Structural and electronic properties of alkyl-trifluoroborate based ionic liquids: A theoretical study

Journal of Fluorine Chemistry 153 (2013) 96–100

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Journal of Fluorine Chemistry journal homepage: www.elsevier.com/locate/fluor

Structural and electronic properties of alkyl-trifluoroborate based ionic liquids: A theoretical study Mehdi Shakourian-Fard a, Zahra Jamshidi b, Ahmad Bayat a, Alireza Fattahi a,* a b

Department of Chemistry, Sharif University of Technology, P.O. Box 11365-9516, Tehran, Iran Chemistry and Chemical Engineering Research Center of Iran, P.O. Box 14335-186, Tehran, Iran

A R T I C L E I N F O

A B S T R A C T

Article history: Received 22 December 2012 Received in revised form 23 April 2013 Accepted 7 May 2013 Available online 23 May 2013

In this study, ionic liquids formed between 1-ethyl-3-methylimidazolium cation ([emim]+) and alkyltrifluoroborate ([RBF3], R = n-CmH2m+1 (m = 1–5)) anions have been investigated theoretically. The interactions between the cation and anions have also been calculated at the MP2/6-311++G(d,p)//B3LYP/ 6-311++G(d,p) level of theory. The calculated interaction energies were found to decrease in magnitude with the increase of side-chain length in anions. The results of energy decomposition analysis (EDA) show that the interaction of these anions with [emim]+ cation is electrostatic in the nature and the side chain length in the anions has an important effect on the contribution of DEelect term. The H-bonds in the most stable ion pairs occur between fluorine atoms of anion and hydrogen atoms of methyl, ethyl groups and the hydrogen atom on the imidazolium ring (C2–H). These H-bonds were also considered by quantum theory of atoms in molecules (QTAIM). ß 2013 Elsevier B.V. All rights reserved.

Keywords: Ionic liquids Cation–anion interaction Alkyl-trifluoroborate QTAIM analysis NBO analysis

1. Introduction Ionic liquids (ILs) have been widely investigated as potential electrolytes for various electrochemical devices including rechargeable lithium cells [1], solar cells [2], actuators [3], and double-layer capacitors (DLCs) [4]. Compared with conventional organic liquid electrolytes, the main advantages of ILs as electrolytes are their nonflammability, nonvolatility, and high thermal stability [5]. Recently, a family of new hydrophobic ionic liquids, namely alkyl-trifluoroborate and perfluoroalkyl-trifluoroborate, has been developed. Zhou et al. [5,6] first prepared ionic liquids including 1ethyl-3-methylimidazolium ([emim]+) cation and perfluoroalkyltrifluoroborate ([RFBF3]) (RF = n-C2F5, n-C3F7, and n-C4F9), alkyl(alkenyl)trifluoroborate ([RBF3], R = n-CmH2m+1 (m = 1–5), CH2CH) [7] anions in high yield. They also synthesized and characterized new cyclic quaternary ammonium salts, composed of N-alkyl-(alkyl ether)-N-methylpyrrolidinium, -oxazolidinium, piperidinium, and -morpholinium cations (alkyl = n-C4H9, alkyl ether = CH3OCH2, CH3OCH2CH2) and perfluoroalkyl-trifluoroborate ([RFBF3], RF = CF3, C2F5, n-C3F7, n-C4F9), [BF4], and [(CF3SO2)2N] anions [8]. The [RBF3] and [RFBF3] anions can form low-melting and low-viscosity ILs. This is mainly attributed to a number of favorable features of these ions, including low

symmetry, good charge distribution, and high conformational degrees of freedom [8]. Thus, these ILs can be used as safe electrolytes (i.e., with nonvolatility and nonflammability) for electrochemical energy devices [5] such as Li batteries. Although understanding the molecular-level interaction is important to predict the physicochemical properties and design of functional ILs, theoretical studies on the influence of the structural variations of anions in ILs is narrow. Many theoretical studies have considered the influence of various anions (organic/ inorganic anions) on the cation–anion interaction [9–18]. Herein, we report a theoretical study on the gas phase ILs based on the 1ethyl-3-methylimidazolium [emim]+ cation and the structurally related alkyl-trifluoroborate ([RBF3], R = n-CmH2m+1 (m = 1–5)) anions. In this study, we focus on the influence of changing the length of the alkyl chain (R) in the anion [RBF3] on the strength of cation–anion interaction. These variations across different anion series are useful for understanding the structural and properties of these ILs in the liquid phase. Moreover, properties extracted from quantum theory of atoms in molecules (QTAIM) were used to determine the nature and strength of intermolecular H-bond interactions in the ILs. 2. Results and discussion 2.1. Computational details

* Corresponding author. Tel.: +98 2166165361. E-mail address: [email protected] (A. Fattahi). 0022-1139/$ – see front matter ß 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jfluchem.2013.05.009

The structures of the [emim][RBF3] (R = n-CmH2m+1 (m = 1–5)) ion pairs and the corresponding monomers were optimized at

M. Shakourian-Fard et al. / Journal of Fluorine Chemistry 153 (2013) 96–100

B3LYP/6-311++G(d,p) level using Spartan 06 software [19]. Singlepoint energy calculations were performed at the MP2/6311++G(d,p) level of theory. Vibrational frequency calculations were also carried out at B3LYP/6-311++G(d,p) level of theory to confirm that the geometries of ion pairs were local minima without any imaginary frequency. The interaction energies of the ion pairs are defined as follows:

Eint ¼ Eðion pairÞ  EðcationÞ  EðanionÞ The Eint is the energy of the ion-pair formation (interaction energy). The zero-point vibrational energy corrections (ZPE) have been obtained within the harmonic approximation, and basis set superposition errors (BSSE) have been determined using the counterpoise method [20]. To better clarify the nature of interactions in the ionic pairs, energy decomposition analysis (EDA) have been carried out on the optimized structures. The energy decomposition analysis was done using the program package ADF (2009.01) [21] at the B3LYP-D3 level of theory. For the quantum theory of atoms in molecules (QTAIM) [22] analysis, the topological properties were obtained by using the AIM2000 package [23] with the wave functions generated at the B3LYP/6311++G(d,p) level of theory. The nature of the H-bond interaction can be predicted from the topological parameters, such as the electron density (r(r)), the Laplacian of electron density (r 2r(r)), and the energy density (H(r)) at the bond critical points (BCPs). The hydrogen bond energies (EH  X) are also calculated using the following equations: EH  X ¼

VðrÞ ¼

1 VðrÞ 2

1 2 r rðrÞ  2GðrÞ 4

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[BF3C5H11] anions. In comparison between the alkyl-trifluoroborate and perfluoroalkyl-trifluoroborate anions, Tsuzuki et al. [26] revealed that the [BF3C3F7] anion has two gauche and trans rotamers which the gauche rotamer is 0.21 kcal/mol more stable than the trans rotamer at MP2/cc-pVTZ//MP2/6-311G** level, although we showed the most stable geometry for [BF3C3H7] anion is trans. They also showed the B–C–C–C bond is trans, and the C–C–C–C bond is gauche in the most stable rotamer of [BF3C4F9] anion, although both of the B–C–C–C and C–C–C–C bonds are trans in the most stable geometry of [BF3C4H9] anion. The most stable geometry of [emim]+ cation is in good agreement with previous results [11,12,27]. The ring is aromatic (four electrons on two N atoms and two electrons on three C atoms) and 6pp electrons shared the 5pp orbitals, and the electrons are fully delocalized on it. Charge analysis also shows that the double bond between C4 and C5 atoms and the other four electrons are delocalized around the N1–C2–N3 group. Obviously, the bond lengths of C2–H, C4–H, and C5–H are almost equal to each other, and the imidazolium ring exhibits a coplanar structure. The dihedral angle of DC2–N1–C7–C8 is also 106.038. Based on previous results [10,27], the high positively charged region in the [emim]+ cation is around the C2–H group followed by the regions around the C4/C5–H atoms attached to the imidazolium ring and the other H atoms in the side chains. Thus, there are five regions around the [emim]+ cation for interaction of [RBF3] (R = n-CmH2m+1 (m = 1–5)) anions via intermolecular hydrogen bonds (Fig. 1). The five regions are methyl-front (S1), ethyl-front (S2), ethyl-back (S3), back (S4), and methyl-back (S5).

(1)

2.3. Effect of the anions on the interaction energy and properties of ion pairs

(2)

2.3.1. Optimized geometries The initial configurations were determined by placing the [RBF3] (R = n-CmH2m+1 (m = 1–5)) anions in the regions of S1–S5 around the [emim]+ cation. A number of subconfigurations in S1– S5 regions are also possible because of the alkyl chain orientation in the anions with respect to ethyl chain in [emim]+ cation in these positions. Fig. 2 shows the most stable geometries of ion pairs in S1–S5 regions optimized at B3LYP/6-311++G(d,p) level. As seen from Fig. 2, the interactions occur between fluorine atoms of anions and several sites of imidazolium ring including hydrogen atoms of methyl and ethyl groups and the two hydrogen atoms on the imidazolium ring (C2–H and C5–H). These geometries will be identified as [emim][RBF3]Sa (R = n-CmH2m+1 (m = 1–5) (interaction through C2–H, methyl, and ethyl groups simultaneously) and [emim][RBF3]Sb (interaction through C5–H and ethyl group). The optimized configuration for the methyl-front (S1), ethyl-front (S2), back (S4), and methyl-back (S5) regions is [emim][RBF3]Sa ion pairs. There was no stable point at the potential energy surface for the methyl-front (S1), ethyl-front (S2), back (S4), and methyl-back (S5) initial configurations. For these configurations, the [RBF3] (R = n-CmH2m+1 (m = 1–5)) anions move

where G(r) (always positive) and V(r) (always negative) are the kinetic and potential energy densities, respectively. Finally, the criterion nature of hydrogen bond was evaluated by means of G(r)/V(r) ratio. When G(r)/V(r) > 1 the hydrogen bond is noncovalent, whereas for 0.5 < G(r)/V(r) < 1 the hydrogen bond is partly covalent [24,25]. 2.2. Structures of [emim]+ cation and [RBF3] (R = n-CmH2m+1 (m = 1–5)) anions In order to find the most stable geometry of [emim]+ cation and [RBF3] (R = n-CmH2m+1 (m = 1–5)) anions, different conformers of the cation and anions on the potential energy surface at the relative energy range of 0–10 kcal/mol are determined using the Merck Molecular Force Field (MMFF) provided in Spartan 06 software. Finally, the obtained conformers are optimized at B3LYP/6311++G(d,p) level. The B–C–C–C bonds are trans in the most stable geometries of [BF3C3H7], [BF3C4H9], and [BF3C5H11] anions, and the C–C–C–C bonds are also trans in [BF3C4H9] and

Fig. 1. 1-Ethyl-3-methylimidazolium cation [emim]+ and alkyl-trifluoroborate ([RBF3], R = n-CmH2m+1 (m = 1–5)) anions.

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Fig. 2. The most stable geometries of [emim][RBF3]Sa/Sb (R = n-CmH2m+1 (m = 1–5)) ion pairs.

below the imidazolium ring and the steric exclusion of the ethyl chain leads to the [emim][RBF3]Sa ion pairs. [emim][RBF3]Sb ion pairs were obtained from the ethyl-back (S3) configuration. Indeed, it is very difficult for the anions to move largely from ethyl-back region due to steric exclusion from ethyl side chain and the repulsion from p electrons between C4 and C5 atoms. It is also interesting to compare the interactions in the [emim][RBF3] (R = n-CmH2m+1 (m = 1–5)) ion pairs with those in the [emim][RFBF3] (RF = n-CmH2m+1 (m = 1–4)) ion pairs reported by Tsuzuki et al. [26]. In the most stable geometries of [emim][RBF3] and [emim][RFBF3] ion pairs, the –BF3 group has contact with the imidazolium ring because of the large negative charge on the –BF3 group. In [emim][RFBF3] ion pairs, perfluoroalkyl-trifluoroborate anion has contact with the C2–H, C4–H, and C5–H bonds of the [emim]+ at the HF/6-311G(d,p) level and the geometries where the perfluoroalkyl-trifluoroborate anion has contact with the C2–H bond are significantly more stable. More stability of perfluoroalkyltrifluoroborate anions in contact with C2–H bond is similar to more stability of alkyl-trifluoroborate anions when they are in contact with C2–H bond in [emim][RBF3] ion pairs. There are three and five H-bonds in the [emim][RBF3]Sb and [emim][RBF3]Sa ion pairs as drawn by dashed lines in Fig. 2, respectively. The equilibrium distances between the protons and the F atoms, referred to as the intrinsically preferred H bond length, are shown in Fig. 2. These distances are in the range of 1.874– 2.457 A˚, longer than the covalent bond distance of H–F (1.07 A˚) and shorter than the van der Waals distance of H  F (2.670 A˚) [28]. The vibrational frequencies of C–H  F3BR H-bonds in the ion pairs show that these vibrations are not isolated but appear in combination modes, and their values are relatively small. Additionally, the vibrational frequencies of the C–H bonds in each ion pair are slightly red-shifted relative to those in the isolated cation up to 23–47 cm1. Compared with amino acid ionic liquids (AAILs), the H-bond distances in the [emim][RBF3] ionic liquids are much larger [10] and the variations of vibrational frequencies are much smaller. Obviously, although the number and fashion are very different, the H-bonds are the explicit structural features in the ion pairs. Moreover, the feature of more than one H-bond between cation and anions is different from the ILs with a single atomic anion, like dialkylimidazolium chloride [emim][Cl], in which there is only one H-bond between the cation and anion [11,29].

2.3.2. Interaction energy The interaction energy is one of the most powerful measurements for estimating the strength of noncovalent interactions. Table 1 lists the interaction energies Eint corrected by BSSE for the ion pairs by MP2/6-311++G(d,p)//B3LYP/6-311++G(d,p) method. For the ion pairs, the interaction energies change from 76.45 to 89.80 kcal/mol. As shown in Table 1, comparing the Eint values for Sa and Sb structures with the same anions shows that these values for [emim][RBF3]Sa structures are more than [emim][RBF3]Sb structures. [emim][RBF3]Sa ion pairs are around 9 kcal/mol more stable than [emim][RBF3]Sb ion pairs, it seems, the higher stability of [emim][RBF3]Sa ion pairs is due to the presence of an additional hydrogen bonding. An increase of the side-chain length in [RBF3] (R = CH3, C2H5, C3H7, C4H9, and C5H11) anion has an important effect on the Eint values of [emim][RBF3]Sa and [emim][RBF3]Sb ion pairs that can be related to decrease in electrostatic interaction as you can find in Table 2. As shown in Table 1, the decrease of interaction energy with the increase of side-chain length for [emim][RBF3]Sa and [emim][RBF3]Sb ion pairs is about 1.5 kcal/mol. Considering the two types of ion pairs indicates that [emim][BF3CH3]Sa and [emim][BF3CH3]Sb ion pairs form the most stable configurations in each type of ion pairs. Also the stability of ion pairs decreases as the side-chain length of anion becomes larger. Despite the decrease of stability with increase of side-chain length, in the interaction between a specific anion with [emim]+ cation, it has been observed that ion pairs of [emim][RBF3]Sa are more stable than [emim][RBF3]Sb. The results of Tsuzuki et al. [26] for [emim][RFBF3] (RF = n-CmH2m+1 (m = 1–4)) ion pairs have indicated that the effects of perfluoroalkyl chain length on the stabilization Table 1 Interaction energy (kcal/mol), and relative interaction energy (kcal/mol) for [emim][RBF3]Sa/Sb (R = n-CmH2m+1 (m = 1–5)) ion pairs at the MP2/6-311++G(d,p)// B3LYP/6-311++G(d,p) level of theory. Structure

Eint

DEint

Structure

Eint

DEint

[emim][BF3CH3]Sa [emim][BF3CH3]Sb [emim][BF3C2H5]Sa [emim][BF3C2H5]Sb [emim][BF3C3H7]Sa [emim][BF3C3H7]Sb

89.80 80.58 89.22 79.99 88.68 79.54

0 9.22 0 9.23 0 9.14

[emim][BF3C4H9]Sa [emim][BF3C4H9]Sb [emim][BF3C5H11]Sa [emim][BF3C5H11]Sb

88.51 79.37 88.26 79.27

0 9.14 0 9.18

M. Shakourian-Fard et al. / Journal of Fluorine Chemistry 153 (2013) 96–100 Table 2 Energy decomposition analysis (in kcal/mol) for [emim][RBF3]Sa/Sb (R = n-CmH2m+1 (m = 1, 5)) ion pairs at B3LYP-D3 level. Structure

[emim][BF3CH3]Sa [emim][BF3CH3]Sb [emim][BF3C5H11]Sa [emim][BF3C5H11]Sb

Energy decomposition analysis

DEpauli

DEelect

DEorb

DEdisp

Eint

20.48 20.20 19.29 19.10

95.76 85.59 83.46 73.53

14.82 15.59 15.18 15.63

5.51 4.36 4.56 4.12

95.61 85.35 83.92 74.18

energies are small, although our results for [emim][RBF3] (R = nCmH2m+1 (m = 1–5)) ion pairs reveal that the alkyl chain length has an important effect on the interaction energy values so that interaction energy values decrease in magnitude with the increase of alkyl chain length in anion. 2.3.3. Energy decomposition analysis The nature of interaction between anions and cations can be determined by energy decomposition analysis (EDA) method [30]. In this method, the interaction energy between two fragments, Eint, is split up into four physically meaningful components:

Eint ¼ DEpauli þ DEelect þ DEorb þ DEdisp

DEpauli gives the repulsive four-electron interactions between occupied orbitals. DEelect gives the electrostatic interaction energy between the fragments, which is calculated with a frozen electron density distribution in the geometry of the complex. It can be considered as an estimation of the electrostatic contribution to the binding energy. In addition, the stabilizing orbital interaction term DEorb is calculated in the final step of the analysis when the Kohn– Sham orbitals relax to their optimal form. The orbital term DEorb can be considered as an estimation of the covalent contributions to the attractive interactions. The associated orbital term DEorb

99

accounts for charge transfer, polarization, and electron-pair bonding and overestimated the covalent by considering the effects of charge polarizations. However, we should mention that as DEelect term is calculated using the frozen charge distribution of the interacting fragments the effects of charge polarization is completely adsorbed by the DEorb term. The latter expression contains also a component, which clearly does not come from covalent bonding between the fragments. It is the relaxation of the orbitals, which is caused by the electrostatic effect of the other fragment. As for cation ([emim]+) and alkyl-trifluoroborate ([RBF3] interactions there is no ion-pair interactions, DEorb associated with polarization or induction energy. In addition, DEdisp has been calculated when the dispersion corrected density functional has been used, this term basically is the difference between total energy based on DFT-D or DFT-D3 and DFT methods [31]. Therefore, by going from DFT to DFT-D or DFTD3 methods the DDEpauli, DDEelect, and DDEorb, values remain unchanged and the dispersion correction appears as an extra term. Recently, some works have shown that dispersion interaction has significant influence on the molecular behavior of ionic liquids [32–34]. For consideration of the most effect of side chain length in anions on the contributions of energy, we selected [emim][RBF3]Sa/Sb (R = n-CmH2m+1 (m = 1, 5)) ion pairs and performed energy decomposition analysis for the ion pairs. Table 2 collects the results of the EDA calculations at the B3LYPD3 level for these ion pairs. The results of Table 2 show that the interaction of [RBF3] (R = n-CmH2m+1 (m = 1, 5)) anions with [emim]+ cation is electrostatic in the nature because the contribution of the electrostatic term to the binding energy is very larger than that of other terms. On the other hand, the side chain length in the anions has an important effect on the contribution of DEelect term. As seen from Table 2, the absolute values of DEelect for the [emim][BF3CH3]Sa and [emim][BF3C5H11]Sa ion pairs are more than those of

Table 3 The electron densities (P(r)), their Laplacians (r2r(r)), kinetic energy densities (G(r)), potential energy densities (V(r)), and electronic energy densities (H(r)) in a.u., G(r)/V(r) ratio at the BCPs, and H-bond energies (EH  F, kcal/mol) in the [emim][RBF3]Sa ion pairs. Structure

BCP

P(r)

r2r(r)

G(r)

V(r)

H(r)

G(r)/V(r)

EH  F

[emim][BF3CH3]Sa

C8–H  F2 C6–H  F1 C2–H  F1 C7–H  F3 C2–H  F3

0.0084 0.0124 0.0126 0.0085 0.0171

0.0359 0.0572 0.0601 0.0374 0.0892

0.0080 0.0128 0.0134 0.0083 0.0198

0.0070 0.0112 0.0118 0.0072 0.0174

0.0010 0.0015 0.0016 0.0011 0.0025

1.143 1.137 1.135 1.145 1.141

2.2 3.5 3.7 2.3 5.5

[emim][BF3C2H5]Sa

C8–H  F2 C6–H  F1 C2–H  F1 C7–H  F3 C2–H  F3

0.0083 0.0124 0.0125 0.0083 0.0169

0.0358 0.0575 0.0592 0.0367 0.0875

0.0079 0.0128 0.0132 0.0081 0.0195

0.0069 0.0113 0.0117 0.0071 0.0171

0.0010 0.0015 0.0016 0.0010 0.0024

1.144 1.137 1.134 1.147 1.140

2.2 3.5 3.7 2.2 5.4

[emim][BF3C3H7]Sa

C8–H  F2 C6–H  F1 C2–H  F1 C7–H  F3 C2–H  F3

0.0079 0.0121 0.0134 0.0087 0.0157

0.0339 0.0555 0.0642 0.0382 0.0794

0.0075 0.0124 0.0143 0.0085 0.0177

0.0066 0.0109 0.0126 0.0074 0.0155

0.0010 0.0015 0.0017 0.0011 0.0022

1.147 1.137 1.135 1.145 1.142

2.1 3.4 4.0 2.3 4.9

[emim][BF3C4H9]Sa

C6–H  F1 C2–H  F1 C2–H  F3 C7–H  F3 C8–H  F2

0.0109 0.0121 0.0177 0.0079 0.0086

0.0574 0.0572 0.0934 0.0347 0.0370

0.0128 0.0128 0.0208 0.0077 0.0082

0.0113 0.0113 0.0183 0.0067 0.0072

0.0015 0.0015 0.0025 0.0010 0.0010

1.136 1.135 1.138 1.151 1.142

3.5 3.5 5.7 2.1 2.2

[emim][BF3C5H11]Sa

C8–H  F2 C6–H  F1 C2–H  F1 C7–H  F3 C2–H  F3

0.0083 0.0124 0.0125 0.0082 0.0168

0.0358 0.0574 0.0592 0.0360 0.0872

0.0080 0.0128 0.0132 0.0080 0.0194

0.0070 0.0113 0.0117 0.0069 0.0170

0.0010 0.0015 0.0016 0.0010 0.0024

1.144 1.136 1.134 1.149 1.141

2.2 3.5 3.7 2.2 5.3

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[emim][BF3CH3]Sb and [emim][BF3C5H11]Sb ion pairs, respectively. 2.3.4. QTAIM analysis of hydrogen bonds The nature of interaction in ionic liquids is determined by the sum of electrostatic interactions, hydrogen bond interactions and van der Waals interactions between cation and anion. The energy decomposition analysis (EDA) results revealed that the interaction between [RBF3] (R = n-CmH2m+1 (m = 1, 5)) anions and [emim]+ cation in the ion pairs has the electrostaticin nature. Although the hydrogen bond interactions are only small section of the total interaction between cation and anion, quantum theory of atoms in molecules (QTAIM) analysis can be a practical tool to better understand the hydrogen bonds between constituents of ionic liquids. A topological analysis of the electron density on the most stable ion pairs ([emim][RBF3]Sa (R = n-CmH2m+1 (m = 1–5)) reveals the existence of bond critical points (BCPs) between the fluorine atoms of anions (donor) and the C2–H, methyl, and ethyl groups. The properties at the BCPs were analyzed in terms of electron densities (P(r)), their Laplacians (r2r(r)), kinetic energy densities (G(r)), potential energy densities (V(r)), and electronic energy densities (H(r)). The results of QTAIM analysis for H-bonds of [emim][RBF3]Sa ion pairs are available in Table 3. It is seen that the values of electron density are in a range of 0.0116–0.0177 a.u. for C2–H  F hydrogen bonds. The values of (r 2r(r)) are all positive with a range of 0.0572–0.0934 a.u. for the hydrogen bonds. The r(r) and (r 2r(r)) values in the BCPs of hydrogen bonds are also within the commonly accepted values for normal H-bonds [35]. According to the ratio of G(r)/V(r) in Table 3, the H-bonds interactions which are minor portion of the total interaction between cation and anion in [emim][RBF3]Sa ion pairs are noncovalent interactions. The hydrogen bond energy values (EH  F in kcal/mol, which are also only minor portion of the total interaction energies between cation and anion) summarized in Table 3 indicate that the C2–H and methyl group have the most interaction with fluorine atoms of anions. 3. Conclusion In this study, a systematic study on the structure, electronic properties, and molecular interaction in alkyl-trifluoroborate based ionic liquids composed of 1-ethyl-3-methylimidazolium cation ([emim]+) and alkyl-trifluoroborate ([RBF3], R = n-CmH2m+1 (m = 1–5)) anions has been investigated. Among all studied ion pairs, [emim][CH3BF3]Sa ion pair was found to form the most stable ion pair by interaction of fluorine atoms of anion and hydrogen atoms of methyl, ethyl groups and the hydrogen atom on the imidazolium ring (C2–H). Also the stability of [emim][RBF3]Sa (R = n-CmH2m+1, m = 1–5) ion pairs decreases as the side-chain length of anion becomes larger. Comparison of vibrational frequencies in isolated cation and ion pairs indicates that the C– H bond vibrational frequencies associated with H-bonds are slightly red-shifted relative to those in the isolated cation due to Hbond interactions. The energy decomposition analysis (EDA) shows that the interaction of anions with [emim]+ cation is electrostatic in the nature and the side chain length in the anions has an important effect on the contribution of DEelect term.

The nature of intermolecular H-bonds in ion pairs is considered by quantum theory of atoms in molecules (QTAIM) analysis. From the QTAIM results, it can be concluded that the H-bond interactions between [emim]+ cation and [RBF3] (R = n-CmH2m+1, m = 1–5) anions (which are minor portion of the total interaction between cation and anion) have the electrostatic nature. Acknowledgement Support from Sharif University of technology is gratefully acknowledged.

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