the stabilization of ionic structure via hydrogen bonding is more pronounced in the ion pairs with the smaller cations. The ionic mobility was also found to decrease with an increase in the size of the alkyl group (methyl, ethyl, and propyl) in tertiary alkylammonium triflate PILs.25 The authors of different works26-31 pointed out the importance of the directionality of hydrogen bonds, entropic effects and the dynamics of the hydrogen bonding network on the determination of PILs melting point and transport properties. In Hunt’s study26, for example, ionic liquids with the higher melting point have a smaller number of stable ion pair configurations, thus suggesting that it has lower entropy. The presence of hydroxyl groups in the structure of ammonium cation (when passing from triethyl- to triethanolammonium cation) leads to an increase in overall energetic stability of the ion pair with anion of phosphorous acid but to no strengthening of hydrogen bond.32 Additionally, it was shown that the NH group of triethanolammonium cation has more proton donor-like character than the OH group that makes the N-H…O hydrogen bonds stronger. Despite the growing amount of experimental work including the synthesis and study of the physicochemical properties of PILs based on triethylamine with benzoic33, 34, phosphorous 35, phosphoric36-39 and sulfonic33,

35-38, 40-47

acids, there are still many questions about the

microstructure of these compounds, interionic interactions and features of H-bonding. It is therefore our aim to investigate the effects of different anions on the structure and interactions in the ion pairs that can forms in triethylammonium - based ionic liquids using


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quantum chemical methods. The obtained data on the stability, structure and strength of hydrogen bond in these compounds will be analyzed together with the results from our previous study of the ion pairs of triethylammonium cation with tosylate and dihydrogen phosphite anions48. The calculation results will be compared to the data of our experimental study of the thermal stability35, 49 of triethylammonium - based PILs.

COMPUTATIONAL METHODS The quantum chemical calculations were performed with the Gaussian 09 program package.50 The geometry of each species were optimized using density functional theory51 based Becke’s three parameter Lee-Yang-Parr exchange and correlation functional (B3LYP) method52,

53

employing the 6-31++G(d,p) basis set. The B3LYP functional is often the

method used for estimating the structural parameters and interionic interaction strengths in the ionic systems because of its higher accuracy and less computational workload.24, 32, 48, 54-57 Addition, an empirical dispersion correction to hybrid functional (B3LYP-GD358) was incorporated due to presence of non-covalent interactions in the investigated compounds. To obtain the structures of the ion pairs, we started from the optimized geometry of the individual molecules of triethylamine and acid. Ten acids of varying H-bond donor strength were selected for this study. The formulas and names of the investigated acids, their anions and the literature data on the dissociation constants in water59-66 are summarized in Table 1. The harmonic vibrational analysis was performed to identify the nature of the stationary points found by geometric optimization and to calculate the thermodynamic properties of the investigated compounds. Each of the reported structures corresponded to the energy minimum without imaginary frequencies.


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The change in Gibbs free energy associated with the ion pair formation (at 298 K and atmospheric pressure) from triethylamine and acid was defined as a difference between the free energies of the optimized ion pair and the optimized acid and amine molecules. The interaction energy corrected by the basis set superposition error (BSSE67) (using the counterpoise method of Boys and Bernardi68) was calculated by evaluating the energetic difference between the ion pair and ions constituting it. The proton affinity (PA) of the anion was determined as the change enthalpy associated with the deprotonation of the conjugate acid. The numerical values for the gas phase proton affinities of the investigated anions are given in Table 1. Table 1. Formulas and names of the investigated acids and anions, acid dissociation constants (pKa in water) and calculated proton affinities of anions (PA, kJ/mol) using different methods at the 631++G(d,p) basis set. Acid name Phosphorous acid, H3PO3 Phosphoric acid, H3PO4 Methanesulfonic acid, CH3HSO3 3-Aminobenzenesulfonic (metanilic) acid, 3-(NH2)C6H4SO3H 4-Toluenesulfonic (tosylic) acid, 4-(CH3)C6H4SO3H Amidosulfonic (sulfamic) acid, NH2SO3H Benzenesulfonic acid, C6H5SO3H Sulfuric acid, H2SO4 3-Nitrobenzenesulfonic acid, 3-(NO2)C6H4SO3H Trifluoromethanesulfonic acid, CF3HSO3

Anion name Dihydrogen phosphite, H2PO3 Dihydrogen phosphate, H2PO4 Mesylate, MsO Metanilate, MTN

PA B3LYP-GD3 B3LYP 1367.58 1366.49

Tosylate, PTSA Sulfamate, SAM Besylate (benzene sulfonate), BSu Hydrogen sulfate, HSO4 3-nitrobenzenesulfonate, NBSu Triflate, TfO

pKa

1360.70

1359.74

1330.64

1328.77

1.3 (I)a 6.70 (II)a 2.16 (I)a 7.21 (II)a -1.92b

1328.54

1326.01

3.74a

1325.11

1322.75

-6.56c

1325.02

1322.61

0.99d

1319.72

1317.40

1290.34

1288.47

0.70a 2.8e 1.99a

1275.97

1273.63

-7.12f

1247.94

1245.63

-14.2g -12h

aTaken

from ref 59. bTaken from ref 60. cTaken from ref 61. dTaken from ref 62. eTaken from ref 63. fTaken from ref 64. gTaken from ref 65. hTaken from ref 66.


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The bonding features in the investigated ion pairs were analyzed according to the “atoms in molecules” (AIM) theory by Bader69. AIM calculations were carried out with AIMAll program (Version 10.05.04)70 on the wave functions obtained at the B3LYP-GD3/631++G(d, p) level. In order to evaluate the possibility of the formation of molecular complexes through the back transfer of the proton to the acid anion within the ion pair, the energy curves along the proton transfer coordinate, δ=rHO–rNH were calculated using the potential energy surface scan method. The scan involved incrementally increasing an N-H bond length in step 0.05 Å with geometry optimizations performed at each step. The energies associated with proton transfer were defined from the difference of the energies of the structure corresponding to the transition state and the global minimum structure.

RESULTS AND DISCUSSION Formation of ion pairs. Before discussing the data, some moments need to be highlighted. When optimizing the geometry of the molecular system (triethylamine and acid), this structure converges to the ionic form (triethylammonium cation and acid anion). This means that the acid donates the proton to the amine to form the ions. The NH proton donating group in the cation is involved in hydrogen bonding to the oxygen atom of the acid anion and, this in turn leads to the formation of the hydrogen bonded [TEA][A] ion pair (see Figure 1). Negative Gibbs free energy values show that the ion pair formations from triethylamine and acid molecules are thermodynamically favorable and that the process proceeds spontaneous. From Figure 2 it can be seen that the free energy of the ion pair formation grows (the value becomes more negative) as the proton affinity of the anion acid decreases. We observe the relationship between the calculated free energies of the ion pair formation and the


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decomposition temperatures35, 49 of triethylammonium-based PILs (see Figure 3). The greater the energy value is, the higher the decomposition temperature is.

Figure 1. Optimized structures of the investigated [TEA][A] ion pairs obtained from DFT(B3LYPGD3) calculations. Dashed lines indicate H-bonds.


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Figure 2. Correlation between the calculated values (at the B3LYP-GD3/6-31++G(d,p) level) of Gibbs free energy of the H-bonded [TEA][A] ion pairs and proton affinity of anion. The labels for each anion are individually assigned.

Figure 3. Dependence of Gibbs free energy of the H-bonded [TEA][A] ion pairs on decomposition temperature of triethylammonium-based PILs. The labels for each anion are individually assigned.

Optimized geometries. From the above Figure 1 it can be seen that all the investigated compounds have similar structures. One of the main factors that stabilize of the structure of these ion pairs is the presence of the interaction of the hydrogen atom in the NH group in the cation with the oxygen atom of the anion, as a result of which the distance between these


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Table 2. Hydrogen bond distance (Å) and bond angles (degrees) in the investigated ion pairs of triethylammonium cation with anions of different acids and their interaction energies (kJ/mol) calculated using B3LYP-GD3 and B3LYP (the values in parentheses) methods at the 6-31++G(d,p) basis set. Ion pair [TEA][H2PO3]а [TEA][H2PO4] [TEA][MTN] [TEA][PTSA]а [TEA][Bsu] [TEA][MsO] [TEA][SAM] [TEA][HSO4] [TEA][NBsu] [TEA][TfO] аData

rNH 1.122 1.112 1.091 1.090 1.089 1.087 1.084 1.078 1.076 1.068

rHO 1.421 1.447 1.504 1.507 1.509 1.520 1.529 1.552 1.551 1.589

rNO 2.542 (2.543) 2.556 (2.560) 2.592 (2.601) 2.595 (2.602) 2.596 (2.603) 2.605 (2.610) 2.606 (2.614) 2.623 (2.631) 2.627 (2.640) 2.648 (2.660)

NHO 176.2 174.9 174.6 174.3 174.4 174.5 172.4 171.8 177.2 170.2

Eint -503.67 (-476.60) -496.98 (-468.61) -469.15 (-434.72) -467.44 (-433.25) -463.80 (-429.53) -474.84 (-442.54) -464.47 (-432.67) -450.45 (-419.45) -434.68 (-401.41) -423.21 (-390.58)

obtained at the B3LYP/6-31++G(d,p) level of theory in our previous study, ref 48.

binding atoms (see Table 2) is much shorter than the sum of the van der Waals radii (O: 1.52 Å and H: 1.20 Å)71. Based on the definition of a hydrogen-bond72, the sum of van der Waals atomic radii of hydrogen and oxygen can be used as a critical value for judging the existence of a hydrogen bond. The N-H…O bond angle is close to 180 degrees that is also the characteristic of the H-bond formation in these compounds. Hydrogen bonding interaction between the cation and the anion in all cases results in an elongation of the covalent N-H bond in the cation compared to its isolated cation value (rNH=1.026 Å from both methods). The trend of lengthening of the N-H bond in the cation with decreasing of the H…O distance in the H-bonded fragment for the studied [TEA][A] ion pairs in the following order of anions: TfO < NBsu < HSO4 < SAM < MsO < Bsu < PTSA < MTN < H2PO4 < H2PO3 is observed. This sequence reflects the change in the H-bond strength between the cation and anion in the series of the investigated compounds indicating a strengthening of the H-bond, consistent with the geometrical hydrogen bond criterion. The order of changing geometric parameters of the H-bonds in the abovementioned anion series in the ion pairs of TEA does not depend on the DFT functionals used (see Table 2).


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AIM analysis. The topological analysis of the electron density further validates the existence of hydrogen bonds in all of the structures. This theory is widely used as a theoretical tool to understand and analyze of different types of interatomic interactions at the qualitative level by using the signs of the Laplacian of the electron density, 2ρ(r), at the bond critical point (BCP) (3, -1) and energy densities, H(r). On the basis of the classification of bonds proposed by Bader and Essen73 and Cremer and Kraka74 all interactions with 2ρ(r)>0 and H(r)0 at the BCPs can be to classified as the so-called closed-shell interactions which include the H-bonding and van der Waals interactions. According to Koch and Popelier75, 76 the electron density and its Laplacian at the BCP of interacting atoms must be in the range 0.002-0.035 a.u. and 0.014-0.139, respectively, to be considered as H-bonding interactions. For van der Waals interactions, the ρ(r) tends to be smaller than for hydrogen bonding interactions, e.g. 0.002-0.009 a.u.77 Table 3 shows that the values of both ρ(r) and 2ρ(r) at the H…O BCPs in the investigated ion pairs are outside the proposed range for hydrogen bonds, thus indicating the formation of strong H-bonding interactions in these compounds. In this, the higher value of ρ(r) implies stronger interactions. The electron density at the H…O BCP correlates well with the hydrogen bond energy and the geometrical parameters of the hydrogen bond. The EHB values calculated based on Espinosa’s equation78 are in all cases within the interval of 60-160 kJ/mol, which is characteristic of strong ionic Hbonds79. The increase in the H-bond strength is primarily due to the strengthening electrostatic interaction between the charged particles. As can be seen, the H-bonding interactions in the ion pairs of TEA with anion of phosphorus acids is the strongest and is in agreement with its highest interaction energy, as we shall discuss later. On the contrary, there exist the weakest H-bonding interactions in the ion pair of [TEA][TfO]. It is also worthwhile to note that the values of AIM parameters are almost independent of the functional used. For comparison, Table 3 shows the results from our previous calculations of the electron-


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Table 3. Topological electron density properties at the critical points (ρ(r), 2ρ(r) and H(r), au; |V(r)|/G(r)) of the N-H and H…O bonds and H-bond energies (EHB, kJ/mol) at the B3LYP-GD3/631++G(d, p) level of theory. Ion pair [TEA][H2PO3]а

Bond H...O N-H

[TEA][H2PO4] [TEA][MTN] [TEA][PTSA]а

H...O N-H H...O N-H H...O N-H

[TEA][Bsu] [TEA][MsO] [TEA][SAM] [TEA][HSO4] [TEA][NBsu] [TEA][TfO]

H...O N-H H...O N-H H...O N-H H...O N-H H...O N-H H...O N-H

ρ(r) 0.096 (0.096) 0.257 (0.252) 0.090 0.264 0.078 0.281 0.077 (0.076) 0.281 (0.279) 0.077 0.282 0.075 0.284 0.073 0.287 0.068 0.292 0.068 0.293 0.062 0.300

2ρ(r) 0.137 (0.133) -1.203 (-1.194) 0.152 -1.270 0.164 -1.400 0.164 (0.162) -1.407 (-1.393) 0.164 -1.413 0.164 -1.426 0.166 -1.453 0.165 -1.496 0.165 -1.504 0.160 -1.565

H(r)=V(r)+G(r)

|V(r)|/G(r)

-0.035

1.505

EHB -137.22 (-138.05)

-0.358 -0.028 -0.373 -0.017 -0.403

1.423

-122.81

1.288

-97.32

-0.016

1.280

-95.74 (-92.33)

1.274

-94.94

1.259

-91.22

1.230

-87.20

1.195

-80.02

1.190

-79.95

1.136

-69.26

-0.405 -0.016 -0.459 -0.014 -0.409 -0.012 -0.414 -0.010 -0.424 -0.010 -0.427 -0.006 -0.440

аThe

numbers in parentheses are the data obtained at the B3LYP/6-31++G(d,p) level of theory in our previous study, ref 48.

topological parameters (obtained from wave functions at “standard” DFT-B3LYP level48) in the H-bonded fragment for the ion pairs of TEA with H2PO3 and PTSA. It is also of interest to look at the energy density at the H…O BCPs, which is the degree of covalency of the Hbond. The positive value of H(r) is, as a rule, indicative of weak H-bonding interactions which are mainly electrostatic in nature. When the value of H(r) is negative, the H-bond possesses an interaction of a dominantly covalent character. For all the ion pairs of TEA the negative values of H(r) are obtained, suggesting that the H-bonds have some degree of covalent character. In addition to the criteria mentioned above, Espinosa et al.80 stated that the bonding interactions can be classiﬁed on the basis of the ratio of the absolute potential


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energy to the kinetic electron energy density, |V(r)|/G(r). In their study, interactions are deﬁned as a closed-shell (H-bonding) if the ratio |V(r)|/G(r)2, and as intermediate if the ratio falls between 1 and 2. Here, the N-H…O hydrogen bond in all cases has the values of |V(r)|/G(r) ratio greater than 1. The critical points of the N-H bonds, in contrast to the BCPs of the H…O interactions, have high values of ρ(r) and negative value of 2ρ(r), which allows us to characterize them as shared bonding (covalent bonds). The energy density values at the N-H BCPs are much more negative than at the Н…О BCPs. Interaction energies. The interaction energy, Eint is the most convenient measure of the strength of the interaction in forming an ion pair. For a stable compound, the value of the interaction energy is always negative. The more negative is Eint, the more strongly bound is the complex, and thus the more stable is the structure. As can be seen in the Table 2 above, the calculated values of the interaction energies of the investigated [TEA][A] ion pairs using B3LYP-GD3 method are 20-30 kJ/mol larger than “standard” B3LYP energies that may represent the missing dispersion interaction from the latter. And in spite of that, both functionals give very high energy values. Thus, it is clear that, the electrostatic interactions make the main contribution to the interaction energy of the considered systems. The hydrogen bonds have larger contributions in the interionic interactions (see Table 3) than the dispersion interactions. It can be observed that the replacement of the hydroxyl group in the hydrogen sulfate anion by the methyl (MsO), amino (SAM) groups and benzene ring (Bsu) leads to the increasing of the interaction energy of triethylammonium mesylat, sulfamate and besylate, correspondingly. The introduction of an electron donating NH2 and CH3 groups in the Bsu benzene ring increases the strength of interionic interactions in the ion pairs of TEA with MTN and PTSA. Herewith, in both these cases the energies are very closely in values. The presence of the NO2 group in the BSu benzene ring, in contrast to the aforementioned


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compounds, decreases the interaction energy between TEA cation and NBsu anion in the ion pair. In general, both methods of calculation (both with and without the GD3 dispersion correction) show the same qualitative trend of increase of the interaction energy in the ion pairs of TEA with the following anions: TfO < NBsu < HSO4 < Bsu < SAM < PTSA < MTN < MsO < H2PO4 < H2PO3. This series correlates well with the proton affinity (PA) parameter change of the considered acid anions, the energy value increases as the PA increases. Proton transfer process. Although the above results clearly indicate that there are ion pairs in triethylammonium-based PILs, we examined the possibility of proton transfer from TEA cation to acid anion along the hydrogen bond within the ion pair. The calculation results show that the potential curves for proton transfer have qualitatively similar shapes when passing from one anion to another within the ion pairs of TEA (some of them are shown in Figure 4). The proton transfer energy in all cases increases with increasing N…H distance. The only minimum on the curve corresponds to the structures, in which the proton is covalently bonded to the nitrogen atom of TEA cation (Figure 1). The changing the anion within the ion pairs of TEA leads to an increase in the energy values in the following sequence for anions: H2PO3 < H2PO4 < MTN < PTSA < Bsu < MsO < SAM < HSO4 < NBsu < TfO. In the fact these results correlate well with the H-bond strength change in the ion pairs, as shown earlier in this paper (see Table 2, 3). It is very possible that the structures of the ion pairs with stronger hydrogen bonds are close to the transition state structure (the H atom is located midway between the N and O atoms), that greatly reduces the energy for proton transfer. Comparison of the present with our previous studies24,

48

shows that the changing the

ammonium cation within the ion pair with the same acid anion has a greater effect on the proton transfer process than the changing the acid anion in the ion pairs of TEA. Figure 5a,


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Figure 4. Potential energy profiles for proton transfer from the nitrogen atom of triethylammonium cation to the oxygen atom of acid anion within the ion pair. The curves for acid anion are assigned in the legend.

Figure 5. Potential energy profiles for proton transfer from ethyl (EA) -, diethyl (DEA) - and triethyl (TEA)-ammonium cation to the oxygen atom of tosylate anion. Fig. 5b also shows the structures of the ion pair and molecular complex resulting from the proton transfer process in the ethylammonium tosytate. The interatomic distances are given in Å.

for example, shows that the proton transfer process from ethylammonium cation (EA) to tosylate anion within ion pair is much easier than the ones in the ion pairs of diethyl- (DEA) and triethylammonium cation with the same acid anion. For latter cases the potential curves show similar shapes, but it is clear that the energies associated with proton transfer within the


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ion pairs with tosylate anion become higher when passing from DEA to TEA. The curve for ethylammonium tosylate has two asymmetric minima with respect to the activation barrier, thus indicating the formation of both molecular complex and ion pair (see Figure 5b). And the potential energy of the molecular complexes is lower than for the ion pair.

CONCLUSIONS In this work, we have analyzed the structural characteristics and interionic interactions of ten ion pairs of triethylammonium (TEA) – based ionic liquids by the B3LYP hybrid density functional method without and with the Grimme empirical dispersion correction. The dispersion correction was found to contribute significantly to the interaction energy, but it has a small effect on the optimized geometries of the studied ion pairs. The interaction triethylamine with each of the investigated acids leads to proton transfer from the acid to the amine and formation of ions held together in the ion pair by the electrostatic interaction, dispersion forces and hydrogen bond. The calculated characteristics of the hydrogen bonding in all the studied ion pairs of TEA completely satisfy the geometrical (interatomic distances in the N-H…O fragment and H-bond angle), energetic (hydrogen bond energy) and electrontopological (electron density and its Laplacian at (3; -1) bond critical point) criteria of Hbonds indicating that they can be identified as strong H-bonds. Among the analyzed systems the ion pair of TEA with anions of phosphorus acid has the strongest hydrogen bonds while there is the weakest H-bonding interaction in triethylammonium triflate. We have obtained data on the proton affinity (PA) of the studied anions. The anions that possess a smaller gas phase proton affinity have the tendency form more thermodynamically stable ion pairs with triethylammonium cation. The relationship between the decomposition temperatures of TEAbased PILs and the calculated free energies of the ion pair formation has been established, particularly: the greater the thermal stability, the higher this energy. It has been shown that


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the interaction energy of the ion pairs of TEA increases with the PA parameter growth in the following order: TfO < NBsu < HSO4< Bsu < SAM < PTSA < MTN < MsO < H2PO4 < H2PO3. The replacement of the hydroxyl group in the hydrogen sulfate anion by the electron donating groups such as CH3 (MsO) and NH2 (SAM) as well as benzene ring (Bsu) strengthens the interionic interaction in triethylammonium mesylat, sulfamate and besylate, correspondingly. The introduction of the NH2 and CH3 groups in the Bsu benzene ring increases the strength of the interaction in the ion pairs of TEA with MTN and PTSA. The interionic interaction in the ion pairs of TEA significantly decreases when going from the electron donating to the electron withdrawing groups like CF3 (TfO) and NO2 (NBsu). For evaluating the possibility of molecular complex formation in these TEA-based PILs, the simulation of proton transfer from TEA cation to acid anion along the hydrogen bond within the ion pair was performed by the scan technique of proton transfer coordinate along the potential energy surface. It has been established that the spontaneous proton transfer in the ion pairs of TEA is very unlikely. The changing the acid anion within the ion pairs of TEA has a minor effect in the energy profile for proton transfer.

AUTHOR INFORMATION Corresponding Author e-mail:

[email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Notes The authors declare that they have no conflict of interest. ACKNOWLEDGMENT The authors gratefully acknowledge the support from the Russian Science Foundation (№ 1613-10371) and the Russian Foundation for Basic Research and the government of the region of the Russian Federation (№ 18-43-370009). REFERENCES 1. Greaves, T. L.; Drummond, C. J. Protic ionic liquids: properties and applications. Chem. Rev. 2008, 108, 206-237. 2. Greaves, T. L.; Drummond, C. J. Protic ionic liquids: evolving structure-property relationships and expanding applications. Chem. Rev. 2015, 115, 11379-11448. 3. Greaves, T. L.; Kennedy, D. F.; Mudie, S. T.; Drummond, C. J. Diversity observed in the nanostructure of protic ionic liquids. J. Phys. Chem. B. 2010, 114, 10022-10031. 4. Sun, X.; Liu, S.; Khan, A.; Zhao, C.; Yan, C.; Mu, T. Ionicity of acetate-based protic ionic liquids: evidence for both liquid and gaseous phases. New J. Chem. 2014, 38, 34493456. 5. Fumino, K.; Peppel, T.; Geppert-Rybczynska, M.; Zaitsau, D. H.; Lehman, J. K.; Verevkin, S. P.; Kockerling, M.; Ludwig, R. The influence of hydrogen bonding on the physical properties of ionic liquids. Phys. Chem. Chem. Phys. 2011, 13, 14064-14075. 6. Hayes, R.; Imberti, S.; Warr, G. G.; Atkin, R. The nature of hydrogen bonding in protic ionic liquids. Angew. Chem., Int. Ed. 2013, 52, 4623-4627.


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TOC Graphic


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Figure 1. Optimized structures of the investigated [TEA][A] ion pairs. 56x86mm (300 x 300 DPI)



Figure 2. Correlation between the calculated values of Gibbs free energy of the H-bonded [TEA][A] ion pairs and proton affinity of anion. 286x199mm (300 x 300 DPI)


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Figure 3. Dependence of Gibbs free energy of the H-bonded [TEA][A] ion pairs on decomposition temperature of triethylammonium-based PILs. 286x199mm (300 x 300 DPI)



Figure 4. Potential energy profiles for proton transfer from the nitrogen atom of triethylammonium cation to the oxygen atom of acid anion within the ion pair. 286x199mm (300 x 300 DPI)


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Figure 5. Potential energy profiles for proton transfer from ethyl-, diethyl- and triethylammonium cation to the oxygen atom of tosylate anion. 227x85mm (300 x 300 DPI)



TOC Graphic 53x27mm (300 x 300 DPI)


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Ab Initio Investigation of the Interionic Interactions in

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