Probing Molecular Interactions in Functionalized Asymmetric

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Probing Molecular Interactions in Functionalized Asymmetric Quaternary Ammonium Based Dicationic Ionic Liquids Prakash L. Verma, Libero J. Bartolotti, and Shridhar P. Gejji J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b07337 • Publication Date (Web): 12 Sep 2016 Downloaded from http://pubs.acs.org on September 17, 2016

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The Journal of Physical Chemistry

.

Probing Molecular Interactions in Functionalized Quaternary Ammonium Based Dicationic Ionic Liquids

Asymmetric

Prakash L. Verma1, Libero J. Bartolotti2 and Shridhar P. Gejji1* 1Department

of Chemistry, Savitribai Phule Pune University, Pune 411 007, India

2Department

of Chemistry, East Carolina University, Greenville, North Carolina 27858-4353, USA

Abstract Electronic structure, binding energies and spectral characteristics of functionalized asymmetric dicationic ionic liquids (DILs) composed of quaternary ammonium cations substituted with the ethoxyethyl and allyl/3-phenylpropyl/methoxyethoxyethyl/pentyl functionalities

on

two

different

nitrogen

centers

of

the

dication

and

the

bis(trifluromethanesulfonyl)imide (Tf2N‾) anion have been derived employing the dispersion corrected density functional theory. DILs based on methoxyethoxyethyl substituted cation reveal stronger binding towards the Tf2N‾ anion. The measured glass transition temperatures are found to be strongly dependent on the cation-anion binding facilitated through non-covalent interactions with dominant contributions from the electrostatics and hydrogen bonding. The manifestations of these interactions to vibrational spectra, in particular to SO2 and CF3 stretchings in the complexes have been presented. It has been demonstrated that the frequency down (red)-shift of the SO2 stretching in these DILs with varying substituent follows the order: methoxyethoxyethyl (35 cm‒1) > allyl (23 cm‒1) > pentyl (20 cm‒1) > 3-phenylpropyl (5 cm‒1), which is 1

ACS Paragon Plus Environment


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consistent with the strength of cation-anion binding. On the other hand, the CF3 stretching of the anion exhibits the frequency shift in the opposite direction with the hierarchy being reversed to that of SO2 stretchings; the largest up-shift (blue shift) of 60 cm‒1 was predicted for the DILs composed of 3-phenlpropyl substituted dications. The direction of such frequency shift has been rationalized through the difference molecular electron density maps in conjunction with the electron density at the bond critical point in the quantum theory of atoms in molecules. The underlying cation-anion binding has been analyzed through charge distribution analysis characterized in terms of molecular electrostatic potential topography. Furthermore the observed decomposition temperatures of DILs are shown to correlate well with the maximum surface electrostatic potential (Vs,max) parameter in quantum theory of atoms in molecules.

2


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Introduction Dicationic ionic liquids (DILs) composed of two singly charged cations linked through a hydrocarbon linkage chains (spacer) and paired up with two singly charged anions have been of growing interest in the recent literature.1, 2 DILs possess high thermal stability3 and larger liquidus ranges4 which makes them suitable for applications as solvent in organic reactions,5 lubricants,6 gas chromatography,7 and electrolytes8 especially at high temperatures compared to the monocation based ILs. The works on DILs was initialized with earlier investigations by Armstrong and co-workers9 who measured thermal stabilities and shear viscosities of germinal DILs based on substituted imidazolium and pyrrolidinium cations. Synthesis and characterization of geminal imidazolium based ILs further demonstrated that DILs possess higher thermal stability.10 Shirota et al.11 compared liquid density, surface tension, and shear viscosity of DILs with those of monocationic based ILs. These authors have shown that the glass transition temperature, surface tension and shear viscosity in DILs was higher in comparison to those based on monocations. Graber et al.12 carried out measurements on physicochemical properties e.g. density, dynamic viscosity, and refractive index of DILs composed of functionalized amino groups of imidazolium ring and the bis(trifluoromethanesulfonyl)imide anion (Tf2N‾). Pursuance to this Pitawala and co-workers13 measured the ionic conductivities and thermal properties of DILs composed of [Tf2N]‾ anion combined with the cation containing two imidazolium rings, connected by pentane or decane hydrocarbon chain and different side groups. These authors concluded that the rigid aromatic side groups increase the glass transition temperature (Tg) significantly. The alkyl chain length on the cation, however, has no, or 3



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weak influence on the observed Tg values. To understand cation-anion binding the interionic vibrations, in particular, the Raman intensities of low frequency components in monocationic and dicationic ILs based on the imidazolium and [Tf2N]‾ counter-anion were analyzed through the femtosecond spectroscopy experiments.14 The oxygen-containing side chain on imidazolium moiety significantly influences thermal behavior and solvent properties of DILs bringing about concomitant improvement in water miscibility and lowering of melting point.15,

16

Besides it has been shown that

incorporation of ether substituent in functionalized ILs composed of quaternary ammonium groups further revealed lower viscosities and melting points for monocation based ILs.17−19 It has recently been realized that the DILs containing quaternary ammonium groups with asymmetric nitrogen centers exhibit interesting physicochemical properties. The asymmetrically quaternized dicationic ammonium based room temperature DILs containing bis(trifluromethanesulfonyl)imide and bromide anions showed large fluid range from 330 to 370 0C owing to their high thermal degradation and low glass transition temperature.20 The electronic spectra and solvation dynamics of ether, alcohol and alkylfunctionalized quaternary ammonium dications combined with [Tf2N]‾ anions have been reported.21 The DILs composed of same (symmetric) and different (asymmetric) substituents attached to quaternized nitrogen centers of dications have emerged with a new class of DILs. In particular, asymmetric22-24 DILs have stimulated growing interest owing to their large thermal stabilities, low viscosities and low melting points compared to monocationic or symmetric DILs.25, 26

4


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The attempts to understand the physicochemical behavior of DILs at the molecular level till to date are, however limited to geminal dicationic imidazolium based ILs. On the theoretical front, molecular dynamics (MD) simulations have been carried out on these DILs which have lead to insights for their structure and transport properties over a wide range of thermodynamic state. Bodo et al.27 concluded that the trans conformer of [Tf2N]‾ is favored in DILs composed of [Tf2N]‾ and germinal dication and the anionic structure is not dependent on the cation or the length of ‘linker’ (spacer). Vitorino et al.28 further showed that gas phase molecules in imidazolium based DILs possess relatively low vaporization enthalpies compared to monocationic ILs, which partly can be attributed to electrostatic interactions in DILs. These inferences have been corroborated by MD simulations. The investigation of dynamic properties of DILs composed of imidaolium cation and [Tf2N]‾ anions by Shirotal and coworkers29 underlined the role of cation-anion or anion-anion interactions. The liquid structure, self diffusion dependence on the alkyl chain length with varying anions have widely been studied in the literature.30 Electronic structure, frontier orbital and 1H NMR spectra of [mim]C3[mim]-2Br‒ have also been characterized through the density functional theory.31 Bodo and Caminiti32 derived the charge distributions and infrared spectra of geminal imidazolium cation and Tf2N‾ based DILs. To assess cation-anion binding in DILs density functional based investigations benchmarking the use of a variety of exchange correlation functional have been carried out. Dispersion corrected DFT incorporating exchange−correlation energy dependence on local spin density, spin density gradient and spin kinetic energy density, through the use of meta-GGA functionals33,

34

(M05, M05-2x, or M06 or variants thereof) has accomplished

remarkable success in predicting reliable structures of the complexes providing molecular 5



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insights for cation-anion interaction and hence, are explored for modeling of physicochemical properties.35-40 In the present endeavor we analyze ion-ion interactions accompanying DILs composed of quaternary ammonium dications viz., N1-(2-Ethoxyethyl)-N3-(2-(2methoxyethoxy)ethyl)-N1,N1,N3,N3-tetramethyl-propane-1,3 diaminium (DRR1), N1-AllylN3-(2-ethoxyethyl)-N1,N1,N3,N3-tetramethylpropane-1,3-diaminium (DRR2) and N1-(2Ethoxyethyl)-N1,N1,N3,N3-tetramethyl-N3 pentylpropane-1,3-diaminium (DRR3), N1-(2Ethoxyethyl)

N1,N1,N3,N3-tetramethyl-N3-(3-phenylpropyl)propane-1,3-diaminium

(DRR4) with the bis-(trifluoromethanesulfonyl)imide (Tf2N‾) anion which are shown in Figure 1. The present work focuses on how the hydrogen bonding network affects the energies and chemical shifts in 1H NMR spectra of these DILs wherein the [Tf2N]‾ via its SNS hinge is shared by dications which renders different conformational structures41-44 for the complexes which marginally differ in energies (up to 2 kJ mol–1) which governs their concentrations in liquid or solid phase and thereby influence physicochemical properties significantly. Further modification of the dication functionality by suitable substitution renders them with “tunable’’ physicochemical properties. In other words the cation–anion binding in the DIL complexes should provide valuable insights for binding energies, glass transition temperatures, viscosity. It should be discernible here that the complexes may as well be characterized in terms of their electronic structure and vibrational spectra derived from the dispersion corrected DFT. The “frequency shifts” of the characteristic vibrations in DILs based on the quaternary ammonium based dications and [Tf2N]‾ anion have further been rationalized through the quantum theory of atoms in molecules (QTAIM).45 Lastly 6


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how the substitution at the quaternary ammonium nitrogen affects the electronic structure, charge distribution and vibrational characteristics of the modified quaternary ammonium dication complexes has been systematically analyzed in the present work. The computational method is outlined below.

7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 [DRR1]2+ [DRR2]2+ 16 17 18 19 20 21 22 23 24 25 26 27 28 29 [DRR3]2+ [DRR4]2+ 30 31 32 33 34 35 36 37 38 39 [Tf2N]– 40 Figure 1. Structures of [DRR1]2+, [DRR2]2+, [DRR3]2+, [DRR4]2+ cations and [Tf2N]‾ anion along with atomic labeling scheme. 41 42 43 44 45 46 ACS Paragon Plus Environment 47 48

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2. Computational Method The atomic numbering scheme for the modified quaternary ammonium based dications and [Tf2N]‾ anion is shown in Figure 1. Structures of individual dications were generated by varying mutual orientation of two quaternary ammonium groups by rotations of the side chain on the quaternary nitrogen atom center. The lowest energy conformers of dications obtained at M05-2x are shown in Figure 2(a). Different [DRR1](Tf2N)2, [DRR2](Tf2N)2, [DRR3](Tf2N)2 and [DRR4](Tf2N)2 conformers

were derived from the

molecular electrostatic potential (MESP) topography of the anion. Topological features of MESP have been summarized in terms of critical points (CPs).46 CPs here are classified as the maxima, minima and the saddle points. The CPs of a molecular system are unique and hence, their existence and nature offer a signature of its structure.47-49 The minima or saddles in the MESP of the transoid and cisoids structures of the [Tf2N]‾ anion, with the transoid structure being merely 0.96 kcal mol–1 lower in energy, are shown in Figure 2(b). MESP mapped on the molecular electron density (MED) isosurface (0.002 au), which has been characterized from the characteristic Vs,max parameters. As may readily be noticed electron-rich regions in the [Tf2N]‾ are observed near two SO2 groups and midway between SNS 'hinge'. Accordingly complex conformers with quaternary ammonium dication coordinating in bidentate fashion with (a) the oxygen and nitrogen atom of the anion or (b) two oxygens of the same SO2 group (c) one oxygen from each of the SO2 groups and lastly (d) one oxygen from SO2 groups and one of the fluorines at the CF3 end of each of [Tf2N]‾ anion conformer were considered. These initial complexes were subjected to optimizations at the B3LYP level of theory using the Pople 6-31G(d,p) basis set employing the Gaussian 9



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09 program.50 Optimized structures of these are shown in Figure S1-S4 of the supporting information along with their relative stabilization energies (in kJmol‒1) given in parentheses. To account for the non-covalent interactions underlying the DILs, we employ the hybrid meta-GGA (generalized gradient approximation) M05-2x exchange correlation functional within the density functional theory. The M05-2x functional which accounts well for dispersive interactions and has proven useful for deriving reliable structures and binding in non-covalently bonded complexes.35, 36 To understand energetics accompanying the DILs we considered different conformers of the dication complexes. The conformers, up to 5 kJ mol–1 higher in energy relative to its lowest energy structure, exhibiting qualitatively different binding patterns, were subsequently optimized within the M05-2x regime of theory using the augmented basis set with the diffuse functions (aug-cc-pVDZ) added on all the atoms. Characteristics Vs, max parameters for different dication complexes have been computed. The vibrational frequencies of the dication–(Tf2N)2 complexes and those of individual ions were derived from the hessian matrix (whose elements refer to the second order partial derivatives) within the M05-2x/aug-cc-pVDZ framework of theory. It has been demonstrated earlier that density functional calculations with the M05-2x functional coupled with augmented cc-pVDZ basis set engender the accurate structures and energetics for ion pairs/complexes in ionic liquid systems.51, 52 Stationary point structures were confirmed to be local minima on the multivariate potential energy surface through the vibrational frequency calculations at the same level of theory, since all the normal vibration frequencies turned out to be real. Potential energy distributions (PED) were computed and the normal vibrations were visualized through the displacement of atoms around their equilibrium (mean) positions through Gaussview-5 program.53 The infrared 10


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spectral plots are derived employing the Gnuplot software54 based on csplines smoothing fitting algorithm.55 Binding energies were calculated by subtracting the sum of zero point corrected SCF energies of individual ions/molecules from those of the individual complexes. The thermodynamic parameters accompanying DILs were also obtained. The bond critical points (BCP) in the MED topography for different hydrogen bonds were located. To rationalize the direction of shifts in frequencies of the characteristic normal vibrations accompanying the dication-(Tf2N)2 complexes relative to the free ions, we compute the difference electron density (∆ρ) by subtracting the sum of electron densities of the dication and anion from the electron density of complex. Positive and negative valued contours of the electron density were visualized through the program Univis56 which allowed tracking a single value within the entire range for a given data set. One may as well employ multiple of values for contours to contrast more than one subset of data. The technique however, can be used with constrained number of such values since the visuals become too crowded to infer any valuable comparison when a range of these values need to be compared. For those cases color-mapping visualization provides a continuous subset of values, which can be shown with a set of colored dots. These dots are plotted wherever a value within the data set is found to fall in the given range. Such points are further rendered with appropriate shades of color according to a domain specific (pertaining to the origin of the data set being visualized like molecular scalar field) colormapping function that brings out spectral features differentiating a range of values for a given specified data.

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[DRR1]2+

[DRR1]2+

[DRR2]2+

[DRR2]2+

[DRR3]2+

[DRR3]2+

[DRR4]2+

[DRR4]2+

Figure 2(a). Optimized structures of isolated cations, [DRR1]2+, [DRR2]2+, [DRR3]2+, [DRR4]2+ and [Tf2N]‾ anion shown with atomic labeling scheme. MESP mapped iso-density (0.002 a.u.) in dications are shown along with.

12


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Transoid-[Tf2N]‾ (0.0)

Cisoid-[Tf2N]‾ (0.96)

Figure 2(b). MESP mapped iso-density surface (0.002 a.u.) in [Tf2N]‾ anion conformers. The arrow points to the deepest MESP minima (in kcal mol–1).

13



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To delve into molecular interactions and their manifestations in the infrared spectral characteristics quantum theory of atoms in molecules (QTAIM)57 approach was used employing the AIMAll software.58 The δH parameters in 1H NMR spectra were calculated by subtracting the nuclear magnetic shielding tensors of protons in the cation, anion and complexes from those in tetramethylsilane (TMS, the reference) using the gaugeindependent atomic orbital (GIAO) method.59,

60

The ‘charge transfer’ accompanying the

complex formation can be envisaged through the natural bond orbital (NBO) analyses.61 Molecular insights for non-covalent interactions were derived using the reduced gradient density (RGD) plots which were visualized through the visual molecular dynamics program.62

3 Results and discussion 3.1 Structure and binding energies It was pointed out in the preceding section that MESP mapped on the isodensity surface brings about effective electron-rich regions in the [Tf2N]‾ anion and its topography emerge with the sites for cation binding. The critical points in the MESP minima were explored to generate the initial structures of [DRR1][Tf2N]2,

[DRR2][Tf2N]2, [DRR3][Tf2N]2,

and

[DRR4][Tf2N]2 conformers. Lowest energy structures obtained from the M05-2x/aug-ccpVDZ level of theory revealed that each of [Tf2N]‾ anion in the complexes are located midway between quaternary ammonium nitrogen centers of dications with the alkyl chain of the cation facilitating attractive interactions. These conformers are rendered with more hydrogen bonding between methyl protons of quaternary nitrogen and those from the propyl linkage of the dication and [Tf2N]‾ anions (cf.

Table S1(a) of supporting 14


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information). On the other hand, the interactions between [Tf2N]‾ anions and ethoxyethyl substituent of dication engender relatively high energy structures for the complex; destabilization up to 10 kcal mol–1 was noticed for the [DRR4](Tf2N)2. As shown in Figure 3 the lowest energy conformers of quaternary ammonium based DILs reveal qualitatively similar cation-anion binding features. Selected structural parameters in [DRR1](Tf2N)2 complex are compared with those of the free dication in Table S1(b) and Table S1(c) of supporting information. As may be discerned the SO* (O* refers to oxygens participating in hydrogen bonding interactions) bond distances are longer relative to those in gas-phase structure of the [Tf2N]‾ anion. Structural parameters in the ethoxyethyl substituent of dications on complexation are nearly unchanged. As shown S=O* bond distances turn out to be ~1.481 Å in [DRR1](Tf2N)2 relative to 1.477 Å in the [Tf2N]‾ anion. As opposed this, the noninteracting SO bonds in these complexes are contracted (1.472 Å) relative to those in the transoid [Tf2N]‾ conformer. C-F bonds too are shortened (~1.333 Å) consequent to the cation binding. Overall the bond angles are nearly unchanged compared to those in the free anion; except that for the 'S1NS2 hinge’ which shows a deviation up to 5°. Furthermore DILs formation brings about a large conformational change through the O3S2NS1 dihedral angle. Similar inferences are drawn for the rest of the complexes from the data summarized in Table S1(c) of supporting information. The cation-anion binding energies (in kcal mol–1) with the varying substituent follow the order: [DRR1] > [DRR2] > [DRR3] > [DRR4], as reported in Table 1. Thus the [DRR1][Tf2N]2 complex bring about electrostatic attractions between methoxyethoxyethyl functionality and SO3 end of the anion. Thus the orientation of methoxyethoxyethyl group relative to two [Tf2N]‾ anions maximize the electrostatic interactions. 15



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[DRR1](Tf2N)2

Page 16 of 50

[DRR2](Tf2N)2

[DRR3](Tf2N)2

[DRR4](Tf2N)2

Figure 3. Optimized structure of dication-(Tf2N)2 complexes at the M05-2x/aug-cc-pVDZ level of theory. The broken line shows Hydrogen bonding interactions.

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Table 1. Zero-point Corrected Binding Energies (∆E) and change in free energies (∆G), enthalpy (∆H) and Vs,max parameters in kcal mol–1 accompanying in the dication-(Tf2N)2 complexes. [DRR1](Tf2N)2

[DRR2](Tf2N)2

[DRR3](Tf2N)2

[DRR4](Tf2N)2

∆E

241.0

236.4

234.2

233.0

∆G

–210.3

–209.9

–205.2

–203.3

∆H

–237.1

–232.9

–230.3

–229.1

Vs,max

40.0

43.9

36.6

34.7

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3.2 Normal Vibrational Analyses Intermolecular hydrogen bonding between donor-acceptor or dication-(Tf2N)2 complexes induce structural changes and show their signature in vibrational frequency shifts in their vibrational spectra compared to their constituents ions. 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 dication-(Tf2N)2 complex systems in regions namely, (a) 3400-3000 cm−1 and (b) 1600-800 cm−1 are depicted in Figure S5 through S7 of the supporting information. The calculated spectra in the region 3400-3000 cm−1 for dication [DRR1](Tf2N)2 complex has been depicted (in black) in Figure 4(a) along with those of individual dication, [DRR1]2+(red) and anion, [Tf2N]‾ (blue), as a proto type example. It may as well be discernible that the formation of [DRR1](Tf2N)2 complex has been accompanied by the wavenumber upshift (blue shift) in the vibrational frequency (in cm–1) of CH2 stretching of propyl linkage compared to the isolated dication (cf. Table 2). As displayed in Figure 4(b) the region 1600-800 cm−1 of Infrared spectra reveals S=O, S-N and C-F stretching vibrations of the [Tf2N]‾ anion. The SO2 stretching was assigned to 1310 cm−1 and 1281 cm−1 bands. Further the underlying dication-(Tf2N)‾ interactions engenders an increase of electron density in the localized antibonding orbital of the anion, as a result of charge transfer to the methylene protons of the propyl linkage of dication, leading to the bond weakening (cf. Table S2 of supporting information). Thus the elongation of the corresponding bond can be observed. The dication [DRR1](Tf2N)2 complex further engenders a downshift (red shift) of 35 cm−1 for the SO2 stretching frequency compared to the isolated [Tf2N]‾ anion. On the other hand, the electron density redistribution cause a transfer of a larger portion of electron density to noninteracting bonds of the [Tf2N]‾ anion 18


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in the complex, accompanied by increase in its bond strength which concomitantly brings about the CF bond contraction. The frequency up-shift (blue shift) of 9 cm−1 for CF3 stretching was noticed compared to the isolated [Tf2N]‾ anion. Similar inferences are drawn for the rest of the dication complexes. The frequency down-shift (red shift) of SO2 stretching in DILs considered here follows the order: methoxyethoxyethyl (35 cm‒1) > allyl (23 cm‒1) > pentyl (20 cm‒1) > 3-phenylpropyl (5 cm‒1) which is in accordance with the cation-anion binding strengths. Remarkably enough, the CF3 stretching of the anion, the blue shifted vibration, exhibits hierarchy which is opposite to that predicted for the SO2 stretchings. The largest up-shift (blue shift) of 60 cm‒1 was predicted for the DILs composed of 3-phenylpropyl substituted dication. To delve further into molecular interactions accompanying the DILs, MED and QTAIM analyses have been carried out which are outlined below.

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Figure 4. Calculated IR frequencies for the cation, [DRR1]2+, anion [Tf2N]‾ and [DRR1](Tf2N)2 complex in regions (a) 3400−3000 cm−1 (b) 1600−800 cm−1. 20


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Table 2. Comparison of Selected Vibrational Frequencies (ν = Stretching, in cm‒1 ) in the Dication-(Tf2N)2 Complexes.

Cation Assignments

[DRR1]2+

[DRR1](Tf2N)2

[DRR2] 2+

[DRR2](Tf2N)2

[DRR3] 2+

[DRR3](Tf2N)2

[DRR4] 2+

[DRR4](Tf2N)2

ν C4-H

3258, 3228 3131 3246, 3238 3138 3245, 3233 3135 3236, 3331 3134 3180, 3126 3150 3152 3214, 3152 3204, 3142

3266, 3244 3144 3246, 3240 3139 3238, 3227 3125 3247, 3224 3132 3223, 3170 3204 3237, 3156 3215, 3142 3202, 3137

3244, 3236 3137 3259, 3239 3132 3247, 3233 3142 3240, 3228 3132 3181, 3126 3215, 3150 3192, 3130 3202, 3138 3204, 3143

3249, 3238 3141 3263, 3234 3138 3256, 3229 3137 3236, 3218 3123 3251, 3174 3224, 3161 3237, 3142 3193, 3132 3202, 3139

3239, 3212 3118 3242, 3235 3136 3225, 3213 3115 3243, 3223 3224 3167, 3112 3195, 3127 3179, 3109 3186, 3121 3199, 3138

3267, 3250 3150 3241, 3233 3135 3244, 3241 3138 3245, 3243 3141 3202, 3144 3220, 3156 3236, 3168 3203, 3132 3199, 3137

3223, 3214 3118 3243, 3234 3130 3225, 3202 3117 3234, 3218 3126 3162, 3108 3194, 3134 3175, 3111 3183, 3114 3198, 3135

3249, 3237 3139 3263, 3232 3136 3251, 3226 3167 3237, 3223 3123 3253, 3176 3218, 3161 3239, 3142 3192, 3127 3202, 3140

ν C5-H ν C6-H ν C7-H ν C1-H ν C2-H ν C3-H ν C8-H ν C15-H

Anion [Tf2N]‾

[DRR1](Tf2N)2

[DRR2](Tf2N)2

[DRR3](Tf2N)2

[DRR4](Tf2N)2

ν S-O ν C-F ν S-N

1310, 1281 1250, 1219 1115, 1028

1275 1256, 1228 1105, 1104, 1023

1287, 1284 1280, 1261 1107, 1087, 1018

1290, 1225 1281, 1268 1114, 1100, 1032

1305, 1228, 1225, 1256 1283, 1279 1108, 1096, 1086, 1081, 1018

ν S’-O’ ν C’-F’ ν S’-N’

1310, 1281 1250, 1219 1115, 1028

1291, 1030 1255 1104, 1111, 1030

1294, 1293 1271, 1260 1110, 1101, 1027

1289 1271, 1260, 1257, 1230 1109, 1090, 1023

1272, 1238 1259, 1257, 1247,1241 1112, 1102, 1028

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3.3. Difference MED-, QTAIM- analysis and charge distributions in complexes In the following, we discuss how difference MED63, 64 can be utilized to explain the direction of frequency shift of the characteristic vibration in the calculated infrared spectra that stem from the redistribution of electron density consequent to dication−anion interactions within the DILs. Difference electron density, Δρ, was calculated by subtracting the sum of electron densities of the individual anion and dication from the corresponding complex. The Δρ contours for [DRR1](Tf2N)2 complex in the range of (±0.001 to ±0.0009 au) are displayed in Figure 5 wherein the blue lines represent positive valued contours and the negative valued contours appear in red. Zero valued contours are depicted as green. To gain deeper insights for molecular electron density redistribution accompanying the complexation, the difference MED contours in the plane passing through the methylene protons of propyl linkage and the –SO functionality of [Tf2N]‾ anion participating in CH---O* interactions (O* refer to anionic oxygens) have been generated. As is evident the difference electron density maps for the [DRR1](Tf2N)2 complex reveal BCPs for the hydrogen bonds in such interactions fall in the region (red) where the electron density is depleted which explains the frequency downshift of the SO* stretching vibrations in the complex compared to the isolated [Tf2N]‾. Interestingly the diminutive densities in these difference MED plots around the SO* region was found to be largest in [DRR1](Tf2N)2 complex; which brings about largest downshift (35 cm‒1) of the S=O stretching vibration. The direction of SO* frequency shifts in the rest of the complexes has further been rationalized on parallel lines. Difference MED plots in the rest of the complexes are shown in Figures 8S through 10S of the supporting information.

22


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Figure 5. Difference MED maps in the [DRR1](Tf2N)2 complex in the plane passing through interacting C−H and S-O bonds. Contours in the range of ±0.001 to ±0.009 a.u. are shown along with BCPs in the MED topography. 23



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To understand bonding properties of DILs the QTAIM analysis was carried out.65-67 An excellent review68 describing how density fundamentally relates to the properties of the molecular systems demonstrating the correlation between molecular descriptors obtained from such a scalar field to a variety of properties for example NMR chemical shifts, electronic transition energies, pKa’s etc. has appeared in the literature. Taking a cue from this in the present work the bond critical points ( ρ BCP ) along with electrostatic potential map in the lowest energy complex of the dication–(Tf2N)2 complexes thus obtained are

depicted in Figure S11 through S14 of the supporting information. The corresponding QTAIM parameters ρ BCP and ∇ 2 ρ BCP values for the dication–(Tf2N)2 are reported in the Table 3. As may readily be noticed ρBCP parameters fall in the range 0.002−0.035 au whereas the corresponding ∇ 2 ρ BCP parameters are in between 0.024−0.139 au. The presence of hydrogen bonding interactions in cation-anion complex thus have been reaffirmed from the QTAIM theory.69 Interestingly the local electron energy density HBCP parameter70-72 herein serves as good index to gauge the weak nonbonded interactions and energetic properties of BCPs can further be characterized from the relation: HBCP = GBCP + VBCP, where respective local kinetic energy (GBCP) and local potential energy density (VBCP) terms counterbalance each other. For the [DRR1](Tf2N)2 complex the −GBCP/VBCP ratio turns out to be in between 1.0 to 1.2 au which suggests that the complex is stabilized by weak hydrogen bonding and van der Waals interactions as well. Since both HBCP and ∇ 2 ρ BCP are > 0 the dominance of the electrostatic contributions in complexes are transparent. Thus above inferences on the [DRR1](Tf2N)2 complex can further be extended to the rest of DILs.

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It may further be concluded that bonding features of DILs are qualitatively different from the monocation based ionic liquids.

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Table 3. AIM Parameters (in atomic units) for the dication–(Tf2N)2 complexes.

ρ BCP

∇ 2 ρ BCP

H1 – O1 H3 – O4 H5 – N H5 – O2 H7-O1 H13-O4 H21 – O1 H2 – O2’ H4 – O4’ H6 – O1’ H16-O3’

0.0112 0.0140 0.0066 0.0098 0.0129 0.0116 0.0073 0.0097 0.0123 0.0135 0.0124

0.0331 0.0409 0.0207 0.0299 0.0391 0.0338 0.0247 0.0293 0.0354 0.0435 0.0367

GBCP [DRR1](Tf2N)2 0.0080 0.0101 0.0045 0.0071 0.0095 0.0084 0.0055 0.0069 0.0087 0.0102 0.0090

H1 – O4 H1 – O3 H3 – O1 H5 – O4 H5 – O1 H21– O3 H2 – O3’ H4 – O2’ H6 – O4’ H10-O3’

0.0104 0.0117 0.0111 0.0116 0.0107 0.0075 0.0141 0.0130 0.0103 0.0111

0.0325 0.0345 0.0430 0.0364 0.0364 0.0253 0.0448 0.0375 0.0298 0.0359

H5 – O1 H3 – O3 H1 – O4 H21 – O4 H2– O2’ H4 – O4’ H6 – O1’

0.0120 0.0112 0.0131 0.0065 0.0104 0.0143 0.0138

H2 – O4 H3 – O1 H1 – O4 H1 – O3 H13 – O1 H21 – O3 H2 – O3’ H4 – O2’ H6 – O4’ H12– O3’

0.0104 0.0111 0.0112 0.0108 0.0118 0.0076 0.0146 0.0132 0.0102 0.0108

VBCP

HBCP

–GBCP/VBCP

–0.0077 –0.0100 –0.0039 –0.0067 –0.0092 –0.0083 –0.0048 –0.0065 –0.0086 –0.0095 –0.0088

0.0003 0.0001 0.0006 0.0004 0.0003 0.0001 0.0007 0.0004 0.0001 0.0007 0.0002

1.04 1.01 1.15 1.06 1.03 1.01 1.15 1.06 1.01 1.07 1.02

[DRR2](Tf2N)2 0.0077 0.0085 0.0095 0.0087 0.0084 0.0056 0.0107 0.0094 0.0073 0.0084

–0.0073 –0.0084 –0.0082 –0.0082 –0.0076 –0.0049 –0.0102 –0.0093 –0.0071 – 0.0078

0.0004 0.0001 0.0013 0.0004 0.0008 0.0007 0.0005 0.0001 0.0002 0.0006

1.06 1.01 1.16 1.06 1.11 1.14 1.05 1.01 1.03 1.08

0.0362 0.0404 0.0376 0.0229 0.0296 0.0421 0.0472

[DRR3](Tf2N)2 0.0088 0.0090 0.0095 0.0050 0.0073 0.0103 0.0110

–0.0085 –0.0080 –0.0094 –0.0043 –0.0072 –0.0101 –0.0102

0.0003 0.0010 0.0001 0.0007 0.0001 0.0002 0.0008

1.04 1.13 1.01 1.16 1.01 1.02 1.08

0.0327 0.0430 0.0344 0.0324 0.0364 0.0256 0.0473 0.0379 0.0292 0.0350

[DRR4](Tf2N)2 0.0077 0.0095 0.0083 0.0079 0.0088 0.0057 0.0112 0.0095 0.0072 0.0081

–0.0072 –0.0083 –0.0079 –0.0076 –0.0084 –0.0050 –0.0107 –0.0094 –0.0070 –0.0075

0.0005 0.0012 0.0003 0.0002 0.0004 0.0007 0.0007 0.0001 0.0002 0.0006

1.07 1.15 1.05 1.04 1.05 1.14 1.05 1.01 1.02 1.08

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3.4 Reduced Density Gradient To derive molecular insights accompanying the DILs we incorporate the non-covalent interactions reduced density gradient (NCIs-RDG) based on the bonding topologies discussed in the preceding subsection. Extended network of hydrogen bonding and other dispersive interactions in quaternary ammonium based DILs are quantified through the NCI analyses. The NCI is characterized in terms of the electron density (ρ), its Laplacian ( ∇ 2 ρ ), and the reduced gradient of density (s). The ∇ 2 ρ is decomposed into three

eigenvalues λ1 + λ2 + λ3 (with λ1 ≤ λ2 ≤ λ3) is located in its Hessian with the second component, λ2, shading light on the redistribution of electron density consequent to noncovalent interactions distinguishing the bonding (λ2 < 0) features from the nonbonding (λ2 > 0). Underlying molecular interactions are analyzed in terms of attractive, moderately strong hydrogen bonds, repulsive, steric, and weak dispersion-type of components. The reduced density gradient and electron density are generated by using the MULTIWFN program.73 The elegance of the NCI lies in elucidating the interactions in real-space and the graphical representation from which the distinct non-covalent interactions can be inferred. The position, strength, and type of an interaction shown as RDG isosurface are represented on a blue−green−red (BGR) scale according to the values and sign of (λ2)ρ ranging from −0.05 to +0.05 au. The RDG isosurfaces in different dication-(Tf2N)2 complexes are displayed in Figure 6 A (i-iv). The regions with positive (λ2)ρ area are portrayed as red which indicates strong repulsive nonbonded overlap. Here the attractive interactions are displayed in blue whereas green regions refer to very weak interactions. As shown in

27



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Figure 6 A(i) the [DRR1](Tf2N)2 complex the green isosurfaces observed midway between dication, [DRR1]2+ and [Tf2N]‾ anion which imply the van der Waals interactions.

A(i)

B(i)

[DRR1](Tf2N)2

A(ii)

[DRR1](Tf2N)2

B(ii)

[DRR2](Tf2N)2

[DRR2](Tf2N)2

Continued… 28


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A(iii)

B(iii)

[DRR3](Tf2N)2

A(iv)

[DRR3](Tf2N)2

B(iv)

[DRR4](Tf2N)2

[DRR4](Tf2N)2

Figure 6. A(i-iv) Color-filled RDG plots showing non-covalent interactions (NCI) regions in different dication complexes. The green and red isosurfaces refer to weak hydrogen bonding and steric contributions. B(i-iv) NCI index plot for complexes: The plot of function 1 (sign(λ2) ρ values) on the X-axis vs. function 2, the reduced density gradient on (RDG) on the Y-axis.

29



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The cation-anion binding accompanying DILs stem from the hydrogen bonding and van der Waals interactions which in turn, have contributions from electrostatic, dispersive and induction terms. For DILs with a large number of dication and anions in the liquid state, the electrostatic interactions become significant.74 The electrostatic interactions reflect in the increase of “spattered” regions between the dication, [DRR1]2+ and [Tf2N]‾ anions as green isosurfaces. As an alternate perspective to real-space plot, Yang et al.75 proposed a method to visualize the interactions types by plotting of reduced gradient of density as a function of electron density multiplied by the sign of λ2 as shown in Figure 6 B(i-iv). Resulting NCI scattered plot of [DRR1](Tf2N)2 has been given in Figure 6 B(i) reveal multiple disperse spikes near low electron density region (near – 0.02 au) between the atomic basins in DILs, which refer to non-covalent bonding sites. For the [DRR1](Tf2N)2 complex the spikes are deeper and finally reach the minimum which imply interaction. Interestingly disperse spikes appears on the opposite side of the scale near +0.02 au. Moreover the inter ring steric repulsions are shown as red RDG isosurface in the region between the dication and anion which are balanced by attractive electrostatic attractions. The attractive as well as repulsive interactions between the [DRR1]2+ dication moiety and two [Tf2N]‾ anions are evident from the region -0.005 to + 0.005. Similar inferences are drawn for rest of the complexes. The sensitive dependence of many body effects on cation-anion binding and nature of accompanying molecular interactions are corroborated further from the histogram shown in Figure S15 of the supporting information.

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3.5 1H and 13C NMR studies Molecular insights for underlying non-covalent interactions accompanying DILs can be derived from the 1H and 13C NMR experiments. In Table 4 the (δH or δC) chemical shifts of the dicationic complexes in acetone are compared with those measured in the experiment.20 The relatively strong hydrogen bonding interactions between methylene protons (H5 and H6) and [Tf2N]‾ anions reflect in deshielded signals at the 3.69 ppm compared to those at 2.90 ppm assigned to H3, H4 protons of the propyl linkage in methoxyethoxyethyl functionalied DILs which can also be deciphered from the experiment. It was conjectured earlier that the hydrogen bonding facilitated by H5, H6 protons is crucial for cation-anion binding in DILs. It therefore, would be intriguing to understand how such interactions reflect in the NMR spectra that capture subtle energy changes accompanying the formation of DILs. With this view the calculated binding energies were plotted as a function of the δH values (in ppm, mean of H3 and H4) in Figure 8. The plot reveals an excellent linear correlation (r2 = 0.967).

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Table 4. 13C NMR and 1H NMR Chemical Shifts (in ppm)in dication–(Tf2N)2 Complexes from the SCRF-PCM Theory.

[DRR1](Tf2N)2

[DRR2](Tf2N)2

[DRR3](Tf2N)2

[DRR4](Tf2N)2

M05-2x

Expt.

M05-2x

Expt.

M05-2x

Expt.

M05-2x

Expt.

C4, 5

54.86

52.96

51.10

51.24

54.11

52.77

52.46

51.89

C6, 7

52.66

52.73

54.65

52.73

49.90

51.63

54.39

52.83

H7, 8, 9, 10, 11, 12

5.01

3.42

3.31

3.32

3.26

3.42

3.11

3.44

H13, 14, 15, 16, 17, 18

5.05

3.42

3.31

3.41

3.04

3.36

3.26

3.39

H1, 2

3.24

3.72

3.42

3.62 - 3.65

3.06

3.66 - 3.69

3.50

3.70 - 3.73

H3, 4

2.90

2.75 - 2.77

2.38

2.71 - 2.78

2.23

2.71 - 2.78

1.91

2.71 - 2.78

H5, 6

3.69

3.76

3.56

3.74 - 3.76

3.11

3.56 - 3.60

3.02

3.76 - 3.79

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Figure 8. Binding Energies (kcal mol–1) as a function of δH (mean of H3 and H4) in ppm.

3.6 Structure Property Relationships with Viscosity, Decomposition and Glass transition Temperature It has earlier76-78 been realized that theoretical investigations on cation-anion interaction energies in the imidazolium cation based ILs can be correlated to different physicochemical properties such as the melting point, viscosity, vapour pressure etc. The decomposition temperature (Td) refers to temperature at which a system undergoes either chemical or physical change of state and provides an upper bound for the operating range. Usually higher Td implies wider potential operating range for the DILs. Recently Gejji et al.40 have established the correlation between cation-anion binding energy and Td in the ILs composed of amino acid cation/their ester derivative and nitrate anion. It therefore, would be interesting to understand how hydrogen bonding interactions affect Td values of the 33



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DILs. It was pointed out earlier that the MESP topography can be characterized in terms of the Vs,max parameters. A plot of measured Td values as a function of Vs,max parameter yield a straight line (correlation 0.998), as shown in Figure 9.

Figure 9. Decomposition temperature, Td (in ⁰C) as a function of the maximum surface electrostatic potential, Vs,max (in kcal mol–1) for the complex.

Besides Td, the glass transition temperature (Tg) is yet another important property of ILs79, which reveal dependence on the cation-anion binding accompanying the DILs which are facilitated through S=O∙∙∙Hα (where Hα -methylene protons from dication moiety) interactions. The shortening of the corresponding S=O covalent counterpart was earlier noticed. It therefore would be interesting to assess whether such cation-anion binding within dication-(Tf2N)2 can be correlated to the measured Tg parameters. A graph showing Tg versus binding energies in DILs is depicted in Figure S16 of the supporting information. 34


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Moreover ΔG parameters accompanying the DILs exhibit parallel trend to the calculated binding energies. Further variation of Tg parameter as a function of ΔG accompanying the DILs has been illustrated in Figure 10.

Figure 10 Glass transition temperatures, Tg (in ⁰C) as a function of change in free energies, ΔG (in kcal mol–1).

The experimental investigation on the imidazolium based monocationic ILs composed of Tf2N‾ anion by Ludwig et al.80 demonstrated that hydrogen bonding interactions facilitating the cation-anion binding engender diminutive viscosities and lower melting points. Furthermore it was concluded that the ΔH revel reciprocal relation with the hydrogen bonding interactions. Thus the change in viscosity can be correlated to cationanion binding underlying the formation of ILs. On the parallel lines we herewith 35



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established a correlation between ΔH and the dynamic viscosity which is shown in Figure S17 of the supporting information. Alternatively the formation of ILs is governed by ΔG parameters (related to ΔH) which may as well be shown to be dependent on the viscosities. With this view we attempt to correlate the measured dynamic viscosities20 of DILs with the ΔG parameters. An excellent linear correlation (r2 = 0.99) thus is evident from the plot displayed in Figure 11. As may be inferred higher viscosities of the DILs imply lower binding energies which in turn, also correlate to change in free energies accompanying the DILs.

Figure 11. Viscosity, η (mPa.s) plotted against the change in change in free energies, ΔG (in kcal mol–1) accompanying DILs.

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Moreover, dependence of measured Tg values on the (mean) δH values of methylene protons facilitating hydrogen bonding has been portrayed in Figure 12. It may be concluded that δH signals from the NMR spectra serve as the signature of cation-anion binding.

Figure 12. Glass transition temperatures (Tg) as a function of (mean) δH (ppm) values for H3 and H4 protons.

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4. Conclusions This work presents a systematic investigations of electronic structure, binding energies and vibrational spectra of quaternary ammonium based functionalized dication-(Tf2N)2 complexes.

The

following

conclusions

may

be

drawn.

(i)

DILs

based

on

methoxyethoxyethyl substituted dication reveal stronger binding towards the Tf2N‾ anion. The lowest energy conformers of the dication-(Tf2N)2 complexes reveals each of [Tf2N]‾ anion located midway between quaternary ammonium nitrogen of dications facilitate hydrogen bonding with methyl groups on the quaternary nitrogen and those from propyl linkage of the dication. Binding energies here follow the order: [DRR1] > [DRR2] > [DRR3] > [DRR4]. (ii) Cation-anion binding in the dication-(Tf2N)2 complexes reveal its signature in the downshift of the characteristic SO2 stretching anionic vibrations compared to the free anion. As opposed to this the frequency shift in the opposite direction was noticed for the CF3 stretching. The wavenumber upshift to 60 cm‒1 was predicted for 3-phenylpropyl substituted dication complex. The direction of these frequency shifts was rationalized through the QTAIM approach combined with the difference MED plots. (iii) GIAO derived δH values from the present DFT agree well with those in the experimental 1H NMR spectra. Theoretical calculations further demonstrated that δH values of methylene protons from the propyl linker in DILs correlate linearly to binding energies. (iv) Observed Tg parameters in the dication-(Tf2N)2 complexes are governed by electrostatics and hydrogen bonding interactions. Moreover the experimental Td values of DILs exhibit parallel trend to the characteristics Vs,max parameters obtained from the QTAIM approach. (v) Finally, dynamic

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viscosities of DILs governed by electrostatics and hydrogen-bonding interactions reveal a linear correlation with the accompanying free energy change. In brief, the present work should serve as an initial step to understand the liquidus behavior of DILs and prove useful in modeling of their physicochemical properties.

Supporting Information Optimized geometries, vibrational frequencies, net atomic charges in ion-pairs, difference MED plots, topological graphics of the critical points, electrostatic potential map and correlation plots in dication-(Tf2N)2 complexes.

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 (SPG and PLV) acknowledge the financial support from the Board of Research in Nuclear Sciences, India for a research grant through the project (37(2)/14/11/2015BRNS). Authors thank the Center for Development of Advanced Computing (CDAC), Pune for providing National Param Supercomputing Facility.

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References (1) Frizzo, C. P.; Bender, C. R.; Gindri, I. M.; Villetti, M. A.; Machado, G.; Bianchi, O.; Martins, M. A. P. Elucidating Anion Effect on Nanostructural Organization of Dicationic ImidazoliumBased Ionic Liquids. J. Phys. Chem. C 2016, 120, 14402-14409. (2) Shirota, H.; Shohei, K. Temperature Dependence of Low-Frequency Spectra in Molten Bis(trifluoromethylsulfonyl)amide Salts of Imidazolium Cations Studied by Femtosecond Raman-Induced Kerr Effect Spectroscopy. J. Phys. Chem. B 2015, 119, 9835−9846. (3) Nakajima, H.; Ohno, H. Preparation of Thermally Stable Polymer Electrolytes from Imidazolium-type Ionic Liquid Derivatives. Polymer 2005, 46, 11499−11504. (4) Anderson, J. L.; Armstrong, D. W. Immobilized Ionic Liquids as High- selectivity/HighTemperature/High-Stability Gas Chromatography Stationary Phases. Anal. Chem. 2005, 77, 6453−6462. (5) Han, X.; Armstrong, D. W. Using Geminal Dicationic Ionic Liquids as Solvents for High Temperature Organic Reactions. Org. Lett. 2005, 7, 4205−4208. (6) Zeng, Z.; Phillips, B. S.; Xiao, J. C.; Shreeve, J. M. Polyfluoroalkyl, Polyethylene Glycol, 1,4Bismethylenebenzene,

or

1,4-Bismethylene-

2,3,5,6-Tetrafluorobenzene

Bridged

Functionalized Dicationic Ionic Liquids: Synthesis and Properties as High Temperature Lubricants. Chem. Mater. 2008, 20, 2719−2726. (7) Qi, M.; Armstrong, D. W. Dicationic Ionic Liquid Stationary Phase for GC-MS Analysis of Volatile Compounds in Herbal Plants. Anal. Bioanal. Chem. 2007, 388, 889−899.

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Probing Molecular Interactions in Functionalized Asymmetric

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