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The Nature of the Interactions in Triethanolammonium - Based Ionic Liquids. A Quantum Chemical Study Irina V. Fedorova, and Lyubov P. Safonova J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b02598 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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The Journal of Physical Chemistry

The Nature of the Interactions in Triethanolammonium − Based Ionic Liquids. A Quantum Chemical Study 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. Structural features and interionic interactions play a crucial role in determining the overall stability of ionic liquids and their physicochemical properties. Therefore, we have performed high-level quantum chemical study of different cation-anion pairs representing the building units of protic ionic liquids based on triethanolammonium cation and anions of sulfuric, nitric, phosphoric and phosphorous acids to provide essential insight into these phenomena at the molecular level. It has been shown that every structure is stabilized through multiple H-bonds between the protons in the N-H and O-H groups of the cation and different oxygen atoms of the anion acid. Using atoms in molecules topological parameters and natural bond orbital analysis, we have determined the nature and strength of these interactions. Our calculations suggest that the N-H group of the cation has more proton donor-like character than the O-H group that makes the N-H…O hydrogen bonds stronger. A close relation between the binding energies of these ion pairs and experimental melting points has been established: the smaller the absolute value of the binding energy between ions, the lower is the melting point.


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INTRODUCTION Protic ionic liquids, PILs (a sub class of ILs) are nowadays highly discussed. This is partly due to the fact that PILs possesses remarkable physicochemical properties suitable for many industrial and technical applications.1-6 Despite pure ionic composition of these liquids, H-bonds play a crucial role in PILs7-13 since donor and acceptor groups are formed on the ions during synthesis through proton transfer from a Brønsted acid to a Brønsted base14-17. Therefore, the Hbonding characteristics in these PILs mainly depend on the ability of acid and base molecules −

+

(AH+B) to form ions (A +BH ). Numerous studies have shown that the structural features and H-bonding interactions between cation and anion within PILs define their physicochemical properties.12, 17-23 In most cases the understanding of the relationship between macro-properties and micro-structures of PILs has been obtained using quantum chemical calculations in the gas phase.24-29 It has been shown30, 31 that the density functional theory (DFT) methods are able to deliver reasonably reliable results for the structure and properties of these ionic systems. Therewith, the quality of DFT results are significantly improved by including correction terms for dispersion interactions.32-35 Nowadays, the number of works devoted to uncovering the relationship between the structural features of PILs composed of triethanolammonium (tris(2-hydroethyl)ammonium, TEOA) cation with different acid anions and their physical properties are rather sparse. Herein, the physicochemical properties of these PILs such as thermal behavior, density, viscosity, specific conductivity, etc. have been reported.36, 37 There are many X-ray diffraction studies that provide information on the crystalline structures of TEOA-based PILs with inorganic anion (Hal38-40, NO341, ClO442, H2PO343, H2PO444) or anions of different organic acids45-48. These works have manifested that the distinctive features of these ionic compounds are that the TEOA cation


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(protatrane49) in contrast to alkylammonium cations has tricyclic structure closed by a trifurcated intraionic N-H(…O)3 hydrogen bonds, also called the endo-configuration. In Odabasoglu’s study50, however, the geometry of this cation in triethanolammonium 2-formylbenzoate differs substantially from the aforementioned structure, since two hydroxyethyl groups are directed along the N-H group (endo-branches) whereas the third CH2CH2OH group (exo-branch) is pointed in a reverse direction. In this case TEOA cation has an exo-endo conformation. A computational study by Chipanina et al.51 have shown some distortion of tricyclic structure of the cation in TEOA-based PILs with different anions of carboxylic acids that is due to the additional interaction between the hydrogen atoms in the O-H groups of the cation and the oxygen atoms in the carboxyl group of organic anion. In the present work, the quantum chemical study of the structure and cation-anion interaction of TEOA-based PILs with anions of sulfuric, nitric, phosphoric and phosphorous acids have been performed. The influence of the anion nature on structure of TEOA cation in these ionic compounds has been investigated. Since triethanolammonium cation (Figure 1) can participate in the formation of multiple H-bonds (both N-H and O-H groups can act as proton donors) in these systems, the individual strength of each H-bond has been estimated by atoms in molecules (AIM) method52-55. The AIM analysis has been also applied for the localization of bond paths and corresponding bond critical points (BCPs) in the H-bonded X-H…O fragment (where X=N, O). Topological parameters such as the electron density, ρ(r), the Laplacian of the electron density, ∇2ρ(r) at the X…H and H…O BCPs, the total electron energy density, H(r) and others have been used to distinguish between shared (covalent) and closed-shell (H-bonding) interactions56, 57. In addition, the natural bond orbital (NBO) analysis58 has been applied for studying the interactions between the lone pair orbitals of the proton acceptor atom and


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antibonding orbital of the proton donor which contribute to the formation of H-bonds and their second-order perturbation stabilization energies. The relationships between geometric characteristics of H-bonds and parameters derived from the AIM and NBO analysis have been examined. The effect of the anion on the melting point of TEOA-based PILs has been discussed.

Figure 1. The geometry of triethanolammonium cation optimized at the B3LYP/aug-cc-pvtz level with atom numbering scheme.

THEORETICAL METHODS Quantum chemical calculations were performed using Gaussian 09 program59. Although all discussions in this paper are based on the B3LYP/aug-cc-pvtz calculations, we also tested the B3LYP functional60, 61 with the following basis sets: 6-31+G(d,p), 6-31++G(d,p), 6-311++G(d,p) and cc-pvtz and no qualitatively different result was found for the investigated systems. The results of these calculations can be found in the electronic supplementary material, Tables S1. The full geometry optimizations of single ions and molecules were carried out for each combination of B3LYP/basis set. For the optimizations of the ion pairs, the individual molecules in their lowest-energy were used as a starting point.


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In order to include an additional dispersion correction in the energy calculations, the reoptimizing the geometry was performed at the B3LYP-GD3 functional62 with the aug-cc-pvtz basis set. For all the optimized geometries the harmonic vibrational analysis was performed to confirm the nature of the stationary points found and also to calculate the thermodynamic properties of the investigated compounds. These data are given in the electronic supplementary material, Tables S2. Table S3 shows the shifts of the N-H and O-H stretching frequencies for the ion pairs studied here. The proton affinity (PA) of the anion was determined as the enthalpy of deprotonation (∆acidH298) of the conjugate acid (i.e., PA(A)=∆acidH298(HA)). The supermolecule approach63 was used in the calculation of interaction energy, Eint between the ions in the H-bonded ion pair. It can be written as

E int = E A...B (A...B) A ∪ B − E A...B (A) A − E A...B (B) B

(1)

in which EA...B (A)A and EA...B (B)B represent the energy of the anion and the cation in the ion pair A…B, respectively, calculated using the basis functions of the whole system (A…B). The binding energy, Ebind, was estimated according to the following

E bind = E int + E def = E A...B (A...B) A∪B − E A (A) A − E B (B) B

(2)

where EA(A)A and EB(B)B are the energies of the optimized ions; Edef is the deformation energy which is always positive since the ions having geometries taken from the ion pair are not in energetic minima. It can be represented as

E def = E A...B (A) A + E A...B (B) B − E A (A) A − E B (B) B

(3)

Basis set superposition error, BSSE was considered by the counterpoise method of Boys and Bernardi64 on all the optimized structures. AIM calculations were carried out with AIMAll program (Version 10.05.04)65 on the wave functions obtained at the B3LYP/aug-cc-pvtz level. The bond critical points of the H…O


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interactions were found, and their topological parameters were used as measure of H-bonding strength. We also employed Espinosa’s equation for the estimation of the H-bonding interaction energies, EHB using the potential energy density, V(r) at the H…O critical point66, as

E HB =

1 ⋅ V(r) 2

(4)

To evaluate the magnitude of the donor-acceptor interactions, the natural bond orbital (NBO) analysis58 was performed at the same level as the one used for AIM analysis using NBO program under Gaussian 09 program package. The amount of charge transfer, qct from the oxygen lone pairs (LP) of the acid anion to the antibonding (BD*) X-H orbital of the cation in the H-bonded ion pairs was calculated in the following way:

q CT

 Fij   = n ⋅   ε − ε j  i

2

(5)

where n is the orbital occupancy; εi, εj are the diagonal elements (orbital energies) and Fij is the off-diagonal NBO Fock matrix element. The stabilization energy, E(2) associated with LPO→BD*X-H orbital interactions was estimated from the second-order perturbation approach as follows: E

( 2)

=−

n ⋅ Fij2

(6)

εi − ε j

The atomic charges were determined in terms of natural population analysis as implement in the NBO method. These data are given in the electronic supplementary material, Tables S4.

RESULTS AND DISCUSSION The interaction of triethanolamine with inorganic acids studied here leads to the formation of free ions by proton transfer from acid to amine (neutralization reaction14-17). And the TEOA


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Figure 2. The optimized geometries of ions pairs composed of TEOA cation and anion of sulfuric, nitric, phosphoric, or phosphorous acid at the B3LYP/aug-cc-pvtz level. Dashed lines indicate H-bonds.

cation and acid anion (A=HSO4, NO3, H2PO3, H2PO4) are held together in the ion pair by the electrostatic interaction and a hydrogen bond. The ionic [TEOA][A] structures obtained from B3LYP/aug-cc-pvtz optimization are shown in Figure 2 together with atom numbering. According to the thermochemical parameters for the ion pair formation (Table S2, supplementary material), it can be concluded that all these compounds are readily formed and energetic stable. As one sees, their overall structures are similar enough. In all cases, the cation and anion are


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associated with one another by four hydrogen bonds. There exists a hydrogen bond between the proton in the N-H group of TEOA cation and the oxygen atom (O2) of acid anion. And there are three H-bonds formed by the hydrogen atoms in three hydroxyl groups of the cation and two different O1 and O2 atoms of the same anion. Before discussing the structural features of the investigated ion pairs, we briefly consider the geometry of TEOA cation itself (see Figure 1, Table 1). The results of our calculations indicate that TEOA cation has a tricyclic structure in which the N-H bond is situated within the protatrane framework. The hydrogen atom in the N-H group of the cation is surrounded by three oxygen atoms from its hydroxyethyl groups. All the carbon atoms in the hydroxyethyl groups and the nitrogen atom in the cation have tetrahedral electronic geometry with bond angles close to 109.5° (the ideal tetrahedral value). Within all three hydroxyethyl groups of the cation the C-C, C-N and C-O bond lengths and corresponding bond angles are identical to each to other. The three distances between the proton in the N-H group and the oxygen atoms in the O-H groups have equal values and these are less than the sums of van der Waals radii of the O and H atoms (2.72 Å)67. This clearly indicates that there exist three intracationic trifurcate H-bonds. The influence of the anion nature on the geometry of triethanolammonium cation can also be seen in Table 1. The symmetrical structure of TEOA cation within all the investigated ion pairs is significantly distorted as a result of multiple H-bonding interactions between the protons in the N-H and O-H groups of the cation and the oxygen atoms of the anion. In the all optimized [TEOA][A] structures all the three C-N-C bond angles are not equal in values but these are very close to that found in the isolated cation. The different values are also observed for the three bond angles of N-C-C and C-C-O and all these are larger compared to TEOA itself. In the presence of acid anion all the three bonds of N-C, C-C and C-O in the cation are of different lengths. The


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Table 1. Comparison of some geometric parameters in the isolated TEOA cation and H-bonded ion pairs composed of the same cation and anions of different inorganic acids calculated by the B3LYP/aug-cc-pvtz level of theory. Atom numbers refer to Figs. 1 and 2. Parameters

+

[TEOA][NO3] [TEOA][H2PO4] [TEOA][H2PO3] Bond lengths, Å N-C 1.512 1.524; 1.518; 1.521; 1.516; 1.508 1.521; 1.517; 1.510 1.522; 1.517; 1.510 1.511 (1.503-1.513) (1.492-1.506) (1.497-1.509) C-O 1.419 1.404; 1.407; 1.404; 1.407; 1.408 1.402; 1.405; 1.408 1.403; 1.405; 1.406 1.407 (1.414-1.419) (1.399-1.414) C-C 1.516 1.527; 1.534; 1.526; 1.534; 1.526 1.529; 1.532; 1.526 1.529; 1.532; 1.527 1.526 (1.491-1.502) (1.496-1.507) H1…O3 (4, 5) 2.250 2.975; 2.743; 2.979; 2.811; 2.778 2.903; 3.009; 2.911 2.911; 3.021; 2.911 2.836 (2.32-2.42) (2.31) (2.31) Bond angles, º C-N-C 112.3 111.0; 112.0; 111.0; 111.9; 110.8 110.7; 112.3; 109.8 110.7; 112.1; 109.9 110.2 (111.4-112.8) N-C-C 110.9 115.0; 115.1; 113.6; 115.2; 114.1 115.1; 115.3; 116.2 115.2; 115.3; 116.0 114.8 (110.5-111.1) C-C-O 105.5 113.2; 111.6; 113.4; 111.6; 111.8 113.4; 112.8; 113.0 113.4;112.5; 113.1 112.4 (106.1-107.5) In parentheses are given the experimental data for crystal structure of triethanolammonium salts with nitrate, dihydrogenphosphite and dihydrogenphosphate reported in refs. 41, 43 and 44, respectively. [TEOA]

[TEOA][HSO4]

intracationic N-H(…O)3 distances being not identical in all the H-bonded [TEOA][A] structures are in the range of 2.743-3.021 Å, and these are significant longer than that in the isolated cation. Based on the obtained results, it can be declared that there is no intracationic H-bonds in the investigated ion pairs. Similar results were obtained when studying the interactions between triethanolammonium cation and different anions of carboxylic acids in the single H-bonded ion pairs51. Therewith, it was shown that the intracationic trifurcate N-H(…O)3 H-bonds are only retained in the crystalline structure of these salts. In like manner, there are trifurcated H-bonds in the crystalline structure of triethanolammonium salts with anions of nitric41, phosphorous43 and phosphoric44 acids.


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The calculated values of the H-bond geometry parameters in [TEOA][A] ion pairs using the “standard” B3LYP and dispersion corrected B3LYP-GD3 methods are given in Table 2. In general, dispersion effects cause minor changes in the relative geometry and orientation of the constituent ions within the investigated ion pairs. In all the cases the H-bond lengths in the XH…O fragment (where X=N, O) decrease slightly for B3LYP-GD3 level of theory. The H-bond angles have close values for both functionals. On the other hand, the use of dispersion correction leads to significant changes in energetic characteristics of these systems, as we shall discuss later. In all of the calculations the H…O distances are much shorter than the sum of van der Waals radii of bonding atoms (H: 1.20 Å; N: 1.55 Å, O: 1.52 Å)67, thus showing the formation of the N-H…O and O-H…O hydrogen bonds. The H-bond formation in all the cases leads to the elongation of the covalent N–H and O-H bonds in the cation. The greater N–H bond elongation is reflected in the shorter H…O distance. The H-bond length in the N-H…O fragment in [TEOA][A] ion pairs increases in the series of the anions: H2PO4

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