Quantum Chemical Modeling of the Structure and H-Bonding in

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Quantum Chemical Modeling of the Structure and H-Bonding in Triethanolammonium-Based Protic Ionic Liquids with Sulfonic acids Irina V. Fedorova, and Lyubov P. Safonova J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b01189 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 7, 2019

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The Journal of Physical Chemistry

Quantum Chemical Modeling of the Structure and H-Bonding in Triethanolammonium-based Protic Ionic Liquids with Sulfonic Acids 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 electronic structure calculations based on density functional theory (DFT) for protic ionic liquids (PILs) consisting of triethanolammonium cation paired with anion of different sulfonic acids are reported. The influence of the anion nature on the structure and interactions in the ion pairs that are formed in these PILs is discussed in detail. Multiple H-bonding interactions exist between the protons in the NH/OH groups of the cation and different oxygen atoms of the acid anion in the ion pairs. The quantum theory of “atoms in molecules” has been used to estimate the individual contributions of each hydrogen bond to the stability of the ion pair. The hydrogen bonding interactions in the ion pairs vary in their strength ranging from weak to moderately strong. In addition to these hydrogen bonds, there are other dispersion and electrostatic-dominant interactions that play an important role in the overall stability of PILs and their physicochemical properties. Aided by results from our previous DFT studies of triethanolammonium class of PILs with inorganic anions, these new data allow us to gain an improved understanding of the structure–property relationships in the studied ionic liquids. Close to linear correlation, in particular, has been found between the melting points and the binding energies of the cation and anion in the ion pairs.


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INTRODUCTION Among the various classes of ionic liquids, an important place is taken by triethanolammonium (TEOA) – based protic ionic liquids (PILs), also known as protatranes1, whose cation contains a protonated nitrogen atom covalently bonded to three hydroxyethyl groups. And most scientific studies are available for TEOA – based PILs with anions of different carboxylic acids due to their significant biological activities.2-9 Trecrezan (triethanolammonium salt of 2-methylphenyloxyacetic acid), for example, is a drug with broad spectrum of activity (immunomodulator, adaptogen, etc.).10, 11 A single-crystal X-ray diffraction study was performed to determine the structure of triethanolammonium class of PILs with inorganic (Hal12-14, NO315, ClO416, H2PO317, H2PO418) and organic2, 5, 8, 9 anions. These studies show that TEOA cation has a tricyclic structure in which the ammonium proton is located inside the “lampshade” formed from three hydroxyethyl groups connected with nitrogen atom. The N-H(…O)3 distances within the cation are typically in the range of 2.3 – 2.5 Å and these are substantially shorter than the sums of van der Waals radii of the oxygen and hydrogen atoms involved (vdWOH=2.72 Å)19, thus showing the formation of a four-center (trifurcated) H-bonds. In very rare cases20, 21, the hydrogen bonds within TEOA cation are described as three-center (bifurcated) H-bonds, since one of the three hydroxyethyl groups of the cation is faced to the side opposite to the NH group. In addition to the intracationic Hbonding interactions, the structure of TEOA – based PILs is stabilized by different types of hydrogen bonds involving hydroxyl protons of TEOA cation and oxygen, halogen or other atoms of acid anions. We showed recently that the simultaneously formation of multiple Hbonds between TEOA cation and inorganic anion (HSO4, NO3, H2PO4, H2PO3) in the single hydrogen bonded ion pair has a great influence on the geometry of TEOA cation.22 The NH…O and O-H…O hydrogen bonds between the cation and anion in these ion pairs are strongly coupled with the electrostatic interactions, and together they cause the loss of the


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structural symmetry of the cation itself. The three N-H(…O)3 distances within the cation being not identical in the ion pair structures are in the range of 2.74 – 3.02 Å that are somewhat larger than the sums of vdWOH. No hydrogen bonds within cation were found in these ion pairs that makes them different structurally from crystalline TEOA – based PILs with the corresponding anions. Similar results were obtained when studying the structure and interactions in the ion pairs of triethanolammonium class of PILs with anions of carboxylic acids.7 This study also shows that both molecular complexes and ion pairs with hydrogen bonds can be formed depending on the nature of alkanolammonium cation and acid anion. In particular, only hydrogen bonded molecular complexes were found to be formed upon interaction of ethanol- and diethanolamine derivatives with these acids. The authors of work23 studied the physicochemical properties of PILs consisting of the primary, secondary and tertiary ammonium cations with the hydroxyethyl and/or the ethyl group and different anions and found that the cation-anion interaction becomes stronger with increasing the number of hydroxyethyl groups on cation. The structure-property relationships were investigated in a series of primary alkanolammonium – based PILs.6 Although some aspects related to the physicochemical and electrochemical properties of PILs composed of TEOA cation with anions of sulfonic24, 25, carboxylic and inorganic26 acids have been already considered, a deeper understanding of the microstructure of these compounds and interactions between the ions that constitute them is urgently needed. In this work, the influence of the anion nature on the structure and ion-ion interactions (especially via H-bonding) in the ion pairs of triethanolammonium class of PILs with anions of different sulfonic acids has been examined using quantum chemical density functional theory (DFT) calculations. Mesylate (MsO), triflate (TfO), besylate (BSu), metanilate (MTN), tosylate (PTSA) and 3-nitrobenzenesulfonate (NBSu) anions were taken for the present study. For the simplicity of quantum chemical modeling, our study was limited to


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single ion pairs. The effect of the anion on TEOA cation geometry in the ion pairs has also been studied. The correlations between hydrogen bond lengths, their energies and electrontopological parameters at the H…O bond critical point are presented. The calculated free energies of the formation of these ion pairs, interaction strength between ions in the pairs and their H-bonding parameters are compared to those obtained for the analogous ion pairs with triethylammonium (TEA) cation27. In contrast to TEOA cation in which both NH and OH groups are simultaneously involved in hydrogen bonding with the chosen anions in the ion pairs, the distinctive feature of TEA cation is the single proton donor function of the NH group. For some purposes the results from our previous theoretical DFT studies of triethanolammonium class of PILs with inorganic anions22 were added. From these studies we are trying to find the relationships between ionic structure and thermal properties (melting and decomposition temperatures) of TEOA – based PILs24, 26.

THEORETICAL METHODS Geometry optimization calculations were performed using the Gaussian 09 suite of programs28. These calculations employed the hybrid B3LYP density functional29, 30 corrected for dispersion interactions using Grimme GD3 empirical term31. The B3LYP functional has been used successfully and extensively to study the structure and interionic interactions in the ion pairs of ionic liquids.7, 22, 27, 32-36 The 6-31++G(d, p) basis set was employed to find the optimal geometries. Our previous calculations22 indicated that the effect of basis set on the magnitude of the interaction energy of the ion pairs with TEOA cation and inorganic anion are insignificant, if much more computationally expensive basis sets such as cc-pvtz and augcc-pvtz are used, and all these give the same qualitative trends for these compounds. Harmonic vibrational frequencies were obtained in order to confirm the existence of a true


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minimum (no imaginary frequencies for all the geometries) and also to calculate the thermodynamic characteristics of the investigated systems. The optimized geometries of both triethanolamine and acid molecules were used as a starting point (according to neutralization reaction37-39) to obtain the structural information about the ion pairs. The change in Gibbs free energy associated with the ion pair formation (calculated at 298 K and atmospheric pressure) from triethanolamine and acid was defined as the difference between the free energies of the ion pair and the sum of the isolated molecules. Geometries of TEOA cation and anions of different sulfonic acids were also optimized in order to give a visual understanding of the interactions between these ions in the ion pairs studied here. Figures S1 and S2 (see supporting information to this paper) show the electrostatic potential (ESP) for the investigated ions and ion pairs calculated from the optimized structure with the electronegative and electropositive regions of them. This parameter is widely used to explain the origin hydrogen bonding and chemical reactivity. In the ESP surfaces, red, blue and green colours represent the regions of the most negative (electrophilic reactivity), most positive (nucleophilic reactivity) and zero electrostatic potential, respectively. Potential increases in the order: red < orange < yellow < green < blue. It may be seen that, in the isolated anions, the highly negative region (red) is located around the oxygen atoms of the sulfonate group showing high activity on these atoms. On the contrary, the highly positive regions (blue) in TEOA cation are localized on the hydrogen atoms of the NH group and, to a greater extent, OH groups, which can be considered as possible sites for nucleophilic attack of the oxygen atoms. In the ESP surfaces of ion pairs, the reduction in the electrostatic potential around the preferred sites indicates the equalization of electrostatic potentials as a result of the N-H…O and O-H…O hydrogen bonding, which we shall discuss in more detail later.


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The interaction energy (Eint) between the ions in the studied ion pairs was calculated by subtracting the sum of the cation and anion energies from the total energy of the ion pair. The basis set superposition error (BSSE40) was corrected for all the interaction energy calculations using the counterpoise method of Boys and Bernardi41. The BSSE counterpoise corrections were applied only to the optimized structures of the ion pairs. The BSSE's calculated at the B3LYP-GD3/6-31++G(d, p) level range between 12.4 and 14.8 kJ/mol and these values could be ignored for all the compounds studied here. The binding energy (Ebind) by the formation of the ion pair from the isolated ions was estimated as the sum of the Eint and the deformation energy (Edef), which is the sum of the increase of the cation and anion energies by deformation of the geometries associated with the ion pair formation. The electronic parameters, such as the highest occupied molecular orbital (HOMO) energy, lowest unoccupied molecular orbital (LUMO) energy and band gap energy (Egap = ELUMO EHOMO) were obtained. The HOMO represents the ability to donate an electron, LUMO as an electron acceptor represents the ability to obtain an electron.42 The results of all these calculations can also be found in the supporting information to this paper (Tables S1). To evaluate the dispersion effects on the structure and interactions in the ion pairs, all the geometries were re-optimized at the B3LYP/6-31++G(d, p) level. The difference in the Eint values calculated by using the “standard” B3LYP and B3LYP-GD3 methods was considered as dispersion energy of the ion pair. And furthermore, it was found that the dispersion correction to B3LYP functional enhances the interionic interactions in the studied ion pairs, but does not reflect the changes in the values of Edef. There is also a minor difference in the geometric parameters of the H-bonds in the ion pairs. In general, these results are in accordance with findings reported for tertiary alkylammonium – based PILs with organic and inorganic anions27, 32, 33. In all cases the both B3LYP functionals with and without including


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the dispersion correction follow the same general trend of change in the geometric and energetic characteristics in the series of the considered ionic liquids. The Baders quantum theory of “atoms in molecules” (QTAIM)43 was applied for calculations of the electron-topological characteristics in the N-H…O and O-H…O hydrogen bonded fragments in the ion pairs. The QTAIM analysis was carried out using the AIMAll program (Version 10.05.04)44 on the wave functions obtained at the B3LYP-GD3/631++G(d,p) level. For a quantitative comparison of the strengths of the N-H…O and O-H…O hydrogen bonds, their energies were calculated according to the Espinosa’s equation45 EHB = ½*V(r), in which V(r) is the potential energy density at the H…O critical point.

RESULTS AND DISCUSSION The two stable structures of the ion pairs with triethanolammonium cation and acid anion ([TEOA][A], where A = MTN, PTSA, BSu, MsO, NBSu, TfO) shown in Figures 1 and S3 (see supporting information to this paper) were found. The hydrogen atom in the NH group of the cation in each of the reported structures has contact with the oxygen atom of the acid anion. And there exist three contacts between the hydroxyl protons of the cation and different oxygen atoms of the same anion. However, if in the ion pairs shown in Figure 1 each oxygen atom in the SO3 group of the anion interacts with hydrogen atom in the OH group of the cation to form three two-center (linear) O-H…O hydrogen bonds, in the ion pair structures in Figure S3, the bonding is characterized by the three-center (bifurcated) hydrogen bonds. The change in Gibbs free energy (ΔG298) associated with the ion pair formation from triethanolamine and acid is negative in all these cases indicating that the process is thermodynamically favorable and occur spontaneously (Table S2). The values of ΔG298 of the formation of the ion pairs shown in Figure 1 are more negative (~ 4 – 14 kJ/mol) than for the


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Figure 1. Optimized hydrogen bonded structure of the ion pairs of triethanolammonium cation with anions of different sulfonic acids. The interatomic distances are given in Å.

ion pairs with bifurcated H-bonds in Figure S3. Further studies will therefore focus on the structures of the ion pairs with the highest thermodynamically stability. As will be shown later, the order of changing geometric parameters of the hydrogen bonds and energy characteristics in the series of the studied PILs are not dependent on the different hydrogen bonded configurations found in the ion pairs. The calculation results show that the strong acids like trifluoromethanesulfonic and 3nitrobenzenesulfonic acids with pKa values of -14.246 and -7.1247, correspondingly (pKa is the negative logarithm of the acid dissociation constant in water) demonstrate a marked ability to act as a proton donor in the interactions with triethanolamine than the other investigated acids. The ΔG298 calculated for the ion pairs of TEOA cation with TfO and NBSu anions are somewhat larger than ACS Paragon Plus Environment

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that for the ion pairs with other anions studied here (Table S2). A similar pattern was observed when we examined the reaction of the interaction of triethylamine with the same acids27. These studies also indicated that the increase in the free energy of the formation of the ion pairs of triethylammonium class of PILs correlates with the increase in their thermal stability (i.e. an increase in the decomposition temperature, Tdec). For the PILs studied here, however, there is no clear dependence between Tdec and ΔG298 (Figure S4) and that is because the Tdec values reported for these PILs24 fall within a fairly narrow range of 210 – 250 ºС. From Table S1, one can find that the order of the HOMO energies for the selected anions is as follows: MsO < MTN < PTSA < BSu < NBSu < TfO. This clearly indicates that the anions with higher HOMO energies easily provide the electrons to the unoccupied orbital of the cation leading to the formation of the ion pairs with greater thermodynamic stability, which is consistent with the aforementioned results. The energy gap between HOMO and LUMO characterizes the chemical stability. The less the energy gap is, the less stable/more reactive the compound is. We note that the ion pairs with smaller anions such as TfO and MsO exhibit a large energy gap, implying high chemical stability. Replacement of the CH3 group in MsO anion by an aromatic group (with different substituents) leads to destabilization of the HOMO and LUMO levels for the ion pairs of TEOA cation and corresponding anion and reduction of their energy gap. The changes in the structure of TEOA cation in the ion pairs compared to the cation geometry itself (Figure S5, Table S3, his in-deep structure analysis was performed earlier22) are as follows: There is the distortion in the structure of TEOA cation in all the investigated ion pairs owing to the presence of multiple hydrogen bonding interactions between the cation and anion in them. None of the optimized ion pair structures have a tricyclic protrane structure within the cation that is characteristic of the solid phase. For all the ion pairs represented in this work, the intracationic N(H…O)3 distances are in the range of 2.738 to 3.024 Å, and all these are significantly longer than that of 2.224 Å found in the TEOA itself (Table S3). The covalent N-H and O-H bonds within the cation in the ion pairs are lengthened due to hydrogen bonding. The covalent bond lengthening ACS Paragon Plus Environment

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(observed in all the ion pairs) is a consequence of the orbital mixing accompanied by the loss of electron density from the NH/OH region of cation. As consequence of HOMO–LUMO mixing, electron density redistribution within the cation and anion occur. A clear contribution of the anion’s HOMO to the ion pair HOMO and of the cation’s LUMO to the ion pair LUMO is observed. This leads to a decrease in the orbital energy of the LUMO and an increase for the HOMO in all the ion pairs studied here (Table S1). The greater lengthening of the N–H bond in the N-H…O fragment (0.020 – 0.012 Å), as compared to the ones in the O-H…O (0.017 – 0.004 Å), is reflected in the shorter H…O distance varying from 1.650 Å to 1.707 Å (Figure 1). The changing the anion in the ion pairs with TEOA cation leads to an increase in the length of the N-H…O hydrogen bond in the following order of anions: MTN < PTSA < BSu < MsO < NBSu < TfO. The H…O distances in the three O-H…O hydrogen bonds which held together the cation and anion in the ion pair indicate that they are rather dissimilar. These distances are generally located within the range of 1.767 to 2.219 Å. The H…O distances in the H-bonded fragments can be used to determine the individual H-bond strength as a first estimate. With geometric criteria, all the H-bonds in the ion pairs are in the moderate category (the rHO ranges from 1.5 to 2.2 Å48). The H-bond angle is also important in determining the H-bond strength. The closer the hydrogen bond is to linear geometry (180º), the stronger the bond. For the N-H…O hydrogen bonds the angles vary from 170.1 to 173.2º indicating that the linear H-bonded fragment is favored. Conversely, there are greater deviations of the OH…O angles from linearity (153.1 – 168.3º). To understand in more detail the role of H-bonding interactions in the structure of the discussed PILs, we employed the Bader’s topological analysis (QTAIM analysis43). Within the QTAIM framework, the presence of the bond path between interacting atoms with the bond critical point (3, -1) (BCP) is evidence of a bonding interaction. Figure S6 presents the molecular graphs of the ion pairs considered in this study. There are different criteria for describing the nature and strength of bonding interactions using certain topological parameters at the BCP. The classification of bonds proposed by Bader and Essén49 is based on the accumulation of the electron density, ρ(r) and the


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Table 1. Topological properties analysis of the N–H…O and O–H…O fragments at the corresponding BCPs (ρ(r), 2ρ(r) and H(r) in au; |V(r)|/G(r)) and H-bond energies (EHB in kJ/mol) for the ion pairs of TEOA cation with anions of different sulfonic acids. Atom numbering is given on Fig. 1. Interaction types

2ρ(r)

ρ(r)

H1…O2 N-H1 H2...O1 O4-H2 H3...O2 O5-H3 H4...O3 O6-H4

0.0537 0.3080 0.0342 0.3419 0.0269 0.3505 0.0379 0.3416


0.0535 0.3083 0.0342 0.3423 0.0270 0.3507 0.0378 0.3419


0.0529 0.3090 0.0341 0.3427 0.0269 0.3509 0.0375 0.3419


0.0503 0.3104 0.0326 0.3435 0.0265 0.3510 0.0341 0.3423


0.0487 0.3138 0.0327 0.3451 0.0259 0.3523 0.0354 0.3439


0.0444 0.3155 0.0290 0.3480 0.0126 0.3576 0.0294 0.3474

[TEOA][MTN] 0.1437 -1.6257 0.1003 -1.9868 0.0868 -2.0430 0.1125 -1.9818 [TEOA][PTSA] 0.1432 -1.6272 0.0999 -1.9890 0.0872 -2.0440 0.1122 -1.9845 [TEOA][BSu] 0.1420 -1.6330 0.1006 -1.9886 0.0868 -2.0444 0.1112 -1.9879 [TEOA][MsO] 0.1409 -1.6464 0.0976 -2.0157 0.0789 -2.0330 0.1001 -2.0116 [TEOA][NBSu] 0.1338 -1.6710 0.0965 -2.0062 0.0836 -2.0503 0.1051 -2.0026 [TEOA][TfO] 0.1296 -1.6899 0.0853 -2.0407 0.0437 -2.0759 0.0862 -2.0436

H(r)

|V(r)|/G(r)

EHB

-0.0036 -0.4537 -0.0003 -0.5680 0.0002 -0.5822 -0.0004 -0.5685

1.0910

-56.6

1.0109

-33.6

0.9880

-27.8

1.0112

-37.9

-0.0036 -0.4541 -0.0003 -0.5685 0.0002 -0.5825 -0.0004 -0.5692

1.0906

-56.4

1.0106

-33.5

0.9868

-27.9

1.0108

-37.6

-0.0034 -0.4554 -0.0003 -0.5685 0.0002 -0.5827 -0.0003 -0.5701

1.0869

-55.5

1.0099

-33.7

0.9867

-27.7

1.0104

-37.3

-0.0023 -0.4580 -0.0002 -0.5754 0.0003 -0.5809 -0.0003 -0.5745

1.0617

-52.3

1.0074

-32.5

0.9865

-27.2

1.0102

-33.9

-0.0024 -0.4641 -0.0002 -0.5721 0.0003 -0.5846 -0.0002 -0.5747

1.0668

-50.2

1.0071

-32.4

0.9863

-26.8

1.0117

-35.1

-0.0009 -0.4672 -0.0003 -0.5822 0.0005 -0.5914 -0.0003 -0.5812

1.0282

-45.0

1.0067

-29.3

0.9488

-12.9

1.0010

-29.8


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sign and magnitude of the Laplacian, 2ρ(r) at the BCP. In their study, any bonding interaction could be classified either as shared (if in the BCP, ρ(r)10-1 and 2ρ(r)0) or closed shell (if in the BCP, ρ(r)10-2 and 2ρ(r)0). Shared interactions usually are found between covalently bonded atoms while closed shell interactions usually correspond to H-bonding and van der Waals interactions. Koch and Popelier50, 51 performed very detailed analysis of these properties for hydrogen bonded compounds and pointed out that the values of ρ(r) and 2ρ(r) must be in the range 0.002 – 0.035 a.u. and 0.014 – 0.139, respectively. As can be seen from Table 1, large magnitude of ρ(r) and negative value of 2ρ(r) at the N-H and O-H BCPs within cation in all the ion pairs show the shared interactions type. The shared character of these bonds increases as the covalent bond length decreases. All the H…O BCPs in the X–H…O fragments (where X=N, O) in the ion pairs, in contrast with the previous ones, are characterized by positive values of 2ρ(r). This is an indication of the closed shell type of interactions between the H and O atoms. At the same time, these H…O binding interactions are different; this difference manifests itself in the value of ρ(r) and 2ρ(r) at the corresponding BCPs, which is also consistent the geometric analysis. These values are always higher at the H…O BCPs for the N-H…O fragments and they are also more significant compared with those of the conventional H-bonds. The energy of the N-H…O hydrogen bond in the ion pairs with TEOA cation grows in the anion series: TfO < NBSu < MsO < BSu < PTSA < MTN. The same trend was observed for the ion pairs of triethylammonium class of PILs with the same acid anions27. The latter, however, have very strong N-H…O hydrogen bonds (the value extends up to 140 kJ/mol). The QTAIM and DFT approaches show linear relationship between the hydrogen bond energy and the donor-acceptor distance (Figure S7), namely that the strength of H-bonding decreases as the hydrogen bond length increases. In order to define the degree of covalency of H-bonding interaction in the ion pairs studied here, we applied another classification of the bonding interactions based on total energy density, H(r) at the BCP introduced by Cremer and Kraka52. The negative value of H(r) indicates the covalent nature of interaction between atoms connected by the corresponding bond path whereas the positive value of H(r) characterizes interactions that are mainly electrostatic in nature. The more negative ACS Paragon Plus Environment

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the H(r) value, the more the interaction will tend to the shared character and thus to the greater the stabilization of the structure. The results of our computations show that the H(r) at the critical points of the N-H and O-H bonds within cation in all cases is the most negative indicating the covalent character of these bonds. The values of H(r) at the BCPs for the N-H…O hydrogen bonding region are still negative but close to zero suggesting that these H-bonds have mixed character, i.e., they are significantly ionic with some covalent component. The different values of H(r) at the H…O BCPs are obtained for the three O–H…O fragments. For two of the three BCPs corresponding to the H2…O1 and H4…O3 interactions in the ion pairs the H(r) is very near zero, with slightly negative values, indicative of weak H-bonding interactions underlining their electrostatic nature. The H(r) at the remaining H3…O2 BCP is positive in all cases. The latter is indicative of weakest interaction which occurs in the considered ion pairs, especially in the ion pair with triflate anion. In addition to the above-mentioned criteria, Espinosa et al.53 provided a way to categorize the bonding interactions of intermediate character based on the ratio of the absolute potential energy to the kinetic electron energy density, |V(r)|/G(r). A bonding interaction is indicated to be closed-shell, when the ratio is smaller than 1, shared when the ratio is larger than 2, and intermediate when the ratio falls in the range between 1 and 2. All the H…O interactions in the ion pairs, with the exception of the H3…O2 interaction, have the values of |V(r)|/G(r) ratio greater than 1 classifying as intermediate interactions. The value of |V(b)|/G(b) ratio for the H3…O2 bond in all cases are less than 1 which may be assigned to weak hydrogen bond. The N–H…O and O–H…O hydrogen bonds were also analyzed in the ionic structures in which there are bifurcated H-bonds (Figure S3 and S8, Table S4). A similar electron density topology is revealed for the bonding interactions in these compounds. The same trend of increase of the strength in the series of the studied PILs is observed; however, the N-H…O hydrogen bond in all these structures is somewhat weaker and is in agreement with the relatively long rHO distances (1.669 – 1.756 Å). All the O-H…O bonds become stronger together in these structures but each of them is always weaker than the N-H…O hydrogen bond. It is interesting to note that the N-H…O


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bonds in the structures S3 are more likely to bent than O-H…O bonds. The angles calculated here are in the range of 156.2 – 157.8 and 153.2 – 177.1º, respectively. Table 2 presents the energetic characteristics for the ion pairs of triethanolammonium class of PILs with anions of the investigated sulfonic acids. The interaction energy, Eint that is the results of the different types of interactions in the ion pair can be considered as one of the effective structural parameters on the properties of PILs. One can see that the strongest interactions exist between the cation and anion in all the ion pairs studied here. It is clear that the electrostatic interaction mainly determines the energetic stable of the ion pairs. The energy contribution of each hydrogen bond is too small but all together they makes a favorable contribution to the overall energy stability (117 – 156 kJ/mol). Due to the additional O-H…O hydrogen bonding between the cation and anion in these ion pairs, the Eint calculated for them are significantly larger than that for the ion pairs with triethylammonium cation and the corresponding anion (Eint ranges from -474.84 to -423.21 kJ/mol)27. The dispersion interaction, Edisp makes a certain contribution to the interaction energy of the considered ion pairs, but always less than H-bonding in general. For the ion pairs with TEOA cation the interaction energy increases in the following sequence for anions: TfO < NBSu < BSu < PTSA < MTN < MsO. This series is in agreement with the increase of the proton affinity (PA) of the chosen anions (Table 2); it means that the anions with greater PA have a strong tendency to bind with the cation in the ion pairs. The formation of the ion pairs with TEOA cation in all cases is connected with the large deformation energies, Edef. Herein, the geometry of the cation is more strongly deformed under the influence of hydrogen bonding than that for the acid anion. The smallest deformation energy associated with the formation of the ion pair from isolated ions is found for the ion pair of TEOA cation with TfO anion. The energy required to deform the cation and anion into the geometries adopted in the ion pair is readily compensated by the stabilization due to hydrogen bonding. The binding energies, Ebind are less negative than their corresponding interaction energies because of the positive contributions of deformation energies included in the binding energies. The order of the magnitude of Eint coincides with the order of Ebind. Similar trends can be seen in the series of the ion pairs with bifurcated H-bonds (Figure S3, Table S5). In these ACS Paragon Plus Environment

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cases, however, the Ebind calculated for them are smaller compared to the considered above structures. The data from Tables 2 and S5 shows that the presence of different substituents in the BSu benzene ring results in changes to the energetic characteristics of the analyzed ion pairs. The introduction of the NH2 and CH3 groups in the BSu benzene ring causes a slight strengthening of the interaction in the ion pairs of TEOA cation with MTN and PTSA anions. Conversely, the presence of the NO2 group in the BSu benzene ring decreases the binding energy of TEOA cation and NBSu anion in the ion pair. The strength of the interionic interactions in the ion pairs of TEOA cation significantly decreases when going from the electron donating (CH3) to the electron withdrawing (CF3) groups in MsO and TfO anions, correspondingly. Table 2. Calculated energetic characteristics of the ion pairs of TEOA cation with anion of different sulfonic acids and proton affinities of these anions (all values in kJ/mol). a

Ion pair

PA

[TEOA][MsO] [TEOA][MTN] [TEOA][PTSA] [TEOA][BSu] [TEOA][NBSu] [TEOA][TfO]

1330.64 1328.54 1325.11 1319.72 1275.97 1247.94

Eint

Ebind

Edef

Edisp

-583.11 -582.71 -579.23 -574.46 -535.01 -492.79

-475.12 -455.58 -453.71 -449.72 -412.90 -411.29

107.99 127.13 125.52 124.74 122.11 81.51

-40.74 -40.55 -40.79 -40.94 -40.96 -23.34

a

Taken from ref 27.

Combined with the results of our previous work on studying the ion pairs of triethanolammonium class of PILs with inorganic anions (H2PO3, H2PO4, HSO4 and NO3)22, we plotted the melting points, Tmelt for triethanolammonium – based PILs24 versus the computed binding energies of the cation and anion in the ion pairs (Figure 2, Table 2). Close to linear correlation is observed between the Tmelt and Ebind parameters. On this basis it can be seen that TEOA – based PILs with anions of phosphorus acids exhibiting higher melting points show higher values of Ebind. However, it is not just the binding energy that has a major effect on the determination of the melting point of PILs. The hydrogen bonding ability, packing properties of the ions (size, symmetry, and conformational flexibility), crystal lattice energy, entropic effects also affects the melting point of ionic liquids, among other properties.54-60 Considering the same anions for triethylammonium – based PILs27, no obvious correlation between the Tmelt and Ebind were found. One of the possible reasons for this is ACS Paragon Plus Environment

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the presence of three mobile ethyl groups in TEA cation leading to larger variety of states and thus indicating the role of entropic contributions on the stabilization of the ion pairs. On the contrary, the ion pairs with TEOA cation have close packed structures. For them there is an entropy loss due to the conformational restriction, which accompanies multiple H-bonding interactions.

Figure 2. Dependence of binding energy of the ions in the ion pairs on melting point of triethanolammonium-based PILs. The labels for each anion are individually assigned.

CONCLUSIONS The structural features and interactions in the ion pairs of triethanolammonium (TEOA) class of protic ionic liquids with anions of different sulfonic acids have been investigated using density functional theory. The following anions were used: mesylate (MsO), triflate (TfO), besylate (BSu), metanilate (MTN), tosylate (PTSA) and 3-nitrobenzenesulfonate (NBSu). The effect of the anion on the formation of the ion pair and hydrogen bonded structure has been analyzed. All PILs studied here have two types of stable structures, in both of which the cation forms multiple hydrogen bonds to the anion. Using the Bader's theory of “atoms in molecule”, we have determined the nature and strength of all these H-bonding interactions. The various correlations have been found between geometrical, energetic and electron-topological characteristics of the H-bonds in the investigated ion pairs. The H-bonding interactions in the ion pairs vary in their strength ranging from weak to ACS Paragon Plus Environment

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moderately strong. The NH group within the cation is a better proton donor than the OH groups as a result of which the N-H…O hydrogen bond in the ion pairs becomes stronger than O-H…O. All hydrogen bonds are stronger for the ion pairs of TEOA cation with MTN and PTSA anions. The weakest hydrogen bonds are found in the ion pair of TEOA cation with TfO anion. In addition to multiple hydrogen bonds, there are other dispersion and electrostatic-dominant interactions that play an important role in the overall stability of PILs and their physicochemical properties. The structure of TEOA cation in the ion pairs has been analyzed in terms of the influence of the anion. In all the ion pairs the distortion of the symmetrical structure of the cation are observed due to the coupling effects of the H-bonding and electrostatic interactions. The interaction strength in the series of the studied ion pairs of TEOA cation increases with the following anions: TfO < NBSu < BSu < PTSA < MTN < MsO. This series also correlates very closely with the proton affinity of the considered acid anions. Combined with the results of our previous work on studying the triethanolammonium – based PILs with inorganic anions, the relationship between the melting points of PILs and the calculated binding energies of the ions in the ion pairs have been found; indicating that higher binding energies lead to higher melting points.

ASSOCIATED CONTENT Supporting Information. The 3D plots of the electrostatic potential surface for TEOA cation and six anions studied in this work and the analyzed ion pairs are given in Figure S1 and S2, respectively. Calculated HOMO energies for the isolated anions as well as HOMO and LUMO energies and their energy gaps for the ion pairs are given in Table S1. Table S2 contains the calculated ΔG298 values of the formation of the ion pairs from triethanolamine and acid molecules. The optimized structures of the ion pairs with bifurcated hydrogen bonds are shown in Figure S3, and their topological and energetic characteristics are reported in Table S4 and S5, respectively. Figure S4 shows the dependence between the decomposition temperatures of triethanolammonium class of PILs with different anions and the ΔG298 calculated for the corresponding ion pairs. Figure ACS Paragon Plus Environment

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S5 and Table S3 contain additional results for TEOA cation. Figure S6 and S8 presents the molecular graphs (based on the electron density distributions derived from the Bader theory) of the ion pairs considered in this study. Figure S7 depicts the correlation between the H-bond energy and donor-acceptor distance for the structures of the ion pairs in Figure 1. The complete text of ref 28 is also given. 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. Notes The authors declare that they have no conflict of interest. ACKNOWLEDGMENT This work was partially funded by the Russian Foundation for Basic Research and the government of the region of the Russian Federation, grant No. 18-43-370009. REFERENCES 1. Verkade, J. G. Main group atranes: Chemical and structural features. Coord. Chem. Rev. 1994, 137, 233-295. 2. Voronkov, M. G.; Albanov, A. I.; Aksamentova, T. N.; Chipanina, N. N.; Adamovich, S. N.; Mirskov, R. G.; Kochina, T. A.; Vrazhnov, D. V.; Litvinov, M. Yu. Tris(2-hydroxyethyl) ammonium salts: 2,8,9-trihydroprotatranes. Russ. J. Gen. Chem. 2009, 79, 2339-2346.


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TOC Graphic


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Figure 1. Optimized hydrogen-bonded structure of the ion pairs of triethanolammonium cation with anions of different sulfonic acids. The interatomic distances are given in Å. 60x54mm (300 x 300 DPI)



Figure 2. Dependence of binding energy of the ions in the ion pairs on melting point of triethanolammoniumbased PILs. The labels for each anion are individually assigned. 286x199mm (300 x 300 DPI)


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TOC Graphic 38x30mm (300 x 300 DPI)


Quantum Chemical Modeling of the Structure and H-Bonding in

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