An investigation on the physicochemical properties of the nanostructured [(4-X)PMAT][N(CN)2] ion pairs as energetic and tunable aryl alkyl amino tetrazolium based ionic liquids

Accepted Manuscript An investigation on the physicochemical properties of the nanostructured [(4-X)PMAT] [N(CN)2] ion pairs as energetic and tunable aryl alkyl amino tetrazolium based ionic liquids Behzad Khalili, Mehdi Rimaz PII:

S0022-2860(17)30199-0

DOI:

10.1016/j.molstruc.2017.02.053

Reference:

MOLSTR 23447

To appear in:

Journal of Molecular Structure

Received Date: 11 September 2016 Revised Date:

11 February 2017

Accepted Date: 13 February 2017

Please cite this article as: B. Khalili, M. Rimaz, An investigation on the physicochemical properties of the nanostructured [(4-X)PMAT][N(CN)2] ion pairs as energetic and tunable aryl alkyl amino tetrazolium based ionic liquids, Journal of Molecular Structure (2017), doi: 10.1016/j.molstruc.2017.02.053. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract

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An investigation on the physicochemical properties of the nanostructured

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[(4-X)PMAT][N(CN)2] ion pairs as energetic and tunable aryl alkyl amino

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tetrazolium based ionic liquids

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Behzad Khalili,*a Mehdi Rimaz b

Department of Chemistry, Faculty of Siences, University of Guilan, P.O. Box 41335-1914,

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a

Rasht, Iran. Email:[email protected]

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b

Department of Chemistry, Payame Noor University, P.O. Box 19395-3697, Tehran, Iran.

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Abstract

In this study the different class of tunable and high nitrogen content ionic liquids termed

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TAMATILs (Tunable Aryl Methyl Amino Tetrazolium based Ionic Liquids) were designed.

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The physicochemical properties of the nanostructured TAMATILs [(4-X)PMAT][N(CN)2]

13

(X=H, Me, OCH3, OH, NH2, NO2, F, CN, CHO, CF3, COMe and CO2Me) were fully

14

investigated using M06-2X functional in conjunction with the 6-311++G(2d,2p) basis set.

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For all of the studied nanostructured ILs the structural parameters, interaction energy, cation’s

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enthalpy of formation, natural charges, charge transfer values and topological properties were

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calculated and discussed. The substituent effect on the interaction energy and physicochemical

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properties also is taking into account. The results showed that the strength of interaction has a

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linear correlation with electron content of the phenyl ring in a way the substituents with electron

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withdrawing effects lead to make more stable ion pairs with higher interaction energies. Some of

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the main physical properties of ILs such as surface tension, melting point, critical-point

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temperature, electrochemical stability and conductivity are discussed and estimated for studying

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ion pairs using quantum chemical computationally obtained thermochemical data. Finally the

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enthalpy and Gibbs free energy of formation for twelve nanostructured individual cations with

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the general formula of [(4-X)PMAT]+ (X=4-H, 4-Me, 4-OMe, 4-OH, 4-NH2, 4-NO2, 4-F, 4-CN,

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4-CHO, 4-CF3, 4-COMe and 4-CO2Me) are calculated.

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Keywords: TAAIL, Phenyl methyl tetrazolium cation, Binding energy, Substituent effect,

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Enthalpy of formation, Physicochemical property

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1. Introduction

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Ionic liquids (ILs) are green and novel class of chemical compounds that their popularity and

31

applications in various fields are still in progress. They composed of ion pairs in which the cation

32

part is exclusively organic molecules otherwise the anion part could be inorganic or organic.

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Properties of ILs are very different from molecular liquids and also ionic solids because of the

34

existence of interactions with quite different natures between cation and anion parts which held

35

them together in an ion pair structure. ILs have more intermolecular interaction energy and low

36

lattice energy, melting point and packing ability than molecular liquids and ionic solids.

37

Therefore, ILs have been classified as ion pairs with melting points less than 100 °C. In fact,

38

many of introduced ionic liquids have melting points below than room temperature [1].

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In recent years a new class of ILs composed of aryl alkyl substituted imidazolium based cations

40

was introduced by Ahrens and coworkers [2-3]. These new ILs showed different properties than

41

that of other ILs because of higher flexibility of those cation parts and termed as Tunable Aryl

42

Alkyl Ionic Liquids (TAAILs). As we know the usual imidazolium based ILs have only

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inductive effects because of caring two alkyl groups (Csp3) on heterocyclic ring, whereas

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TAAILs which are composed of aryl (Csp2) alkyl (Csp3) substituted heterocyclic ring could

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have mesomeric effects as well as inductive ones. These alterations on cationic part make

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TAAILs more flexible than standard imidazolium based ILs with two alkyl groups and vary the

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most of the main characteristics of TAAILs in comparison to the usual ILs. Various anions

48

similar to the standard ILs could be chosen as anionic part for TAAILs without restrictions [4].

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These classes of ILs in addition to van der Waals interactions could make π-π interactions with

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other materials, which makes them more suitable candidates for applications as separation

51

solvent and also as active metal catalyst stabilizers [5-6].

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ILs have many unique physicochemical properties such as negligible vapor pressure[7-8], high

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solvation ability, non-flammability, resistant to oxidation[9], controllable hydrophobicity, high

54

chemical and thermal stability [7-11] and applications as green solvents for various reactions

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[11-16], extraction solvents [10,17-18], as electrolyte in batteries [19-21], in solar and fuel cells

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[22, 23], in electrochemistry [24] nanotechnology [25, 26] and etc.

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The main properties of ILs are affected by the nature and properties of both cation and anion

58

parts. Recent reports show that the physical and chemical properties of ILs mainly influenced by

59

their anionic constituent [27] whereas the cationic constituent of ILs is important to the thermal

60

stability of them [5]. There for it is obvious that one of the simplest ways to design and make

61

new ILs with different properties is to change or modulate one of cation or anion part or both of

62

them.

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Physical properties of ILs are mainly controlled by the nature of the intermolecular forces as

64

most of other chemical materials. In many cases the physicochemical properties of ILs could be

65

tuned by little structural changes at one or both parts of ion pairs [28-29].

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Getting comprehensive information about structures and interactions nature in various IL

67

systems to make better understanding about them to design and choose of suitable candidate for

68

special application is necessary. There are two ways to achieve this comprehensive information:

69

first time-consuming and cost-intensive experimental study and second making use of the

70

quantum chemical computations. Nowadays using of computational chemistry is widespread in

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various fields because of its low cost and high reliability.

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Altering the electronic properties of substituted molecules, especially aromatic ones by the

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nature of attached groups on the various positions is well understood. For example, electron

74

donating and accepting groups attached to the para position of the phenyl ring increase and

75

decrease the electron content of the ring respectively [30-31]. As a result the interactions

76

between ionic parts, including such substituted molecules are affected from this matter from both

77

strength and nature point of view. There for significant changes in the properties of ion pairs

78

could be expected by changing on the substitution.

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In present study work, we first explore the effect of different substituents (electron-donor and

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electron acceptor) on the phenyl ring of the nanostructured aryl methyl amino tetrazolium

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(AMAT) cation on the nano structural and physicochemical characteristics of model TAAILs

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with different AMAT cations as a high nitrogen content and energetic cationic part and N(CN)2−

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anion. The selected twelve substituents cover the whole set of substituents from strong electron

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donating to strong electron attracting groups and are as 4-H, 4-Me, 4-OMe, 4-OH, 4-NH2, 4-

85

NO2, 4-F, 4-CN, 4-CHO, 4-CF3, 4-COMe and 4-CO2Me.

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At recent years, many research works focus on exploration of ion pair interactions of ionic

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liquids using both experimental [32-40] and theoretical [6, 41-53] methods.

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To the best of our knowledge٫ the interaction of [4X-phenyl methyl tetrazolium]+ cations and

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[N(CN)2]- anion at a model ion pair structure as a nanostructured IL has not been yet

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investigated. In addition, we could not find detailed studies in which the influence of different

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substituents on the cations upon the strength of interactions between their components has been

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considered. Only in one research work the same study has done on an imidazolium cation based

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ILs [6].

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In continue the interaction energies between ion pairs, structural parameters of obtaining ILs,

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cations gas phase enthalpy and Gibbs free energy of formation and topological properties at bond

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critical points have been computed. In addition, for more detailed insight on the nature of

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intermolecular interactions in model nanostructured [(4-X)PMAT][N(CN)2] ILs the natural bond

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orbital theory (NBO) [54] and Bader's quantum theory of atoms in molecules (QTAIM) were

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used [55-56].

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2. Computational Methods

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M06-2X [57-58] method in conjunction with the 6-311++G(2d,2p) basis set [59-60] were used to

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perform all of calculations appear in this study. One of the most widely used methods for

103

investigation and exploration about the energetic, structural and physicochemical properties of

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ionic liquids during the past ten years was Density functional theory (DFT) [61-67]. Its various

105

functional performances in providing ionic liquids interaction energies and etc. have been

106

analyzed recently and the results verified that the DFT methods have adequate reliability to

107

exploring ILs theoretically by them [46, 68-74].

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Initially structure of the all ion pairs were fully optimized using M06-2X/6-311++G(2d,2p)

109

method at gas phase. In continue to verify that all of the structures are true minima the

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vibrational frequencies have been calculated and then zero-point vibrational energy (ZPVE)

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corrections extracted from their results.

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Basis set superposition errors (BSSE) were obtained using the counterpoise method (CP) [75]

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during interaction energy calculations and results showed that the chosen basis set is big enough

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to reduce BSSE energy less than 0.5 kcal/mol at most cases. All of the calculations were carried

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out using GAMESS [76] and Gaussian03 [77] program packages.

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The nature of interactions between ion pairs were examined using natural bond orbital (NBO)

117

analyses [78] which is carried out on the M06-2X/6-311++G(2d,2p) wave functions. Quantum

118

theory of atoms in molecules (QTAIM) analysis [79] used to illustration of bond characteristics

119

for the considered configurations. The topological properties were obtained with the help of the

120

AIM2000 package [80] using the wave functions obtained from the M06-2X/6-311++G(2d,2p)

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calculations.

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3. Results and Discussion

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In this study twelve IL molecules composed of N(CN)2- as an anion and various para substituted

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phenyl methyl amino tetrazolium cations including 4-H, 4-Me, 4-OMe, 4-OH, 4-NH2, 4-NO2, 4-

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F, 4-CN, 4-CHO, 4-CF3, 4-COMe and 4-CO2Me substituents as cation part are considered.

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Various sites around the nanostructured cation part [(4-X)PMAT]+ which is used in this study are

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examined from the interaction point of view with N(CN2)- as an anion part. The obtained most

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stable, optimized structure for each ion pair (IP) composed of N(CN)2- and various para

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substituted nanostructured cations of [(4-X)PMAT]+ are shown in Fig. 1. In these ion pair series,

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proton acceptors and proton donors are nitrogen (N) atoms of N(CN)2- anion and N-H and C-H

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bonds of the nanostructured [(4-X)PMAT]+ cations respectively.

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Accordingly, there are three H-bonds in the nanostructured [(4-X)PMAT][N(CN)2] IPs which are

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formed between N23 atom and H16 atom of the phenyl ring, N27 atom and H21atom of the amino

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group and N27 and H5 atom of the methyl group as depicted by dashed lines in Fig. 1. In general

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the hydrogen bond distance is defined as the distance between a hydrogen atom of the proton

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donor group and the acceptor atom one [6].The hydrogen bonds which are appearing here are of

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N---HN(C) types. Sum of the van der Waals radii of nitrogen and hydrogen atoms is 2.750 Å

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[81] which is higher than the hydrogen bond distances discussed in this study.

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Fig. 1. Optimized structures of the nanostructured [(4-X)PMAT][N(CN)2] (X=4-H, 4-Me, 4-

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OMe, 4-OH, 4-NH2, 4-NO2, 4-F, 4-CN, 4-CHO, 4-CF3, 4-COMe and 4-CO2Me) ion pairs at the

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M06-2X/6 311++G(2d,2p) level of theory.

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3.1. Interaction energy

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The electronic and Gibbs Free interaction energies of the studied ion pairs are calculated at M06-

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2X/6-311++G(d,p) level of theory and are collected in Table 1. During the interaction energy

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calculation, the basis set superposition errors (BSSE) and zero-point vibrational energies (ZPVE)

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also were taking into account. Below equations are used to calculation of various corrected

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interaction energies (-∆Eele) which are listed in Table 1:

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∆Eele =EIP - (ECation + EAnion)

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∆E0 =∆Eele + ∆ZPVE

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∆EBSSE = ∆E0 + BSSE

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Table 1 shows the relatively large interaction energies for studying ion pairs which are ranging

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from 88.43 to 98.24 kcal/mol at M06-2X/6-311++G(2d,2p) level of theory. The obtained values

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for Gibbs free energies of interactions are also considerable and are ranging from 72.83 to 82.79

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kcal/mol at the same level of theory. Change in electrostatic strength of the phenyl ring which is

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occurred due to the delocalization of the charges around it may be responsible for the observed

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large interaction energies.

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Table 1

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Interaction energies (kcal/mol) for most stable configurations of the nanostructured [(4-

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X)PMAT][N(CN)2] (X=4-H, 4-Me, 4-OMe, 4-OH, 4-NH2, 4-NO2, 4-F, 4-CN, 4-CHO, 4-CF3, 4-

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COMe and 4-CO2Me) ion pairs obtained at M06-2X/6-311++G(2d,2p) level. BSSE

∆Ea

∆Eb

∆Ec

∆Ed

∆Gd

4-H

1.2

0.54

-92.33

-91.16

-91.79

-90.62

-78.12

4-Me

1.6

0.53

-91.07

-89.49

-90.55

-88.96

-75.21

4-OMe

1.5

0.53

-90.42

-88.94

-89.89

-88.41

-74.93

4-OH

1.5

0.53

-91.51

-90.00

-90.98

-89.47

-76.62

4-NH2

1.7

0.53

-88.43

-86.76

-87.90

-86.24

-72.83

4-NO2

1.5

0.58

4-F

1.4

0.53

4-CHO

1.4

0.58

4-CN

1.5

0.54

4-CF3

1.4

0.59

4-COMe

1.4

0.58

4-CO2Me

1.7

193 194 195 196

-97.66

-96.20

-82.79

-94.14

-92.70

-93.61

-92.18

-79.16

-95.08

-93.67

-94.50

-93.09

-79.81

-97.55

-96.05

-97.01

-95.51

-82.25

-96.04

-94.61

-95.45

-94.02

-80.89

-93.78

-92.43

-93.20

-91.85

-79.16

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-91.62

-92.70

-91.05

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In comparison to the nearly same tunable aryl alkyl imidazolium based ILs ([x-phenyl methyl

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imidazolium tetrafluoro borate) which are investigated by Roohi et al [6], the systems

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investigated herein have slightly higher interaction energies. In addition, in agreement with

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Roohi et al [6] reports, the electron-donor substituted cations present lower interaction energies

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than that of the electron-accepting substituted ones as can be seen in Table 1.

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It’s well-known that the substituent constants (Hammett's constants) are well correlated with

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several electronic or physicochemical properties [82].

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As part of Hammett's equation, the sum of the total electronic effects composed of resonance

208

and inductive field effects is represented by substituent constant (σp) [83]. The substituent

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constant could have positive or negative value when an electron-withdrawing or electron-

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donating substituent attached to the phenyl ring respectively.

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Fig. 2 shows the linear correlation between the binding energies of studying nanostructured ILs

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which are obtained at M06-2X/6-311++G(2d,2p) level of theory with Hammett's constants (σp)

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which are available from literature for some substituents.

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Fig. 2. Correlation between the interaction energy values and Hammett's constants (σp) of

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nanostructured [(4-X)PMAT][N(CN)2] (X=4-H, 4-Me, 4-OMe, 4-OH, 4-NH2, 4-NO2, 4-F, 4-

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CN, 4-CHO, 4-CF3, 4-COMe and 4-CO2Me) ion pairs.

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As shown on Fig. 2 the slope of the plot of interaction energy versus Hammett's substituent

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constant is positive which indicate that the interaction energies increased/decreased with the

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increasing/decreasing of electron accepting power of the substituent attached to the phenyl ring.

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It can also be concluded from the data in Table 1 that the values of BEs for ion pairs having

236

electron withdrawing substituents are greater than those of electron donating ones or in summary

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the binding strength between cation and anion parts of studied ILs are sensitive to the nature of

238

the attached substituent to the phenyl ring on cation and it is affected by the electron donation or

239

the acceptation power of the substituent.

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Based on the basis set supper position error and zero point vibrational energy corrected

241

interaction energies, the following order could be obtained for strength of the cation-anion

242

interactions within the studied nanostructured ILs: [(4-NO2)PMAT][N(CN)2]

243

CN)PMAT][N(CN)2]

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F)PMAT][N(CN)2] > [(4-COMe)PMAT][N(CN)2] > [(4-CO2)MePMAT][N(CN)2] > [(4-

245

H)PMAT][N(CN)2]

246

OMe)PMAT][N(CN)2] > [(4-NH2)PMAT][N(CN)2] .

247

The above order could be interpreted as of increasing of the electron content of the cation by

248

electron donating substituent on the phenyl ring caused to decrease the overall net charge on the

249

cation ring through electron donation in comparison to the electron-withdrawing substituents,

250

and as a result some weakening of the anion-cation interactions which are affected by charge

251

transfer from anion to the cation were occurring.

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3.1.1. Correlation between binding energy and physical properties

253

The melting point is one of the main and important characteristics of ILs that determines the

254

lower limit of their liquidity and thermal stability and also usefulness of them in variant

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> [(4-

[(4-OH)PMAT][N(CN)2]

>

[(4-Me)PMAT][N(CN)2]

>

[(4-

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applications. As we know the melting point is related to the some structural parameters and the

256

strength of intermolecular forces. Molecules or configurations with the higher intermolecular

257

forces have usually higher melting points then; we can expect that we could correlate the binding

258

energy of ILs with their melting points. The same correlation was found by Dong [84], Strassner

259

[85] and Roohi [6] et al during the investigation of various IL systems. They found that the

260

melting point of ILs increases as the BE increases. As Strassner et al [85] illustrated electron

261

accepting substituents on the phenyl ring cause to increase melting points because of their

262

influence in increasing of the binding energy between ion parts. For example, in the case of

263

TAAILs they showed that the ILs having electron withdrawing groups such as NO2, CHO on the

264

phenyl ring have higher melting points than ILs having electron donating groups such as Me,

265

OMe on the phenyl ring. Therefore , based on above mentioned explanation and obtained

266

relative stability for ion pairs, we can expect that the melting point of our studied nanostructured

267

ILs decreases in the following order: [(4-NO2)PMAT][N(CN)2] > [(4-CN)PMAT][N(CN)2] >

268

[(4-CF3)PMAT][N(CN)2] > [(4-CHO)PMAT][N(CN)2] > [(4-F)PMAT][N(CN)2] > [(4-

269

COMe)PMAT][N(CN)2] > [(4-CO2)MePMAT][N(CN)2] > [(4-H)PMAT][N(CN)2] > [(4-

270

OH)PMAT][N(CN)2]

271

NH2)PMAT][N(CN)2] .

272

Another important characteristic of ILs is electrical conductivity which is the main factor for

273

their application in electrochemical reactions and applications as electrolytes in batteries and so

274

on. Electrical conductivity is a physical property that determines with how facility one material

275

conducts electricity. Materials composed of ion pairs are conductance whereas molecular

276

materials are insulator. ILs as ionic materials could have electrical conductivity. Their

277

conductivity depends on the facility of ion pair’s motion. Loosely held ion pairs have higher

[(4-Me)PMAT][N(CN)2]

>

[(4-OMe)PMAT][N(CN)2]

>

[(4-

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amount of motion and show the higher amount of conductivity in comparison to the other ones

279

that within them the ion pairs attract each other strongly and reduces the number of ions in

280

solution. Therefore it can be concluded that the order of IL,s electrical conductivity is in contrast

281

to the order of their melting points and ILs with electron-donating substituents(with lower Bes)

282

have the higher conductivity than the ones with electron withdrawing substituents(with higher

283

BEs)

284

Me)PMAT][N(CN)2]

285

CO2)MePMAT][N(CN)2] > [(4-COMe)PMAT][N(CN)2] > [(4-F)PMAT][N(CN)2] > [(4-

286

CHO)PMAT][N(CN)2]

287

NO2)PMAT][N(CN)2].

288

As could be obtained from the literature the surface tension property of ILs has the same

289

variation trend with the relative interaction energies which are determined by mass spectrometry

290

or calculated by DFT methods [6, 46]. There for by considering the BEs it could be expected that

291

the surface tension of TAAILs having electron withdrawing substituents is greater than those of

292

electron donating ones.

293

“The critical temperature of a substance is the temperature at and above which vapor of the

294

substance cannot be liquefied, no matter how much pressure is applied [86].”

295

Usually increasing of the interaction energy between ion parts is accompanied by an increase in

296

the critical-point temperature of the ion pairs. Therefore it can be expected that the same order as

297

of melting points is achieved for the critical points and TAAILs having electron withdrawing

298

substituents have the greater critical points than those of electron donating ones.

299

3.2. Geometrical analysis

>

>

[(4-OH)PMAT][N(CN)2]

[(4-CF3)PMAT][N(CN)2]

>

>

[(4-H)PMAT][N(CN)2]

[(4-CN)PMAT][N(CN)2]

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>

[(4-OMe)PMAT][N(CN)2]

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[(4-NH2)PMAT][N(CN)2]

>

[(4-

>

[(4-

>

[(4-

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The main geometrical parameters for twelve nanostructured ILs with the general formula of [(4-

301

X)PMAT][N(CN)2] calculated at M06-2X/6-311++G(2d,2p) level of theory are collected in

302

Table 2. The most stable, optimized configuration for all twelve studied ion pairs were the same

303

and corresponds to a geometry with the N(CN)2- anion located above the amino tetrazole ring.

304

The above mentioned geometry includes three main interactions as follow: anion interacting with

305

the CH group of Me, NH group of amine branch and CH group of the phenyl ring. Two of

306

interactions are of hydrogen bond of type C-H---N and the third one is hydrogen bond of C-H---

307

N type. The distance and angle of the N20-H21---N27 hydrogen bond which is the strongest

308

interaction within ionic parts in all of the twelve studied ILs is as: is 1.849(157.8), 1.849(157.9),

309

1.853(157.3), 1.852(157.5), 1.853(158.0), 1.857(155.1), 1.854(156.9), 1.849(156.2), 1.858(

310

155.4), 1.855(156.1), 1.846(156.7) and 1.848(156.7) Å(˚) for [(4-X)PMAT][N(CN)2] (X = H,

311

CH3, OCH3, OH, NH2, NO2, F, CN, CHO, CF3,COMe and CO2Me) ILs respectively. At the free

312

cations the tetrazole and the phenyl ring are nearly coplanar with the dihedral angle of about 179

313

˚ in all twelve optimized free cations whereas upon ion pair formation this coplanarity lost and

314

the dihedral angle between tetrazole and the phenyl ring decreases. The value of C12-C10-N9-N3

315

dihedral angles are 140.0, 141.1, 140.8, 138.4, 158.0, 155.1, 156.9, 156.2, 142.9, 142.0, 145.5

316

and 144.9 (˚) for nanostructured [(4-X)PMAT][N(CN)2] (X = H, CH3, OCH3, OH, NH2, NO2, F,

317

CN, CHO, CF3,COMe and CO2Me) ILs respectively (Fig 3a).

319 320

SC

M AN U

TE D

EP

AC C

318

RI PT

300

321 322

16

ACCEPTED MANUSCRIPT

323

Table 2

324

The optimized geometrical parameters for nanostructured [(4-X)PMAT][N(CN)2] at M06-2X/6-

325

311++G(2d,2p) level of theory.

327 328 329 330

4H 1.324 1.281 1.321 1.355 1.343 1.007 1.458 1.088 1.439 1.388

Cation 44OMe OH 1.328 1.327 1.282 1.282 1.334 1.321 1.321 1.353 1.344 1.343 1.007 1.007 1.457 1.457 1.088 1.088 1.429 1.431 1.394 1.392

RI PT

Anion 4- [N(CN)2]NO2 1.314 1.280 1.324 1.364 1.346 1.033 1.459 1.085 1.429 1.385 1.386 1.385 1.079 1.162 1.164 1.312 1.311 1.300 1.311 1.167 1.164 2.599 2.700 1.857

4Me 1.325 1.281 1.321 1.354 1.344 1.007 1.457 1.088 1.436 1.388

SC

4NH2 1.319 1.279 1.324 1.360 1.349 1.032 1.457 1.085 1.426 1.387 1.385 1.380 1.080 1.162 1.314 1.301 1.166 2.602 2.664 1.853

M AN U

Ion Pair 44OMe OH 1.318 1.318 1.278 1.278 1.324 1.324 1.360 1.361 1.348 1.348 1.032 1.033 1.458 1.457 1.085 1.085 1.428 1.428 1.381 1.384 1.389 1.387 1.387 1.384 1.080 1.081 1.162 1.162 1.314 1.313 1.301 1.301 1.166 1.166 2.589 2.535 2.684 2.662 1.853 1.852

119.9 118.0 119.3 121.5 119.7 112.8 116.6 116.6 116.8 116.6 116.8 116.4 157.8 157.9 157.3 157.5 158.0 155.1 140.0 141.1 140.8 138.4 140.2 144.3

AC C

326

4Me 1.317 1.278 1.324 1.361 1.348 1.033 1.458 1.085 1.430 1.384 1.384 1.385 1.081 1.162 1.314 1.301 1.166 2.628 2.668 1.849

EP

N1-N2 N2-N9 N3-C8 C8-N1 C8-N20 N20-H21 N1-C4 C4-H5 N9-C10 C10-C12 C10-C11 C12-C15 C12-H16 N23-C24 C24-N25 N25-C26 C26-N27 N23---H16 N27---H5 N27---H21 Bond Angle C12-H16-N23 C24-N25-C26 N20-H21-N27 Dihedral Angle C12-C10-N9-N3

4H 1.316 1.278 1.324 1.362 1.347 1.033 1.458 1.084 1.432 1.384 1.385 1.387 1.080 1.162 1.313 1.301 1.166 2.589 2.659 1.849

TE D

Parameter Bond length

331 332 333

17

4NH2 1.330 1.283 1.335 1.350 1.346 1.007 1.456 1.088 1.424 1.393

4NO2 1.321 1.281 1.331 1.358 1.339 1.007 1.459 1.088 1.439 1.388

1.079 1.079 1.079 1.079 1.080 1.079

120.3

179.1 179.6 179.4 179.3 179.7 178.7

ACCEPTED MANUSCRIPT

Table 2. Continued

336 337 338 339 340 341

Anion 4- [N(CN)2]CO2Me 1.315 1.279 1.324 1.361 1.347 1.033 1.458 1.085 1.430 1.386 1.385 1.384 1.079 1.162 1.164 1.313 1.311 1.300 1.311 1.167 1.164 2.666 2.677 1.848

4F 1.324 1.281 1.321 1.355 1.342 1.007 1.458 1.088 1.436 1.389

4CHO 1.322 1.281 1.321 1.357 1.341 1.007 1.459 1.088 1.439 1.391

Cation 44CN CF3 1.322 1.322 1.281 1.281 1.321 1.321 1.357 1.357 1.340 1.341 1.007 1.007 1.459 1.459 1.088 1.088 1.438 1.439 1.388 1.388

4COMe 1.322 1.281 1.321 1.356 1.342 1.007 1.458 1.088 1.439 1.389

4CO2Me 1.323 1.281 1.321 1.356 1.342 1.007 1.458 1.088 1.439 1.389

RI PT

4COMe 1.315 1.279 1.323 1.363 1.347 1.033 1.459 1.085 1.430 1.387 1.383 1.382 1.079 1.162 1.313 1.300 1.167 2.676 2.680 1.846

SC

Ion Pair 44CN CF3 1.314 1.315 1.279 1.279 1.324 1.324 1.364 1.363 1.346 1.346 1.033 1.033 1.459 1.459 1.085 1.085 1.429 1.430 1.385 1.383 1.385 1.386 1.384 1.386 1.080 1.079 1.162 1.162 1.312 1.313 1.301 1.301 1.167 1.167 2.577 2.576 2.704 2.706 1.858 1.855

119.4 112.8 115.1 116.5 112.2 113.1 116.4 116.6 116.4 116.5 116.5 116.4 156.9 156.2 155.4 156.1 156.7 156.7 139.6 145.2 142.9 142.0 145.2 144.9

AC C

335

4CHO 1.315 1.279 1.324 1.363 1.346 1.033 1.459 1.085 1.430 1.389 1.384 1.382 1.079 1.162 1.313 1.301 1.166 2.655 2.688 1.849

EP

N1-N2 N2-N9 N3-C8 C8-N1 C8-N20 N20-H21 N1-C4 C4-H5 N9-C10 C10-C12 C10-C11 C12-C15 C12-H16 N23-C24 C24-N25 N25-C26 C26-N27 N23---H16 N27---H5 N27---H21 Bond Angle C12-H16-N23 C24-N25-C26 N20-H21-N27 Dihedral Angle C12-C10-N9-N3

4F 1.317 1.279 1.324 1.362 1.347 1.033 1.458 1.085 1.429 1.386 1.385 1.385 1.080 1.162 1.313 1.301 1.167 2.548 2.670 1.854

1.079 1.079 1.079

1.078 1.079 1.079

M AN U

Parameter Bond length

TE D

334

342 343 344

18

120.3

179.2 178.9 179.1

179.6 179.0 179.0

ACCEPTED MANUSCRIPT

As obtained from Table 2 the bond length of the N20-H21 and C12-H16 bonds, which are involved

346

in hydrogen bond interactions significantly increase in the IPs with respect to the free ions.

347

Correlations between C12-H16 bond length and Hammett's substituent constants (σp) for both ion

348

pairs and free cations are depicted in Fig. 3b.

RI PT

345

349 350

SC

351 352

M AN U

353 354 355 356

360 361 362 363

EP

359

AC C

358

TE D

357

19

SC

RI PT

ACCEPTED MANUSCRIPT

M AN U

364

367

AC C

366

EP

TE D

365

368

Fig. 3. C12-C10-N9-N3 dihedral angle (a) as well as C12-H16 bond length (b) versus Hammett's

369

substituent constant (σp) in both nanostructured ion pairs (♦) and free cations (▲).

370 371 372 20

ACCEPTED MANUSCRIPT

The high BEs obtained for nanostructured ILs proposed that the contribution of other attraction

374

forces (electrostatic attraction between cation and anion) than hydrogen bond interactions are

375

responsible for or contribute with hydrogen bonding for those high interaction energies.

376

Although the structural parameters which are collected in Table 2 indicate that the hydrogen

377

bond interactions are the major intermolecular interactions between cations and anions but, the

378

slight changes on structural parameters of hydrogen bond involving bonds verifies above

379

mentioned suggestion about the interaction energy values.

380

Therefore we can conclude that there is a cooperativity between hydrogen bond and electrostatic

381

interactions during formation of ion pairs of isolated ion parts which leads to higher net

382

interaction energies for formation of nanostructured ILs. The same situation has been reported

383

for [EMim][BF4] and [PhMim][BF4] by Dong [84] and Roohi [6]et al respectively.

384

The change in C10-N9, C8-N1, C8-N3 and C8-N20 bond lengths against the Hammett's substituent

385

constants (σp) are shown on Fig. 4. The N9-C10 which is the bond that connects the phenyl and

386

tetrazole rings of the cation is contracted upon formation of ion pairs; it’s also worth to note that

387

this contraction for electron withdrawing substituents is greater than other ones and in the case of

388

4-NH2 which is the strongest electron donating substituent the C10-N9 bond length increased

389

instead of decreasing (Fig. 4a).

391 392 393

SC

M AN U

TE D

EP

AC C

390

RI PT

373

21

ACCEPTED MANUSCRIPT

SC

RI PT

4a

M AN U

394 395 396

398 399

AC C

EP

TE D

397

400 401 402 22

SC

RI PT

ACCEPTED MANUSCRIPT

M AN U

403 404

406 407

AC C

EP

TE D

405

408

Fig. 4. Correlation between the N9-C10 (a), N1-C8 (b), N3-C8 (c) and N8-C20 (d) bond lengths and

409

Hammett's substituent constants (σp) in nanostructured [(4-X)PMAT][N(CN)2] ion pairs(♦)

410

and[(4-X)PMAT] cations (▲) .

23

ACCEPTED MANUSCRIPT

As can be seen on Fig.4 (b, c and d), the C10-N9, C8-N1, C8-N3 and C8-N20 bond lengths in all ILs

412

including both electron donating and electron withdrawing substituents increased with respect to

413

that of corresponding free cation.

414

3.3. Vibrational frequencies

415

Generally some changes in vibrational frequencies of an isolated molecule upon formation of a

416

complex system were observed. This means that IR spectroscopy could be a powerful technique

417

to investigate intermolecular interactions. Usually the amount of red or blue shift in vibrational

418

frequencies of bonds, which are involved in interaction, depends on the strength of interaction

419

between monomers during complex formation process. For example the distance between proton

420

donor atom and a hydrogen atom (D-H) increased because of hydrogen bond interaction, and

421

then as a result a meaningful red shift occurred in D-H stretching vibrational frequencies,

422

therefore the higher interaction energies lead to higher red shift values.

423

Analysis of the vibrational frequencies reveals the significant changes in vibrational frequencies

424

of N-H bonds, whereas vibrational frequencies of C-H bonds do not change significantly as of N-

425

H bonds upon ion pair formation.

428 429 430 431

SC

M AN U

TE D

EP

427

AC C

426

RI PT

411

432 433

24

ACCEPTED MANUSCRIPT

434

Table 3

435

Vibrational frequency of some selected bonds in both ion pairs and corresponding cations.

436 437 438

443 444 445 446

EP

442

AC C

441

TE D

439 440

N20-H21 3599.8 3599.9 3605.2 3601.6 3601.2 3597.4 3599.2 3598.0 3595.2 3597.3 3601.0 3604.6

RI PT

C26-N27 2323.1 2325.9 2325.4 2322.7 2325.6 2321.6 2323.5 2321.6 2324.6 2322.4 2323.5 2322.3

Monomer C12-H16 C26-N27 3235.6 2313.3 3230.6 3246.3 3236.2 3227.5 3247.4 3240.7 3237.7 3239.5 3246.5 3239.0 3247.9

SC

N20-H21 3165.1 3177.5 3200.1 3183.8 3183.4 3173.8 3180.7 3182.7 3185.8 3179.5 3177.8 3177.6

Ion Pair C12-H16 3241.3 3255.4 3240.8 3252.1 3247.2 3259.5 3253.7 3243.8 3246.2 3252.9 3260.2 3263.1

M AN U

IR(cm-1) Bond 4-H 4-Me 4-OMe 4-OH 4-NH2 4-NO2 4-F 4-CN 4-CHO 4-CF3 4-COMe 4-CO2Me

447 448

25

ACCEPTED MANUSCRIPT

The stretching vibrational frequencies of the N20-H21 bond in the cations are 3599.8, 3599.9,

450

3605.2, 3601.6, 3597.4, 3599.2, 3598.0, 3595.2, 3597.3, 3601.0 and 3604.6 cm-1 for (X = H,

451

CH3, OCH3, OH, NH2, NO2, F, CN, CHO, CF3, COMe and CO2Me) respectively, that change to

452

3165.1, 3177.5, 3200.1, 3183.8, 3183.4, 3173.8, 3180.7, 3182.7, 3185.8, 3179.5, 3177.8 and

453

3177.6 cm-1 upon ion pair formation.

454

In addition, as obtained from data in Table 3 the ion pair formation is accompanied by a little

455

change in the vibrational frequency of C12-H16 bond. This is because the H-bonding interactions

456

of C12-H16---N are weak in comparison to the N20-H21---N strong hydrogen bond ones.

457

3.4. Quantum Theory of Atoms in Molecules (QTAIM) analysis

M AN U

SC

RI PT

449

Quantum theory of atom in molecules (QTAIM) [79], explain the nature of bonding

459

interactions in terms of the electron density features and its derivatives. This is also a useful tool

460

to investigate hydrogen bond type interactions of all type [87-89].

461

Some AIM parameters such as electron density, ρ(r), Laplacian of the electron density, ∇2ρ(r),

462

and electronic energy density, H(r), at the bond critical points (BCPs) at M06-2X /6-311++G

463

(2d,2p) level of theory that are important in describing the bonding nature of our studied ILs are

464

collected in Table 4.

467 468

EP

466

AC C

465

TE D

458

469 470 471

26

ACCEPTED MANUSCRIPT

472

Table 4

473

Topological properties of the BCPs (au) in the nanostructured [(4-X)PMAT][N(CN)2] ion pairs

474

calculated at M06-2X /6-311++G (2d,2p) level of theory.

0.0336 0.0089 0.0122 0.0338 0.0096 0.0126 0.0337 0.0094 0.0130

H(r)

ρ(r)

-0.0013 0.0013 0.0019

0.0339 0.0084 0.0123

-0.0011 0.0014 0.0018

0.0338 0.0097 0.0124

-0.0013 0.0015 0.0019

0.0340 0.0084 0.0130

-0.0012 0.0015 0.0020

475

479 480 481 482 483 484

EP

478

AC C

477

0.0337 0.0093 0.0128

TE D

476

∇2ρ(r) 4-Me 0.0988 0.0280 0.0462 4-OH 0.0984 0.0332 0.0463 4-CHO 0.0993 0.0283 0.0491 4-CF3 0.0982 0.0320 0.0487

H(r) 0.0013 0.0013 0.0019 -0.0013 0.0015 0.0019

485 486

27

ρ(r)

∇2ρ(r) 4-NH2 0.0985 0.0290 0.0454 4-NO2 0.0981 0.0319 0.0500 4-COMe 0.0994 0.0273 0.0490 4-CO2Me 0.0989 0.0276 0.0492

RI PT

0.0339 0.0089 0.0125

∇2ρ(r) 4-H 0.0987 0.0299 0.0470 4-OMe 0.0988 0.0300 0.0454 4-F 0.0981 0.0330 0.0478 4-CN 0.0980 0.0325 0.0493

0.0335 0.0086 0.0122

0.0337 0.0092 0.0131

SC

ρ(r)

-0.0013 0.0013 0.0020

0.0343 0.0081 0.0129

-0.0012 0.0015 0.0020

0.0342 0.0082 0.0129

M AN U

Parameter Bond N27 ---H21 N23---H16 N2---N23 Bond N27 ---H21 N23---H16 N2---N23 Bond N27 ---H21 N23---H16 N2---N23 Bond N27 ---H21 N23---H16 N2---N23

H(r)

-0.0011 0.0013 0.0018 -0.0012 0.0015 0.0020 -0.0014 0.0012 0.0020 -0.0014 0.0012 0.0020

ACCEPTED MANUSCRIPT

Fig. 5 shows the molecular graphs, including the bond critical points(BCPs), ring critical points

488

(RCPs), cage critical points (CCPs) and bond paths of the nanostructured [(4-X)PMAT][N(CN)2]

489

ion pairs at the ground states. The molecular graphs for all IPs show at least four BCPs and three

490

RCPs at inter-ionic region.

RI PT

487

491 492

SC

493 494

M AN U

495 496 497

AC C

EP

TE D

498

28

499

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 5. Molecular graphs of the nanostructured [(4-X)PMAT][N(CN)2] ion pairs. Critical points

501

are represented by small red (bond), yellow (ring) and green circles (cage).

503 504 505

AC C

502

EP

500

506 507 508 29

ACCEPTED MANUSCRIPT

Fig. 6(a) represents a correlation between the sum of the electron densities in inter-ionic BCPs

510

and BE. As appears from this figure as the sum of the electron densities in inter-ionic BCPs

511

increases the binding energy also increases in consistent with it. It is also worth noting that the

512

electron density increases as the substituent electron withdrawing power increases.

RI PT

509

513 514

SC

515 516

M AN U

517 518 519 520

524 525 526 527 528 529

EP

523

AC C

522

TE D

521

530 531

30

ACCEPTED MANUSCRIPT

532 533

6a

0.06620

534

RI PT

536

Ʃρ(r)/au

0.06540

535

0.06460

537

SC

538 0.06380

539

88

92

96

BE(kcal/mol)

M AN U

540 541

543 544

AC C

EP

TE D

542

545 546 547

31

100

ACCEPTED MANUSCRIPT

M AN U

SC

RI PT

548

549 550

Fig. 6. Correlation of the BE with (a) Sum of the electron densities at BCPs in inter-ionic region

552

of all IPs. (b) Sum of electron densities at C26-N27 and C24-N23 BCPs of all IPs and (c) electron

553

densities at N2-N23 BCPs of all IPs with the exception of the IPs including CHO, COMe and

554

CO2Me substituents.

557 558 559 560

EP

556

AC C

555

TE D

551

561 562 563

32

ACCEPTED MANUSCRIPT

Another useful linear correlation was found between the sum of electron densities at C26-N27 and

565

C24-N23 BCPs and BEs (Fig. 6b) on which the sum of electron densities at C26-N27 and C24-N23

566

BCPs decreases as the BEs increase which are in accordance with the participation of nitrogen

567

atoms of anion in an ion pair formation through hydrogen bonding interactions.

568

Fig. 6c represents an extra linear relationship between the BEs and electron densities at N2-N23

569

BCP in all IPs with the exception of the IPs including the substituents CHO, COMe and CO2Me.

570

As is revealed from Fig. 6c the electron density at N2-N23 BCP increases as the BE and as well as

571

the electron withdrawing power of substituents increase.

572

3.5. NBO analysis

573

The amount of electronic charge on both of the monomers which are involved in ion pair

574

formation through hydrogen bonding is changed. During hydrogen bond interactions some of

575

electronic charge transfers from lone pairs of the proton acceptor to the antibonding orbitals of

576

the proton donor [51, 60-61].

577

The difference between the net charge on each of cation or anion before and after of formation of

578

an ion pair system could be defined as charge transfer (CT) value. Here we get N(CN)2- as a

579

selected monomer for acquiring of the CT amounts.

580

The results of the NBO analysis for the ion pairs of [(4-X)PMAT] and N(CN)2- at M06-2X/6-

581

311++G (2d,2p) level of theory are collected in Table 5.

583 584

SC

M AN U

TE D

EP

AC C

582

RI PT

564

585 586

33

ACCEPTED MANUSCRIPT

Table 5

588

Natural bond orbital data for nanostructured [(4-X)PMAT][N(CN)2] ion pairs obtained at M06-

589

2X/6-311++G (2d,2p) level of theory.

σ* C12-H16 Charge(au) H21 N20 H16 N23

σ* C12-H16

N20 H16

590

AC C

Charge(au) H21

4-OH

4-NH2

4-NO2

0.0496 (0.0051) 0.0129 (0.0124)

0.0493 (0.0051) 0.0126 (0.0123)

0.0485 (0.0051) 0.0127 (0.0121)

0.0493 (0.0051) 0.0127 (0.0120)

0.0485 (0.0052) 0.0124 (0.0119)

0.0497 (0.0048) 0.0124 (0.0121)

0.4587 (0.4203) -0.8287 (-0.7892) 0.2642 (0.2399) -0.5837 (-0.5856) -0.6319 (-0.5856) -0.0555

0.4584 (0.4193) -0.8295 (-0.7901) 0.2616 (0.2386) -0.5807

0.4582 (0.4185) -0.8297 (-0.7907) 0.2632 (0.2402) -0.5815

-0.6318

-0.6316

-0.6324

-0.6316

-0.6343

-0.0547

-0.0540

-0.0555

-0.0539

-0.0573

12.11 0.45 6.24 4-CHO

12.15 0.57 5.86 4-CN

11.75 0.80 6.37 4-CF3

11.98 0.55 6.02 4-COMe

11.30 0.38 8.53 4-CO2Me

0.0498 (0.0050) 0.0126 (0.0120)

0.0496 (0.0050) 0.0128 (0.0128)

0.0496 (0.0049) 0.0124 (0.0119)

0.0495 (0.0049) 0.0126 (0.0121)

0.0502 (0.0050) 0.0128 (0.0128)

0.0489 (0.0050) 0.0100 (0.0126)

0.4587 (0.4209) -0.8285 (-0.7884) 0.2690 (0.2435) -0.5909 -0.6329 -0.0565

0.4598 (0.4216) -0.8276 (-0.7878) 0.2637 (0.2430) -0.5855 -0.6329 -0.0558

0.4599 (0.4229) -0.8269 (-0.7865) 0.2711 (0.2471) -0.5948 -0.6337 -0.0569

0.4596 0.4594 (0.4222) (0.4211) -0.8273 -0.8285 (-0.7872) (-0.7884) 0.2691 0.2620 (0.2449) (0.2418) -0.5920 -0.5840 -0.6332 -0.6340 -0.0563 -0.0561

0.4637 (0.4207) -0.8623 (-0.7887) 0.2460 (0.2421) -0.5841 -0.6346 -0.0551

12.33 0.30 6.02

11.36 0.47 8.34

12.10 0.56 6.27 4-F

EP

CT(au) E(2)/kcal. mol-1 LPN27 → σ*(N20-H21) LPN23 → σ*(C12-H16) σ(C26-N27) → σ*(N20-H21) 4-X Occupancy(au) σ* N20-H21

4-OMe

TE D

N27

4-Me

N23 N27 CT(au) E(2)/kcal. mol-1 LPN27 → σ*(N20-H21) 11.35 LPN23 → σ*(C12-H16) 0.72 σ(C26-N27) → σ*(N20-H21) 6.70 (Data in parenthesis are for monomers)

SC

Occupancy(au) σ* N20-H21

4-H

0.4581 0.4577 (0.4193) (0.4171) -0.8294 -0.8303 (-0.7901) (-0.7921) 0.2676 0.2626 (0.2408) (0.2384) -0.5875 -0.5792

M AN U

4-X

RI PT

587

34

11.65 0.51 6.33

12.32 0.28 6.29

0.4601 (0.4234) -0.8268 (-0.7860) 0.2701 (0.2477) -0.5949

12.29 0.26 6.39

ACCEPTED MANUSCRIPT

As appeared from the NBO analysis the LP(N) → σ*(N-H), LP(N) → σ*(C-H) and σ (C26-N27)

592

→ σ*(N20-H21) are the main intermolecular interactions between anion and cation during ion pair

593

formation.

594

Charge transfer energy E(2) corresponding to the LP(N27) → σ*(N20-H21) and (LP(N23) →

595

σ*(C12-H16)) interactions are 12.10(0.56), 12.11(0.45), 12.15(0.57), 11.75(0.80), 11.98(0.55),

596

11.30(0.38), 11.35(0.72), 12.33(0.30), 11.36(0.47), 11.65(0.51), 12.32(0.28) and 12.29(0.26)

597

kcal/mol for nanostructured [(4-X)PMAT][N(CN)2] (X = H, CH3, OCH3, OH, NH2, NO2, F,

598

CN, CHO, CF3,COMe and CO2Me) ILs respectively. In addition, the significant amount of

599

charge transfers from anion to cation through σ(C26-N27) → σ*(N20-H21) with the energy E(2) of

600

6.27, 6.24, 5.85, 6.37, 6.02, 8.53, 6.70, 6.02, 8.33, 6.33, 6.34 and 6.39 kcal/mol for [(4-

601

X)PMAT][N(CN)2] (X = H, CH3, OCH3, OH, NH2, NO2, F, CN, CHO, CF3,COMe and CO2Me)

602

ILs respectively.

603

As obtained from the NBO analysis the LP(N27) → σ*(N20-H21) interaction is the stronger and

604

the σ(C26-N27) → σ*(N20-H21) interaction is the weaker one. The sum of LP(N) → σ*(N(C)-H)

605

donor acceptor interaction energies for nanostructured [(4-X)PMAT][N(CN)2] (X = H, CH3,

606

OCH3, OH, NH2, NO2, F, CN, CHO, CF3,COMe and CO2Me) ILs is 12.66, 12.56, 12.72, 12.55,

607

12.53, 11.68, 12.07, 12.66, 11.83, 12.16, 12.60 and 12.55 kcal/mol respectively. As results

608

revealed the sum of LP(N) → σ*(N(C)-H) donor acceptor interaction energies in the case of ILs

609

including CN and NO2 substituents is lower than the other ones whereas the BE is greater for

610

those ion pairs . At first glance, there is an inconsistency between donor acceptor interaction

611

energies and BEs which are obtained for those ILs but considering of the donor acceptor

612

interaction energies for the σ(C26-N27) → σ*(N20-H21) interaction which is the highest in the case

613

of ILs including CN and NO2 substituents(see Table 5) this inconsistency could be resolved. In

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591

35

ACCEPTED MANUSCRIPT

other word the net donor acceptor interaction energies including all of three of above mentioned

615

interactions are larger for ILs including CN and NO2 substituents than others.

616

The occupancy of σ*(N20-H21) gets increased upon ion pair formation which is in good

617

agreement with the elongation of N20-H21 bond distance with respect to the N20-H21 bond

618

distance in isolated cation. Occupancy of σ*(N20-H21) is 0.0496, 0.0493, 0.0485, 0.0493, 0.0485,

619

0.0497, 0.0498, 0.0496, 0.0496, 0.0495, 0.0502 and 0.0489 au in nanostructured [(4-

620

X)PMAT][N(CN)2] (X = H, CH3, OCH3, OH, NH2, NO2, F, CN, CHO, CF3,COMe and CO2Me)

621

ILs respectively as appeared in Table 5.

622

As obtained from Table 5, the change in the amount of electron density at σ*(C-H) is less than

623

that of σ*(N20-H21) one and in agreement with the lower interaction energies for σ*(C-H)

624

included interactions in comparison to the σ*(N20-H21) included ones.

625

As of σ*(N20-H21), the electronic density of σ*(C12-H16) is greater in ion pairs with respect to

626

free cations.

627

The electronic density of σ*(C12-H16) for nanostructured [(4-F)PMAT][N(CN)2] ion pair is

628

higher than other ion pairs, which is in good agreement with the its higher E(2) energy with

629

respect to other substituents as resulted from Table 5.

630

The increase in electron density of σ*(C12-H16) bonds leads to bond elongation in the ion pairs as

631

compared to those in the free cation (see Table 2).

632

Considering of the change in atomic charges, we could find that the charge on hydrogen atoms

633

which are involved in hydrogen bond interactions during ion pair formation becomes more

634

positive with respect to those on the free cation. It is in accordance with the changes on charge of

635

atoms involved at conventional hydrogen bonding. Within our studied ion pairs the positive

636

charge of H21 and H16 increased upon hydrogen bonding. In addition, this increasing at positive

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614

36

ACCEPTED MANUSCRIPT

charges is higher at ion pairs with electron withdrawing substituents than those with electron

638

donating ones due to the incorporation of substituent and hydrogen bond interactions at

639

increasing of the positive charge of H21 and H16 atoms.

640

The natural charges calculated at M06-2X/6 311++G(2d,2p) level of theory are collected in

641

Table 5.

642

The C12 atom is one that involved in the C-H---N hydrogen bond interaction and acts as a proton

643

donor. The value of negative charge on C12 atom of cation decreases upon IP formation and this

644

decrease for electron withdrawing substituents is bigger than other ones.

645

The natural bond orbital analysis shows that the charge is transferred from N(CN)2− anion to

646

[(4X)PhMAT]+ cation. The CT values of -0.05548, -0.05475, -0.05403, -0.05555, -0.05390, -

647

0.05733, -0.05652, -0.05576, -0.05694, -0.05631, -0.05606 and -0.0551 au are obtained for

648

nanostructured [(4-X)PMAT][N(CN)2] (X = H, CH3, OCH3, OH, NH2, NO2, F, CN, CHO,

649

CF3,COMe and CO2Me) ion pairs respectively.

650

As could be seen, the amount of CT for ILs with electron withdrawing substituents are larger

651

than that for ILs with electron donating substituents which is in consistence with the BE values.

652

Correlation between BEs and CT values for nanostructured [(4-X)PMAT][N(CN)2] (X = H, CH3,

653

OCH3, OH, NH2, NO2, F, CN, CHO, CF3,COMe and CO2Me) ion pairs is depicted on Fig. 7. As

654

obtained from this figure decrease/increase in CT is accompanied by decrease/increase in BE.

656

SC

M AN U

TE D

EP

AC C

655

RI PT

637

37

SC

RI PT

ACCEPTED MANUSCRIPT

M AN U

657 658 659

Fig. 7. Correlation between the BEs and CT values of nanostructured [(4-X)PMAT][N(CN)2] (X

660

= H, CH3, OCH3, OH, NH2, NO2, F, CN, CHO, CF3,COMe and CO2Me) ion pairs.

664 665 666 667 668 669

EP

663

AC C

662

TE D

661

670 671 672

38

ACCEPTED MANUSCRIPT

3.6. Frontier molecular orbital insight

674

Analysis of frontier orbitals using the molecular orbital theory is useful approach to the depth

675

understanding of the molecular interactions specially charge transfers ones. In an ion pair series

676

generally HOMO of anion and LUMO of cation are of great importance because the charge

677

transfers from the anion HOMO orbital to the cation’s LUMO one. Therefore as the energy

678

difference between those HOMO and LUMO orbitals becomes smaller the charge transfer

679

becomes faster and as a result the stronger interaction will be occurred.

680

Within our studied ion pairs the anion part is N(CN)2- with the HOMO energy of -0.1149 au

681

whereas cation parts are different and then have different LUMO energies. Therefore the values

682

for LUMO of cations could be enough to determine of facility of charge transfer from anion part

683

to the cation one and as well as strength of interactions according to the frontier molecular orbital

684

analysis. HOMO and LUMO energies for ion pairs and isolated ion parts which are calculated at

685

M06-2X/6-311++G(2d,2p) level of theory are collected in Table 6.

689 690 691 692 693

SC

M AN U

TE D

688

EP

687

AC C

686

RI PT

673

694 695

39

ACCEPTED MANUSCRIPT

Table 6

697

HOMO and LUMO energies for studied structures obtained at M06-2X/6-311++G(2d,2p) level

698

of theory.

699

703 704 705 706 707 708

EP

702

AC C

701

TE D

700

709 710 711 40

∆EHOMO-LUMO/au -0.2063 -0.2078 -0.2109 -0.2114 -0.2123 -0.1810 -0.2071 -0.1875 -0.1907 -0.1977 -0.1915 -0.1942

SC

EHOMO/au -0.2679 -0.2664 -0.2659 -0.2674 -0.2636 -0.2769 -0.2707 -0.2724 -0.2760 -0.2735 -0.2706 -0.2697

Ion pair ELUMO/au -0.0616 -0.0585 -0.0550 -0.0560 -0.0513 -0.0958 -0.0636 -0.0848 -0.0852 -0.0758 -0.0791 -0.0755

M AN U

4-X 4-H 4-Me 4-OMe 4-OH 4-NH2 4-NO2 4-F 4-CHO 4-CN 4-CF3 4-COMe 4-CO2Me

Cation EHOMO/au -0.4397 -0.4230 -0.4005 -0.4096 -0.3784 -0.4649 -0.4360 -0.4420 -0.4471 -0.4560 -0.4299 -0.4412

RI PT

696

ACCEPTED MANUSCRIPT

712

Fig 10. Shows the correlation between binding energies and the LUMO orbital’s energy of the

713

cations. As revealed from Fig. 10 as the LUMO energies of the cations decrease (be more

714

negative) the binding energies between ion parts increase.

RI PT

715 716 717

SC

718 719

M AN U

720 721 722 723

727 728 729

EP

726

AC C

725

TE D

724

41

SC

RI PT

ACCEPTED MANUSCRIPT

M AN U

730 731 732

Fig. 10. Correlation between LUMO energy of cations and binding energies.

733

737 738 739 740 741 742

EP

736

AC C

735

TE D

734

743 744 745

42

ACCEPTED MANUSCRIPT

746

The

frontier

molecular

orbital

(FMO)

energy

diagram

for

nanostructured

747

X)PhMAT][N(CN)2] (X = OMe, H and NO2) ion pairs are depicted in Fig. 11.

748

RI PT

749 750 751

SC

752 753

M AN U

754 755 756 757

AC C

EP

TE D

758

43

[(4-

ACCEPTED MANUSCRIPT

LUMO(Ion pair) -0.0550au

LUMO(Ion pair) -0.0617au

∆E= -0.2109au

∆E= -0.2063au

HOMO (Anion) -0.1149au

SC

LUMO (Cation) -0.1813au

LUMO (Cation) -0.1911au

HOMO(Ion Pair) -0.2659au

HOMO(Ion pair) -0.2680au

[(4-OMe)PMAT][N(CN)2]

[(4-H)PMAT][N(CN)2]

TE D

759 760

∆E= -0.1810au

HOMO (Anion) -0.1149au

M AN U

HOMO (Anion) -0.1149au

RI PT

LUMO(Ion pair) -0.0959au

LUMO (Cation) -0.2126au

HOMO(Ion pair) -0.2769au

[(4-NO2)PMAT][N(CN)2]

Fig. 11. Frontier molecular orbital (FMO) energy diagrams for nanostructured [(4-

762

X)PhMAT][N(CN)2] (X = OMe, H and NO2) ion pairs

764 765 766 767

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761

768 769 770 44

ACCEPTED MANUSCRIPT

As shown on Fig. 11, HOMO of anion consists of three p orbitals (EHOMO= -0.1149au) localized

772

in the three N atoms and LUMO of cations is a π* MO in nature with the energy of -0.0550, -

773

0.0616 and -0.0958 au for [(4-X)PhMAT]+ (X = OMe, H and NO2) cations respectively.

774

Considering of the molecular orbitals of ion pairs revealed that the HOMOs are localized on the

775

cation part, especially on tetrazolium and the phenyl rings rather than anion one. Therefore the

776

cation part acts as an electron acceptor during ion pair formation indicating that CT occurs from

777

the anion to cations. Distribution of LUMO orbitals is different from HOMO ones within ion

778

pairs. For example, in [(4-OMe)PhMAT][N(CN)2], the LUMO orbital is located only across the

779

phenyl ring, whereas in [(4-NO2)PhMAT][N(CN)2], it distributed on both phenyl and tetrazolium

780

rings.

781

The electrochemical stability of ionic liquids could be evaluated as how the cation and anion

782

parts are resistance against to the reduction and oxidation respectively. It’s well-known that the

783

electrochemical stability is related to the electron affinity which it has good correlation with the

784

energy of LUMO orbitals. The electron accepting ability of the molecule increases as the energy

785

of it’s LUMO orbital decreases [90-93, 6].

786

Since the LUMO orbitals of our studied ILs are centered on the cation part, there for it can

787

conclude that there is a direct relation between the resistance of the cation part and the stability

788

of the ionic liquid with respect to reduction.

789

There is also a good correlation between the LUMO energy of cations and Hammett's constant

790

(σp). As depicted in Fig. 12 the LUMO energy of cations decreases as the Hammett's constant

791

(σp) decreases (be more negative) therefore it can be concluded that the cations having electron

792

donating substituents are more resistant against reduction in comparison to the cations with

793

electron withdrawing substituents due to lower LUMO energies. Thus ILs having electron

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771

45

ACCEPTED MANUSCRIPT

794

donating groups on cation part are more stable and suitable than the others with electron

795

withdrawing groups from the electrochemical point of view and application as an electrolyte in

796

electrochemical cells.

RI PT

797 798 799

SC

800 801

M AN U

802 803 804 805

809 810 811 812

EP

808

AC C

807

TE D

806

46

SC

RI PT

ACCEPTED MANUSCRIPT

M AN U

813 814 815

Fig. 12. Correlation between the LUMO energies of [(4X)-PMAT]+ and Hammett's

816

constants(σp)

820 821 822 823 824 825

EP

819

AC C

818

TE D

817

826 827 828

47

ACCEPTED MANUSCRIPT

3.7. Cations heat of formation

830

In this part of our study, we employed CBSQB3 composite method [94-95] for calculation of

831

both combustion and formation enthalpy and Gibbs free energies of cations involved in the

832

formation of ILs which are discussed during this work.

833

The following reactions are used for calculation of the above mentioned thermodynamic

834

quantities according to the combustion process:

835

TE D

M AN U

SC

RI PT

829

The standard enthalpy and Gibbs free energy for the combustion reaction ( ∆Η 0c, g ,298 and ∆Gc0, g ,298 )

837

and formation of cations ( ∆Η 0f , g ,298 and ∆G 0f , g ,298 ) were computed using the calculated enthalpy

838

changes of the above combustion reactions and experimental ∆Η 0f , g ,298 of H2O, CO2 and HF

839

molecules. The CBS-QB3 method computed thermodynamic data are collected in Table 7.

AC C

840

EP

836

841 842 843

48

ACCEPTED MANUSCRIPT

844

Table 7

845

Thermodynamic (kcal/mol) values obtained for cations by using the CBS-QB3 composite

846

method.

3

[(4-OMe)-PMAT]+

6

852 853 854

-1823.5 580.2

646.3

-1729.1

-1784.5 534.8

607.3

551.9

-1619.7

-1672.2 548.4

616.7

+

-1523.8

-1580.8 510.3

580.0

[(4-NH2)-PMAT] [(4-NO2)-PMAT] +

-1536.5

-1583.8 487.1

544.9

-1672.5

-1721.4 536.0

550.0

-1714.7

-1760.8 607.1

665.6

-1605.0

-1667.3 389.6

458.1

-1855.6

-1909.3 567.1

637.8

[(4-CO2Me)-PMAT] -1792.9

-1852.3 504.4

580.7

+

[(4-CN)-PMAT]

+

[(4-CF3)-PMAT]

+

[(4-COMe)-PMAT]

EP

TE D

+

AC C

851

-1774.6

-1580.0 487.8

[(4-CHO)-PMAT]+

12

576.5

-1530.2

8

11

-1632.0 545.8

+

[(4-F)-PMAT]

10

850

[(4-OH)-PMAT]

7 9

849

+

-1588.2

RI PT

[(4-Me)-PMAT]

5

848

+

2 4

847

[(4-H)-PMAT]+

∆Gc0, g ,298 ∆Η 0f , g ,298 ∆G 0f , g ,298

M AN U

1

∆Η 0c , g ,298

SC

Entry Cation

855 856 857

49

ACCEPTED MANUSCRIPT

The amount of combustion and formation enthalpies of cations are ranged from -1523.8

859

(X=NO2) to -1855.6 (X=COMe) kcal/mol and from 389.6 (X=CF3) to 607.1 (X=CN) kcal/mol,

860

respectively. The ∆Gc0, g ,298 and ∆G 0f , g ,298 values for the studied cations were also calculated using

861

the same method and are collected in Table 7. As obtained from these results the formation of

862

studying cations from elements that are composed of them, are not spontaneous at 298 K.

863

4. Conclusions

864

The ionic liquids based on aryl methyl amino tetrazolium cation and dicyanimidium anion as a

865

tunable nanostructured ILs were studied and the influence of various substituents at the para

866

position of the phenyl ring on the main physicochemical properties of them were analyzed with

867

the help of M06-2X level of quantum chemical calculations.

868

The results showed that the main interactions are of hydrogen bond types of C-H---N and N-H---

869

N in which the latter one is more effective than the former.

870

The higher interaction energies were obtained for ILs which are contained electron withdrawing

871

substituents at the phenyl ring in comparison to those with electron donating substituents.

872

Electron density properties, natural charge and charge transfer values are in good agreement with

873

interaction energy changes due to substituent exchange.

874

Some of physical properties including surface tension, melting point, critical-point temperature,

875

electrochemical stability and conductivity are estimated for studying ion pairs and the effect of

876

substituent exchange on them were discussed. In brief within our studied ILs the electron

877

donating substituent including ones have the lower melting point, critical point and higher

878

electrical conductivity in comparison to the other ones. Therefore [(4-NH2)PMAT][N(CN)2]

879

nanostructured IL is more suitable for use as electrolyte due to its higher electrical conductivity

AC C

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M AN U

SC

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858

50

ACCEPTED MANUSCRIPT

and in contrast the [(4-NO2)PMAT][N(CN)2] one is more suitable for applications at high

881

temperatures because of it’s higher thermal stability.

882

Finally the enthalpy and Gibbs free energy of formation for twelve individual cations with the

883

general formula of [(4-X)PMAT]+ (X= 4-H, 4-Me, 4-OMe, 4-OH, 4-NH2, 4-NO2, 4-F, 4-CN, 4-

884

CHO, 4-CF3, 4-COMe and 4-CO2Me) are calculated using their corresponding combustion

885

reactions with the help of CBS-QB3 composite method.

886

Acknowledgment

887

We thank the Research Council of University of Guilan for supporting this work.

888

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889

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890

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Highlights:
Tunable nanostructured ILs based on [AMAT]+ cation and [N(CN)2]- anion were studied.
Substituents effects on the physicochemical properties of the title ILs were analyzed.

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Some physical properties including surface tension, conductivity and etc. were estimated.
The nature of interactions between ion pairs illustrated using NBO and QTAIM theory.

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The enthalpy and Gibbs free energy of formation for [AMAT]+ cations were calculated.