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Electronic Structure, NMR, Spin−Spin Coupling, and Noncovalent Interactions in Aromatic Amino Acid Based Ionic Liquids Soniya S. Rao and Shridhar P. Gejji* Department of Chemistry, Savitribai Phule Pune University, Pune 411 007, India

J. Phys. Chem. A 2016.120:5665-5684. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/13/18. For personal use only.

S Supporting Information *

ABSTRACT: Noncovalent interactions accompanying phenylalanine (Phe), tryptophan (Trp), and tyrosine (Tyr) amino acids based ionic liquids (AAILs) composed of 1-methyl-3butyl-imidazole and its methyl-substituted derivative as cations have been analyzed employing the dispersion corrected density functional theory. It has been shown that cation−anion binding in these bioionic ILs is primarily facilitated through hydrogen bonding in addition to lp---π and CH---π interactions those arising from aromatic moieties which can be probed through 1H and 13C NMR spectra calculated from the gauge independent atomic orbital method. Characteristic NMR spin−spin coupling constants across hydrogen bonds of ion pair structures viz., Fermi contact, spin−orbit and spin−dipole terms show strong dependence on mutual orientation of cation with the amino acid anion. The spin−spin coupling mechanism transmits spin polarization via electric field effect originating from lp---π interactions whereas the electron delocalization from lone pair on the carbonyl oxygen to antibonding C− H orbital is facilitated by hydrogen bonding. It has been demonstrated that indirect spin−spin coupling constants across the hydrogen bonds correlate linearly with hydrogen bond distances. The binding energies and dissected nucleus independent chemical shifts (NICS) document mutual reduction of aromaticity of hydrogen bonded ion pairs consequent to localization of πcharacter. Moreover the nature and type of such noncovalent interactions governing the in-plane and out-of-plane NICS components provide a measure of diatropic and paratropic currents for the aromatic rings of varying size in AAILs. Besides the direction of frequency shifts of characteristic CO and NH stretching vibrations in the calculated vibrational spectra has been rationalized.

1. INTRODUCTION Room temperature ionic liquids (RTILs) exclusively composed of organic cations and organic/inorganic anions exhibit liquidus behavior with melting points well below 100 °C and find ubiquitous applications owing to their characteristic properties such as nonvolatility, high viscosity, high ion conductivity, and thermal and chemical stability.1,2 Naturally occurring carboxylate salts,3,4 amino acids,5−9 and sugar derivatives10−12 are been explored as economical and clean precursors for the synthesis of ILs. Early works of Fukumoto and Ohno13 on 1ethyl-3-methylimidazolium [Emim] and natural amino acids have led to a new class of amino acid ionic liquids (AAILs). Amino acids comprising both amino and carboxyl groups with a chiral center and varying alkyl side chains have been harnessed for designing and synthesis of novel functional materials.14,15AAILs are liquids at ambient or far below ambient temperatures serve as suitable media and thus have emerged as an alternative to volatile organic solvents in a variety of chemical reactions. The zwitterionic amino acids allows their use as either cation or anion facilitating multitudes of combinations for ILs.16−19 AAILs have been employed as © 2016 American Chemical Society

chiral solvents or reactants for dissolution and stabilization of cellulose, nucleic acids, carbohydrates, and other species of primary biological importance.20−25 On the theoretical front, molecular dynamics (MD) simulations on the ILs composed of deprotonated amino acid anions and choline cation revealed that their formation is facilitated through proton transfer involving the hydrogen bond effectively neutralizing cation and anion.26 Fileti and coworkers27 consequently carried out atomistic force field simuations to determine shear viscosities and cluster compositions of imidazolium and amino acid salts containing alanine, methionine and tryptophan anions in aqueous solutions and further demonstrated how structural change of amino acid modulates the dispersion of AAILs in water for varying concentrations. A molecular level understanding of noncovalent interactions encompassing hydrogen bonding, steric repulsion, aromatic Received: April 20, 2016 Revised: June 23, 2016 Published: June 23, 2016 5665

DOI: 10.1021/acs.jpca.6b03985 J. Phys. Chem. A 2016, 120, 5665−5684

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The Journal of Physical Chemistry A

Figure 1. Structures of (a) [Bmim] and (b) [MBmim] cations and amino acid (c) [Phe], (d) [Trp], and (e) [Tyr] anions. The atomic labeling scheme is shown along with the structures.

media for stabilizing bimolecules are rather scanty. With this perspective a more complete picture of noncovalent interactions involving Aromatic AAILs should prove pivotal for rational design of task specific ILs in a myriad of chemical applications. The present work precisely focuses on analyzing underlying noncovalent interactions accompanying aromatic AAILs composed of [Phe], [Trp], and [Tyr] anions with 1methyl-3-butylimidazolium cation [Bmim] and its substituted methyl derivative [MBmim] employing the dispersion corrected density functional theoretic calculations. It would be intriguing to understand how aromatic rings in the cation and anion render different noncovalent interactions in turn affecting structure and spectral characteristics of pure ILs. The following questions have been addressed. How methyl substitution on the imidazolium cation influences the ion-pair structures? How interactions arising from π-cloud of the aromatic rings and interas well as intramolecular hydrogen bonding govern the local aromaticities in these aromatic AAILs? How such interactions reflect in spin−spin coupling constants, NMR and vibrational spectra? The computational method is outlined below.

ring stacking and electrostatic interactions should prove useful for designing of task specific ILs. To this direction interactions arising from aromatic rings or π-stacking have received significant attention in recent years.28−30 The origin of highly directional electrostatic attractions (with possible induction and dispersive contributions) between rings involving positive electrostatic potential region on the molecular surface of aromatic or nonaromatic moieties was discussed in terms of πhole interactions by Politzer et al.31−33 Consequent anion/lonepair-π or π−π interactions are usually referred as π-hole bonds. Further Hunt and co-workers described π−π interactions in imidazolium based ILs through energetic landscapes of π+-π+ stacked motifs, hydrogen bonding and anion-π+ interactions.29 Dias et al. examined solubilities of aromatic molecules, in particular those of benzene and its fluorinated derivatives in [Emim][NTf2] based ILs, using NMR spectroscopy experiments combined with MD simulations.34 The solubilities of aromatic molecules in ILs further are shown to be dependent on diamagnetic characteristics of the aromatic ring. The aromatic amino acids phenylalanine ([Phe]), tryptophan ([Trp]), and tyrosine ([Try]) primary constituents of biomolecules, are fluorescent aromatic chromophores used as probe to unravel protein structure environment from fluorescence and NMR experiments.34−38 It has also been shown that reaction between tryptophan and [Bmim][PF6]based ILs occurs via the proton transfer.39 Despite the aforementioned works, which are concerned with the interaction of amino acids or aromatic moieties with ILs, attempts to exclusively explore aromatic AAILs as storage

2. METHODOLOGY The atomic numbering scheme for [Bmim] cation and amino acid anions (AA) (AA= Phe, Trp and Tyr) is shown in Figure 1. The initial structures of ion pairs were generated by carrying out appropriate transformations combining the rotation and translation of [Bmim+] and [AA−] around the minima and saddle points in the molecular electrostatic potential (ESP) 5666


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Figure 2. Optimized structures of (a) [Bmim][Phe], (b) [Bmim][Trp], and (c) [Bmim][Tyr] ion pairs. Hydrogen-bonding interactions are depicted as broken lines.

topography40−42 followed by subsequent optimizations employing the GAUSSIAN-09 program.43 The structures thus derived were subjected to density functional calculations based on Becke’s three parameter exchange combined with the correlation functional due to Lee, Yang, and Parr (B3LYP).44,45 The internally stored 6-311G++(d,p) basis with diffuse functions added to heavy atoms and hydrogens were used. Optimized ion pair conformers destabilized up to 5 kJ mol−1 relative to the lowest energy conformer, possessing qualitatively different binding patterns were subjected to optimizations employing the hybrid meta-GGA (generalized gradient approximation) exchange correlation M05-2X functional subsequently.46−51 Binding energies of [Bmim][AA] were obtained by subtracting the sum of energies of individual cations and anions making up the ion pairs in their free state (isolated) from those of the ion pair systems. Stationary point structures obtained were confirmed to be local minima on the multivariate potential energy surface since of all the normal vibration frequencies turned out to be real. The potential energy distribution (PED) was derived. The normal modes were assigned through visualization of displacement of atoms around their equilibrium (mean) positions using the GAUSSVIEW-5 program.52 To delve into underlying molecular interactions and their manifestation in vibrational spectral characteristics we use the quantum theory of atoms in molecule (QTAIM)53,54 approach employing the AIMAll software.55 The underlying noncovalent interactions are deciphered in terms of the bond critical points (bcp) in molecular electron density topography, its Laplacian (∇2ρ) and the corresponding reduced density gradients (RDG) defined through

1

|∇ ρ(r)|

2(3π 2)1/3 ρ(r)4/3

the QTAIM model.56 The NCI-RDG isosurfaces were obtained employing a uniform spatial grid with a step size of 0.1 au The δH parameters in 1H NMR spectra were calculated by subtracting the nuclear magnetic shielding tensors of protons in the cation, anion and ion pair complexes from those in tetramethylsilane (reference) using the gauge-independent atomic orbital (GIAO) method57,58 within the framework of M05-2x based density functional theory. Further the nucleus independent chemical shifts (NICS) were derived.59,60 For nonplanar systems studied in the present work the local aromaticities were gauged from the NICS profiles. The ring critical points (rcp) within the MED topography were located. This was followed by identifying the points up to 3 Å both above as well as below the aromatic ring with a step size of 0.5 Å along the unit normal vector perpendicular to the ring plane with rcp as origin. The magnetic shielding tensors were computed at these points. To explain the frequency shifts of characteristic vibrations in the calculated vibrational spectra, natural bond orbital (NBO) analyses were carried out.61

3. RESULTS AND DISCUSSION 3.1. Structure and Noncovalent Interactions. It is known that the ion pair refers to the smallest representative unit of ILs that simulates the structural and binding features and widely have been employed to model their physicochemical properties and liquidus behavior.62,63 The isolated anions were used to generate initial ion pair structures employing the ESP topography of the anion. The optimized lowest energy structures of [Bmim][AA] and [MBmim][AA] using the M05-2x/6-311++G(d,p) level of theory are shown in Figures 2 and 3. As shown in Figure 2 the acidic H1 proton of the

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Figure 3. Optimized structures of (a) [MBmim][Phe], (b) [MBmim][Trp], and (c) [MBmim][Tyr] ion pairs. Hydrogen-bonding interactions are depicted as broken lines.

[Bmim] cation facilitates bif urcated CO---H hydrogen bonds with carboxylate oxygens O1 and O2 of the anion rendering Tshaped structure to the conformer. The [Bmim][Phe] conformer possesses C−H---O interactions (corresponding bond distances being 2.004 and 2.288 Å) from alkyl group of the [Bmim] cation in addition to bifurcated hydrogen bonds from the H1 proton of imidazole ring as shown in Figure 2a. These inferences can be extended to explain the stabilities of [Bmim][Trp] and [Bmim][Tyr] ion-pairs. Selected geometrical parameters including the hydrogen bond distances in [Bmim][AA] ion-pairs are compared with the free ions in Table 1. The C1−H1---O interactions engender an elongation up to 0.06 Å for C1H1 bond in the [Bmim][Tyr] ion-pair. The CN bonds from the imidazole ring are contracted by ∼0.1 Å compared to the free cation. A shortening of ∼0.06 Å for the N3−H12 bond was further predicted for [Bmim][Phe] and [Bmim][Trp] ion-pairs. The carbonyl bond distances are elongated up to 0.009 Å in these ion-pairs. The bifurcated hydrogen bond distances range from 1.990 to2.497 Å in [Bmim][Trp] and [Bmim][Tyr] ion pairs. Generally bond

angle parameters of the anion do not vary significantly on interaction with the [Bmim] cation, the only exception being the ∠HNH of the amide group for which an increase of % scharacter of the amide nitrogen from the anion and results in opening of such angle from 104° to 109° in the [Bmim][Phe] ion pair. The imidazole ring orients near normal (deviation of ∼14°) to carboxyl group of the anion renders T-shaped structure to the [Bmim][Phe] ion pair. The intramolecular N3−Ha---O1 hydrogen bonding is evident from the N1′C2′C1′O1 dihedral angle parameters reported in Table 1. The [Bmim][Trp] and [Bmim][Tyr] ion pair led to similar inferences. It should further be remarked that the [Tyr] ion pair reveals shorter cation−anion separations with the imidazole ring placed near normal (marginal deviation of 2°) to carboxylate group. As mentioned earlier, H1 is the most reactive center in imidazolium cation and substitution at C1 position with the electron-donating methyl group significantly affects the ion pair structure consequent to π-hole interactions from the imidazole or phenyl ring. The π-hole refers to a region with positive 5668


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30.24 −138.39 −100.29

1.091 1.336 1.254 1.251 1.012 −35.34 1.089 1.335 1.256 1.253 1.014 4.10 1.090 1.334 1.250 1.258 1.014 13.13 1.329 1.257 1.252 1.012 −19.98 −75.43 1.329 1.258 1.252 1.014 −17.44 −76.02

1.082 1.081

1.250 1.246 1.018 −5.9

1.081

electrostatic potential in direction perpendicular to σ-framework of the molecular system and attributes to induction effect from electron withdrawing groups or electronegative atoms bound to the π-system.31−33 Nonconjugated as well as πconjugated aromatic or nonaromatic molecules display positive electrostatic potential perpendicular to the plane of a cyclic molecular framework.64,65 Further lp---π interactions can be envisaged as π-hole interactions. In this perspective, ESP mapped isodensity surfaces can be employed to understand the redistribution of molecular electron density in composite systems. ESP mapped isodensity surfaces (V = −265.5 kJ mol −1 ) in isolated cations, anions, [Bmim][Phe] and [MBmim][phe] ion pairs are portrayed in Figure 4a−g. The deep blue region in the isolated [Bmim]+ cation display positive π regions (π+) which are further intensified upon methyl substitution on the cation. As shown in parts c−e of Figure 4, the electron-rich regions (red) around the carboxylate group and relatively less electron-rich phenyl rings (π−) have further been inferred. The cation−anion binding in [Bmim][Phe] ion pair renders electronic charge transfer from the COO− functionality to the cation through CO---H interactions. The extent of polarization of phenyl and imidazole rings is different which renders distinct electronic distributions within the π-rings as evident from the ESP isosurface in the [MBmim][Phe] system. The [MBmim][Phe] ion pair brings about the polarization of electron density toward the edge of the molecule with concomitant increase of the electron population near periphery of the ring as depicted in Figure 4g. Thus, the imidazole ring of aromatic AAILs possesses π+ character contrary to the π− phenyl ring thereby engendering distinct π-interactions. The binding of [MBmim] to [Phe] anions therefore, can be envisaged as a competition between the interactions arising from the oxygen atoms of carboxylate group, lp---π interactions from the imidazole ring and CH---π interactions facilitated through the phenyl moiety. Here the anion preferentially orients in a way that the carboxylate group lies above the center of C1N1 bond bringing about lp---π interactions with the corresponding separation being 2.883 Å. Noteworthy enough lp---π interactions belong to multicenter class of interactions with relatively small individual electronic delocalizations. The phenyl ring brings about CH---π interactions from alkyl chain and renders a parallel displaced stacked structure with the butyl group placed above phenyl ring of the anion (3.197 Å) in [MBmim][Phe]. This is evident from Figure 3(a). Noteworthy enough, the CH---π interaction occurs between the center of CC bond and the methyl group. A partial negative π-electron density above and below the plane of the imidazole ring and partial positive charge on periphery of the rings is evident (cyan regions) in such arrangements. The anisotropic charge distribution leads to favorable electrostatic interactions for parallel stacked structures. These inferences further are extended to [MBmim][Tyr] ion pair wherein the lp--−π (2.930 Å) interactions are noticed (cf. Figure 3c).The [MBmim][Trp ] ion pair is void of CH---π interactions due to relatively less electron-rich phenyl ring and second bulkier pyrrole and phenyl groups render a T-shaped geometry to this ion pair as depicted in Figure 3b. The carbonyl bonds in [MBmim] ion pairs are longer (∼0.012 Å) compared to the free anion(s) or their unsubstituted analogues (0.006 Å) (cf. Table 1) as a result of coexistence of hydrogen bonding and lp-−π interactions. The NHN bond angle does not vary significantly in the [MBmim] ion pairs. The ESP isosurface

1.329 1.258 1.252 1.013 −26.08 −77.23

[MBmim][Trp] [MBmim][Phe] [Bmim][Tyr] [Bmim][Trp] [Bmim][Phe]

1.249 1.245 1.018 10.37

tyr

1.249 1.246 1.018 11.52

trp phe

1.330

C1−H1 C1−H3 C1−N1 C1′O1 C1′O2 N1′−Ha N1′C2′C1′O1 C1H1O1C1′ C1N1O2C1′

1.076

1.090 1.337

MBmim bmim

Table 1. Selected Bond Distances (in Å) of [Bmim]+, [MBmim]+ Cations, and [AA]− Anions (AA = Phe, Trp, Tyr) along with the Respective Ion Pairs

[MBmim][Tyr]


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Figure 4. ESP mapped isodensity surface (0.001 a.u) in isolated (a) [Bmim] and (b) [MBmim] cations, as well as (c) [Phe], (d) [Trp], and (e) [Tyr] anions along with (f) [Bmim][Phe] and (g) [MBmim][Phe] ion pairs.

Figure 5. Angular scatter plot of CO---H angles as a function of O---H separations. See text for details.

H-C interactions along with lp---π interactions from the imidazole ring. The combined effect of hydrogen bonding and lp---π interactions in these ion pairs transforms the Tshaped structure to parallel stacked conformer with an increase in the van der Waals contributions. Politzer et al.33 suggested that bond angle between the interacting π-hole and anion or lone pair deviates from 90°. For [MBmim] ion pairs the bond angle for lp---π interactions spans a range from 77° to 108°

for the rest of the ion pairs have been summarized in Figure S1 of the Supporting Information. Substitution of the methyl group is accompanied by a partial transition of hydrogen bond to van der Waals interactions concomitantly resulting in diminutive electrostatic component. As a result the van der Waals contribution increases with angular characteristics transforming from directional to isotropic thus can be deciphered from directional CO--− 5670


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Table 2. Inter-Molecular Distances and Corresponding ρbcp and Laplacian ∇2ρ Parameters (in au) in (a) [Bmim][AA] and (b) [MBmim][AA] Ion Pairs (a) [Bmim][AA] Ion Pairs [Bmim][Phe] O1---H1 O2---H1 O1---H6 O2---H7 N1′---H9 N1′---H13 O2---Ha

[Bmim][Trp]

ρbcp

∇2ρ

r

ρbcp

∇2 ρ

r

ρbcp

∇2 ρ

2.004 2.288 2.211 2.386 2.740 2.809

0.0179 0.0257 0.0155 0.0124 0.0072 0.0068

0.0685 0.0961 0.0570 0.0424 0.0195 0.0174

2.292 2.013 2.208 2.392 2.802

0.0180 0.0253 0.0156 0.0123 0.0060

0.0687 0.0950 0.0576 0.0423 0.0172

2.291 1.990 2.409 2.223 2.771

0.0179 0.0263 0.0119 0.0153 0.0069

0.0687 0.0983 0.0405 0.0556 0.0185

2.117 0.0224 (b) [MBmim][AA] Ion Pairs [MBmim][Phe]

O1---H6 O1---H16 O2---H16 O2---H9 O2---H7

[Bmim][Tyr]

r

0.0970

[MBmim][Trp]

[MBmim][Tyr]

r

ρbcp

∇2 ρ

r

ρbcp

∇2ρ

r

ρbcp

∇2 ρ

2.218 2.258

0.0162 0.0150

0.0517 0.0498

2.340 2.115

0.0142 0.0189

0.0459 0.0736

2.378

0.0127

0.0390

2.217 2.132

0.0156 0.0222

0.0620 0.0924

2.453 2.497 2.161 2.460 2.473

0.0131 0.0109 0.0178 0.0122 0.0107

0.0461 0.0357 0.0660 0.0461 0.0364

Figure 6. Color-filled RDG isosurfaces depicting Noncovalent interaction (NCI) regions in (a) [Bmim][Phe] and (c) [MBmim][Phe] ion pairs. Green regions denote O---H and lp---π interactions while the red isosurface refers to steric effects. The NCI index plot of function 1 (sign (λ2) ρ values) on the x-axis versus function 2, the reduced density gradient (RDG) on the Y-axis for the same have also been shown in (b) [Bmim][Phe] and (d)[MBmim][Phe] ion pairs.

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Figure 7. Dispersive, orbital, and steric contributions along with total binding energies accompanying cation−anion binding in the regime of M05-2x level of theory.

Laplacian (∇2ρ) in the regime of QTAIM theory reported in Table 2. On the basis of ρbcp values, Popelier et al.65 categorized hydrogen bonding interactions those fall in the range of 0.002− 0.035 au; the corresponding Laplacian values being 0.014− 0.139 au. NCI is characterized in terms of electron density (ρ), its Laplacian (∇2ρ), and the reduced gradient of density (s). The ∇2ρ is decomposed into three eigenvalues λ1 + λ2 + λ3 (with λ1 ≤ λ2 ≤ λ3) located in its Hessian. The second component, λ2, provides insights for redistribution of the electron density consequent to such interactions distinguishing bonding (λ2< 0) features from the nonbonding (λ2 > 0) ones. These interactions are described in terms of attractive, repulsive steric and weak dispersive components. The elegance of NCI method lies in elucidating such interactions in real-space and as graphical representations. The position, strength and type of an interactions shown as RDG isosurface are represented on a blue−green−red (BGR) scale according to the values and sign of (λ2)ρ located in between −0.05 and +0.05 au. Regions with positive (λ2)ρ for the area are portrayed as red aqnd indicate strong repulsive nonbonded overlap; the attractive interactions displayed in blue and the green regions refer to electrostatic interactions. NCI surfaces in [Bmim][Phe] and [MBmim][Phe] ion pairs are shown in Figure 6. As shown in Figure 6a, the green regions between the [Bmim][Phe] ion pairs stem from hydrogen-bonding interactions. The green isosurface indicate weak attractive interactions between H7, H9, and H11 protons of the alkyl chain with the anion. As suggested earlier,66−68 the hydrogen bond is considered to be composed of components representing covalent as well as electrostatic and van der Waals interactions to a varying degree. The presence of donor/acceptor groups or alternatively substitution of the electron donating or withdrawing groups engenders mutual existence of hydrogen bond along with other noncovalent interactions possibly encompassing the van der Waals interactions as well.69 Consequently additional lp---π and hydrogen bonding interactions accompanying [MBmim][Phe] ion pairs emerge with isosurfaces between imidazole ring and the carboxylate group shown as green imply weak hydrogen

consequent to interacting carbonyl oxygens which facilitate hydrogen bonding simultaneously. The CH---π interactions revealed the interacting angle to be nearly 90°. The main structural feature that distinguishes the hydrogen bond from the lp---π interaction is its preference for the linearity. The angular scatter plots of hydrogen bond angles versus the C1 O1---HX − CY distances are shown in Figure 5. Each point in this plot represents the hydrogen bond. As may be noted, the short distances occur for the relatively linear angles, whereas longer bonds are observed for relatively large angles. The hydrogen bond facilitated via the most acidic H1 atom turns out to be ∼2.0 Å are rendered with a large covalent character (marked as green in the figure) while those arising from methylene group fall in the range ∼2.3−2.5 Å with the corresponding angle being 78−120°; these directional hydrogen bonds are electrostatic in nature. It is further evident that the hydrogen bonds facilitated through the alkyl chain nearly linear are either electrostatic or dispersive kind. In case of [MBmim] ion pairs parallel shaped structure is attained with the hydrogen bond angle approaching 90° compared to 78° in the T-shaped [Bmim][Phe] ion pairs (marked as red in the figure). The interactions from methyl group of the cation reveal hydrogen bond angles to be ∼130°. As pointed out in the preceding section, the zero point corrected binding energies of ion-pairs are obtained by subtracting the sum of individual ions from those of ion pair energies. Calculated binding energies for the [Bmim][AA] (AA = Phe, Tyr) ion pairs turn out to be ∼9 kJ mol−1 larger than those for the [Bmim][Trp]. The [MBmim][Phe] and [MBmim][Trp] ion-pair binding energies (384 kJ mol−1) are comparable, whereas for the [MBmim][Tyr] ion pair it was predicted to be 392 kJ mol−1. The interplay of intermolecular hydrogen bonding and πinteractions in AAILs can be envisaged through the noncovalent interaction reduced density gradient (NCI-RDG) method based on the bonding topologies described through QTAIM theory. These interactions are further deciphered from the electron density at the bond critical point (ρbcp) and its 5672


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The Journal of Physical Chemistry A Table 3. 1H NMR in [Bmim][AA] and [MBmim][AA] Ion Pairs δH

bmim

H1 H2 H3 H4 H5 H6 H7 H8 H9 H10 H11 H12 H13 H14 H15 H16 H17 H18 Ha Hb Hc Hd He Hf Hg Hh Hi Hj Hk

8.0 7.8 7.9 3.9 3.9 3.8 4.1 4.2 1.8 2.0 1.4 1.4 1.0 1.5 1.0

MBmim

phe

trp

tyr

7.9 7.5 4.0 4.0 4.1 4.0 4.0 2.1 2.1 1.6 1.6 1.4 2.0 1.4 2.8 2.8 3.2 −0.7 3.5 2.8 4.6 2.2 8.0 7.6 7.5 7.9 10.0

−0.7 3.8 3.0 4.2 2.3 8.5 7.6 7.6 7.6 8.9 7.0

−0.4 3.7 2.5 3.8 2.1 7.9 6.8 7.4 8.6 3.3

[Bmim][Phe]

[Bmim][Trp]

[Bmim][Tyr]

12.9 7.1 7.1 3.3 3.4 6.4 5.3 3.5 2.6 1.2 1.5 1.0 1.7 0.9 0.5

12.7 7.2 7.1 3.3 3.5 6.4 5.4 3.6 2.4 1.5 1.5 1.0 2.2 1.0 0.7

12.8 7.1 7.0 3.4 3.2 6.2 5.2 3.5 2.8 1.3 1.7 1.0 2.4 1.2 0.7

0.4 1.5 3.1 3.3 2.2 8.2 8.1 8.0 8.0 7.9

2.6 0.9 3.8 2.6 3.3 8.4 8.1 8.0 8.0 7.4 7.3

2.4 0.1 3.2 2.2 3.6 7.7 7.2 7.4 8.1 3.8

bonding interactions in conjuction with the lp---π interactions; the latter being dominant. Thus, the underlying cation anion binding stem from the electrostatic and van der Waals type of interactions (cf. Figure 6c). Furthermore, a distinct NCI surface between the phenyl ring and terminal methyl group of the cation further suggests the presence of CH---π interactions. The red regions as shown in figure arise as a result of depletion of electron density which is counterbalanced by steric contributions. As an alternate perspective to real-space NCI isosurface for cation−anion binding the reduced gradient of density is plotted as a function of electron density multiplied by the sign of λ2. The NCI plot of [Bmim][Phe] ion-pair shown in Figure 6b reveals spikes around ±0.010 au consequent to strong bifurcated hydrogen bonding. The reduced gradient at low electron density values shows deep spikes around ±0.020 au on the abscissa, implying the presence of attractive interactions with the alkyl protons. NCI plot of [MBmim][Phe] has been portrayed in Figure 6d. The region between ±0.005 au shows the appearance of new spikes which refers to lp---π interactions. As a result narrowing of the region separating low and high electron densities in vicinity of 0.00 a.u can be observed. The NCI plots of [Bmim] and [MBmim] AAILs portrayed in Figure S2 of the Supporting Information also corroborate these conclusions. To delve further into the sensitive dependence of many body effects on cation−anion binding and accompanying molecular interactions we carried out the energy decomposition analyses (EDA). EDA gives contributions from steric (electrostatic + exchange terms, ΔEsteric), orbital (ΔEorb), and dispersion (ΔEdisp) energies to overall binding energy (ΔEtotal) etc. and

[MBmim][Phe]

[MBmim][Trp]

[MBmim][Tyr]

9.7 8.1 7.9 7.8 7.7 7.1 7.1 6.9 4.6 4.0 3.7 3.5 3.4 3.2 2.8 2.8 2.7 0.0 −0.5 0.2 0.8 0.4 2.4 2.0 1.5 1.4 0.8

8.4 8.2 8.1 7.8 7.6 7.5 7.1 6.9 6.0 5.7 4.4 3.8 3.7 3.3 3.3 3.2 3.1 0.9 0.0 0.9 1.3 1.3 2.3 2.8 1.7 1.7 1.6 1.6

8.0 7.7 7.5 7.3 7.2 7.1 5.5 4.9 4.7 3.7 3.5 3.4 3.2 3.2 3.1 2.8 2.6 1.1 0.8 1.1 1.2 1.8 1.8 1.9 2.0 2.1 0.1

provide insights for charge distributions in noncovalent bonded systems. Figure 7 illustrates the contributions toward the cation anion binding constituting the ion pair in the form of a histogram. The methyl substitution strongly influences the molecular interactions as a result of contributions from noncovalent components to the ion pair. The underlying interactions in [MBmim] ion pairs are dominated by dispersion effects and are simulated well via the inclusion of the M0x exchange correlation functionals those incorporate a large set of parameters.46 The parallel displaced stacked [MBmim][AA] structures can be distinguished those for [Bmim][AA] ion pairs. In other words, the interplay of electrostatic/dispersive effects from the π-cloud and hydrogen bonding interactions dictate structural characteristics of aromatic AAILs. The hydrogen bonding and lp--π interactions with large electrostatic and polarization components, further are rationalized in terms of dispersive contributions. The electron donating methyl substituent in the [MBmim] ion pair enhances the donor− acceptor ability and exhibits comparable binding strengths to its unsubstituted analogues. It should be discernible here that [MBmim][Trp] ion pair void of CH--π interactions emerge with relatively less dispersion contributions as evidenced from the histogram. 3.2. 1H and 13C NMR. It is well known that the cation−anion binding can be monitored through 1H NMR experiments. GIAO computed δH and δC values of the individual cation or anion and those in the ion pair from the M05-2x/6-31++G(d,p) theory are summarized in Table 3. The accurate determination of NMR parameters is a challenging task and requires simulation of electron correlation and 5673


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The Journal of Physical Chemistry A Table 4. 13C NMR in [Bmim][AA] and [MBmim][AA] Ion Pairs δC

bmim

MBmim

C1 C2 C3 C4 C5 C6 C7 C8 C9 C1′ C2′ C3′ C4′ C5′ C6′ C7′ C8′ C9′ C10′ C11′

152.4 144.8 145.3 39.5 59.2 41.2 23.4 14.6

166.7 139.5 144.3 39.8 55.8 34.1 23.9 16.8 12.2

phe

196.7 61.7 48.9 178.6 153.3 147.2 142.9 150.1 156.2

trp

196.5 61.9 36.4 146.2 154.6 142.2 137.1 139.2 127.5 156.9 146.2

tyr

194.2 66.3 52.2 170.6 153.4 129.4 174.9 132.8 158.8

[Bmim][Phe]

[Bmim][Trp]

[Bmim][Tyr]

[MBmim][Phe]

[MBmim][Trp]

[MBmim][Tyr]

168.7 137.5 140.1 38.3 56.3 38.5 24.1 14.0

166.8 137.5 140.1 38.6 23.1 39.0 23.1 15.0

168.5 136.8 139.9 38.0 57.3 38.9 24.7 14.4

204.6 66.8 52.2 170.5 153.2 152.1 147.5 151.7 153.9

205.8 60.9 39.9 139.1 152.5 140.0 139.9 144.0 130.2 158.4 139.3

203.2 66.1 48.1 161.6 152.1 133.0 178.0 135.2 156.7

174.4 136.4 141.0 40.1 56.2 36.7 24.9 14.8 12.8 205.6 67.1 46.4 173.8 153.2 149.9 146.9 152.1 154.6

173.1 138.1 137.8 38.7 55.6 38.1 23.0 15.5 17.9 205.0 61.0 37.0 136.1 152.7 140.0 139.4 141.6 130.0 158.1 143.6

170.6 135.8 143.5 39.0 54.3 36.8 23.0 15.0 18.4 200.0 65.3 47.5 162.0 153.0 132.8 178.1 136.0 155.8

extended basis set.70−72 The NMR shielding constants, interaction energies and spin−spin coupling constants (SSCCs) are sensitive to choice of the basis set. It has further been known that for noncovalently bonded systems the shielding constants and NMR parameters for example, indirect or direct nuclear SSCCs are critically dependent on the electronic structure and their computations requires good description of core as well as diffuse orbitals.73−75 It should be remarked here that the density functional theoretic calculations incorporating the dispersion corrected M05-2x or related functionals combined with the Huzinaga and further extended correlated basis set have been carried out for water clusters and clatherates.76,77 Deriving the electronic structure and NMR spectral characteristics with such level of theory becomes computationally demanding for large molecular systems. As an alternative, the M05-2x/6-311++G(d,p) level of theory has widely been employed to derive the NMR parameters in the recent literature78−85 and therefore the M05-2x/6-311++G(d,p) approach has been identified to be economical and convenient in terms of quality to cost ratio for obtaining NMR chemical shifts. We therefore employ this level of theory for deriving NMR characteristics of aromatic AAILs studied in this work. The protons in the [Bmim] cation are classified as the methine proton H1, aromatic H2 and H3 protons on the imidazole ring, and those of methyl or butyl group. The H1 proton from the bifurcated hydrogen bonds reveals a large deshielding (∼4.9 ppm) compared to the cation in the [Bmim][Phe] ion pair. The alkyl protons participating in the C−H---O interaction fall in the range 2.6−4.9 ppm and can be distinguished from those which are noninteracting. These inferences are borne out for the [Bmim][Trp] and [Bmim][Tyr] ion pairs as well. The amide Hb protons on the anion emerge with downfield NMR signals owing to intramolecular hydrogen bonding and follows the order: [Bmim][Trp] (2.6 ppm) > [Bmim][Tyr] (2.4 ppm) > [Bmim][Phe] (1.5 ppm). A large deshielding for [Bmim][Phe] points to stronger intramolecular hydrogen bonding. The signals near 7.2−8.4 ppm arise from protons occupying diamagnetic positions in close

vicinity of the aromatic ring of the anion(s) and are attributed to ring current effects. The shielding/deshielding effects within the ion pair can be probed using the 13C NMR data reported in Table 4. The C1, C2, and C3 carbon atoms of imidazole ring penetrate deep inside diamagnetic zone of the [Bmim] cation, while aliphatic carbons place farther from the ring experience a large paramagnetic influence. The C1 carbon shows signal at 152 ppm in the isolated [Bmim] cation that corresponds to the ∼169 ppm signal for the [Phe] ion pair. Similar conclusions are drawn for [Bmim][Trp] and [Bmim][Tyr] ion pairs. The polar network accompanying the ILs is influenced strongly by substitution at the (most reactive) H1 proton and has profound influence on the structure and NMR spectra of ion pairs. The substituent (the functional group) at this site introduces lp---π or CH---π interactions in addition to hydrogen bonding. The downfield signals at 9.7 and 8.4 ppm assigned to H2 and H3 protons on imidazole ring in the [MBmim][Phe] ion pair. Further protons participating in inter as well as intramolecular hydrogen bonding interactions emerge with downfield signals. Subsequently orientations of phenyl ring and carboxylate group of the anion relative to the imidazole ring influence magnetic shielding largely and appear as distinct paramagnetic zones for imidazole and phenyl rings. This is accompanied by deshielded signals for C1 in the [MBmim] ion pairs. Moreover, deshielding of 13C signals in the anion was attributed to lp---π interactions and follows the order: [MBmim][Phe] (206 ppm) > [MBmim][Trp] (205 ppm) > [MBmim][Tyr] (200 ppm). The CH---π interactions in the [MBmim][Phe] and [MBmim][Tyr] engender downfield signals for aromatic carbons of the anion as opposed to those in the [MBmim][Trp] ion pair. Thus, the methyl substitution gives rise to paratropic ring currents to the aromatic ring facilitating different noncovalent interactions with their direction being strongly dependent on the π-conjugation. Lastly 15N chemical shifts reported in Table S2 of the Supporting Information turned out to be nearly the same for [Bmim] and [MBmim] substituted ion pairs. 5674


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Table 5. Calculated Intermolecular Spin−Spin Coupling Constants J( C1′,C1) across the C1′O1H1C1 Dihedral Angle in [Bmim][AA] and [MBmim][AA] Ion Pairs [Bmim][Phe] [Bmim][Trp] [Bmim][Tyr] [MBmim][Phe] [MBmim][Trp] [MBmim][Tyr]

PSO

DSO

FC

SD

J( C1′,C1)

distance (C1′−C1)

−0.137 −0.136 −0.136 −0.055 −0.108 −0.103

0.115 0.116 0.116 0.070 0.088 0.141

−0.661 −0.661 −0.682 −0.003 −0.011 0.056

−0.002 −0.002 −0.001 −0.010 −0.019 −0.011

−0.684 −0.684 −0.703 0.002 −0.051 0.083

3.023 3.095 3.015 3.318 3.224 3.044

3.3. Spin−Spin Coupling Constants. The direct and indirect spin−spin coupling constants in NMR spectra provide better understanding of the electronic structure and in turn, the molecular interactions.86,87 Calculated spin−spin coupling constants (SSCCs) are formulated using the Ramsey nonrelativistic theory having contributions from the Fermi contact (FC), spin-dipole (SD), paramagnetic spin−orbit (PSO), and diamagnetic spin−orbit (DSO) components reported in Table 5. For the purpose of decoding the spin−spin coupling mechanism FC, SD, DSO, and PSO coupling is discussed in details. NMR chemical shifts or SSCCs sensitively depend on the structure of a molecule.88 A knowledge of SSCCs across hydrogen bonds and their characteristics enable one to unravel the noncovalent interactions accompanying the ILs which subsequently can be explored for modeling of physicochemical properties in task specific ILs. The inferences drawn from indirect coupling constants across C1′O---H1−C1 hydrogen bond in [Bmim] and [MBmim] ion pairs are outlined below. Intermolecular equilibrium C1′−C1 distances, total coupling constant (J(C1′,C1)), and contributions from other components to J(C1′,C1) are reported in Table 5. The total SSCC in the [Bmim][Phe] ion pair turned out to be −0.7 Hz. Likewise [Bmim][Trp], [Bmim][Tyr], and [MBmim][Trp] ion pairs exhibit qualitatively similar J(C1′,C1) values in terms of sign and their magnitudes with values −0.05 to −0.7 Hz range. On the other hand, the parallel displaced stacked [MBmim][Phe] and [MBmim][Tyr] ion pairs reveal the total SSCCs values which are> 0. Change in sign of J from nehative to positive and can be attributed to the induced spin polarization. The J(C1′,C1) in the [MBmim][Phe] turns out to be 0.01 Hz. which is nearly ten times smaller compared to 0.1 Hz for [MBmim][Tyr] ion pair. It may therefore, be conjectured that CH---π interactions from rear end of the anion engender large C1′−C1 separations subsequent to mutual orientation of imidazole and carboxylate groups. The histogram displayed in Figure 8, shows the FC term contributes largely to total SSCC parameters. To elucidate further the nature of hydrogen bond indirect SSCCS across both intermolecular CO---H−C as well as intramolecular N−H---OC hydrogen bonds were obtained (cf. Table 6). The relative contributions of various components to J(O,H) for the [Bmim][Phe] ion pair are reported. As pointed out earlier the total SSCCs shows sensitive dependence on the cation anion intermolecular separation and their mutual orientation. Accordingly the relatively shorter separations of the bifurcated hydrogen bonds engender relatively high SSCCs (∼2.7 Hz) which reaffirm predominant electrostatic interactions. On the contrary hydrogen bonding interactions arising from the alkyl chain on the cation reveal distances leading to negative J values (in the range of −0.02 to −0.7 Hz) and thus covalent nature of the hydrogen bond can be inferred. The

Figure 8. Fermi contact (FC), spin−dipole (SD), paramagnetic spin− orbit (PSO), and diamagnetic spin−orbit (DSO) components contributing to total spin−spin coupling for ion pairs.

Table 6. Spin-Spin Coupling Constants for J(O,H)for Interas well as Intramolecular Hydrogen Bonding in (a) [Bmim][Phe] and (b) [MBmim][Phe] Ion Pairsa (a) [Bmim][Phe] Ion Pairs PSO

DSO

total J

−0.61 −0.03 0.49 2.46 −0.05 0.83 2.48 −0.09 0.65 0.70 −0.07 0.38 −0.19 0.01 −0.17 −0.06 0.01 −0.14 1.11 0.09 0.56 (b) [MBmim][Phe] Ion Pairs

−0.37 −0.54 −0.48 −0.45 0.19 0.16 −0.41

−0.46 2.69 2.56 0.70 −0.16 −0.02 1.35

distances O1---H1 O2---H1 O1---H6 O2---H7 N1′---H9 N1′---H13 O2---Ha

O1---H6 O1---H16 O2---H6 O2---H7 O1---Ha O2---Hd a

2.004 2.288 2.211 2.386 2.740 2.809 2.242

FC

SD

distances

FC

SD

PSO

DSO

total J

2.218 2.257 2.677 2.378 2.325 2.269

1.66 1.12 −0.22 0.31 1.05 0.38

−0.21 0.08 −0.01 0.03 0.09 0.09

0.62 0.33 0.36 0.48 0.55 0.55

−0.46 −0.44 −0.30 −0.51 −0.35 −0.40

1.61 0.11 −0.16 0.32 1.39 0.64

See text for details.

reciprocal relation describing the dependence of covalent character of intermolecular hydrogen bonding distances on the SSCCs can readily be noticed. The dominance of strongly distance dependent FC term toward total SSCC values across the hydrogen bond is found to be sensitive to the interatomic distance, s-character of the atomic hybrids involved in the bond and last on the extent of induction of spin density transmitted through the nuclear spin magnetic moment. As evident from Table 6, the total J across C1′−H1---O1−C1 turns out to be a small negative (−0.4 Hz.) where the PSO contributions dominate. As compared to this the corresponding J value for 5675


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The Journal of Physical Chemistry A the C1′−H1---O2−C1 turns out to be ∼2.3 Å with dominant FC component. The total J(O,H) is negative for symmetric hydrogen bonds and positive for asymmetric hydrogen bonds. It may therefore, be concluded that O1 centers render large contributions to lp---π interactions through increased proton shared character to the cation. Moreover, the negative magnetogyric ratio of O1 or O2 engendering opposite signs for J(O,H) and K(O,H) suggests asymmetric (conventional) hydrogen bond those provide stability to the ILs. The variation of total J(O,H) as a function of O---H distances is portrayed in Figure 9a. The intramolecular N−H---O interactions are

As far as direct coupling constants are concerned, the one bond J(C,O) coupling constant serve as a probe to understand the nature of hydrogen as well as other noncovalent interactions accompanying ILs. The carboxylate oxygens pointing toward CN bond of the imidazole ring bring about lp---π interactions and accordingly separation of CN midpoint and carbonyl oxygen of the anion turns out to be ∼2.9 Å. Noteworthy enough, the lone pair of carbonyl oxygens facilitating such interactions have dominant contributions from the PSO component corresponding to C1′O1 (PSO1) and C1′O2 (PSO2) (cf. Table 7). On the other hand, the magnitude of DSO was predicted to be merely −0.17 Hz. with the corresponding FC components being 7.283 and 5.156 Hz., respectively. A delicate balance of these components in conjunction with the relatively small SD term plays an important role in determining the total coupling constant. Higher PSO contributions toward total J imply relatively large lp---π contributions that brings about cation anion binding. A comparison of [Bmim] and [MBmim] ion pairs reveals total J(C1,O2) (J2) for the intermolecular hydrogen bond to be large. An increase of J2 parameters across C1O2---H1−C1′ hydrogen bond concomitantly reflects in elongation of the C1O2 bond. Contrary to this, a decrease of J(C1,O1) (J1) was observed for the methyl substituted ion pair systems owing to lp---π interactions. These arguments further can be extended to rest of the ion pairs and the corresponding parameters are summarized in Table S3 of the Supporting Information. 3.4. Nucleus Independent Chemical Shifts. The magnetic field of a nucleus locally induces a magnetic moment in the electronic system, which results from either increase/ decrease of polarizabilities or by inducing orbital ring currents. The electronic motion in a many body electron system is coupled with electrostatic Coulombic interactions. Thus, the locally induced electronic magnetic moment has a profound influence on the electronic structure of a molecule/ion pair and the effect of applied magnetic field on ring currents intrinsically can be correlated to π-electron delocalization, which has been quantified through NICS describing the “local aromaticity” in a polycyclic molecule. The π-cloud contributions of the aromatic rings can be accessed through the standard NICS(1)zz parameters and accordingly the NICS profiles are usually derived by scanning over the distances in the range from 0 to 3 Å. The isotropic NICS parameters are resolved into the in-plane and out-of-plane components. In the present work NICS profiles of imidazole ring of the cation along with those for the phenyl and pyrrole rings of the AA anion were derived independently. The NICS scans for imidazole and phenyl rings in [Bmim][Phe] ion pairs are shown in parts a and b of Figure 10. Aromatic molecules are controlled by the in-plane and out-

Figure 9. Plot showing correlation of indirect J(O,H) spin−spin coupling constant versus hydrogen bond distances distinguishing (a) inter- and (b) intra-molecular hydrogen bonding. See text for details.

transparent from Figure 9b. In brief both FC and PSO mechanisms govern indirect J(H---O) parameters consequent to either hydrogen bonding or π-interactions responsible for strong deshielding of the imidazolium protons in these systems.

Table 7. Direct spin−spin coupling constants for J(C1′,O1) (J1) and J(C1′,O2) (J2) in [Bmim][AA] and [MBmim][AA] Ion Pairsa [Bmim][Phe] [Bmim][Trp] [Bmim][Tyr] [MBmim][Phe] [MBmim][Trp] [MBmim][Tyr] a

PSO1

PSO2

DSO1

DSO2

FC1

FC2

SD1

SD2

D1

D2

J1

J2

13.90 14.00 13.87 14.25 13.93 13.81

14.14 13.81 14.02 13.71 13.65 14.03

−0.176 −0.176 −0.174 −0.166 −0.177 −0.182

−0.170 0.663 −0.171 −0.185 −0.174 −0.174

7.28 7.20 7.25 5.41 5.42 6.07

5.15 0.24 5.19 7.10 5.78 5.07

−0.26 −0.27 −2.27 −0.46 −0.14 −0.29

−0.35 −0.08 −0.35 −0.19 −0.31 −0.34

1.258 1.258 1.258 1.259 1.256 1.255

1.252 1.252 1.253 1.250 1.253 1.252

20.74 20.52 20.68 20.43 18.95 19.42

18.78 18.31 18.68 19.04 18.86 18.58

See text for details. 5676


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Figure 10. Isotropic, in-plane and out-of-plane NICS components as a function of distance (in Å) for (i) imidazole ring and (ii) phenyl ring for [Bmim][AA] ion pairs. See text for details.

of-plane components of the magnetic tensors.89 Both imidazole and phenyl rings show the in-plane components profiles which are similar. The in-plane contribution to the isotropic NICS for imidazole ring reveals the minimum near −19.7 ppm at 1 Å whereas the corresponding minimum (−9.7 ppm) of the phenyl ring was located near 0.5 Å. The shifts are negative throughout the distance range considered. The out-of-plane components of chemical shifts of the imidazole ring are

observed to be relatively less diatropic compared to the phenyl ring. NICS scans of these components of imidazole and phenyl rings in the [Bmim][Trp] ion pair are depicted in parts c and d of Figure 10. The imidazole ring NICS profiles in [Bmim][Trp] and [Bmim][Phe] are qualitatively similar. As far as the out of plane components are concerned they exhibit a minimum corresponding to phenyl rings whereas that for the imidazole ring steadily increases to maximum in the NICS 5677


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Table 8. NICS Parameters, ρrcp and ∇2ρ, (in au) for Imidazole and Phenol Rings in [Bmim][AA] and [MBmim][AA] [Bmim][Phe] [Bmim][Trp]

[Bmim][Tyr] [MBmim][Phe] [MBmim][Trp]

[MBmim][Tyr]

imidazole phenyl imidazole phenyl pyrol imidazole phenyl imidazole phenyl imidazole phenyl pyrol imidazole phenyl

NICS(0)

NICS(1)

NICS(0)zz

NICS(1)zz

ρrcp

∇2 ρ

−13.47 −7.35 −13.95 −9.02 −11.66 −13.58 −8.87 −12.99 −7.53 −13.14 −9.00 −11.93 −12.78 −8.78

−10.57 −10.17 −10.27 −10.95 −9.20 −9.31 −9.85 −8.98 −10.27 −8.82 −10.59 −9.98 −10.55 −9.83

−6.54 −5.11 −7.71 −9.23 −9.92 −8.40 −11.45 −7.87 −5.10 −11.05 −11.00 −9.01 −8.55 −7.55

−8.00 −6.15 −7.40 −14.79 −10.69 −5.46 −23.45 −0.41 −4.57 −0.21 −20.66 −18.13 −13.95 −5.21

0.056 0.023 0.056 0.022 0.051 0.056 0.023 0.056 0.023 0.056 0.022 0.050 0.0561 0.023

0.418 0.164 0.418 0.016 0.351 0.418 0.007 0.414 0.164 0.415 0.159 0.351 0.416 0.162

profiles shown. Thus, a complementary behavior of the in-plane and out-of-plane component can be noticed. Interestingly the inplane components of the phenyl ring engender diatropic shifts up to 2 Å beyond which a slight paratropic behavior can be observed (cf. Figure 10d). NICS components for pyrrole ring in [Trp] shows qualitatively similar behavior to the phenyl ring. A comparison of out-of-plane NICS components and the isotropic values above the center of imidazole ring show unconvincing shallow minima for [Bmim][Phe] and [Bmim][Trp], which was not observed for the [Bmim][Tyr] system (cf. Figure 10e) owing to the presence of the lp---π interactions. The in-plane components in [Bmim][Tyr] system for phenyl ring reveal the deepest minimum near −25 ppm at 1 Å as shown in Figure 10f. The relatively electron-deficient oxygen atoms in [Bmim][Tyr] ion pair are evident from residual NBO charges, which further corroborate these inferences (cf. Table S1 of the Supporting Information). Table 8 show NICS(0) values of the imidazole ring correlate well to ion pair binding energies and follows the hierarchy: [Phe] > [Tyr] > [Trp]. The substitution of an electron donating group significantly affects the orientation and binding patterns of the ion pair. It has earlier been recognized that the negative charge significantly influences the paratropic behavior 90 and emerge as signature for aromatic π-hole interactions. NICS components for [MBmim] aion pairs have been shown in Figure 11a−f. The [MBmim][Phe] ion pair underlines diatropic nature of the imidazole NICSzz below 1.0 Å and beyond 2 Å as shown in Figure 11a. On the other hand, the region from 1 to 2 Å reveals paratropic behavior and shows the maximum near 1.5 Å. The imidazole ring of [MBmim][Tyr] is qualitatively similar to aromatic NICS scan with its minimum being −13.9 ppm at 1 Å. Resolving the isotropic chemicals shifts as in-plane and out-ofplane contributions of the phenyl ring points to their distinct behavior. A comparison of NICS components indicate relatively strong diamagnetic currents in phenyl rings at longer distances within [MBmim][Phe] and [MBmim][Tyr] ion pairs can be attributed to CH---π interactions. It has been recognized that substitution on aromatic ring alters the charge distribution in the π-cloud and the diatropic effect can be experienced at different distances from the plane of the ring. The general shape of the NICSzz scan plots for [Bmim], [MBmim] ion pairs with varying anions for imidazole and phenyl rings are depicted in Figure 12, parts a and b. The computed NICS(1)zz values of −24.9 ppm for the methylimidazole ring in the isolated [MBmim] cation become less

negative (diatropic) upon interaction with the AA anion viz., −0.4 ppm in [MBmim][Phe], −0.2 ppm in [MBmim][Trp] and −13.9 ppm in [MBmim][Tyr] consequent to C−H---O as well as N−H---O hydrogen bonding interactions in these ion pairs. The hydrogen bond induced decreased aromatic sextet character of the rings involved, hence can be inferred. A significant reduction of aromaticity was noticed for the [MBmim] ion pairs than their unsubstituted analogues. As shown in the figure the out-of-plane components for [Bmim][Phe] and [Bmim][Trp] are qualitatively similar to those for the isolated cation or their [MBmim] analogues. A complementary feature for the imidazole ring participating in in the lp---π interactions can be observed for [Tyr] anion based systems. NICS scan for the phenyl rings herein are shown in Figure 12b. The out-of-plane components of the phenyl rings in these ion pairs, except those predicted for the [MBmim][AA] (AA = Phe, Tyr), are qualitatively similar to their respective isolated anions. The [MBmim][AA] (AA = Phe, Tyr) ion pairs possess CH---π interactions. Further the NICS(1)zz parameters data given in Table 8 suggests the phenyl and pyrrole rings of [MBmim][Trp] ion pair have large aromatic character compared to their unsubstituted counterpart. 3.5. Infrared Spectra. Intermolecular hydrogen bonding between donor−acceptor complexes, for example ion pairs or composites induces structural changes, show their signatures in characteristic vibration frequency shifts compared to their constituents. Cation−anion binding can be probed through normal coordinate analysis. The formation of ion pairs engenders a down-shift (red shift) in the vibrational frequency of carbonyl stretching compared to the isolated cation of the ion pair. As opposed to this, a shift toward higher wavenumber (blue shift) can be characterized in terms of bond contraction accompanied by diminutive IR intensity arising from the negative dipole moment derivative with respect to stretching coordinate. Underlying molecular interactions are probed subsequently through the normal vibration analysis based on the potential energy distribution computed for each ion-pair. 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 [Bmim][AA] and [MBmim][AA] systems are depicted in Figure 13 and Figure S3 of the Supporting Information. The spectra of the [Phe] (anion), [Bmim] and the ion-pair are marked as blue, red and black (on line version), respectively, in Figure 13a. The following inferences are drawn. The region 3700−2900 cm−1 shows the 5678


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Figure 11. Isotropic, in-plane and out-of-plane NICS components as a function of distance (in Å) for (i) imidazole ring and (ii) phenyl ring for [MBmim][AA] ion pairs. See text for details.

(1647 cm−1) from imidazole ring of the cation emerges with a downshift of 19 cm−1 with the enhanced intensity accomanying the ion-pair formation. The 1468 cm−1 vibrations in the ionpair arising from [Bmim] and [Phe] respectively, invoke a strong coupling from C2C1H1 deformation and NH2 rocking. Analyses of vibration spectra of [Trp] and [Tyr] ion-pairs with [Bmim] lead to similar inferences. Selected normal vibrations are summarized in Table S4 of the Supporting Information. Similar inferences are drawn for the [MBmim] based ion pairs

ion pair formation is accompanied by a frequency upshift of 48 cm−1 for the N−H stretching vibration in the free anion (3574 cm−1). As may be noted, the 3305 cm−1 band assigned to C1− H1 stretching of the free cation appears as an intense band at 3239 cm−1 in the ion pair. The aromatic CH stretch for the phenyl moiety occurs in the 3253−3239 cm−1 range whereas the imidazole ring vibrations are noticed in the 3353−3335 cm−1 region. The most intense carbonyl stretching of the anion assigned to the 1713 cm−1 vibration shifts to 1677 cm−1 with diminutive intensity. On the other hand, CN stretching 5679


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Figure 12. Graph showing correlation between NICSzz scan and distances (Å) for (a) imidazole and (b) phenyl rings in [Bmim] and [MBmim] ion pairs.

The direction of frequency shifts further is rationalized through natural bond orbital analysis. Thus an increase of antibonding π*(CO) natural orbital population accompanying the [Bmim][AA] ion pair formation relative to the free anion thus explains the red (down) shift of the corresponding vibration. A large change in the [Bmim][Tyr] ion pair is consistent with a large frequency shift (nearly ∼29 cm−1) of the carbonyl vibration. As opposed to this, the wavenumber upshift of the NH stretching in such ion pairs can be rationalized from a decrease of population in the corresponding σ*(NH) natural orbitals.

with methyl vibrations appearing in the range of 3205−3071 cm−1 as shown in Figure 13b. The low frequency aromatic ring liberation modes usually invoke strong coupling of different twisting or bending internal coordinates from the bulky cation and anion. As shown in Figure 13a, the Raman active interionic vibrations in aromatic AAILs appear below 200 cm−1. The aromatic ring liberation modes in the neutral [Bmim][Phe] ion pair was predicted near 80 cm−1 compared to the 65 cm−1 vibration of the charged aromatic ring in the isolated [Bmim]+ cation. Moreover the correponding vibration in respective [Bmim][Trp] and [Bmim][Tyr] ion pairs assigned to 56 and 51 cm−1 bands stem from strong interionic interactions. These inferences are consonance with those drawn earlier for the [MBmim][AA] ion pairs.

4. CONCLUSIONS Structure, binding energies and noncovalent interactions in [Bmim][AA] and [MBmim][AA] (AA = [Phe], [Trp], and 5680


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Figure 13. Infrared spectra for the (a) [Bmim][Phe] and (b) [MBmim[Phe] ion pairs.

[Tyr] have been analyzed. [Bmim]-based ILs attain T-shaped structures owing to bifurcated hydrogen bonding from the most reactive proton of the cation and AA anion. The methyl substitution on cation renders electron-deficient π holes leading to lp---π interactions with concomitant CH---π interactions from phenyl ring of the anion engendering parallel displaced

stacked structures for the [MBmim][Phe] and [MBmim][Tyr] ion pairs. On the other hand, the [MBmim][Trp] ion pair void of CH---π interactions shows a T-shaped structure. Displaced parallel structures of [MBmim] ion pairs have dominant dispersion components contrary to a T-shaped structure in which the electrostatic contributions prevail. The ramifications 5681


Article

The Journal of Physical Chemistry A of these interactions to 1H NMR spectra manifest in strong deshielded signals as a result of an effective induction of paramagnetic current density by the external magnetic field. Isotropic chemicals shifts resolved into in-plane and out-of-plane NICS components corresponding to phenyl and imidazole rings are indicative of diamagnetic and paramagnetic ring currents consequent to underlying noncovalent interactions. Thus, hydrogen bond induced reduction in aromaticity of the ion pairs can be deciphered. Furthermore, the spin−spin coupling constants serve as “fingerprint” for hydrogen bonding interactions with FC components contributing largely to total SSCCs. On the other hand, lp---π interactions have dominant PSO components. The present work demonstrates that the cation anion separations turn out to be a crucial in the determination of indirect coupling constants. The direction of frequency shifts of characteristic CO and N−H stretching in the vibrational spectra accompanying the ion pair formation has further been rationalized.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b03985. Optimized geometries, spin−spin coupling constants, net atomic charges in ion-pairs, and vibrational frequencies for ion-pairs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(S.P.G.) E-mail: [email protected]. Fax: +91-20225691728. Telephone: +91 020 25601225. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.P.G. acknowledges the support from the Board of Research in Nuclear Sciences (BRNS) India through the research project 37(2)/14/11/2015-BRNS. S.S.R. is grateful to University Grants Commission, New Delhi, for award of the meritorious research fellowship. The authors thank the Centre for Development of Advanced Computing (CDAC), Pune, for providing the National Param Supercomputing Facility.

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