Journal of the Iranian Chemical Society

, Volume 10, Issue 4, pp 733–744

Electronic and topological properties of interactions between imidazolium-based ionic liquids and thiophenic compounds: a theoretical investigation

Authors

    • College of ScienceChina University of Petroleum (East China)
  • Dong Liu
    • College of Chemical EngineeringChina University of Petroleum (East China)
  • Yukun Lu
    • College of ScienceChina University of Petroleum (East China)
  • Jin Lin
    • College of Chemical EngineeringChina University of Petroleum (East China)
Original Paper

DOI: 10.1007/s13738-012-0207-z

Cite this article as:
Lü, R., Liu, D., Lu, Y. et al. J IRAN CHEM SOC (2013) 10: 733. doi:10.1007/s13738-012-0207-z

Abstract

To deepen the understanding the interactions of thiophenic compounds in ionic liquids, we have performed a systemic study on the electronic structures, and topological properties of interactions between N-ethyl-N-ethylimidazolium diethyl phosphate ([EEIM][DEP]) ionic liquid and 3-methylthiophene (3-MT), benzothiophene (BT), or dibenzothiophene (DBT) using density functional theory. From NBO atomic charges and electrostatic potential analyses, most of the positive charge is located on C2–H2 in the [EEIM] cation, and the negative charge is focused on oxygen atoms in [DEP] anion, implying oxygen atoms in [DEP] should easily attack C2–H2 in [EEIM]. The electrostatic interaction between anion and cation may be dominant for the formation of the [EEIM]–[DEP] ion pair. The large stabilizing effect is due to the strong orbital interactions between the antibonding orbital of proton donor σ*(C2–H2) in [EEIM] cation and the lone pairs of proton acceptor LP(O) in [DEP] anion. A common feature of [EEIM][DEP], [EEIM][DEP]-3-MT/BT/DBT complexes is the presence of hydrogen bonds between [EEIM] cation and [DEP] anion. This work has also given the interacting mechanism of 3-MT, BT, and DBT adsorption on [EEIM][DEP] ionic liquid. Both [EEIM] cation and [DEP] anion are shown to play important roles in interactions between 3-MT, BT, DBT and [EEIM][DEP], which has been corroborated by NBO and AIM analyses. The π···π, π···C–H and hydrogen bonding interactions occur between [EEIM][DEP] and 3-MT, BT, DBT. The strength of sulfur involved interactions between 3-MT, BT, DBT and [EEIM][DEP] follows the order of 3-MT > BT > DBT. The order of interaction energies between [EEIM][DEP] and 3-MT, BT, DBT is 3-MT < BT < DBT, in agreement with the order of extractive selectivity from fuel oils (DBT > BT > 3-MT) in terms of sulfur partition coefficients.

Keywords

Ionic liquidDesulfurizationDensity functional theoryElectronic propertiesNBO analysesTopological properties

Introduction

The desulfurization of fuel has received worldwide attention because of the increasingly stringent environment regulation of the sulfur limit for fuels. Conventional hydrodesulfurization (HDS) is more effective for the removal of aliphatic sulfur compounds than that of sulfur containing aromatic compounds, such as thiophene, benzothiophene, and dibenzothiophene. HDS requires high temperature and high hydrogen pressure to eliminate the alicyclic sulphur compounds [1]. Therefore, alternative sulphur removal techniques are being explored. In the past years, ionic liquids have gained increasing interest due to its unique properties both as extractant and also as catalyst [2]. Ionic liquids have been classified as ionic compounds that have melting points at temperatures of 100 °C or lower. The first attempt of deep desulfurization using ionic liquids was made by the group of Wasserscheid and Jess [3]. The application of ionic liquids as the media for liquid–liquid extractions is growing rapidly, since their hydrophobic or hydrophilic nature can be modulated by modifications in both cation and anion. Lo et al. [4] firstly investigated removal of sulfur-containing compounds from light oils by a combination of both chemical oxidation and solvent extraction using room temperature ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate and 1-butyl-3-methylimidazolium tetrafluoroborate. Therefore, the proper combination of imidazolium cations and anions can lead to highly selective materials for the extraction of aromatic compounds. Moreover, different types of hydrogen bonds may be operative depending on the structural and electronic properties of both the ionic liquids and the sulfur-containing aromatic compounds.

Sulfur partition coefficients for the sulfur-containing compounds 3-methylthiophene (3-MT), benzothiophene (BT), and dibenzothiophene (DBT) between N-methyl-N-methylimidazolium dimethyl phosphate ([MMIM][DMP]), N-ethyl-N-methylimidazolium diethyl phosphate ([EMIM][DEP]), N-butyl-N-methylimidazolium dibutyl phosphate (BMIM][DBP]), N-ethyl-N-methylimidazolium diethylphosphate ([EMIM][DEP]), N-butyl-N-methylimidazolium dibutylphosphate ([BMIM][DBP]), N-ethyl-N-methyl-imidazolium dimethylphosphate ([EMIM][DMP]), N-ethyl-N-ethyl-imidazolium diethylphosphate ([EEIM][DEP]), N-butyl-N-ethyl-imidazolium dibutylphosphate ([BEIM][DBP]) and gasoline have been determined over a wide range of sulfur content [57]. It was shown that the order of extractive selectivity from fuel oils was DBT > BT > 3-MT in terms of sulfur partition coefficients. The results suggest that [EEIM][DEP] might be used as a promising solvent for the extractive desulfurization of fuel, considering its higher sulfur extractive ability, lower solubility for fuel and thus negligible influence on the constituent of fuel, and the ease of regeneration for the spent ionic liquid via water dilution process. Therefore, it is interesting to investigate the most appropriate site for interactions of various molecules with [EEIM][DEP] ionic liquid and compare their electronic and topological properties. Understanding the interactions at molecular level is important for the design of functional ionic liquids.

Nowadays, to further promote the development and application of ionic liquids and to guide the design of novel ionic liquids, researchers have made great efforts in investigating their structures, properties, and structure–property relationships, through which valuable information has been obtained. However, the knowledge of the microstructure for ionic liquids obtained from experiments is indirect. An explicit and complete understanding toward the structures and properties of ionic liquids appeals to the assistance of theoretical research. Therefore, large numbers of investigations on these subjects have been performed by means of simulations. The quantum chemical-based COSMO-RS was used to predict the non-ideal liquid phase activity coefficient for mixtures containing 1-ethyl-3-methyl imidazolium thiocyanate [EMIM][SCN], thiophene, quinoline, pyridine, indoline, pyrrole, and water [8]. The use of mixed ionic liquids as possible alternatives for the removal of aromatics has been studied via COSMO (COnductor like Screening MOdel) [9]. Kumar and Banerjee [10] used the COSMO-RS predictions to evaluate the performance of 264 possible cation–anion pairs in the removal of thiophene from diesel oil. The simultaneous separation of thiophene and pyridine from isooctane were investigated by the non-random two liquid (NRTL) and UNIversal QUAasi-Chemical (UNIQUAC) models with 1-ethyl-3-methylimidazolium acetate [EMIM][OAc], 1-ethyl-3-methylimidazolium ethylsulfate [EMIM][EtSO4], and 1-ethyl-3-methylimidazolium methylsulfonate [EMIM] [MeSO3] as green solvents [11]. Aznar et al. [12] used the UNIQUAC model to correlate the liquid–liquid equilibrium (LLE) of fifty ternary systems involving twelve ionic liquids with different activity coefficient. Molecular dynamics simulations of solutions of benzene in dimethylimidazolium chloride and dimethylimidazolium hexafluorophosphate have been performed to explain the better solubility of aromatic compounds as compared to aliphatic compounds in the ionic liquids of dimethylimidazolium [13]. Extraction of thiophene or pyridine from n-heptane using ionic liquids was modeled by both NRTL and UNIQUAC approaches [14]. Quantum chemical calculations, including natural bond orbital (NBO) analyses have also been carried out to investigate the simultaneous interactions of thiophene and pyridine with different ionic liquids, including 1-butyl-1-methyl pyrrolidinium tetrafluoroborate ([Pyr14][BF4]), 1-butyl-1-methyl pyrrolidinium hexafluoro-phosphate ([Pry14][PF6]), 1-butyl-4-methyl pyridinium tetrafluoroborate ([BPY][BF4]), 1-butyl-4-methylpyridinium hexafluorophosphate ([BPY][PF6]), and 1-benzyl-3-methylimidazolium tetrafluoroborate ([BeMIM][BF4]) [15]. Zhang et al. [16] employed multinuclear NMR spectroscopy and ab initio calculations to study the interactions between thiophene and the ionic liquids of 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]+[PF6]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]+[BF4]). The interaction between ethanethiol molecule and either anhydrous FeIII chloride anions or 1-butyl-3-methylimidazolium ([BMIM]+) cations of ionic liquids was investigated using density functional theory approach, ionic liquids containing anionic FeIII species suggested excellent performance to remove sulfur compounds from natural gasoline [17]. Molecular dynamic simulations were performed to screen suitable ionic liquid for desulfurization. DBT and DBTO2 were used as model compounds to study the mechanism of desulphurization [18]. The structures, acidities and interactions between the cations and the anions of a series of task-specific acidic ionic liquids have been investigated by density functional theory method [19]. The interactions between N, N-dialkylimidazolium dialkylphosphate ionic liquids and aromatic sulfur compound were investigated [20].

To the best of our knowledge, there is no theoretical report on the investigation of the interactions between N-ethyl-N-ethyl-imidazolium diethylphosphate ([EEIM][DEP]) ionic liquid and 3-MT, BT, DBT. Our primary purpose is to try to study the interactions between cation–anion pair and 3-MT, BT, DBT by means of density functional approach. The present results would provide some useful information, such as geometrical, electronic, topological properties, and interaction mechanisms between [EEIM][DEP] and 3-MT, BT, DBT, which is essentially important for the continuous exploitation and application of ionic liquids for desulfurization.

Models and computational details

Specification of initial geometries

The structures of 1-ethyl-3-ethylimidazolium cation ([EEIM]), diethylphosphate anion ([DEP]), 3-methyl-thiophene (3-MT), benzothiophene (BT), and dibenzothiophene (DBT) are shown in Fig. 1. The [DEP] anion and 3-MT/BT/DBT have been gradually placed in different regions around imidazolium cation to form [EEIM][DEP]-3-MT, [EEIM][DEP]-BT, and [EEIM][DEP]-DBT for optimization. The most stable structures were further employed for NBO and AIM analyses.
https://static-content.springer.com/image/art%3A10.1007%2Fs13738-012-0207-z/MediaObjects/13738_2012_207_Fig1_HTML.gif
Fig. 1

The illustration of a 1-ethyl-3-ethylimidazolium cation ([EEIM]), b diethylphosphate anion ([DEP]), c 3-methyl-thiophene (3-MT), d benzothiophene (BT), and e dibenzothiophene (DBT)

Computational details

In this study, we model gas-phase interactions by calculating the energy-minimized electronic structures using density functional theory (DFT). DFT has also been successfully employed to calculate the structural properties of some of ionic liquids. All the geometric optimizations reported here were performed with DMol3 program package [21, 22]. The double numerical basis sets plus polarization functional (DNP) was employed. For the exchange correlation term of the energy functional, the generalized gradient corrected functional GGA and PW91 functional [23] as implemented in the DMol3 program, were used for all the geometry optimizations. Although PW91 functional is unable to provide a good description of dispersion interactions, GGA/PW91/DNP can give good results of interactions between conjugated systems [24]. No restrictions on symmetries were imposed on the initial structures. A frequency analysis was performed on all structures to ensure the absence of imaginary frequency and verify the existence of a true minimum. To quantify contributions to the binding interaction, the second-order perturbation energy lowering (or stabilization energy) E(2) output in the natural bond orbital (NBO) was investigated [25]. These non-local donor–acceptor–orbital interactions are associated with the delocalization of electron density between states i and j in the NBO basis, as given by
$$ E(2) = \Updelta E_{ij} = n_{i} \frac{{(F_{ij} )^{2} }}{{\varepsilon_{j} - \varepsilon_{i} }} $$
where ni is the donor orbital occupancy, εi and εj are the diagonal elements, and Fi,j is the off-diagonal NBO Fork matrix element. Intermolecular interactions, such as lone pair → anti-bonding orbital mixtures are representative of donor–acceptor bonding, whereas non-Lewis-type (highly delocalized) interactions such as anti-bond → anti-bond orbital mixtures represent effects like resonance stabilization. Atoms in molecules (AIM) analyses were calculated by AIM2000 to provide topological properties [26, 27].
DMol3 uses numerical functions that are far more complete than traditional Gaussian functions, and therefore we expect BSSE contribution to be small [28]. The interaction energies are defined as the differences between the energies of [EEIM][DEP]- 3-MT/BT/DBT and the sum of the energies of [EEIM][DEP] and 3-MT, BT, DBT.
$$ \Updelta E = -\, \{ E(\left[ {\text{EEIM}} \right]\left[ {\text{DEP}} \right] - 3 - {\text{MT}}/{\text{BT}}/{\text{DBT}}) - [E(\left[ {\text{EEIM}} \right]\left[ {\text{DEP}} \right]) + E(3 - {\text{MT}}/{\text{BT}}/{\text{DBT}})]\} $$

Results and discussion

Geometries of [EEIM][DEP], [EEIM][DEP]-3-MT/BT/DBT complexes

In this section, the most stable geometry of isolated ion pair [EEIM][DEP] is discussed firstly. We have considered various possible relative orientations between cation and anion to find the most stable conformer. As shown in Fig. 2, the [DEP] anion can occur on the front, back, top, bottom or side sites of [EEIM] cation. By comparing the optimized energy values, the most stable geometry was selected. Figure 3a shows the most stable geometry of [EEIM][DEP] ion pair optimized at the PW91/DNP level. It is found that the [DEP] anion appears primarily near the imidazolium ring, particularly the positions around the C2–H2 of the [EEIM] cation. The H···O distances are 1.649, 1.927, and 2.703 Å, shorter than or equal to the sum of van der Waals radii of hydrogen (1.20 Å) and oxygen (1.52 Å) [29], indicating preferential localization of [DEP] anion in the C2–H2 of [EEIM] cation according to the shortest distance of H2···O1. The most possible region of [DEP] anion around [EEIM] cation shown above is consistent with those found in the 1-butyl-3-methylimidazolium chloride [30], where Cl tends to locate near C2–H2 of 1-butyl-3-methylimidazolium cation. This can be explained by the relatively larger positive charges on the C2–H2 unit than on the other part of imidazolium cation. Hydrogen bonds also occur between oxygen atoms on [DEP] anion and the C–H on the ethyl groups (H61 and H81) of imidazolium ring. The bond lengths of C–H in configuration are lengthened, when they are involved in the formation of the C–H···O interactions, especially the C2–H2 bond that is connected to the newly formed C2–H2···O1 hydrogen bond. The formation of hydrogen bonds gives rise to lengthening of P–O bond lengths, and shortening of the other P–O bond in accordance with the principle of bond order conservation. The imidazolium ring still maintains the coplanar characteristic after the formation of [EEIM][DEP] ionic pair.
https://static-content.springer.com/image/art%3A10.1007%2Fs13738-012-0207-z/MediaObjects/13738_2012_207_Fig2_HTML.gif
Fig. 2

Possible positions of a [DEP] anion and 3-MT/BT/DBT around an [EEIM] cation

https://static-content.springer.com/image/art%3A10.1007%2Fs13738-012-0207-z/MediaObjects/13738_2012_207_Fig3_HTML.gif
Fig. 3

The optimized structures and some interacting distances of a [EEIM][DEP], b [EEIM][DEP]-3-MT, c [EEIM][DEP]-BT, and d [EEIM][DEP]-DBT

The sketches of electrostatic potential of [EEIM] cation and [DEP] anion are shown in Fig. 4. As can be seen in Fig. 4, the high positively charged region in the [EEIM] cation is around the C2–H2 group followed by the regions around the C4–H4/C5–H5 atoms attached to the imidazolium ring and the other C–H atoms in the ethyl chains, which is in good agreement with the previous results [30]. This may be attributed to the electron-withdrawing effect of two nitrogen atoms. It can be concluded that the electrostatic attractions play a crucial role in the cation–anion interaction. The interactions between [EEIM] cation and [DEP] anion take place in those regions that possess more positive part of [EEIM] and more negative part of [DEP].
https://static-content.springer.com/image/art%3A10.1007%2Fs13738-012-0207-z/MediaObjects/13738_2012_207_Fig4_HTML.gif
Fig. 4

The electrostatic potentials (ESP) of a [EEIM] and b [DEP]

The initial structures were designed by placing [DEP] anions and 3-MT/BT/DBT on the front, back, top, bottom or side sites of the [EEIM] cations. For [EEIM], all the possible hydrogen bonding sites on the C2–H2, C4–H4, C5–H5 and the hydrogens on two ethyl groups have been taken into consideration for the initial geometry design. As for [DEP], the two terminal oxygen atoms and two bridge oxygen atoms were the main consideration for initial geometry design. Possible π (imidazolium)-π (3-MT/BT/DBT) interactions were considered by placing [DEP] anions and 3-MT/BT/DBT in different positions. The most stable structures of [EEIM][DEP]-3-MT, [EEIM][DEP]-BT, [EEIM][DEP]-DBT are shown in Fig. 3b–d. It is found that the interaction manners between [EEIM] cations and [DEP] anions are similar, where [DEP] anions are situated on the C2–H2 sides of the imidazolium rings. The [DEP] anions generally form multiple contacts with the C–H units of the [EEIM] cations. The H···O distances are in the range of 1.604–2.692 Å as shown in Fig. 3b–d. These data clearly indicate the formation of hydrogen bonds between [EEIM] cation and [DEP] anion. Among all the interactions, the shortest hydrogen bonds (1.671 Å in [EEIM][DEP]-3-MT, 1.604 Å in [EEIM][DEP]-BT, 1.678 Å in [EEIM][DEP]-DBT) are to the C2–H2, indicating the dominant role of the attractive electrostatic term in the ion pairs. In [EEIM][DEP]-3-MT, the interacting distances between [EEIM][DEP] and 3-MT are 2.236 Å (H4’’···O1), 3.358 Å (C4’’···C2), 2.808 Å (S1···H82), and 3.204 Å (H6···C5). As indicated in Fig. 3c, the interactions between [EEIM][DEP] and BT are H3’’···O1 (2.236 Å), H4’’···O1 (2.789 Å), C7a···H82 (2.892 Å), C3’’···H81 (2.879 Å), and C4’’···C2 (3.638 Å). In [EEIM][DEP]-DBT, the interactions between [EEIM][DEP] and DBT are H9’’···O4 (2.446 Å), H1’’···O2 (2.586 Å), C2’’···H61 (2.797 Å), C1’’···C2 (4.035 Å) and https://static-content.springer.com/image/art%3A10.1007%2Fs13738-012-0207-z/MediaObjects/13738_2012_207_Figa_HTML.gif . The Bondi’s van der Waals radii of carbon, oxygen, sulfur and hydrogen are 1.70, 1.52, 1.80 and 1.20 Å [29], some interacting distances are longer than that of sum of atomic radii, but the bond paths with bond critical points exist. The S1···H82 distance is shorter than that of the sum of van der Waals radii of sulfur and hydrogen, suggesting that the sulfur atom in 3-MT is involved in the interactions between [EEIM][DEP] and 3-MT remarkably. In [EEIM][DEP]-3-MT, [EEIM][DEP]-BT, and [EEIM][DEP]-DBT, the [EEIM] rings and 3-MT, BT, DBT ring planes are parallel to each other, implying occurrence of the π–π interactions. The π stacking (also called π–π stacking) refers to attractive, non-covalent interactions between aromatic rings [3133]. Despite their frequent occurrence, there is no unifying picture of the factors that contribute to the interaction, which include electrostatic (quadrapole–quadrapole and quadrapole–dipole, and dipole–dipole), hydrophobic, and van der Waals interactions. This is complicated by the fact that aromatic rings interact in several different conformations, each of which is favored by a different combination of forces [32, 33]. The face–face stacked, edge–face stacked, and offset stacked geometries are three representative conformations of π–π interactions. The edge–face stacked and offset stacked configurations can be observed in the crystal structures of many simple aromatic compounds and proteins. From Fig. 3b–d, the parallel stacking interactions occur between [EEIM] rings and 3-MT, BT, DBT rings in the gas phase. It seems that H4’’···O1 interaction in [EEIM][DEP]-3-MT, H3’’···O1 and H4’’···O1 interactions in [EEIM][DEP]-BT, H9’’···O4 and H1’’···O2 interactions in [EEIM][DEP]-DBT may pronouncedly influence the formation of π–π interactions [33]. The interactions between [EEIM][DEP] and 3-MT, BT, DBT are dependent not only on hydrogen bonds but also on π–π interactions. The above DFT calculations have provided useful information for understanding the strength of the [EEIM]-[DEP] interaction, [EEIM][DEP]-3-MT/BT/DBT interactions in the systems.

Interaction energies

The interaction energy between [EEIM][DEP] and 3-MT/BT/DBT is an important index for evaluating the stability of [EEIM][DEP]-3-MT/BT/DBT complexes. These energy values can provide reasonable explanations for the efficiency of sulfur removal. The interaction energy is defined as the difference between the energy of [EEIM][DEP]-3-MT/BT/DBT and the sum of the energies of [EEIM][DEP] and 3-MT, BT, DBT. The interaction energies between [EEIM][DEP] and 3-MT, BT, DBT are 7.63, 11.29, 13.58 kcal/mol, demonstrating that the magnitude of the interacting energies follows the trend [EEIM][DEP]-3-MT < [EEIM][DEP]-BT < [EEIM][DEP]-DBT. The sulfur removal selectivity of 3-MT, BT and DBT by [EEIM][DEP] ionic liquid was investigated by experiments. The results suggested that the selective extraction followed the order of DBT > BT > 3-MT [57]. The order of interaction energies between [EEIM][DEP] and 3-MT, BT, DBT is in agreement with the selective extraction trend of experimental results.

NBO analyses

NBO analysis, which can give a better description of the electron distribution and bonding characteristics in compounds, has been performed to get more qualitative evidence of the above structural analysis. The NBO charges of the [EEIM][DEP]-3-MT, [EEIM][DEP]-BT and [EEIM][DEP]-DBT complexes have been calculated. Table 1 lists the sum of charges (Σq) of 3-MT, BT and DBT in the monomers and the sum of charges (Σq) upon formation of [EEIM][DEP]-3-MT, [EEIM][DEP]-BT and [EEIM][DEP]-DBT complexes. The positive/negative values indicate decrease/increase in electron density of the respective groups. Clearly, the sum of charges of 3-MT in [EEIM][DEP]-3-MT has positive Σq values, while those of BT and DBT in [EEIM][DEP]-BT and [EEIM][DEP]-DBT has negative Σq values, demonstrating a charge-withdrawing effect of BT and DBT upon the formation of [EEIM][DEP]-BT and [EEIM][DEP]-DBT complexes. The charge transfer may be due to the hydrogen bonding interactions between the oxygen atoms of [DEP] and the C–H on the BT/DBT molecules as well as strong π–π interactions between imidazolium rings and BT/DBT rings.
Table 1

The charge transfers (Σq) of 3-MT, BT, and DBT in [EEIM][DEP]-3-MT/BT/DBT complexes and the interaction energies (ΔE)

 

3-MT

BT

DBT

Monomer

Complex

Monomer

Complex

Monomer

Complex

Σq (|e|)

0

+0.18659

0

−0.00977

0

−0.01348

ΔE (kcal/mol)

7.63

11.29

13.58

NBO method can provide information about the interactions in both filled and virtual orbital spaces that facilitates analysis of the intra- and intermolecular interactions. A second-order perturbation theory analysis of the Fock matrix was carried out to evaluate the donor–acceptor interaction in the NBO basis. In this analysis, a stabilization energy E(2) related to the delocalization trend of electrons from donor to acceptor orbitals was calculated via perturbation theory. If the stabilization energy E(2) between a donor bonding orbital and an acceptor orbital is large, then there is a strong interaction between them. Table 2 lists the selected donor–acceptor interactions in [EEIM][DEP], [EEIM]DEP]-3-MT, [EEIM][DEP]-BT, [EEIM][DEP]-DBT and their second-order perturbation stabilization energies. The calculated results of structure show that the largest E(2) value is 42.62 kcal/mol for the interaction between the antibonding orbital of C2–H2 bond and the lone pair orbital of O1 in [EEIM][DEP]. Thus, the LP(O1) → σ*(O2–H2) interaction determines the stability of the ion pair, which accords with the shortest hydrogen bonding distance (1.649 Å) discussed above. Such a LP(O1) → σ*(O2–H2) interaction results in the electron transfer from the proton acceptor (LP(O1)) to the proton donor (σ*(O2–H2)) and thereby stabilizes the ion pair [EEIM][DEP]. The second largest E(2) value is 7.86 kcal/mol for the LP(O1) → σ*(C6–H61) interaction, associating with the formation of the O1···H61–C6 hydrogen bond. The results reveal that NBO donor–acceptor interactions which mainly involve an oxygen lone-pair of electrons and anti-bond σ*(C2–H2) are important in forming cation–anion pairs in [EEIM][DEP]-3-MT/BT/DBT complexes. The interaction energies of σ*(P–O1) → σ*(C4’’–H4’’), LP(O1) → σ*(C4’’–H4’’), π(C4–C5) → σ*(C6’’–H61’’), π(C2’’–C3’’) → σ*(C8–H82), π(C4’’–C5’’) → σ*(C8–H82), and LP(S1) → σ*(C8–H82) are 1.02, 5.80, 0.22, 0.28, and 1.25 kcal/mol between [EEIM][DEP] and 3-MT. In [EEIM][DEP]-BT complex, the interactions are LP(O1) → σ*(C4’’–H4’’), σ*(P–O1) → σ*(C3’’–H3’’), σ(C2–H2) → σ*(P–O4), π(C3’’–C2’’) → σ*(C8–H81), LP(S’’) → σ*(C8–H82), π(C4’’–C5’’) → π*(N3–C2). The interaction energies of LP(O4) → σ*(C9’’–H9’’), σ*(P–O4) → σ*(C9’’–H9’’), LP(O2) → σ*(C1’’–H1’’), π(C7’’–C8’’) → σ*(C8–H82), π(C1’’–C2’’) → σ*(C7–H73), and π(C4’’–C3’’) → σ*(C7–H73) are 2.06, 0.32, 1.09, 0.18, 0.35, and 0.21 kcal/mol between [EEIM][DEP] and DBT. The NBO charges of sulfur atoms in 3-MT, BT, and DBT are +0.45119, +0.43055, and +0.34395, suggesting that the delocalization trend of lone pair electrons is 3-MT > BT > DBT. Although the lone pair electrons in 3-MT are more delocalized than those of BT/DBT, the largest of the NBO interactions occurring within lone pair of sulfur atoms (Table 2) is LP(S1) → σ*(C8–H82) in [EEIM][DEP]-3-MT, while the interaction energy of LP(S’’) → σ*(C8–H82) in [EEIM][DEP]-BT is 0.23 kcal/mol. The least lone pair electrons of sulfur involved in interaction of [EEIM][DEP]-DBT may be ascribed to the steric hindrance.
Table 2

Selected donor–acceptor interactions in [EEIM][DEP], [EEIM]DEP]-3-MT, [EEIM][DEP]-BT, [EEIM][DEP]-DBT and their second order perturbation stabilization energies, E(2) (kcal/mol)

Donor

Acceptor

E(2) (kcal/mol)

Donor

Acceptor

E(2) (kcal/mol)

[EEIM][DEP]

LP(O1)

σ*(C2–H2)

42.62

LP(O1)

σ*(C6–H61)

7.86

LP(O4)

σ*(C8–H81)

0.98

   

[EEIM][DEP]-3-MT

σ(P–O4)

σ*(C2–H2)

1.78

LP(O1)

σ*(C7–H71)

2.91

σ*(P–O1)

σ*(C4’’–H4’’)

1.02

σ(C2–H2)

σ*(P–O4)

0.30

π(C4–C5)

σ*(C6’’–H61’’)

0.22

LP(O4)

σ*(C2–H2)

39.84

LP(O3)

σ*(C6–H61)

1.99

LP(O4)

π*(N3–C2)

0.25

LP(O4)

σ*(C9–H92)

1.76

σ*(P–O3)

σ*(C6–H61)

0.22

σ*(P–O1)

σ*(C6–H62)

0.10

σ*(P–O4)

σ*(C9–H92)

0.89

σ*(P–O1)

σ*(C7–H71)

0.84

σ*(P–O2)

σ*(C2–H2)

0.35

σ*(P–O4)

σ*(C2–H2)

8.30

σ*(P–O2)

σ*(C6–H61)

0.27

π(N3–C2)

π*(C4’’–C5’’)

0.12

LP(O1)

σ*(C4’’–H4’’)

5.80

π*(N3–C2)

π*(C4’’–C5’’)

0.17

π(C4’’–C5’’)

π*(N3–C2)

0.12

π(C2’’–C3’’)

σ*(C8–H82)

0.28

π(C4’’–C5’’)

σ*(C8–H81)

0.06

π(C4’’–C5’’)

σ*(C8–H82)

0.56

LP(S1)

σ*(C8–H82)

1.25

[EEIM][DEP]-BT

σ(P–O4)

σ*(C2–H2)

1.34

σ*(P–O4)

σ*(C2–H2)

10.27

LP(O3)

σ*(C6–H61)

2.19

LP(O4)

σ*(C2–H2)

49.09

LP(O4)

σ*(N3–C2)

0.13

LP(O4)

π*(N3–C2)

0.13

LP(O1)

σ*(C3’’–H3’’)

5.03

σ*(P–O1)

σ*(C2–H2)

0.17

LP(O1)

σ*(C4’’–H4’’)

0.45

σ*(P–O3)

σ*(C6–H61)

0.16

σ*(P–O1)

σ*(C3’’–H3’’)

0.86

π(C4’’–C5’’)

π*(N3–C2)

0.15

σ*(P–O2)

σ*(C4’’–H4’’)

0.18

σ*(P–O1)

σ*(C4’’–H4’’)

0.19

π*(N3–C2)

π*(C4’’–C5’’)

0.15

σ(C2–H2)

σ*(P–O4)

0.49

π(C3’’–C2’’)

σ*(C8–H81)

0.51

LP(S’’)

σ*(C8–H82)

0.23

[EEIM][DEP]-DBT

σ(P–O1)

σ*(C2–H2)

1.78

σ*(P–O4)

σ*(C9’’–H9’’)

0.32

LP(O1)

σ*(N3–C2)

0.22

LP(O1)

σ*(C2–H2)

37.33

LP(O1)

σ*(C7–H71)

0.36

LP(O1)

π*(N3–C2)

0.40

LP(O1)

σ*(C8–H81)

0.10

LP(O4)

σ*(C8–H81)

11.73

σ*(P–O1)

σ*(C8–H81)

0.10

σ*(P–O1)

σ*(C2–H2)

3.90

σ*(P–O4)

σ*(C8–H81)

2.37

σ*(P–O1)

σ*(C7–H71)

0.45

π(C4’’–C3’’)

σ*(C7–H73)

0.21

LP(O4)

σ*(C9’’–H9’’)

2.06

LP(O3)

σ*(C2–H2)

0.20

σ(C2–H2)

σ*(P–O1)

0.25

LP(O3)

σ*(C1’’–H1’’)

0.15

σ(C7–H73)

π*(C1’’–C2’’)

0.10

LP(O2)

σ*(C1’’–H1’’)

1.09

π(C1’’–C2’’)

σ*(C7–H73)

0.35

σ(C8–H81)

σ*(P–O4)

0.19

π*(C1’’–C2’’)

σ*(C7–H73)

0.13

π (C7’’–C8’’)

σ*(C8–H82)

0.18

   

AIM analysis

The atoms in molecules (AIM) theory [34] can provide a tool for the classification of the bonding interactions that take place in molecular systems, even inside supermolecules. To obtain more information about the intermolecular interactions, the bond properties between each pair of atoms were systematically analyzed using AIM theory which is based on the topological analysis of electron density (ρ) and its Laplacian (∇2ρ) at the bond critical points (BCPs). The Laplacian ∇2ρ at the BCP is the sum of the three curvatures of the density at the critical point, the two perpendicular to the bond path, λ1 and λ2, being negative whereas the third, λ3, lying along the bond path, is positive. The negative curvatures measure the extent to which the density is concentrated along the bond path and the positive curvature measures the extent to which it is depleted in the region of the interatomic surface and concentrated in the individual atomic basins. In covalent bonding, the two negative curvatures are dominant and ∇2ρ < 0. Although in closed-shell bonding, the interaction is characterized by a depletion of density in the region of contact of the two atoms and ∇2ρ > 0. The ρ is used to describe the strength of a bond, a stronger bond associated with a larger ρ value.

Hydrogen bonds are commonly accepted in chemistry as distinct interactions. However, its definition is somewhat arbitrary. Depending on the definition adopted, interactions that span a wide range of energies can be classified as hydrogen bonds. The nature of the hydrogen bond is mainly electrostatic, although some degree of charge transfer and other quantum effects can contribute significantly, thereby making the interaction partially covalent. Geometric, energetic and spectroscopic criteria have been used to classify them into strong, moderate and weak, but the limits between such categories are diffuse. The first evidence of hydrogen bonding according to the AIM approach is the existence of a bond path between two atoms and the existence of a bond critical point (BCP) [35, 36]. The BCPs are found for O1···H61, O1···H2, and O4···H81 in [EEIM][DEP], O3···H61, O4···H2, O4···H92, O1···H71 and O1···H4’’ in [EEIM][DEP]-3-MT, O3···H61, O4···H2, O1···H4’’ and O1···H3’’ in [EEIM][DEP]-BT, O1···H71, O1···H2, O4···H81, O4···H9’’ and O2···H1’’ in [EEIM][DEP]-DBT. From the values of electron density listed in Table 3, it can be concluded that the interactions between the [EEIM] cation and [DEP] anion are all closed-shell systems (hydrogen-bonding interactions). The values of ∇2ρ between [EEIM][DEP] and 3-MT, BT, DBT are all positive, indicating the typical closed-shell interactions of these complexes. A second AIM criterion to define hydrogen bond is that electron density (ρ) and the Laplacian of electron density (∇2ρ) at BCP, must be within 0.002–0.035 au and 0.024–0.139 au ranges, respectively [35, 36]. This second criterion is fulfilled for O1···H2, and O4···H81 in [EEIM][DEP], O3···H61, O4···H2, O4···H92, O1···H71 and O1···H4’’ in [EEIM][DEP]-3-MT, O3···H61, O4···H2, and O1···H3’’ in [EEIM][DEP]-BT, O1···H2, O4···H81, O4···H9’’ and O2···H1’’ in [EEIM][DEP]-DBT. So far, most hydrogen bonds considered here, the ρ and ∇2ρ values lie in the relative proposed ranges. As seen in Table 3, the values of electron density for hydrogen bonding interactions in [EEIM][DEP], [EEIM][DEP]-3-MT/BT/DBT decrease with the elongation of H···O distances. For hydrogen bonds, there is a correlation between the electron density (ρ) at the BCPs and hydrogen bonding distances. Figure 5 presents the linear correlation between the O···H distances and their corresponding ln(ρb) of [EEIM][DEP], [EEIM][DEP]-3-MT, [EEIM][DEP]-BT, and [EEIM][DEP]-DBT, confirming the dependence between hydrogen bonding strength and their distances. So the topological properties are useful descriptors for the strength of hydrogen bonds.
Table 3

Topological properties of electron density (ρ), Laplacian of density (∇2), eigenvalues of the Hessian matrix (λ1, λ2, λ3) of 3-MT, BT, DBT, [EEIM][DEP], [EEIM][DEP]-3-MT, [EEIM][DEP]-BT, and [EEIM][DEP]-DBT (atomic units)

X···Y

cp type

D/Å

ρ

2

λ1

λ2

λ3

3-MT

TS ring

(3, +1)

···

0.03772

0.26503

−0.03558

0.13954

0.16106

BT

TS ring

(3, +1)

···

0.03614

0.25133

−0.03434

0.12985

0.15581

BZ ring

(3, +1)

···

0.02011

0.15754

−0.01571

0.08253

0.09072

DBT

TS ring

(3, +1)

···

0.03443

0.23656

−0.03273

0.11950

0.14979

BZ ring1

(3, +1)

···

0.02024

0.15772

−0.01580

0.08324

0.09028

BZ ring2

(3, +1)

···

0.02024

0.15772

−0.01580

0.08324

0.09028

[EEIM][DEP]

O1···H61

(3, −1)

2.703

0.00584

0.02339

−0.00474

−0.00176

0.02989

O1···H2

(3, −1)

1.649

0.05418

0.14805

−0.09214

−0.08952

0.32971

O4···H81

(3, −1)

1.927

0.02826

0.08352

−0.03581

−0.03476

0.15409

IM ring

(3, +1)

···

0.05166

0.42390

-0.05944

0.22820

0.25514

[EEIM][DEP]-3-MT

O3···H61

(3, −1)

2.468

0.01040

0.03174

−0.01075

−0.01008

0.05257

O4···H2

(3, −1)

1.671

0.05099

0.14017

−0.08422

−0.08216

0.30655

O4···H92

(3, −1)

2.395

0.01050

0.03626

−0.01003

−0.00884

0.05513

O1···H71

(3, −1)

2.368

0.01231

0.03788

−0.01213

−0.01189

0.06190

O1···H4’’

(3, −1)

2.236

0.01536

0.04340

−0.01655

−0.01581

0.07577

C4’’···C2

(3, −1)

3.585

0.00373

0.01086

−0.00194

−0.00032

0.01312

S1···H82

(3, −1)

2.808

0.00844

0.02853

−0.00672

−0.00384

0.03909

C5···H61’’

(3, −1)

3.204

0.00315

0.00883

−0.00225

−0.00124

0.01233

IM ring

(3, +1)

···

0.05177

0.42384

−0.05957

0.22745

0.25596

TS ring

(3, +1)

···

0.03781

0.26286

−0.03556

0.13837

0.16005

[EEIM][DEP]-BT

O3···H61

(3, −1)

2.355

0.01245

0.03986

−0.01333

−0.01194

0.06513

O4···H2

(3, −1)

1.604

0.05808

0.16243

−0.10417

−0.10174

0.36834

O1···H4’’

(3, −1)

2.789

0.00561

0.01972

−0.00442

−0.00415

0.02828

O1···H3’’

(3, −1)

2.236

0.01519

0.04454

−0.01649

−0.01563

0.07665

C4’’···C2

(3, −1)

3.638

0.00342

0.00959

−0.00153

−0.00068

0.01181

C3’’···H81

(3, −1)

2.879

0.00554

0.01741

−0.00366

−0.00209

0.02316

C7a···H82

(3, −1)

2.892

0.00569

0.01792

−0.00345

−0.00253

0.02391

IM ring

(3, +1)

···

0.05165

0.42376

−0.05939

0.22799

0.25515

TS ring

(3, +1)

···

0.03616

0.24944

−0.03423

0.12895

0.15473

BZ ring

(3, +1)

···

0.02029

0.15678

−0.01595

0.08201

0.09073

[EEIM][DEP]-DBT

O1···H71

(3, −1)

2.692

0.00581

0.02320

−0.00487

−0.00179

0.02986

O1···H2

(3, −1)

1.678

0.05019

0.14104

−0.08258

−0.07994

0.30356

O4···H81

(3, −1)

2.017

0.02365

0.06749

−0.02825

−0.02737

0.12312

O4···H9’’

(3, −1)

2.446

0.01091

0.03357

−0.01056

−0.00996

0.05408

O2···H1’’

(3, −1)

2.586

0.00773

0.02542

−0.00750

−0.00715

0.04007

C2’’···H61

(3, −1)

2.797

0.00657

0.01938

−0.00518

−0.00237

0.02693

C1’’···C2

(3, −1)

4.035

0.00160

0.00463

−0.00069

−0.00032

0.00563

H82···C8’’–C9’’

(3, −1)

···

0.00408

0.01177

−0.00224

−0.00074

0.01475

IM ring

(3, +1)

···

0.05163

0.42246

−0.05930

0.22839

0.25337

TS ring

(3, +1)

···

0.03441

0.23541

−0.03263

0.11896

0.14908

BZ ring1

(3, +1)

···

0.02046

0.15792

−0.01607

0.08328

0.09071

BZ ring2

(3, +1)

···

0.02047

0.15811

−0.01609

0.08347

0.09073

https://static-content.springer.com/image/art%3A10.1007%2Fs13738-012-0207-z/MediaObjects/13738_2012_207_Fig5_HTML.gif
Fig. 5

Correlations between the O···H distances and their corresponding ln(ρb) of a [EEIM][DEP], b [EEIM][DEP]-3-MT, c [EEIM][DEP]-BT, and d [EEIM][DEP]-DBT

When comparing the topological properties of the ring critical points of 3-MT, BT, DBT, [EEIM][DEP], [EEIM][DEP]-3-MT, [EEIM][DEP]-BT, and [EEIM][DEP]-DBT, the changes of topological properties may be ascribed to the interactions between [EEIM][DEP] and 3-MT, BT, DBT.

Conclusions

In this work, possible conformations of the complexes consisting of [EEIM] cation and [DEP] anion, with or without 3-MT/BT/DBT, have been investigated by DFT calculations to characterize the interactions between [EEIM][DEP] and 3-MT/BT/DBT. The most stable geometries of [EEIM][DEP], and [EEIM][DEP]-3-MT/BT/DBT complexes were obtained. The configurations, electronic and topological properties as well as interaction energies have been investigated systematically. In [EEIM] cation, the ESP suggests that the C2–H2 group is more positive than C4–H4, C5–H5 and C–H groups in the two ethyl groups of [EEIM] cation, and the positive charge decreases in the order of C2–H2 > C4–H4/C5–H5 > C–H (ethyl groups). This indicates that C2–H2 in the imidazolium cation is a better hydrogen bond donor than the other C–H groups. The negative regions of [DEP] anion are on the electronegative atoms of oxygen. As expected, the formation of ionic liquids between [EEIM] cation and [DEP] anion should occur in those regions possessing more positive charges and more negative charges. In the [EEIM][DEP]-3-MT/BT/DBT complexes, it is found that multiple hydrogen bonds can exist between the O atoms of [DEP] anion and C2–H2 or even ethyl C–H groups on [EEIM] cation. Partial charge calculations showed that the total charges of 3-MT, BT and DBT was significantly affected by the formation of [EEIM][DEP]-3-MT/BT/DBT complexes. It was found that the tendency to delocalization of lone pair electrons of sulfur atom follows the order of 3-MT > BT > DBT. The steric hindrance of the DBT may prevent the sulfur-involved interaction between sulfur of DBT and [EEIM][DEP] ionic liquid.

From the AIM results, the interactions between the [EEIM] cation and [DEP] anion, [EEIM][DEP] and 3-MT/BT/DBT are closed-shell systems. The results of AIM show that there is a linear correlation between the O···H distances and their corresponding ln(ρb). Electron density can be an excellent indicator for determination of the relative strength of hydrogen bonds.

Although the environment may be different from the gas phase, the present calculation can provide insight into non-bonding interactions which stem from the specifics of electronic structures of ionic liquids and organosulfur compounds.

Acknowledgments

The authors gratefully acknowledge financial support from the Natural Science Foundation of China (21176259). Awarded foundation for excellent young and middle-aged scientist of Shandong Province, China (BS2010NJ024) and the Natural Science Foundation of Shandong Province (ZR2011BQ004), China.

Copyright information

© Iranian Chemical Society 2012