Unveiling Noncovalent Interactions in Imidazolium, Pyrrolidinium or

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Unveiling Noncovalent Interactions in Imidazolium, Pyrrolidinium or Quaternary Ammonium Cation and Acetate Anion Based Protic Ionic Liquids: Structure and Spectral Characteristics Prakash L. Verma, and Shridhar P. Gejji J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b04303 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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The Journal of Physical Chemistry

Unveiling Noncovalent Interactions in Imidazolium, Pyrrolidinium or Quaternary Ammonium Cation and Acetate Anion Based Protic Ionic Liquids: Structure and Spectral Characteristics Prakash L. Verma and Shridhar P. Gejji* Department of Chemistry, Savitribai Phule Pune University, Pune 411 007, India

Abstract In the present work protic ionic liquids (PILs) composed of imidazolium-, quaternary ammonium- or pyrrolidinium-dications and acetate (OAc‾) anion have been modeled as the dication-anion complexes through the M06-2x based density functional theory. It has been shown that cation-anion interaction energies are larger for the PILs containing the quaternary ammonium cation, which can be attributed to strong hydrogen bonding from the terminal ammonium proton. Underlying N-H∙∙∙O and C-H∙∙∙O hydrogen bonding, electrostatic and van der Waals interactions are unraveled using the natural bond orbital analyses in conjunction with the quantum theory of atoms in molecules (QTAIM) and noncovalent interaction index reduced density gradient methods. The ramifications of noncovalent binding to 1H NMR and vibrational spectra are presented. The calculations further demonstrate a linear correlation of the kinetic energy density parameter G(r) in QTAIM analysis with the characteristic frequency shift of -NH3+ stretching accompanying the ion pair complexes. Moreover, the chemical shifts (δH) in 1H NMR spectra from theory reveal larger deshielding; the corresponding δH value correlates well with proton affinities and the cation-anion binding energies as well. Effect of solvent (DMSO) on structure, binding energies and 1H NMR are presented. The vibrational frequencies reveal that frequency shifts of the characteristic carbonyl and the terminal ammonium stretching

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vibrations accompanying formation of the dication-anion complexes are in consonance with the self-consistent reaction field theory.

1. Introduction An ease of synthesis, the labile (transferrable) proton and hydrogen bonding network are interesting features of the protic ionic liquids (PILs) which have been fascinating for quite some time.1-6 PILs based on tertiary amines display the improved transport properties and find applications as proton-conducting nonaqueous electrolytes in fuel cells.7 Compilation of experimental data on viscosities8, densities9,10 and ionic conductivities11-13 have proven useful for the rational design of task specific ionic liquids. In particular, the acetate anion based PILs possessing low toxicity; non-mutagenicity14 and ready biodegradability15 find diverse applications such as CO2 capture16, fuel processing17 and biomass dissolution.18 A proton transfer accompanying the PILs has been probed through the1H or

15N

NMR, infrared and Raman spectroscopy experiments.19-22 The

synthesis and characterization PIL, triethylammonium acetate was carried out by Li et al.23 using the total reflectance infrared (ATR-IR) experiments. It was found that the equimolar synthesized triethylammoniumacetate can be separated into two layers which suggested the chemical as well as phase equilibrium exist in the solution which were also confirmed from the existence of molecular species further inferred through 1H NMR as well as infrared spectroscopy experiments. Likewise, the

15N

NMR experiments on the PILs

composed of ammonium and acetate anion by Burrell et al.24 suggested the coexistence of neutral ammonia and ionized ammonium ions which ascertained the proton transfer in these systems. The infrared and NMR spectroscopy thus provide a simple and effective approach to determine the PILs compositions. Moreover the ATR-IR and

1H

NMR 2


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spectroscopy experiments on the tertiary ammonium acetate based PILs by Walker et al.25 established certain structure-property relationships. It was observed that PILs with the hydroxyl substituted cations facilitate the easier proton transfer compared to those based on alkyl or amino-functionalized PILs. Despite of the above experimental investigations, theoretical studies on the acetate based PILs are rather limited. On theoretical front, the transport phenomena within the PILs earlier were modeled through the molecular dynamics simulations.26-28 The electronic structure, thermodynamics and transport properties in ammonium based PILs were simulated using the molecular dynamics simulations.29 The work was further extended to hydroxyl-functionalized PILs namely 2-hydroxyethylammoniumacetate, ethylammoniumand 2-hydroxyethylammonium-hydroxyacetate.30 It was concluded that the hydroxyl substituent on the cation renders slower dynamics consequent to strong hydrogen bonding interactions from the acetate anion. Alternatively, the density functional theory (DFT) has successfully been employed to unveil noncovalent interactions in the task specific ionic liquids.31-33 Noncovalent interactions index34,35 was explored for mapping the contributions from noncovalent interactions such as hydrogen bonding, van der Waals interactions and steric repulsions for the cation-anion binding accompanying imidazolium based ionic liquids with varying anions. Pursuance to this Dominguez et al.36 analyzed the effect of hydrogen bonding and entropic contributions on solubility of hydrofluoric acid in imidazolium based ionic liquids. On parallel lines structure and binding energies of the 1butyl-3-methylimidazolium and multiatomic perfluorinated anions ion pairs or their dimers have been studied by Marekha et al.37 These authors showed that dispersive interactions become crucial in the absence of directional hydrogen bonding. Further the 3



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NCI results were shown to be in excellent agreement with the QTAIM analysis. An extensive review describing thermal properties viz., glass transition, melting-or boiling-points and decomposition temperatures of the PILs and their structure-property relationships has appeared in the literature.38,39 Strategies for design of the PILs made up of modified cation or varying alkyl substituents has been of growing interest. In this light the present endeavor focuses on deriving molecular insights into noncovalent binding accompanying the PILs composed of imidazolium-, ammonium-, pyrrolidinium- cation and acetate anion. The present work focuses on understanding the spectral characteristics of dication-anion complexes. Discernibly the polar acetate anion (OAc‾) with its hydrophilic carboxylate head group and strong H-bond-accepting ability facilitates diverse molecular interactions with the dication engendering PILs having desirable physicochemical properties. The ramifications of these noncovalent binding to electronic structure and vibrational as well as 1H

NMR chemical shifts are presented. The effect of solvent on electronic structure, 1H NMR

chemical shifts and frequencies of characteristic carbonyl and NH vibrations of dication anion complexes has been analyzed. Lastly the correlations of proton affinities with the 1H NMR chemical shifts and cation-anion binding have also been established.

2. Computational Method The atomic numbering scheme for the dication and acetate (OAc-) anion is shown in Figure 1. Optimized structures of the isolated N-butyldimethylammoniumpropylammonium ([C4qm]2+), N-methylpyrrolidiniumpropylammonium ([C1pyr]2+), N-butylpyrrolidinium propylammonium ([C4pyr]2+) or butylimidazoliumpropylammonium ([C4im]2+) dications and [OAc‾] anion were derived from the B3LYP theory employing the GAUSSIAN-09 program.40 The internally stored 6-311++G(d,p) Gaussian basis set with polarization as 4


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well as diffuse fusions being added on the heavier and hydrogen atoms, was chosen for optimization.41 The initial structures of the ion pair conformers underlying the PILs were generated by placing the acetate anions (OAc−) in vicinity of the cation as described in Figure 2. Taking a cue from the guidelines on binding sites derived from the topography of the molecular electrostatic potentials42,43 (cf Figure S1of the supporting information) the acetate was placed in different positions which facilitates a variety of interactions those include, intermolecular interactions from (i)terminal ammonium group (S1), (ii) between the methine C1-H1 group and the propylammonium chain (Front Propylammonium, S2) (iii) front of the cation (S3) (iv) between the butyl chain and the methine C1-H1 group (Front Butyl S4) (v) the terminal CH3 group of butyl chain (Side Butyl, S5) (vi) the butyl chain and the C3-H3 group (Back Butyl S6), (vii) back of the cation (Back S7) (viii) between the propylammonium chain and the C2-H2 group (Back Propylammonium, S8), (ix) top of the cation toward butyl side chain (top Butyl S9), (x) top of the cation towards propylammonium side chain (top propylammonium, S10) and (xi) from bottom of the imidazolium cation (S11). These conformers were optimized using the B3LYP/6-31+G(d,p) level of theory (cf. Figure S2 through S5 of the supporting information). Their relative stabilization energies are given in parentheses. Subsequently the optimized structures of ion pair conformers destabilized up to 5 kJ mol‒1 relative to the lowest energy conformer, exhibiting qualitatively different cation-anion binding patterns, were subjected to optimization employing the hybrid meta-GGA (generalized gradient approximation) exchange correlation M06-2X functional. This level of theory is known to simulate hydrogen bonding, adequately typifying other weak interactions, as demonstrated by extensive benchmarking in the recent literature.44,45 Stationary point structures obtained 5



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were confirmed to be the local minima on the multivariant potential energy surface by computing the vibration frequencies (all of which turned out to be real). The normal vibrations were assigned by visualizing the displacements of the atoms around their equilibrium (mean) positions with the help of the Gaussview-5 program.46 Binding energies of the dication-anion complexes were calculated by subtracting the sum of zero point corrected energies of the individual ions/molecules from those of the ion pair complexes. The accompanying thermodynamic parameters were computed using the partition functions computed from the vibrational frequency calculations. The proton affinities of the ion pair complexes were derived using the same level of theory. To elucidate further nature and strength of cation-anion binding we explore the quantum theory of atoms in molecules (QTAIM) approach47 implemented in the AIMALL-2000 program.48 Furthermore noncovalent interaction (NCI) reduced density gradient (RDG) method34 was explored through the multiwfn program.49 To delve further into the dication and OAc anion binding natural bond orbital (NBO) analyses50 were carried out. Lastly 1H NMR chemical shifts (δH) were calculated by subtracting the nuclear magnetic shielding tensors of protons in the individual ions and their complexes from those in tetramethylsilane (reference) within the framework of the gauge-independent atomic orbital (GIAO) method.51 Effect of solvent (DMSO) on electronic structure, binding energies, vibrational frequencies of characteristic normal vibrations and 1H NMR chemical shifts was simulated through the self-consistent reaction field (SCRF) theory incorporating the polarization continuum model (PCM).52

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H14

H13

H3C

C5

C H2

H7

C4

H2 C

N2 C6

C2 C3

H17

H12

H1 H6

H13

N1

H8

C5

H4 H1

H8 C1

C4

C3

C2

H14

C6

C8

C7 H16

H2 H3

H7 H20

H15

H4

N1

H6

N2

H3 H5

H11

H9

10

H2

C1

H9 H16

H5

H

H10

H15 H2 C

H12 H11

H18 H19

H17

O1

O2

(b)[C1pyr]2+

(a) [C4qm]2+

C C H

H9

H15

H14

H11

H16

H1'

H10

C4

C7 C8

H17 H18

H134

N3 H12 H3'

H9

C6

H8 C3

N2 H7

C5

H4

C5

H14

C6

C1

C4

C3

H6

C7

C1 C2

N1

H6

H3

C2

N2

H5 H1 H2

H16

H17 H

(e) OAcH2

H3

H7

C8

H15

N1

H

H

H1

H8

H12 H13

H4

H5

H11 H10

H19 H

H18

H

H

H2' H H

(c) [C4im]2+

H

(d) [C4pyr]2+

Figure 1. Structures of (a) [C4qm]2+, (b) [C1pyr]2+, (c) [C4im]2+, (d) [C4pyr]2+ dications and acetate (e) (OAc‾) anion. The atomic labeling scheme is shown along with the structures.

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Figure 2. Schematic representation of the cation-anion binding sites in [C4im][OAc]2 ion pair complex as prototype example.

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3. Results and discussion It is known that the ion pairs in PILs refer to the smallest representative unit which simulates their structural and cation-anion binding adequately well and employed to model the liquidus behavior and physicochemical properties.53,54 The lowest energy structures of dication-anion complexes from the M06-2x/6-311++G(d,p) theory are shown in Figure 3. The lowest energy ion pair complexes reveal multiple hydrogen bonding between the dication and (OAc‾) anions. Moreover the hydrogen bonding interactions between methylene group from the propyl linkage and terminal –NH3+ can be noticed. The N-H∙∙∙O and C-H∙∙∙O hydrogen bonding interactions engender longer N1-H1 bonds with the hydrogen bond distance therein compare well with the covalent O-H bond (0.962 Å) in the free acetic acid, which ascertains the proton transfer. As may further be inferred the butyl substituent do not exhibit close contacts with the anion. A closer approach of [OAc‾] anions to the butyl substituent led to ion pair complexes 10–15 kcal mol–1 higher in energy. Selected structural parameters in the lowest energy ion pair complexes are compared with the free cation in Table S1 and Table S2 of supporting information. The inferences with the [C4qm][OAc]2 as a prototype example, are outlined below. Consequent to N-H∙∙∙O interactions N1-H1 and N1-H2 bonds distances in the complex turn out to be 1.111 Ǻ and 1.066 Ǻ respectively, compared to the free dication, 1.027 Ǻ (cf. Figure 3). A deviation up to 3° was noticed for the C1-N1-H1 bond angle. As far as the anion is concerned, the hydrogen bonded C-O1 and C-O2 bonds are elongated upon complexation with the [C4qm]2+ cation. A closure of 0 the dominance of the electrostatic contributions in the dication-anion complexes is transparent. As may readily be noticed the bcps for the N-H∙∙∙O interactions between anionic oxygen and terminal ammonium proton (involved in proton transfer) and those of the CH∙∙∙O interactions arising from the propyl linkage are discernible; the corresponding hydrogen bond energies (EHB) are further estimated from EHB = -0.5 V(r).58 Figure S6 shown in the supporting information displays EHB as a function of ρ BCP as well as the G(r) and V(r)

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components showing exponential dependence on the hydrogen bond distances. Stronger cation-anion binding in [C4qm][OAc]2 ion pair complex is evident. It has been realized that hydrogen bonding interactions should not necessarily be evidenced from the presence of bond critical point in the MED topography. Interestingly the kinetic energy density G(r) corresponding to Smin(equivalent bond critical point in AIM theory) offers certain advantages over the more conventional approach of analyzing electron density properties in the sense that hydrogen-bonding interactions not exhibiting the bond critical points can be envisaged. G(r) is directly proportional to the mobility of the electron density at bond critical point (BCP), which is further reflected in the pressure exerted by electron density accumulated at the BCP on other electrons. Apart from the binding energy perspective, a comprehensive understanding of the complete pattern of non-covalent interactions between the constituting ions and the related electron density distribution is clearly a fundamental prerequisite before attempting to characterize and rationally design such a material for a specific application. Moreover the G(r) was shown to correlate well with the frequencies of OH-stretching in substituted amino alcohols possessing intramolecular hydrogen bonding.59 Recently Lane and et al.60 demonstrated that G(r) can be used to estimate the strength of the intermolecular hydrogen bonds and further the linear correlation with the frequency down shift of OH-stretching was established. In the perspective of the present work the correlation of G(r) with the strength of interaction showing strong dependence on -NH stretching is evident and the plot of frequency shift (blue) of the corresponding stretching as a function of the kinetic energy density G(r) (cf. Figure 5) corroborate these inferences. Moreover the NCI-RDG index was explored for mapping the contributions from noncovalent interactions such as hydrogen 17



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Figure 5 Frequency shift (in cm-1) of NH-stretching vibration as function of kinetic energy density G(r) (in au).

bonding, van der Waals interactions and steric repulsions for the dication-anion binding accompanying these ion pair complexes (cf. Figure S7 and Figure S8 of supporting information). The noncovalent binding accompanying the ion pair complexes induces structural perturbations which reflect in their characteristic vibration frequency shifts compared to their constituents. Cation-anion binding can be probed using the normal coordinate analyses. Calculated vibrational spectra portraying the molar absorption coefficient (or, molar absorptivity in units of 0.1 m2 mol−1) versus the frequency (in cm−1) of the [C4qm][OAc]2 ion-pair complex in (a) 3600–2000 cm−1 and (b) 1800–400 cm−1 are displayed in Figure 6 comparing the ion pair (black) with the individual cation (blue) and anion (red). The following inferences are drawn. The region 3482 to 3400 cm−1 reveal N-H stretching vibrations from the terminal ammonium group. 18


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Figure 6. Computed vibrational frequencies in free [C4qm]2+, [Ac]− and [C4qm][Ac]2 ion pair complex in range (a) 3600–2000 cm−1 (b) 1800–400 cm−1.

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As shown in Figure 6(a) the N1-H vibrations are assigned to 3482 cm−1 which shows upshift of 67 cm−1 upon the binding with the acetate anion. Strongly coupled C1-H, C2-H and C3-H stretching vibrations arising from the alkyl chain show up in 3194 to 3040 cm−1 in the spectra. Figure 6(b) displays the anionic C=O (1814 cm−1 and 1665 cm−1) stretching vibrations. As shown the [C4qm][OAc]2 ion-pair complex the increased electron density in the antibonding natural orbital brings forth charge transfer to terminal ammonium group and the alkyl chain. Consequent the C=O bond weakening and hence, the frequency down shift of the corresponding vibration thus can be explained. A redistribution of the electron density consequent to hydrogen bonding further engenders the bond strengthening and the frequency up-(blue) shift for the non-interacting N-H bond can be rationalized. As displayed in Figure 6(a) the highly intense band at 2677 cm−1 and 2063 cm−1 are assigned to the N-H stretching associated with the proton transfer. Similar inferences are drawn for the rest of the ion pairs and their spectra are given in Figure S7 through Figure S9 of the supporting information. To simulate effect of solvent on the infrared spectra of the dication-anion complexes, in particular the characteristic carbonyl and NH stretching, the normal vibrations within regime of the SCRF-PCM model were obtained in the presence of DMSO. As shown in Table 3, the –NH3+ stretching engenders distinct bands assigned to hydrogen bonded and that arising from the nonbonded protons; the latter exhibiting the frequency blue (up) -shift compared to the free cation; the largest shift amounting to 125 cm-1 being predicted for the [C4pyr][OAc]2 complex. On the other hand, hydrogen bonded NH stretching reveals the shift in its frequency in the opposite direction. A red-shift of ~234 cm-1 was predicted for the [C4im][OAc]2 complex. On parallel lines the C=O stretching 1711 cm-1 facilitating transfer of proton to Ac‒ anion reveals the largest downshift (by 67 20


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cm-1) which consequently emerge with a band near 1665 cm-1 in the [C4qm][OAc]2 complex. Discernibly the ion pair complexes offer signature as frequency blue shift for the carbonyl (not involved in the proton transfer) stretching. Accordingly the [C1pyr][OAc]2 complex has been accompanied by a blue-shift of 104 cm-1. Selected vibrational frequencies in different ion pair complexes in gas phase are compared with the solvated ion pairs in Table 3. The inferences on the NH and carbonyl stretching derived from the SCRF-PCM model generally are in consonance with the gas phase structures. Underlying hydrogen bonding interactions accompanying PILs show their signature in the 1H NMR experiments. Thus 1H NMR is a valuable tool in the study of PILs due to its ability to investigate the properties of the transferred proton. The chemical shifts (δH) computed in hitherto PILs using DMSO as solvent within the framework of the M06-2x/6311++G(d,p) theory are reported in Table 4. The following inferences are drawn. The H1 proton not participating in hydrogen bonding of the terminal group reveals an upfield signal in 1H NMR spectra of ion pair complexes. The [C1pyr](OAc)2 complex in DMSO however, reveals shielded signals in 1H NMR. The rest of the ion pair complexes engender either a slight downfield signal as in [C4qm](OAc)2 and [C4im](OAc)2 or are unchanged as in case of [C1pyr][OAc]2. As opposed to this, the strongly hydrogen bonded H2 proton reveals a large downshift in the gas phase as well as in the presence of DMSO (solvent). The signals of propyl protons H4 to H7 or the H12 to H16 ring protons are nearly insensitive to the solvent. Moreover, the pyrrolidinium ion pair complexes on solvation show a marginal downfield in δH signals corresponding to the H10 and H11 protons.

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Table3. Selected Vibrational Frequencies (in cm-1) in Ion Pair Complexes Computed at M06-2X/6-311++g(d,p) Level of Theory. Assignments

[OAc]-

[C4qm]2+

[C4qm](OAc)2

[C1pyr]2+

[C1pyr](OAc)2

[C4im]2+

[C4im](OAc)2

[C4pyr]2+

[C4pyr](OAc)2

ν (NH)a

3482 (3514)

3549 (3540)

3475 (3521)

3548 (3549)

3474 (3526)

3565 (3554)

3484 (3513)

3609 (3596)

ν (NH)b

3474 (3503)

-

3461 (3507)

3412 (3406)

3469 (3513)

3235 (3321)

3476 (3505)

3525 (3520)

v(NH3)s

3400 (3427)

2062 (2427)

3388 (3431)

2732 (2168)

3392 (3437)

2549 (2144)

3402 (3427)

2977 (2403)

2677 (2653) ν (CO)

1711 (1615)

1665 (1623)

1815 (1748)

1792 (1755)

1804 (1762)

1644 (1612)

1663 (1627)

1672 (1617)

1658 (1620)

Note: a-noninteraction (NH), b-hydrogen bonded (NH), s-symmetric stretching of -NH3 group and values in parenthesis represents DMSO computed frequencies.

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Table 4. Comparison of 1H NMR Chemical Shifts (in ppm) of Ion Pair Complexes in Gas with DMSO (solvent) Optimized Structures.

[C4qm][OAc]2

[C1Pyr][OAc]2

[C4Pyr][OAc]2

[C4im][OAc]2

Gas

SCRF

Gas

SCRF

Gas

SCRF

Gas

SCRF

H1

2.5

2.9

1.9

0.9

1.2

1.4

0.7

1.2

H2

12.5

12.9

18.0

18.4

14.5

17.5

16.6

18.5

H3

17.0

14.8

5.3

5.1

1.7

1.7

6.9

6.1

H4

4.0

3.7

3.1

3.3

3.1

3.2

3.0

3.0

H5

2.7

2.6

2.4

2.3

2.4

2.7

2.9

2.9

H6

2.6

2.5

2.1

2.2

2.1

2.1

2.4

2.1

H7

1.8

1.7

1.6

1.8

1.6

1.8

2.5

2.3

H8

3.0

5.1

3.1

3.1

3.1

3.0

4.1

4.2

H9

3.0

2.8

6.4

3.1

6.4

4.9

5.4

5.1

H10

4.7

3.8

2.6

3.4

2.6

3.0

12.3

11.0

H11

4.5

3.9

5.1

4.7

5.1

5.4

7.6

7.6

H12

2.5

2.6

2.0

2.1

2.0

2.1

7.7

7.7

H13

2.8

2.8

2.6

2.5

2.6

2.1

5.0

4.7

H14

3.1

3.1

2.7

2.6

2.7

2.1

4.0

4.0

H15

2.4

2.5

2.2

2.1

2.2

2.2

2.4

2.6

H16

2.6

2.7

4.7

4.5

4.7

4.4

1.7

1.7

H17

5.1

3.8

3.3

3.3

3.3

3.5

-

-

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As may be realized the kinetic energy density component G(r) from the QTAIM analysis provides a measure of deshielding and in turn, can be correlated to δH value corresponding to the terminal -NH3+ protons in hitherto ionic liquids. A correlation showing G(r) as a function of the δH (in ppm) is transparent from Figure 7 which turns out to be linear with its correlation being 0.970.

Figure 7. Chemical shift (in ppm) of the terminal -NH3+ proton as a function of the Kinetic energy density G(r) (in au). As may further be gauged the cation-anion binding which brings forth a large perturbation in the electron density distribution should possibly also reflect in chemical shift parameter of the most reactive (exchangeable) proton. A shift of the exchangeable proton has information about how strongly proton has associated with the base. We thus calculate proton affinity (PA) of the ion pair complexes. The proton affinity (PA) defined to be the negative enthalpy for the reaction A − + H + → AH in the gas phase can be obtained −

+

from PA = E A + E H − E AH . Figure 8 displays the correlation of chemical shift δH value with the proton affinities which turns out to be linear. In other words, large proton affinity 24


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in case of quaternary ammonium based PILs imply strong cation-anion binding which reflects in its corresponding δH parameter.

Figure 8. Chemical shift (in ppm) of the terminal -NH3+ proton versus proton affinity (in kcal mol-1).

It was earlier inferred that the binding energies in the dication-anion complexes have large contributions from hydrogen bonding arising from the most reactive proton at the terminal -NH3+. It therefore would be reasonable to understand whether these binding energies can be correlated directly to the proton affinity of the PILs. The correlation is transparent from Figure 9. To summarize the PA and kinetic energy density G(r) parameters of the hitherto PILs should prove useful for design of proton conductive materials in fuel cells or other applications.

25



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Figure 9. Binding energy (in kcal mol-1) verses proton affinity (in kcal mol-1).

4. Conclusions A conformational search, electronic structure and the binding energies of the dication anion complexes accompanying the imidazolium-, ammonium- and pyrrolidiniumdication and acetate (OAc) anion based PILs have been carried out employing the M06-2x based density functional theory. The formation of dication-anion pair complex formation is accompanied by interplay of electrostatic, hydrogen bonding and van der Waals interactions where N-H∙∙∙O interactions prevail which render large binding to the quaternary ammonium complexes. A dication-anion complex brings forth charge transfer from anionic oxygen to the antibonding N1-H natural orbital and emerges with a strong band in 2977-2549 cm-1 region in its calculated vibrational spectra. QTAIM analyses in conjunction with the EDDM plots explain direction of frequency shift of the characteristic NH2 and C=O vibrations. The present theory further reveals that calculated binding energies or largely deshielded δH value of the terminal -NH3 + proton in the 1H NMR spectra correlate well with the kinetic energy density component G(r) obtained from the QTAIM theory and the proton affinity of PILs. 26


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Supporting Information Optimized structures, Geometrical parameters, NBO charges, Second order perturbation energies, Vibrational spectra, and Correlation plots in hitherto ion pairs. Author Information *(S.P.G.) E-mail: [email protected]. Fax: +91-20-225691728. Telephone: +91 020 25601225. Notes The authors declare no competing financial interest. Acknowledgments The authors acknowledge the financial support from the Board of Research in Nuclear Sciences, India for a research grant through the project (37(2)/14/11/2015-BRNS). Authors thank the Center for Development of Advanced Computing (CDAC), Pune for providing National Param Supercomputing Facility.

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228x109mm (96 x 96 DPI)


Unveiling Noncovalent Interactions in Imidazolium, Pyrrolidinium or

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