Impact of Size and Electronegativity of Halide Anions on Hydrogen

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Impact of Size and Electronegativity of Halide Anions on Hydrogen Bonds and Properties of 1-Ethyl-3-Methylimidazolium-Based Ionic Liquids Paridhi Sanchora, Deepak Kumar Pandey, Debkumar Rana, Arnulf Materny, and Dheeraj Kumar Singh J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b04116 • Publication Date (Web): 14 May 2019 Downloaded from http://pubs.acs.org on May 15, 2019

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The Journal of Physical Chemistry

Impact of Size and Electronegativity of Halide Anions on Hydrogen Bonds and Properties of 1-Ethyl-3-Methylimidazolium-Based Ionic Liquids Paridhi Sanchora†#, Deepak K. Pandey†#, Debkumar Rana$, Arnulf Materny$, Dheeraj K. Singh†*

†Department

of Physics, Institute of Infrastructure Technology Research & Management, Ahmedabad, India $Physics and Earth Sciences, Jacobs University Bremen, Bremen, Germany *Corresponding author: [email protected] #

Both authors contributed equally

ABSTRACT:

The effect of the anion size and electronegativity of halide-based anions (Cl-, Br-, I-, and BF4-) on the interionic interaction in 1-ethyl-3-methylimidazolium-based ionic liquids (ILs) C2mim X (X = Cl, Br, I, and BF4) is studied by a combined approach of experiments (Raman, IR, UV-Vis spectroscopy) and quantum chemical calculations. The finger-print region of the Raman spectra of these C2mim X ion-pairs provides evidence of the presence of the conformational isomerism in the alkyl chain of the C2mim+ cation. The Raman and IR bands of the imidazolium C2-H stretch vibration for C2mim X (X = Cl, Br, I, and BF4) were noticeably blue shifted with the systematic change in size of anions and the electronegativity. The observed blue shift in the C2-H stretch vibration follows the order C2mim BF4 > C2mim I > C2mim Br > C2mim Cl, which essentially indicates the strong hydrogen bonding in the C2mim Cl ion-pair. DFT calculations predict at least four configurations for the cation-anion interaction. Based on relative optimized energies and base-set-superposition-error (BSSE) corrected binding energies for all ion-pair configurations, the most active site for the anion interaction was found at the C2H position of the cation. Besides information about the C2H position, our DFT results give insights into the anion interaction with the ethyl and methyl chain of the cation, which was also confirmed experimentally [Chem. Commun. 2015, 51, 3193–3195]. The anion interaction at the C2H site of the cation favors a planar geometry in C2mim X for X = Cl, Br, and I, however, for BF4 the system prefers a non-planar 1 ACS Paragon Plus Environment

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geometry where the anion is located over the imidazolium ring. TD-DFT results were used to analyze the observed UV-Vis absorption spectra in a more adequate way giving insights into the electronic structure of the ILs. Overall, a reasonable correlation between the observed and the DFT-predicted results is established. 1. INTRODUCTION Room temperature ionic liquids (RTILs) are an exciting class of chemical compounds that has many interesting potential applications in modern industry e.g. energy storage, material science, and pharmacy and are also considered to be as “green solvents.”1–6 Therefore, research on ionic liquids (ILs) is of both fundamental and applied interest. ILs (a subset of molten salts) are a class of materials having bulky asymmetric cations and various possible anions. They have unique and fascinating properties like low vapour pressure, non-flammability, non-volatility, low melting points, and good thermal and chemical stability.7–11 Because of these unique properties, ILs may be the key to a new generation of electrochemical equipments as they can provide high ionic conductivity12 and a wide electrochemical window.5 ILs are also useful in separation technology13,14 and could be used for CO2 capturing.15,16 Molecular interactions between cations and anions by means of hydrogen bonds play a crucial role and determine physicochemical properties such as viscosity or the low melting point of ILs.1722 To

understand the mechanisms of interionic interactions in ILs and their molecular properties,

a combined approach of experimental and theoretical studies is essential. To gain an understanding of the molecular level properties of ILs, a considerable number of experimental23–43 and theoretical23–32,44–53 investigations have been carried out. The 1-ethyl-3-methylimidazolium (C2mim) cation has been extensively studied with a variety of different anions like halide-based anions,23,24,33–36,44–48 bis(trifluoromethanesulfonyl)imide (NTf2-),27,28,38,40,50,51 ethyl sulphate (EtSO4-),25,26,37–39,49 and others. Recently, quantum computations based on density functional theory (DFT) were applied to come to a better understanding of the properties of the C2mim halide anion ILs in methanol environment.46 The assignment of vibrational bands detected in IR and Raman spectra of Cnmim PF6 (n = 2, 3, 4) has been achieved based on DFT and restricted HartreeFock (RHF) calculations.23 The molecular structure and hydrogen bond dynamics of C2mim BF4 were studied by employing Raman and IR spectroscopy together with DFT calculations.48 Raman 2 ACS Paragon Plus Environment

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spectroscopy in combination with X-ray structure analysis has been applied to investigate the phase behavior of C2mim BF4 under high-pressure conditions. It was found that C2mim BF4 can work as a superpressurized glass whose local structure is completely different from the quenched glass made by rapid cooling.34 All these results clearly demonstrate the importance of vibrational spectroscopy for the investigation of basic properties of ILs, which is ideally combined with an interpretation based on theoretical methods like DFT calculations. Electronic absorption spectra of C2mim BF4 have been reported in earlier studies35,36 and hydrogen-bonding interaction in this IL was studied at the electronic level by DFT calculations.24 The combination of an investigation of structural and electronic properties gives access to a rather detailed picture of ILs and their inter- and intramolecular interactions. The first FTIR analysis in the far-IR spectral region of C2mim+-cation-based ILs combined with various anions (SCN-, N(CN)2-, EtSO4-, and NTf2-) was reported by Ludwig and coworkers.25 Kiefer et al., who have used both IR and Raman spectroscopy, were the first to report a vibrational study of C2mim EtSO4.38 To understand the electronic structure of C2mim EtSO4, the Kiefer group used DFT calculations and correlated these results with IR, Raman, and UV-Vis absorption spectra.26 The ion-pair formation and the structures of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ILs were studied by vibrational spectroscopy (Raman and FTIR) and DFT calculations.27,38 The results pointed to an ion-pair formation in neat IL, which increases with temperature. For the understanding of the electronic structure and interactions, quantum mechanical methods (classical molecular dynamic simulations and DFT) along with FTIR and Raman spectroscopy were also applied to this IL.28,50 The application of femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS) gave access to vibrational dynamics and yielded new insights into the role of hydrogen bonding and energy transfer in C2mim NTf2 ILs.40,51 The intramolecular interactions in 1-ethyl-3-methlimidazolium thiocyanate (C2mim SCN) ion-pairs were investigated by far-infrared spectroscopy.25 The effect of dispersion on the structure and dynamics of these ion-pairs were also examined by DFT calculations using the PBE (Perdew–Burke–Ernzerhof) functional. This theoretical analysis demonstrated that the neglect of the dispersion can result in an incorrect description of hydrogen-bonding dynamics.53 To determine the degree of charge organization in the 1-ethyl-3-methylimidazolium trifluoromethansulfonate IL, (C2mim TfO), transmission and polarized-ATR IR spectroscopy

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were used, which demonstrate that the amount of the charge organization decreases as the length of the alkyl chain increases from ethyl to octyl.41 The overview given above shows that using a combination of different optical spectroscopy techniques and theoretical simulations, detailed information about intra- and intermolecular interactions has obtained for various types of ILs. However, a thorough literature survey reveals that a systematic and comparative study of halide anions (Cl-, Br-, I-, and BF4-) with the C2mim+ cation still has not been presented in literature and a combined approach of vibrational and electronic spectroscopy as well as DFT methods would be desirable. Therefore, we were interested to see the effect of the anion size and electronegativity on the intermolecular interaction in C2mim-based ILs. The ion-pair molecular structure confirms interionic interactions and the resulting electronic and vibrational spectra are investigated by a combined approach of Raman scattering, IR absorption, UV-Vis absorption spectroscopy and DFT calculations. The excited electronic states of the ion pairs were calculated by TD-DFT and compared with the UV-Vis absorption spectra. The change in conformation and vibrational frequencies of the cation due to the interaction with the halide anions (Cl-, Br-, I-, and BF4-) are studied by determining the geometric parameters, bond-length and bond angles, and also the natural bond orbital (NBO) electron density parameters.

2. EXPERIMENTAL METHODS 2.1 Chemical and Sample Preparations. The IL C2mim Cl (C6H11ClN2, purity better than 98%, CAS No. 65039-09-0, water content confirmed through Karl Fischer Titration < 0.1%) was purchased from Alfa Aesar. The other three ILs C2mim Br (C6H11BrN2, purity higher than 97.0%, CAS No. 65039-08-9, water content < 0.02%), IL C2mim I (C6H11IN2, purity higher than 97.0%, CAS No. 35935-34-3, water content < 0.5%) and IL C2mim BF4 (C6H11BF4N2, purity better than 98%, CAS No. 143314-16-3, water content < 0.02 %) were purchased from Sigma Aldrich. All the samples (ILs) were stored and manipulated under nitrogen atmosphere without the inclusion of the moisture (VAC glovebox, Br > I > BF4. The higher electronegativity and the smaller anion (Cl-) results in a stronger interaction with the cation compared to less electronegative, large anions. ATR-IR spectra of C2mim X (X = Cl, Br, I, and BF4) ion-pair interactions in the range of 27003300 cm-1 confirm the observations made in the Raman spectra. The peaks from 2800-3050 cm-1 can be attributed to the alkyl C-H stretching vibration and the range 3050-3200 cm-1 is attributed to the C-H ring stretching modes. The peaks at 3060, 3067, 3077 and 3124 cm-1 are assigned to the C2-H9 ring stretching vibrational modes for C2mim X (X = Cl, Br, I, and BF4), respectively. Again, significant spectral changes are observed in this region and while the modes in the alkyl chain region (2800-3050 cm-1) do not change. It is evident from Figure 2 (right panel) that the magnitude of the shifts of the C2-H9 stretching bands, indicate that the strengths of interactions decreases in the order Cl > Br > I > BF4.

Table 1. Wavenumbers of vibrational modes for C2mim+ and the energetically optimized configuration of C2mim Cl calculated using DFT and observed in the Raman spectra. Vibration

νs (C4-H10, C5-H11) νas (C4-H10, C5-H11) ν (C2-H9) νas (H12,13-C6- H14) νas (H12-C6-H13) νas (H15-C7-H16, H17,18-C8H19) νas (H15-C7-H16, H17,18-C8H19) νas (H15-C7-,H16); νas (H17,18C8-H19) νs(H15-C7-H16)

Computational C2mim+ (cm-1) 3308 (10)

C2mim Cl (cm-1) 3171 (2)

3290 (16) 3292 (29) 3189 (0.3) 3178 (0.1) 3160 (8)

3152 (6) 2544 (1420) 3047 (1) 3028 (6) 3020 (4)

3154 (5)

3012 (8)

3138 (2)

2997 (14)

3098 (6)

2954 (33)


Experimental C2mim Cl (cm-1) IR Raman 3143 (M) 3154 (W)

3060 (S)

3136 (W) 3061 (M)

2980 (M)

2985 (M)

2964 (M) 2940 (W)

2946 (S)

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νs (H12,13-C6-H14) νs (H17-C8-H18,19) νas (H17-C8-H18,19) νas (H12,13-C6- H14) r (N1-C2-H9, N1-C5-H11); νs (C2-N1); νs(C4 =C5) ν (C4=C5) sci (H12,13-C6- H14) sci (H15-C7- H16); sci (H17C8- H18) sci (H17-C8- H18,19) sci (H15-C7- H16); sci (H17C8- H18) t (H12,13-C6-H14) δas(H17-C8-H18,19) sci (H12,13-C6- H14); νas (C2N1-C4); t(H15-C7- H16) wag (H12,13-C6- H14) νs (Ring ip); ν (N)H15-C7H16; ν (N)H17,18-C8- H19 wag (H17,19-C8- H18) νs (C7-C8); wag (H17,19-C8H18); sci (H12-C6- H13) (N)H15-C7-H16; νas (C6-N1C5); νas (Ring ip) νs (Ring ip); ν (N)CH2CN; νs (C7-C8) r (C4-H10, C5-H11); t (H15-C7- H16) t (C7- C8); t (H15-C7- H16); δ(C2-H9) ν (N)CH2CN; ν (N)C8H17,18,19; δ(C2-H9) t (H12,13-C6-H14) t (C7-C8); wag (H17,19-C8- H18) sci (C4-H10, C5-H11) ν (N)CH2CN; ν (N)C8H17,18,19; t(N)(H12-C6H13,14); νs (Ring ip) sci (C4-H10, C5-H11) ν (C2-N1); wag (H12,13-C6H14); δ(C2-H9) νs (C7-C8); νs (Ring ip) ν (C-N); γ(C2-H9) γ [δ(C2-H9)] δas (Ring ip); νs (C7-C8) tor (C4-H10, C5-H11) δ[γ(C2-H9)]; ν (C-C) r (H15-C7- H16); wag (H17C8-H18,19) γ [wag (C4-H10, C5-H11)] δas (Ring ip); ν CH2(N), CH3(N)CN γ [δas(C2-N3-C4), δas(N3-C7C8)] γ [δ (C2-N1); (C4-C5)]

3085 (4) 3063 (4)

2946 (22) 2920 (24)

2930 (S) 2864 (W) 2886 (M) 2844 (M)

1639 (68)

1630 (19)

1631 (33) 1520 (11) 1514 (16)

1618 (75) 1520 (39) 1508 (12)

1498 (17) 1496 (6)

1500 (11)

1569 (S)

1568 (W)

1475 (5)

1496 (2) 1486 (2) 1480 (24)

1452 (W)

1494 (W) 1456 (W)

1467 (1) 1444 (7)

1469 (4) 1432 (12)

1427 (W)

1420 (M)

1490 (17)

1432 (9) 1422 (10) 1395 (9)

1397 (8)

1387 (W)

1390 (M)

1380 (12)

1392 (4)

1335 (W)

1342 (S)

1322 (0.7)

1322 (54)

1278 (0.06)

1295 (26)

1200 (104)

1218 (115)

1157 (0.03) 1146 (2) 1133 (7) 1114 (4)

1162 (0.4) 1150 (5)

1111 (5) 1061 (2) 1053 (0.7)

1267 (W) 1171 (S)

1124 (4)

1125 (W)

1115 (1) 1105 (46) 1063 (5) 1044 (23) 1030 (62) 975 (8) 868 (0.2)

1092 (W) 1029 (W)

1091 (M) 1023 (S)

958 (W)

958 (M)

811 (6)

802 (W)

769(33) 716 (7)

748 (50) 725 (10)

761 (W) 703 (W)

668 (16)

671 (16)

639 (12)

637 (5)

977 (3) 897 (0.05) 857 (32) 808 (1)

902 (W)


649 (W)

698 (M)


δ(N1-C2-N3)ip; νs (Ring ip); νs (C7-C8) δ (N1-Me); δ (N3-Et) δ (N3-Et.) r (H15-C7- H16); tor (H17-C8H18,19); wag (H12,13-C6- H14) δ(N1-Me) t(H17-C8-H18,19) tor(Cat-an) δ (Cat-an)

606 (2)

610 (1)

434 (0.4) 384 (0.4) 296 (0.2)

433 (5) 402 306

234 (2) 211 (0.2)

232 (4) 219 188 69

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596 (W)

598 (S) 435 (W)

The C-H region for C2mim+ and C2mim Cl is scaled by 0.958.

The IR intensities are shown in km/mol in parentheses. W - weak; M - medium; S - strong; ν - stretch; νs - symmetric stretch; νas - asymmetric stretch; δ - bend; δas - asymmetric bend; wag - wagging; t - twisting; r - rocking; γ - out-ofplane; ip - in plane; tor - torsion; Cat - cation; an - anion; sci -scissor; Me- methyl; Et- ethyl.

Table 2. Wavenumbers of vibrational modes for C2mim+ and the energetically optimized configuration of C2mim Br calculated using DFT and observed in the Raman spectra. Vibration νs (C4-H10, C5-H11) νas (C4-H10, C5-H11) ν (C2-H9) νas (H12,13-C6- H14) νas (H12-C6- H13) νas (H15-C7-H16, H17,18-C8H19) νas (H15-C7-H16, H17,18-C8H19) νas (H15-C7-H16, H17,18-C8H19) νs (H15-C7-H16) νs (H12,13-C6-H14) νs (H17,18-C8-H19) νas (H17,18-C8-H19) νas (H12,13-C6- H14) r (N1-C2-H9); r (N1-C5-H11) ; νs (C2-N1 and C4 =C5) ν (C4=C5) sci (H12,13-C6- H14) sci (H15-C7- H16); sci (H17C8- H18) sci (H17-C8- H18,19) sci (H15-C7- H16); sci (H17C8- H18) t (H12,13-C6-H14) δas(H17-C8-H18,19) sci (H12,13-C6- H14); νas (C2N1-C4); t(H15-C7- H16) wag (H12,13-C6- H14) νs (Ring ip); ν (N)H15-C7H16; ν (N) H17,18-C8- H19 wag (H17,18-C8- H19) wag (H17,19-C8- H18); νs (C7C8); sci (H12-C6- H13)


C2mim Br (cm-1) 3308 (2)

3290 (16) 3292 (29) 3189 (0.3) 3178 (0.1) 3160 (8)

3289 (5) 2773 (1277) 3179 (0.9) 3161 (5) 3151 (4)

3154 (5)

3142 (5)

3138 (2)

3127 (15)

3098 (6) 3085 (4) 3063 (4)

3080 (36) 3074 (21) 3048 (24)

Experimental C2mim Br (cm-1) IR Raman 3144 (M) 3144 (W) 3067 (S)

3074 (S)

2980 (M)

2980 (M)

2956(W) 2936 (W)

2942 (M)

2863 (W) 2874 (M) 2827 (M)

1639 (68)

1632 (22)

1631 (33) 1520 (11) 1514 (16)

1622 (74) 1521 (34) 1508 (12)

1499 (17) 1496 (6)

1500 (11)

1490 (17) 1475 (5)

1496 (12) 1488 (1) 1481 (22)

1467 (1) 1444 (7)

1470 (4) 1435 (10)

1432 (9) 1425 (9)


1569 (S)

1567 (W)

1455 (W)

1482 (M) 1445 (S) 1427 (S)

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(N)H15-C7-H16; νas (C6-N1C5); νas (Ring ip) νs (Ring ip); ν (N)CH2CN; νs (C7-C8) r (C4-H10, C5-H11); t (H15-C7- H16) t (C7- C8); t (H15-C7- H16); δ(C2-H9)

1395 (9)

1398 (8)

1381 (W)

1390 (M)

1380 (12)

1390 (4)

1341 (W)

1348 (M)

1322 (0.7)

1324 (33)

1278 (0.06)

1292 (16)

ν (N)CH2CN; ν (N)C8H17,18,19; δ(C2-H9) t (H12,13-C6-H14) t (C7-C8); wag (H17,19-C8H18) sci (C4-H10, C5-H11) ν (N)CH2CN; ν (N)C8H17,18,19; t(N)(H12-C6H13,14); νs (Ring ip) sci (C4-H10, C5-H11) ν (C2-N1); wag (H12,13-C6H14); δ(C2-H9) νs (C7-C8); νs (Ring ip) ν (C-N); γ(C2-H9)

1200 (104)

1216 (107)

1157 (0.03) 1146 (2)

1162 (0.3) 1150 (4)

1133 (7) 1114 (4)

1123 (3)

1111 (5)

1117 (0.3) 1110 (37)

1168 (S)

1114 (S)

1061 (2) 1053 (0.7)

1063 (3) 1045 (35) 992 (22)

1086 (W) 1034 (W)

1091 (M) 1023 (S)

977 (3) 897 (0.05) 857 (32) 808 (1)

975 (8) 871 (0.2)

955 (W)

956 (W)

812 (7)

803 (W)

769 (33)

750 (50)

788(W)

716 (7)

724 (9)

698 (W)

668 (16)

670 (15)

649 (W)

653 (W)

639 (12)

637 (6)

606 (2)

610 (0.6)

597 (W)

598 (M)

434 (0.4) 384 (0.4) 296 (0.2)

433 (2) 399 (0.5) 302 (0.3)

234 (2) 211 (0.2)

232 (2) 215 138 (33) 58 (5)

γ [δ(C2-H9)] δas (Ring ip); νs (C7-C8) tor (C4-H10, C5-H11) δ[γ(C2-H9)]; ν (C-C) r (H15-C7- H16); wag (H17C8-H18,19) γ [wag (C4-H10, C5-H11)] δas (Ring ip); ν CH2(N), CH3(N)CN γ [δas (C2-N3-C4), δas(N3-C7C8)] γ [δ (C2-N1); (C4-C5)] δ(N1-C2-N3)ip; νs (Ring ip); νs (C7-C8) δ (N1-Me); δ (N3-Et) δ (N3-Et.) r (H15-C7- H16); tor (H17-C8H18,19); wag(H12,13-C6-H14) δ (N1-Me) t(H17-C8-H18,19) tor (Cat-an) δ (Cat-an)

1258 (W)

889 (M)

The C-H region for C2mim+ and C2mim Br is scaled by 0.999.

408 (M)




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Table 3. Wavenumbers of vibrational modes for C2mim+ and the energetically optimized configuration of C2mim I calculated using DFT and observed in the Raman spectra. Vibration


C2mim I (cm-1) 3158 (1)

νas (C4-H10, C5-H11) νs (C2-H9) νas (H12,13-C6- H14) νas (H12-C6-H13) νas (H15-C7-H16, H17,18-C8-H19) νas (H15-C7-H16, H17,18-C8-H19) νas (H15-C7-,H16); νas (H17,18C8-H19) νs (H15-C7-H16) νs (H12,13-C6-H14) νs (H17-C8-H18,19) νas (H17-C8-H18,19) νas (H12-C6-H13) r (N1-C2-H9); (N1-C5-H11); νs (C2-N1); νs (C4 =C5) ν(C4=C5)

3290 (16) 3292 (28) 3189 (0.3) 3178 (0.2) 3160 (8) 3154 (5) 3137 (2)

3140 (5) 2774 (1147) 3051 (0.8) 3036 (6) 3023 (6) 3015 (4) 2999 (16)

3098 (6) 3085 (4) 3063 (3)

2946 (36) 2935 (29) 2907 (28)

νs (C4-H10, C5-H11)

3080 (S)

3090 (S)

2983 (M)

3005 (M) 2978 (S)

2943 (W)

2951 (W) 2936 (M)

2877 (W) 2874 (W) 2821 (W)

1639 (68)

1644 (41)

1631 (33)

1638 (64)

sci (H12,13-C6- H14)

1520 (11)

1540 (40)

sci (H15-C7- H16); sci (H17-C8H18) sci (H17-C8- H18,19)

1514 (16)

1529 (12)

sci (H15-C7- H16); sci (H17-C8H18) t (H12,13-C6-H14)

1496 (6)

1521 (14)

1490 (17)

1519 (11)

1571 (S)

1566 (M)

1499 (17)

δas (H17-C8-H18,19) sci (H12,13-C6- H14); νas (C2N1-C4); t(H15-C7- H16) wag (H12,13-C6- H14) νs (Ring ip); ν (N)H15-C7-H16; (N)H17,18-C8- H19 wag (H17,19-C8- H18) νs (C7-C8); wag (H17,19-C8H18); sci (H12-C6- H13) (N)H15-C7-H16; νas (C6-N1C5); νas (Ring ip) νs (Ring ip); ν (N)CH2CN; νs (C7-C8) r (C4-H10, C5-H11); t (H15-C7- H16) t (C7- C8); t (H15-C7- H16); δ(C2-H9) ν (N)CH2CN; ν (N)C8H17,18,19; δ(C2-H9) t (H12,13-C6-H14) t (C7-C8); wag (H17,19-C8- H18) sci (C4-H10, C5-H11) ν (N)CH2CN; ν (N)C8H17,18,19; t(N)(H12-C6- H13,14); νs (Ring ip) sci (C4-H10, C5-H11)

Experimental C2mim I (cm-1) IR Raman 3145 (M) 3142 (M)

1512 (2)

1478 (M)

1475 (5)

1494 (23)

1462 (W)

1450 (M)

1467 (1) 1444 (7)

1481 (6) 1452 (9)

1428 (W)

1426 (S)

1432 (9) 1443 (7) 1395 (9)

1405 (8)

1385 (W)

1389 (M)

1380 (12)

1399 (6)

1335 (W)

1346 (S)

1322 (0.6)

1332 (15)

1278 (0.06)

1297 (9)

1200 (104)

1220 (96)

1157 (0.03) 1146 (2) 1133 (7) 1114 (1)

1170 (1) 1158 (3)

1168 (S)

1129 (3)

1110 (W)

1122 (14)


1258 (W)

1111 (M)

Page 15 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


νs (C2-N1); wag (H12,13-C6H14); δ(C2-H9) νs (C7-C8); νs (Ring ip) ν (C-N); γ(C2-H9)

1111 (5)

1119 (6)

1061 (2) 1053 (0.7)

1067 (4) 1051 (23) 983 (1)

1090 (W) 1029 (W)

1091 (M) 1023 (S)

δas (Ring ip); νs (C7-C8) tor (C4-H10, C5-H11) δ[γ(C2-H9)]; ν (C-C) r (H15-C7- H16); wag (H17-C8H18,19) γ [wag (C4-H10, C5-H11)] δas (Ring ip); ν CH2(N), CH3(N)CN γ [δas (C2-N3-C4), δas(N3-C7C8)] γ [δ (C2-N1); (C4-C5)] δ(N1-C2-N3)ip; νs (Ring ip); νs (C7-C8)

977 (3) 897 (0.05) 857 (32) 808 (1)

931 (65) 858 (0.1)

956 (W)

958 (M)

816 (5)

825 (W)

769 (33)

737 (9)

770 (W)

774 (M)

716 (7)

724 (9)

702 (W)

703 (M)

668 (16)

668 (20)

639 (12)

636 (11)

606 (2)

611 (0.2)

δ (N1-Me); δ (N3-Et) δ (N3-Et.) r (H15-C7- H16); tor (H17-C8H18,19); wag(H12,13-C6-H14) δ (N1-Me) t(H17-C8-H18,19) tor (Cat-an) δ (Cat-an)

434 (0.4) 384 (0.4) 296 (0.2)

435 (2) 398 (0.8) 304 (0.1)

234 (2) 211 (0.2)

232 (2) 221 (0.1) 131 (6) 48 (4.3)

γ [δ(C2-H9)]

854 (M)

620 (S)

596 (W)

598 (S) 493 (W)

The C-H region for C2mim+ and C2mim I is scaled by 0.947.


Table 4. Wavenumbers of vibrational modes for C2mim+ and the energetically optimized configuration of C2mim BF4 calculated using DFT and observed in the Raman spectra. Vibration

νs (C4-H10, C5-H11) νas (C4-H10, C5-H11) ν (C2-H9) νas (H12,13-C6- H14) νas (H12-C6-H13) νas (H15-C7-H16, H17,18-C8-H19) νas (H12-C6-H13,14) νas ( H15-C7-H16); νas ( H17,18-C8-H19) νas (H17-C8-H18,19) νs (H15-C7-H16) νs (H12,13-C6-H14) νs (H12,13-C6-H14)


C2mim BF4 (cm-1) 3111 (1)

3290 (16) 3292 (29) 3189 (0.3) 3178 (0.1) 3160 (8)

3092 (6) 3087 (87) 2986 (10)

3154 (5) 3138 (2) 3098 (6) 3085 (4) 3063 (4)

2976 (0.7) 2972 (5) 2967 (1) 2940 (17) 2896 (28) 2884 (34) 2869 (22)


IR

Experimental C2mim BF4 (cm-1) Raman

3165 (S)

3185(M)

3124 (S)

3125 (W)

2990 (W)

2976 (S)

2955 (M) 2934 (W)


νs (H17-C8-H18,19) r (N1-C2-H9); (N1-C5-H11); νs (C2-N1);(C4 =C5) ν (N)CH2, (N)CH3CN sci (H12-C6- H13); δs ( C6- H14) sci (H15-C7- H16); sci (H17-C8- H18) sci (H17-C8- H18,19) sci (H12,13-C6-H14) sci (H15-C7- H16); sci (H17-C8- H18) sci (H17-C8- H18,19) t (H12,13-C6- H14) wag (H12-C6- H13); νs (C2-N1-C4); t (H15-C7- H16) δas(H12-C6-H13,14) wag (H15-C7- H16); νas (C2-N1-C5, C2-N3-C4); sci (H12,13-C6H14) ν (N)CH3CN wag (H15-C7-H16); ν (N)H17,18-C8-H19; wag (H12-C6- H13) wag (H15-C7- H16); νs (N1-CH3, N3CH2CH3) r (N3-C2-H9); r (H10-C4, H11-C5) δ (H17,19-C8- H18); r (C4-H10, C5-H11); t (H15-C7-H16) ν (BF4) νas (CH3-N1, N2-C7-H15,16); r (C4-H10, C5-H11); r (C2-H9) t (H12-C6- H13,14); νs (BF4) wag (C8-H18-H19); νs (BF4) r (H15-C7-H16); wag (H17-C8-H18,19) νs (C7-C8); t (N)(C6- H12,13,14);νs (Ring ip) νs (H11-C5=C4-H10); δ (N1-C2-H9) νas (N1-C4-H10-C5-H11) δ (Ring) r (C4-H10; C5-H11 ) νas (Ring ip), ν(N-Me) νas (Ring ip); t (N1-CH3) δ (Ring ip); νs (N1-C6) νas (BF4) νs (C7-C8); νas (F23-B20-F24) νs (C7-C8) r (C2-H9); νas (F23-B20-F24); ν (C7-C8) γ (C2-H9) tor (H11-C5=C4-H10) wag (H17-C8-H18,19); wag (H15-C7-H16) γ [wag( H11-C5=C4-H10)] νs (BF4) δ (N3-Et); δ (N1-Me) γ [δas (C2-N3-C4), δas(N3-C7-C8)] γ [δ (C2-N1-C5)] νs (N1-Me); νs (N3-Et) νs (BF4) νas (BF4)

1639 (68)

1640 (70)

1631(33) 1520 (10) 1514 (16) 1498 (17)

1633 (19) 1520 (13) 1514 (8)

1496 (6)

Page 16 of 35

2890 (W)

2892 (M)

1576 (W)

1575 (W)

1507 (15) 1502 (10) 1494 (8)

1489 (17) 1475 (5)

1483 (4)

1467 (1) 1445 (6)

1470 (5) 1439 (5)

1432 (8) 1394 (9)

1429 (8) 1396 (4)

1430 (W) 1392 (W)

1431 (M) 1398 (W)

1380 (12)

1384 (14)

1336 (W)

1342 (M)

1322 (0.7) 1278 (0.06)

1325 (0.5) 1281 (2)

1200 (104)

1205 (131)

1157 (0.03) 1146 (2)

1170 (375) 1164 (180) 1153 (2)

1463 (M)

1285 (W) 1261 (W)

1170 (M) 1121 (W)

1133 (7) 1114 (3) 1111 (3) 1061 (2)

1131 (10) 1122 (10) 1117 (14) 1058 (13)

1090 (S) 1024 (M) 1052 (S)

1053 (0.7) 994 (283) 980 (105)

1017 (S)

977 (3) 969 (215) 857 (31) 808 (1) 769 (33) 716 (7) 668 (16) 639 (12) 606 (2)

897 (80) 875 (0.2) 811 (8) 749 (33) 737 (10) 723 (9) 670 (40) 642 (18) 615 (1) 510 (19) 502 (4)


960 (W)

754 (M) 700 (W) 647 (M) 624 (M) 597 (M) 521 (M)

763 (S) 704 (W)

598 (S) 522 (W)

Page 17 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


δ (BF4) r (N1-Me); r (N3-Et) δ (N3-Et) δ (BF4) sci (F21-B20-F22); sci (F21-B20-F22) wag (N3-Et); wag (N1-Me) δ (N1-Me) tor (H17-C8-H18,19) tor(Cat-an) δ (Cat-an)

434 (0.3) 384 (0.3)

296 (0.1) 234 (2) 211 (0.1)

500 (1) 445 (0.9) 395 (2) 348 (0.1) 345 (0.08) 305 (1) 243 (1) 218 (1) 179 (6) 84

The C-H region for C2mim+ and C2mim BF4 is scaled by 0.938.


From the Raman and IR discussion, we conclude that the Raman spectra are of special importance for the analysis of the isomeric conformation of the alkyl chains. The ATR-IR spectra are helpful for the investigation of the hydrogen-bond formation between cation and anion. 4.1.2 Theoretical Results. DFT-calculated vibrational wavenumbers of C2mim X (X = Cl, Br, I, and BF4) ILs together with the optimized geometries help to understand the molecular structures and interionic interactions. The numbering scheme for the C2mim+ cation and the anions is shown in Figure 3. DFT-computed Raman (top) and IR (bottom) spectra in the fingerprint region 4501700 cm-1 for C2mim X (X = Cl, Br, I, and BF4) are depicted in Figure S1. The fingerprint region does not show significant changes in frequency going from C2mim Cl to C2mim BF4 and reproduces the same spectral range in the spectra obtained from the experiments (compare Figure 1). The cation-anion interaction can be examined by analyzing the DFT-calculated C-H stretch region between 2500 and 3400 cm-1 shown in Figure S2. The anion interaction is preferentially at the C2H9 position, which will be discussed in section 4.2 (see Figure 4). In the neat C2mim+ cation, the C2-H9 frequency was calculated as 3292 cm-1. Upon halide interaction, the C2-H9 frequency of the cation shifts to 2544, 2773, 2774, and 3087 cm-1 for C2mim X (X = Cl, Br, I, and BF4), respectively. The C2-H9 frequency shows a decreasing red shift in the order of C2mim Cl > C2mim Br > C2mim I > C2mim BF4. To understand the variation of the red shift observed for the C2-H9 stretch vibration (marker band for hydrogen bonding) of C2mim X (X = Cl, Br, I, and BF4), an analysis of the geometrical parameters, bond length and bond angle (see Table 5), and NBO charge (see Table S2) has been 17 ACS Paragon Plus Environment


performed. A significant change in the bond length of C2-H9 was calculated when the anion was varied (see Table 5). The bond length of C2-H9 in the C2mim+ cation is calculated to be 1.0781 Å. Upon the halide interaction, the bond length increases to 1.1213, 1.1122, 1.1043, and 1.0782 Å for C2mim X (X = Cl, Br, I, and BF4), respectively. The increasing bond length in C2-H9 shows the reduction in force constant of the vibrational mode and results in the observed red shifts. The results of the NBO charge analysis for the cation, anion, and cation-anion are summarized in Table S2. In the cation case, the average charge density on C2-H9 was calculated to be Δρ = 0.54. However, upon the interaction with the anions, the charge density changed to Δρ = 0.57, 0.58, 0.59 and 0.61 for C2mim X (X = Cl, Br, I, and BF4), respectively. It is evident that due to the significant electron-density transfer from Cl to the C2-H9 bond (in neat Cl the charge was -1 and upon interaction with the cation the charge on Cl was found to be -0.86) the elongation of C2-H9 is relatively large, which results into the observed red shift. The charge transfer from the anions to the C2-H9 bond decreases in the order Cl > Br > I > BF4 (see Table S2). These findings support the assumption that the charge transfer is closely linked to the hydrogen bonding, which plays an important role for the interionic interactions, facilitating an efficient charge transfer between the ionic entities. In conclusion the vibrational analysis results in two interesting points: (i) the contribution of hydrogen bonding in the ILs increases with increasing electronegativity of the anion and (ii) as the anion size increases the cation-anion hydrogen bonding becomes weaker.

4.2 Molecular Structure, Optimized Geometries and Energetic Stability of Ion-Pairs Figure 3 shows the atomic numbering scheme of the DFT-computed (dispersion-corrected wB97XD method) geometrically minimized electronic structure of cation and the BF4- anion. Since the alkyl chain of the cation is short, its rotation around the C-N bond is not hindered,32,72,73 therefore results into two different C2mim+ cation conformers as nonplanar and planar, presented in Figure 3 a and b, respectively. The experimental results also indicate that planar and nonplanar conformations exist in the ILs.72 The nonplanar conformer (gauche) is energetically more stable than the planar conformer as it has a lower energy.59 However, the difference between both conformational energies was found to be less than the thermal energy kBT, which leaves the possibility of having both conformations in the equilibrium state.


Page 18 of 35

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Figure 3. DFT-optimized structures with atomic numbering scheme. (a) C2mim+ cation gauche conformer (non-planar), (b) C2mim+ cation trans conformer (planar), (c) BF4- anion.

In order to get the minima of the energy landscape, all the possibilities of ion-pair interactions were examined and calculated. To investigate the interactions between cation-anion, the different anions were positioned at the C2, (C4 +C5), (alkyl + C4), and (methyl + C5) units of the C2mim+ cation (see Figure 3 for atomic numbering). In order to investigate the strength of the ion-pair interaction at various sites, the binding energy was calculated by the following expression EBE = [Eion-pair - (Ecation + Eanion)] + ΔEBSSE

(1)

where ΔEBSSE corresponds to the energy correction taking into account the basis set superposition error (BSSE) using the counterpoise method, which is applied to all configurations of the ionpairs in the different ILs. The calculated optimized energies, relative energies, BSSE-corrected energies, and binding energies (BE) for all the ILs and the various configurations are listed in Table 6. Based on the energetic stabilities at least four configurations for each ILs were identified. The configuration 1 was found to represent the energetically most stable geometry having strong bond strength at the C2-H9 position (see Figure 4). This trend was observed for all the ILs, C2mim X 19 ACS Paragon Plus Environment


(X = Cl, Br, I, and BF4) as depicted in Figure 4. The ion-pair interactions of C2mim X (X = Cl, Br, and I) involve a specific hydrogen bond between the cation and anion at the C2-H9 position of the imidazolium ring, whereas in C2mim BF4 multiple (non-specific) hydrogen bonds between the cation and the anion contribute. Also for C2mim BF4, configuration 1 was energetically favored, however in all the configurations of the C2mim BF4 ion pair, the BF4- anion remained near the C2-H9 position and was located above the imidazolium ring. Figure 4 shows the interionic interactions in the ILs through C-H•••X (X = Cl, Br, I, and F in BF4) type hydrogen bonds. In literature, the details of the C-H•••X-type hydrogen bonding are still debated.

Figure 4. DFT-computed optimized molecular structures and geometries of the C2mim X (X = Cl, Br, I, and BF4) ion-pairs in four configurations (compare Table 6).


Page 20 of 35

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For assuming intermolecular hydrogen bonds, we have adopted a criterion already used in an earlier study.74 We consider an interaction to be strong if the bond length is less than 2.3 Å and weak if this length is varying between 2.3 and 2.6 Å. Based on this, the C-H•••X (X = Cl, Br, I, and F) interaction between the halide and BF4- anions and the most acidic hydrogen atom of the imidazolium ring of the cation at C2-H9 is found to be strong. In C2mim Cl, molecular dynamics simulations suggested that the interaction between methyl / ethyl group of the cation and the anion together with the C2H-anion interaction is also possible. 45 The measurement of the

19F

NMR

chemical shift provides experimental evidence for the alkyl-chain interaction in BF4-based ILs.75 Here, in the presented work, the interaction between the methyl and alkyl group of the imidazolium ring and the anions has been confirmed. There is also a C4/5-X interaction, though it is comparatively weak. Table 5 summarized the geometry analysis (bond lengths, bond angles, and dihedral angles) of the free cation and lowest-energy geometries of the ion-pairs of all the ILs discussed here. The hydrogen bond lengths between cation and anion at the C2-H9 position are calculated as 1.99, 2.18, and 2.45 Å for C2mim Cl, C2mim Br, and C2mim I, respectively. Interestingly, the Cl- and Br- anions are found to be in the plane of the cation imidazolium ring while the I- anion is positioned slightly off the plane. This is confirmed by the dihedral angles of type ∠N3-C2-H9-X (X = Cl, Br, I), which are listed in Table 5. The crystal structure of the C2mim Cl ion pair has been studied earlier and it was demonstrated that the C2mim+ cation is associated with the three nearest Cl- anions.76 The average distance between Cl- and the imidazolium ring carbon atoms was found to be 3.55 Å. The arrangement of the Br- anion near the C2mim+ cation demonstrates a significant similarity between liquid and crystal phase; the anions are located somewhat more symmetrically near to the ring in the liquid.77 The X-ray structure of C2mim I reveals the presence of a bifurcated hydrogen bonding r(C2-H9...I) with a bond length of 2.93 Å.78 Table 5. DFT-computed geometries of the lowest-energy configurations of the ion-pairs C2mim X (X = Cl, Br, I, and BF4) in the gas phase at the wB97XD/6-311++G(d,p) level of theory.

C2mim+

C2mim Cl

N1−C2

1.3309

1.3327

C2−N3

1.3295

1.3303

C2mim Br

C2mim I

C2mim BF4

1.3321

1.3338

1.3281

1.3295

1.3315

1.3270

Bond Length (Å)



Page 22 of 35

N3−C4

1.3761

1.3776

1.3775

1.3794

1.3772

C4−C5

1.3562

1.3557

1.3558

1.3599

1.3549

C5−N1

1.3766

1.3807

1.3803

1.3814

1.3769

C2−H9

1.0781

1.1213

1.1122

1.1043

1.0782

C5−H11

1.0771

1.0765

1.0765

1.0786

1.0762

C4−H10

1.0770

1.0766

1.0766

1.0787

1.0760

N1−C6

1.4636

1.4569

1.4575

1.4598

1.4600

N3−C7

1.4746

1.4730

1.4732

1.4736

1.4701

C6−H12

1.0892

1.0902

1.0901

1.0897

1.0888

C6−H13

1.0891

1.0900

1.0899

1.0893

1.0904

C6−H14

1.0882

1.0888

1.0889

1.0886

1.0892

C7−H15

1.0906

1.0913

1.0916

1.0910

1.0887

C7−H16

1.0909

1.0916

1.0916

1.0915

1.0923

C8−H17

1.0917

1.0920

1.0920

1.0919

1.0902

C8−H18

1.0918

1.0936

1.0936

1.0935

1.0931

C8−H19

1.0909

1.0916

1.0915

1.0913

1.0915

B20-F21

1.4228

B20-F22

1.4299

B20-F23

1.3720

B20-F24

1.4286

H9-F23

2.2298

H9-F24

2.2292

H14-F23

2.1870

Cl20-H9

1.990

Br20-H9

2.1807

I20-H9

2.4513 Angle (deg)

∠N1C2H9

125.52

129.58

128.67

127.35

125.46

∠N3C2H9

125.51

122.39

123.08

123.95

125.44

∠C2N1C6

125.98

124.20

124.24

124.29

124.70

∠C2N3C7

125.87

123.73

123.97

124.18

124.73

∠N3C7C8

111.82

111.40

111.38

111.39

111.10

∠C2N3C7C8

-105.74

-87.175

-90.89

-95.01

-107.04

∠C4N3C7C8

71.53

88.189

84.20

78.80

62.65

Dihedral Angle (deg)

∠N3C2H9Cl ∠N3C2H9Br

13.7 20 44.05

∠N3C2H9I

It is found that the hydrogen bond is stronger for the small anion Cl- than for the larger anions Brand I-. Obviously, the bond strength scales inversely with the atomic size of the anion. The C2mim BF4-case is different since here instead of an F- anion a tetrafluoroborate anion (BF4-), is involved. Here, the charge is distributed over a wider range such that the electrostatic force between the ion-pair is relatively weak, resulting in a longer bond length compared to the Cl-- and Br--based ILs. For the most stable geometries, the BE was calculated to be -93.61, -88.49, -43.77, and 22 ACS Paragon Plus Environment

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143.98 kcal/mol for C2mim X (X = Cl, Br, I, and BF4), respectively. For all the ILs, configuration 1 shows the largest BE and thus the strongest interaction. Table 6. Optimized energy (in Hartree), relative optimized energy (in kcal/mol), and binding energy (in kcal/mol) of the configurations of the ion pairs C2mim X (X = Cl, Br, I, and BF4). Without BSSE Ionic Liquids

C2mim Cl

C2mim Br

C2mim I

C2mim BF4

With BSSE

Configurations

Optimized Energy

Relative Energy

Optimized Energy

Relative Energy

BE

Conf.1 Conf.2 Conf.3 Conf.4

-804.9525 -804.9382 -804.9400 -804.9285

(ΔE = 0) (ΔE = +8.98) (ΔE = +7.82) (ΔE = +15.05)

-804.9512 -804.9371 -804.9388 -804.9275

(ΔE = 0) (ΔE =8.86) (ΔE =7.75) (ΔE =14.84)

-93.61 -84.51 -85.73 -78.36


-2918.9238 -2918.9099 -2918.9119 -2918.9017

(ΔE = 0) (ΔE = +8.72) (ΔE = +7.49) (ΔE = +13.87)

-2918.9234 -2918.9097 -2918.9116 -2918.9015

(ΔE = 0) (ΔE = 8.64) (ΔE = 7.45) (ΔE = 13.79)

-88.49 -79.72 -80.96 -74.54


-7264.6975 -7264.6836 -7264.6862 -7264.6774

(ΔE = 0) (ΔE = +8.67) (ΔE = +7.09) (ΔE = +12.56)

-7264.6969 -7264.6831 -7264.6855 -7264.6768

(ΔE = 0) (ΔE = 8.63) (ΔE = 7.10) (ΔE = 12.5)

-43.77 -35.05 -36.67 -31.19


-769.2176 -769.2174 -769.2161 -769.2174

(ΔE = 0) (ΔE = +0.12) (ΔE = +0.91) (ΔE = +0.11)

-769.2132 -769.2132 -769.2132 -769.2143

( ΔE = 0) ( ΔE = 0) ( ΔE = 0) ( ΔE = -0.7)

-143.98 -143.74 -142.15 -143.03

To understand the formation of clusters of IL ion-pairs, two ion-pairs of each type of IL were considered and their geometry calculated using dispersion-corrected the wB97XD / 6311++G(d,p) level of theory. This cluster-model can provide a better understanding of the aggregated form of the ILs. Figure 5 depicts the energy-optimized ion-pair clusters together with the hydrogen bond distances for all ILs considered in this work.



Figure 5. DFT-computed optimized structures for clusters formed from two ion pairs of C2mim X (X = Cl, Br, I, and BF4). (a) C2mim Cl, (b) C2mim Br, (c) C2mim I, and (d) C2mim BF4.

Like for single ion pairs, also the ion-pair clusters show strong hydrogen-bonding interaction of the anions with the most acidic cation site at the imidazolium ring (C2H9). However, the formation of weak hydrogen bonding seems also to be possible between the C5, C6, and C7-H sites and the anions. In the case of C2mim BF4, the anion is situated above the C2H9 group in such a way that the anion forms multiple hydrogen bonds through C2H9•••F23, C2H9•••F24, and C6H13•••F23. Overall, the optimization protocol of clusters on ILs reflects the same pattern of interactions like the one obtained for single ion pairs for all the ILs in the present study. The ESP mapped surfaces for conformers of the cation, ion-pairs and clusters of ion-pairs are presented in the Figure S3 and S4, respectively. From mapped ESP surfaces, the distribution of charge on the surface of molecule can be understood. UV-Vis absorption spectroscopy is one of the traditional tools for probing the electronic structure of a molecular system such as ILs. In this direction, Samanta and co-workers extensively studied79-81 optical properties of these molecular systems. In order to learn more about the electronic properties of these ILs, UV-Vis spectra are also investigated for all the studied ILs. We found that these imidazolium based ILs are not transparent in the entire UV region and a long absorption tail in the visible region which is due to π-π* transitions of the imidazolium ring. This is in accordance with the results published by Samanta and co-workers who found that the long 24 ACS Paragon Plus Environment

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tail present in the absorption spectra is due intrinsically to the imidazolium cation ring of these ILs and not due to any kind of impurities.79-81 Rest of the details about UV-Vis spectra, figures and the description of the results are elaborated in the supporting information (Figure S6-S9 and Table S3).

5. Correlation between Experimental and Theoretical Results The comparison between observed experimental vibrational frequencies and DFT calculated vibrational frequencies of the most stable ion-pair conformers (by using wB97XD/6311G++(d,p)) are depicted in Figure 6 in the spectral range of 400-1700 cm-1. The vibrational spectra of the C-H stretch region in the range of 2500-3300 cm-1 are shown in Figure S5. It is well established that the DFT-calculated Raman line intensities cannot be compared in a direct way to the experimental values. The Raman scattering cross sections are directly related to the Raman intensity via ∂𝜎/∂Ω. The calculations of the Raman scattering amplitude for each normal mode of the C2mim X (X = Cl, Br, I, and BF4) vibrational spectra can be done by using the following relationship:59,82,83 𝛿𝜎𝑗

𝛿𝛺 =

24𝜋4

(

( ) 45

(𝑣0 ― 𝑣𝑗)4 1 ― exp

[

―ℎ𝑐𝑣𝑗 𝑘𝑏𝑇

( ])

ℎ 8𝜋2𝑐𝑣𝑗

)𝑆

𝑗

(2)

where 𝑣0indicates the exciting frequency, 𝑣𝑗indicate the vibrational frequency of the jth normal mode, Sj represents the theoretical Raman scattered amplitude, which was computed from DFT approach. Here, h, c, and kB are the Planck constant, speed of light, and Boltzmann constant, respectively. The Raman intensities calculated from these relation perfectly match the intensities observed experimentally. The experimental and theoretical spectra in the fingerprint region (Figure 6) demonstrate a good agreement between theoretical and experimental results without applying a scaling factor. Therefore, DFT calculations are assumed to be able to predict the cation-anion interactions in a reliable way. Generally, in the C-H stretch region (see Figure S5), the harmonic vibrational frequencies obtained from the DFT calculations overestimated the observed anharmonic vibrational frequencies. Therefore, a scaling factor was employed to achieve a better correlation. The scaled theoretical vibrational frequencies of the C2mim X (X = Cl, Br, I, and BF4) ILs now are in close agreement with the experimentally observed Raman lines. 25 ACS Paragon Plus Environment


To achieve comparable spectra in the C-H stretch region, the most intense peak was used to scale the vibrational frequencies (Tables 1-4 and Figure S5). The vibrational bands at 2946 cm-1 assigned to νs (H15-C7-H16), 3074 cm-1 assigned to νs(H12-C6- H14), 3090 cm-1 assigned to νas (H12C6-H13), and 2976 cm-1 assigned to νas(H15-C7-H16);(H17,18-C8-H19) were used to scale the frequencies for C2mim X (X = Cl, Br, I, and BF4), respectively. The scaling factors were calculated to be 0.958, 0.999, 0.947, and 0.938 for C2mim Cl, Br, I, and BF4, respectively. These calculated scaling factors are comparable to the standard scaling factor 0.957 used for the wB97XD/6-311++G(d,p) level of theory.65

Figure 6. Comparison of experimental and theoretical Raman spectra (calculated for the most stable geometry) of the C2mim X (X = Cl, Br, I, and BF4) ILs in the range of 450-1700 cm-1. (a) C2mim Cl, (b) C2mim Br, (c) C2mim I, and (d) C2mim BF4. The DFT calculations predict the C2H9 stretch vibrational wavenumber at 2543, 2773, 2776, and 3090 cm-1 for C2mim X (X = Cl, Br, I, and BF4), respectively. As shown in Figure 4, short distances C2-H•••X (X = Cl, Br, I and BF4) are characteristic of most stable ion-pair optimized 26 ACS Paragon Plus Environment

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configurations. This close proximity to the H-atoms of the imidazolium ring not only induces a red shift of the corresponding νs(C2-H9) bands, but also a substantial increase of the observed Raman intensities, which is considered to be the most reliable marker for the formation of hydrogen bonds.71,84 Therefore, both computational analysis and experimental results point to the strong hydrogen bond formed in the four ILs. The correlation of the experimental UV-Vis absorption spectra of the C2mim X (X = Cl, Br, I, and BF4) ILs with the TD-DFT calculations are shown in Figure S6. Here, except for the BF4anion, a basic agreement was achieved giving further insight into the ion-pair properties.

6. CONCLUSIONS In the present work, the electronic structures and interionic interactions in C2mim X (X = Cl, Br, I, and BF4) ILs were investigated by the combined use of experimental Raman, ATR-IR, and UVVis spectroscopy, and theoretical dispersion-including DFT calculations. The fingerprint region of the Raman spectra yields the alkyl-chain geometry, identifying trans and gauche confirmation of the C2mim+ cation. Halide anion size and electronegativity are also reflected in the C-H stretch region of Raman and IR spectra where information about the hydrogen bonding between anion and cation at the C2-H9 position of the imidazolium ring can be obtained. The wavenumber position of the C2-H9 stretch mode for C2mim Cl suggests the strongest hydrogen-bond between C2-H9 and the Cl- anion compared to the other three ILs investigated. DFT calculations have predicted and simulated various ion-pair configurations by means of hydrogen-bond interactions. DFT-computed geometries revealed that the Cl- and Br- anions are positioned in the imidazolium ring plane at the C2-H9 site whereas the I- anion is located at C2-H9 site with a larger dihedral angle. However, the results indicate that in the C2mim BF4, the BF4- anion is located on top of the imidazolium ring and forms non-specific hydrogen bonds with the C2-H9 site. On basis of the experimental and theoretical results, the strengths of the hydrogen-bond formed between cation and anion are estimated to follow the order C2mim Cl > C2mim Br > C2mim I > C2mim BF4. The UV-Vis absorption spectra of C2mim X (X = Cl, Br, I, and BF4) provide information related to the electronic properties. Theoretical TD-DFT analysis yields the absorption bands in the UV spectral region. Electron transfer from the anion to the imidazolium cation occurs by σ-type orbital overlap and is located near the C2-H9 position of cation. 27 ACS Paragon Plus Environment


In our future work, we will investigate the impact of pressure, temperature, and solvent on the hydrogen bond dynamics. For this, we plan to use femtosecond time-resolved coherent antiStokes Raman scattering (CARS) techniques.

Acknowledgements DKS acknowledges the financial support from SERB-DST ECR project “ECR/2016/001289”. DKP is grateful for the support by the DST INSPIRE fellowship no. IF170625.

Supporting Information Available The figures show (S1) DFT-calculated Raman and IR spectra of C2mim X (X = Cl, Br, I and BF4)

in the range of 450-1700 cm-1, (S2) DFT-calculated Raman and ATR-IR spectra in the C-H stretch region of C2mim X (X = Cl, Br, I, and BF4), (S3) electrostatic potential (ESP) mapped electron density surfaces of C2mim X (X = Cl, Br, I, and BF4), (S4) electrostatic potential (ESP) mapped electron density surfaces of clusters of C2mim X (X = Cl, Br, I, and BF4), (S5) comparision of experimental and theoretical Raman spectra (after scaling) in the C-H stretch region of C2mim X for X = Cl, Br, I, and BF4, (S6) UV-Vis absorption spectra of C2mim X (X = Cl, Br and BF4) in the range of 250-650 nm. For C2mim I the range is reduced to 350-650 nm, (S7) HOMO-LUMO electronic transitions of the C2mim X (X = Cl, Br, I, and BF4) ion-pairs, (S8) HOMO-LUMO electronic transition of C2mim+ cation with orbital energies, and (S9) molecular orbitals HOMO2, HOMO-1, LUMO+1, LUMO+2, LUMO+3 of C2mim Cl, C2mim Br, C2mim I, and C2mim BF4. For the ESPs shown in Figures S3 and S4 a short explanation is given. UV-Vis results is given in Figure S6, S7, S8, S9 and Table S3 and also the explanation is given. The tables present (S1) DFT-calculated vibrational assignments of C2mim X (X = C1, Br, I, and BF4) without scaling in the C-H spectral region (2500-3300cm-1), (S2) natural bond orbital (NBO) electron density in the cation and four configurations of C2mim X (X = Cl, Br, I, and BF4) calculated at the


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wB97XD/6-311++G(d,p) level of theory, (S3) DFT-computed oscillator strength, wavelength (λ), main initial state, and main final state in the 150-400-nm-wavelength region for C2mim X (X = Cl, Br, I, and BF4) calculated at the wB97XD/6-311++G(d,p) level of theory. This information is available free of charge by the Internet at http://pubs.acs.org.

References (1) Rogers, R. D.; Seddon, K. R. Ionic Liquids--Solvents of the Future? Science 2003, 302, 792–793. (2) Welton, T. Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chem. Rev. 1999, 99, 2071–2084. (3) Seddon, K. Ionic Liquids for Clean Technology. J. Chem. Techn. & Biotechn. 1997, 68, 351-356. (4) Smiglak, M.; Pringle, J. M.; Lu, X.; Han, L.; Zhang, S.; Gao, H.; MacFarlane, D. R.; Rogers, R. D. Ionic Liquids for Energy, Materials, and Medicine. Chem. Commun. 2014, 50, 9228–9250. (5) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Ionic-Liquid Materials for the Electrochemical Challenges of the Future. Nature Materials 2009, 8, 621–629. (6) Zakeeruddin, S. M.; Grätzel, M. Solvent‐Free Ionic Liquid Electrolytes for Mesoscopic DyeSensitized Solar Cells. Adv. Funct. Mater. 2009, 19, 2187–2202. (7) Wasserscheid, P.; Keim, W. Ionic Liquids-New “Solutions” for Transition Metal Catalysis. Angew. Chem., Int. Ed. 2000, 39, 3772-3789. (8) Hagiwara, R.; Ito, Y. Room Temperature Ionic Liquids of Alkylimidazolium Cations and Fluoroanions. J. Fluorine Chem. 2000, 105, 221-227. (9) Seddon, K. R.; Stark, A. Selective Catalytic Oxidation of Benzyl Alcohol and Alkylbenzenes in Ionic Liquids. Green Chem. 2002, 4, 119-123. (10) Huddleston, J. G.; Visser, A. E.; Reichert, M. W.; Willauer, H. D.; Broker, G. A.; Rogers, R. D. Characterization and Comparison of Hydrophilic and Hydrophobic Room Temperature Ionic Liquids Incorporating the Imidazolium Cation. Green chem. 2001, 3, 156-164. (11) Garcia, B.; Lavallée, S.; Perron, G.; Michot, C.; Armand, M. Room Temperature Molten Salts as Lithium Battery Electrolyte. Electrochim. Acta 2004, 49, 4583-4588. (12) Galiński, M.; Lewandowski, A.; Stępniak, I. Ionic Liquids as Electrolytes. Electrochim. Acta 2006, 51, 5567–5580. (13) Larriba, M.; Navarro, P.; García, J.; Rodríguez, F. Liquid–Liquid Extraction of Toluene from Heptane Using [Emim][DCA], [Bmim][DCA], and [Emim][TCM] Ionic Liquids. Ind. Eng. Chem. Res. 2013, 52, 2714–2720. (14) Larriba, M.; Navarro, P.; García, J.; Rodríguez, F. Liquid–Liquid Extraction of Toluene from nHeptane by {[Emim][TCM]+[Emim][DCA]} Binary Ionic Liquid Mixtures. Fluid Phase Equilib. 2014, 364, 48–54. (15) Cevasco, G.; Chiappe, C. Are Ionic Liquids a Proper Solution to Current Environmental Challenges? Green Chem. 2014, 16, 2375-2385. (16) Karadas, F.; Atilhan, M.; Aparicio, S. Review on the Use of Ionic Liquids (ILs) as Alternative Fluids for CO2 Capture and Natural Gas Sweetening. Energy Fuels 2010, 24, 5817–5828.



(17) Jeffrey, G. A. An Introduction to Hydrogen Bonding. Oxford University Press, New York, 1997. (18) Hunt, P. A. Why Does a Reduction in Hydrogen Bonding Lead to an Increase in Viscosity for the 1Butyl-2, 3-Dimethyl-Imidazolium-Based Ionic Liquids? J. Phys. Chem. B 2007, 111, 4844–4853. (19) Fumino, K.; Peppel, T.; Geppert-Rybczyńska, M.; Zaitsau, D. H.; Lehmann, J. K.; Verevkin, S. P.; Köckerling, M.; Ludwig, R. The Influence of Hydrogen Bonding on the Physical Properties of Ionic Liquids. Phys. Chem. Chem. Phys. 2011, 13, 14064–14075. (20) Peppel, T.; Roth, C.; Fumino, K.; Paschek, D.; Köckerling, M.; Ludwig, R. The Influence of Hydrogen-Bond Defects on the Properties of Ionic Liquids. Angew. Chemie., Int. Ed. 2011, 50, 6661– 6665. (21) Lehmann, S. B. C.; Roatsch, M.; Schöppke, M.; Kirchner, B. On the Physical Origin of the Cation– Anion Intermediate Bond in Ionic Liquids Part I. Placing a (Weak) Hydrogen Bond between Two Charges. Phys. Chem. Chem. Phys. 2010, 12, 7473–7486. (22) Brehm, M.; Weber, H.; Pensado, A. S.; Stark, A.; Kirchner, B. Proton Transfer and Polarity Changes in Ionic Liquid – Water Mixtures: A Perspective on Hydrogen Bonds from Ab Initio Molecular Dynamics at the Example of 1-Ethyl-3-Methylimidazolium Acetate – Water Mixtures—Part 1. Phys. Chem. Chem. Phys. 2012, 14, 5030–5044. (23) Talaty, E. R.; Raja, S.; Storhaug, V. J.; Dölle, A.; Carper, R. W. Raman and Infrared Spectra and Ab Initio Calculations of C2-4MIM Imidazolium Hexafluorophosphate Ionic Liquids. J. Phys. Chem. B 2004, 108, 13177–13184. (24) Dong, K.; Song, Y.; Liu, X.; Cheng, W.; Yao, X.; Zhang, S. Understanding Structures and Hydrogen Bonds of Ionic Liquids at the Electronic Level. J. Phys. Chem. B 2012, 116, 1007–1017. (25) Fumino, K.; Wulf, A.; Ludwig, R. The Cation–Anion Interaction in Ionic Liquids Probed by FarInfrared Spectroscopy. Angew. Chemie., Int. Ed. 2008, 47, 3830–3834. (26) Dhumal, N. R.; Kim, H. J.; Kiefer, J. Electronic Structure and Normal Vibrations of the 1-Ethyl-3Methylimidazolium Ethyl Sulfate Ion Pair. J. Phys. Chem. A 2011, 115, 3551–3558. (27) Köddermann, T.; Wertz, C.; Heintz, A.; Ludwig, R. Ion‐Pair Formation in the Ionic Liquid 1-Ethyl3-Methylimidazolium Bis(Triflyl)Imide as a Function of Temperature and Concentration. Chem. Phys. Chem. 2006, 7, 1944–1949. (28) Dhumal, N. R.; Noack, K.; Kiefer, J.; Kim, H. J. Molecular Structure and Interactions in the Ionic Liquid 1-Ethyl-3-Methylimidazolium Bis(Trifluoromethylsulfonyl)Imide. J. Phys. Chem. A 2014, 118, 2547–2557. (29) Ding, Z.-D.; Chi, Z.; Gu, W.-X.; Gu, S.-M.; Wang, H.-J. Theoretical and Experimental Investigation of the Interactions between [Emim]Ac and Water Molecules. J. Mole. Struct. 2012, 1015, 147–155. (30) Thomas, M.; Brehm, M.; Hollóczki, O.; Kelemen, Z.; Nyulászi, L.; Pasinszki, T.; Kirchner, B. Simulating the Vibrational Spectra of Ionic Liquid Systems: 1-Ethyl-3-Methylimidazolium Acetate and Its Mixtures. J. Chem. Phys. 2014, 141, 024510. (31) Kiefer, J.; Noack, K.; Penna, T. C.; Ribeiro, M. C. C.; Weber, H.; Kirchner, B. Vibrational Signatures of Anionic Cyano Groups in Imidazolium Ionic Liquids. Vib. Spectrosc. 2017, 91, 141-146. (32) Mao, J. X.; Lee, A. S.; Kitchin, J. R.; Nulwala, H. B.; Luebke, D. R.; Damodaran, K. Interactions in 1-Ethyl-3-Methyl Imidazolium Tetracyanoborate Ion Pair: Spectroscopic and Density Functional Study. J. Mol. Struct. 2013, 1038, 12–18.


Page 30 of 35

Page 31 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(33) Fournier, J. A.; Wolke, C. T.; Johnson, C. J.; McCoy, A. B.; Johnson, M. A. Comparison of the Local Binding Motifs in the Imidazolium-Based Ionic Liquids [EMIM][BF4] and [EMMIM][BF4] through Cryogenic Ion Vibrational Predissociation Spectroscopy: Unraveling the Roles of Anharmonicity and Intermolecular Interactions. J. Chem. Phys. 2015, 142, 064306. (34) Yoshimura, Y.; Abe, H.; Takekiyo, T.; Shigemi, M.; Hamaya, N.; Wada, R.; Kato, M. Superpressing of a Room Temperature Ionic Liquid, 1-Ethyl-3-Methylimidazolium Tetrafluoroborate. J. Phys. Chem. B 2013, 117, 12296–12302. (35) Tanabe, I.; Kurawaki, Y.; Morisawa, Y.; Ozaki, Y. Electronic Absorption Spectra of ImidazoliumBased Ionic Liquids Studied by Far-Ultraviolet Spectroscopy and Quantum Chemical Calculations. Phys. Chem. Chem. Phys. 2016, 18, 22526–22530. (36) Paul, A.; Mandal, P. K.; Samanta, A. On the Optical Properties of the Imidazolium Ionic Liquids. J. Phys. Chem. B 2005, 109, 9148–9153. (37) Jacquemin, J.; Husson, P.; Mayer, V.; Cibulka, I. High-Pressure Volumetric Properties of Imidazolium-Based Ionic Liquids: Effect of the Anion. J. Chem. Eng. Data 2007, 52, 2204–2211. (38) Kiefer, J.; Fries, J.; Leipertz, A. Experimental Vibrational Study of Imidazolium-Based Ionic Liquids: Raman and Infrared Spectra of 1-Ethyl-3-Methylimidazolium Bis(Trifluoromethylsulfonyl)Imide and 1Ethyl-3-Methylimidazolium Ethylsulfate. Appl. Spectrosc. 2007, 61, 1306–1311. (39) Kiefer, J.; Namboodiri, M.; Kazemi, M. M.; Materny, A. Time‐resolved Femtosecond CARS of the Ionic Liquid 1-Ethyl-3-Methylimidazolium Ethylsulfate. J. Raman Spectrosc. 2015, 46, 722–726. (40) Namboodiri, M.; Kazemi, M. M.; Khan, T. Z.; Materny, A.; Kiefer, J. Ultrafast Vibrational Dynamics and Energy Transfer in Imidazolium Ionic Liquids. J. Am. Chem. Soc. 2014, 136, 6136–6141. (41) Burba, C. M.; Janzen, J.; Butson, E. D.; Coltrain, G. L. Using FT-IR Spectroscopy to Measure Charge Organization in Ionic Liquids. J. Phys. Chem. B 2013, 117, 8814–8820. (42) Oulevey, P.; Luber, S.; Varnholt, B.; Bürgi, T. Symmetry Breaking in Chiral Ionic Liquids Evidenced by Vibrational Optical Activity. Angew. Chemie., Int. Ed. 2016, 55, 11787–11790. (43) Scheers, J.; Johansson, P.; Jacobsson, P. Anions for Lithium Battery Electrolytes: A Spectroscopic and Theoretical Study of the B(CN)4- Anion of the Ionic Liquid C2mim [B(CN)4]. J. Electrochem. Soc. 2008, 155, 628-634. (44) Dong, K.; Zhang, S.; Wang, D.; Yao, X. Hydrogen Bonds in Imidazolium Ionic Liquids. J. Phys. Chem. A 2006, 110, 9775–9782. (45) Skarmoutsos, I.; Dellis, D.; Matthews, R. P.; Welton, T.; Hunt, P. A. Hydrogen Bonding in 1Butyl- and 1-Ethyl-3-Methylimidazolium Chloride Ionic Liquids. J. Phys. Chem. B 2012, 116, 4921– 4933. (46) Zhu, X.; Sun, H.; Zhang, D.; Liu, C. Theoretical Study on the Interactions between Methanol and Imidazolium-Based Ionic Liquids. J. Mol. Model. 2011, 17, 1997–2004. (47) Tsuzuki, S.; Tokuda, H.; Mikami, M. Theoretical Analysis of the Hydrogen Bond of Imidazolium C 2 –H with Anions. Phys. Chem. Chem. Phys. 2007, 9, 4780–4784. (48) Heimer, N. E.; Sesto, R. E. D.; Meng, Z.; Wilkes, J. S.; Carper, R. W. Vibrational Spectra of Imidazolium Tetrafluoroborate Ionic Liquids. J. Mol. Liq. 2006, 124, 84–95. (49) Malberg, F.; Pensado, A. S.; Kirchner, B. The Bulk and the Gas Phase of 1-Ethyl-3Methylimidazolium Ethylsulfate: Dispersion Interaction Makes the Difference. Phys. Chem. Chem. Phys. 2012, 14, 12079–12082.



(50) Brela, M. Z.; Kubisiak, P.; Eilmes, A. Understanding the Structure of the Hydrogen Bonds Network and Its Influence on Vibrational Spectra in a Prototypical Aprotic Ionic Liquid. J. Phys. Chem. B 2018, 122, 9527–9537. (51) Chatzipapadopoulos, S.; Zentel, T.; Ludwig, R.; Lütgens, M.; Lochbrunner, S.; Kühn, O. Vibrational Dephasing in Ionic Liquids as a Signature of Hydrogen Bonding. ChemPhysChem 2015, 16, 2519–2523. (52) Dhumal, N. R.; Kim, H. J.; Kiefer, J. Molecular Interactions in 1-Ethyl-3-Methylimidazolium Acetate Ion Pair: A Density Functional Study. J. Phys. Chem. A 2009, 113, 10397–10404. (53) Pensado, A. S.; Brehm, M.; Thar, J.; Seitsonen, A. P.; Kirchner, B. Effect of Dispersion on the Structure and Dynamics of the Ionic Liquid 1-Ethyl-3-Methylimidazolium Thiocyanate. ChemPhysChem 2012, 13, 1845–1853. (54) Chai, J.-D.; Head-Gordon, M. Long-Range Corrected Hybrid Density Functionals with Damped Atom–Atom Dispersion Corrections. Phys. Chem. Chem. Phys. 2008, 10, 6615–6620. (55) Salzner, U.; Aydin, A. Improved Prediction of Properties of π-Conjugated Oligomers with RangeSeparated Hybrid Density Functionals. J. Chem. Theory Comput. 2011, 7, 2568–2583. (56) Minenkov, Y.; Singstad, Å.; Occhipinti, G.; Jensen, V. R. The Accuracy of DFT-Optimized Geometries of Functional Transition Metal Compounds: A Validation Study of Catalysts for Olefin Metathesis and Other Reactions in the Homogeneous Phase. Dalton Trans. 2012, 41, 5526–5541. (57) Becke, A. D. Density-functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648–5652. (58) Grimme, S. Semiempirical GGA-type Density Functional Constructed with a Long‐range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787–1799. (59) Singh, D. K.; Rathke, B.; Kiefer, J.; Materny, A. Molecular Structure and Interactions in the Ionic Liquid 1-Ethyl-3-Methylimidazolium Trifluoromethanesulfonate. J. Phys. Chem. A 2016, 120, 6274– 6286. (60) Singh, D. K.; Cha, S.; Nam, D.; Cheong, H.; Joo, S.; Kim, D. Raman Spectroscopic Study on Alkyl Chain Conformation in 1-Butyl-3-Methylimidazolium Ionic Liquids and Their Aqueous Mixtures. ChemPhysChem 2016, 17, 3040–3046. (61) Singh, S.; Srivastava, S. K.; Singh, D. K. Hydrogen Bonding Patterns in Different Acrylamide–Water Clusters: Microsolvation Probed by Micro Raman Spectroscopy and DFT Calculations. RSC Adv. 2014, 4, 1761–1774. (62) Singh, S.; Srivastava, S. K.; Singh, D. K. Raman Scattering and DFT Calculations Used for Analyzing the Structural Features of DMSO in Water and Methanol. RSC Adv. 2013, 3, 4381–4390. (63) Tsuzuki, S.; Katoh, R.; Mikami, M. Analysis of Interactions between 1-Butyl-3-Methylimidazolium Cation and Halide Anions (Cl−, Br− and I−) by Ab Initio Calculations: Anion Size Effects on Preferential Locations of Anions. Mol. Phys. 2008, 106, 1621–1629. (64) Shukla, M.; Srivastava, N.; Saha, S. Theoretical and Spectroscopic Studies of 1-Butyl-3Methylimidazolium Iodide Room Temperature Ionic Liquid: Its Differences with Chloride and Bromide Derivatives. J. Mol. Struct. 2010, 975, 349–356. (65) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H., et al., Gaussian 16, Revision B.01; Gaussian, Inc.; Wallingford CT, 2016.


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(66) Dennington, R.; Keith, T.; Millam, J.; Eppinnett, K.; Hovell, L. W.; Gilliland, R. GaussView, version 6; Semichem Inc.: Shawnee Mission, KS, 2016. (67) Hayashi, S.; Ozawa, R.; Hamaguchi, H.-O. Raman Spectra, Crystal Polymorphism, and Structure of a Prototype Ionic-Liquid [Bmim]Cl. Chem. Lett. 2003, 32, 498–499. (68) Hamaguchi, H.-O.; Ozawa, R. Structure of Ionic Liquids and Ionic Liquid Compounds: Are Ionic Liquids Genuine Liquids in the Conventional Sense? Advances in Chemical Physics. 2005, 131, 85–104. (69) Ozawa, R.; Hayashi, S.; Saha, S.; Kobayashi, A.; Hamaguchi, H.-O. Rotational Isomerism and Structure of the 1-Butyl-3-Methylimidazolium Cation in the Ionic Liquid State. Chem. Lett. 2003, 32, 948– 949. (70) Yamada, T.; Mizuno, M. Characteristic Spectroscopic Features Because of Cation–Anion Interactions Observed in the 700–950 Cm–1 Range of Infrared Spectroscopy for Various ImidazoliumBased Ionic Liquids. ACS Omega 2018, 3, 8027–8035 (71) Joseph, J.; Jemmis, E. D. Red-, Blue-, or No-Shift in Hydrogen Bonds: A Unified Explanation. J. Am. Chem. Soc. 2007, 129, 4620–4632. (72) Umebayashi, Y.; Fujimori, T.; Sukizaki, T.; Asada, M.; Fujii, K.; Kanzaki, R.; Ishiguro, S. Evidence of Conformational Equilibrium of 1-Ethyl-3-Methylimidazolium in Its Ionic Liquid Salts: Raman Spectroscopic Study and Quantum Chemical Calculations. J. Phys. Chem. 2005, 109, 8976–8982. (73) Turner, E. A.; Pye, C. C.; Singer, R. D. Use of Ab Initio Calculations toward the Rational Design of Room Temperature Ionic Liquids. J. Phys. Chem. A 2003, 107, 2277–2288. (74) Arunan, E.; Desiraju, G. R.; Klein, R. A.; Sadlej, J.; Scheiner, S.; Alkorta, I.; Clary, D. C.; Crabtree, R. H.; Dannenberg, J. J.; Hobza, P.; Kjaergaard, H. G.; Legon, A. C.; Mennucci, B.; Nesbitt, D. J. Definition of the Hydrogen Bond (IUPAC Recommendations 2011). Pure Appl. Chem. 2011, 83, 1637– 1641. (75) Khrizman, A.; Cheng, H.; Bottini, G.; Moyna, G. Observation of Aliphatic C–HX Hydrogen Bonds in Imidazolium Ionic Liquids. Chem. Commun. 2015, 51, 3193–3195. (76) Dymek, C. J.; Grossie, D. A.; Fratini, A. V.; Adams, W. W. Evidence for the Presence of HydrogenBonded Ion-Ion Interactions in the Molten Salt Precursor, 1-Methyl-3-Ethylimidazolium Chloride. J. Mol. Struct. 1989, 213, 25–34. (77) Aoun, B.; Goldbach, A.; Kohara, S.; Wax, J.-F.; González, M. A.; Saboungi, M.-L. Structure of a Prototypic Ionic Liquid: Ethyl-Methylimidazolium Bromide. J. Phys. Chem. B 2010, 114, 12623–12628. (78) Abdul-Sada, A. K.; Greenway, A. M.; Hitchcock, P. B.; Mohammed, T. J.; Seddon, K. R.; Zora, J. A. Upon the Structure of Room Temperature Halogenoaluminate Ionic Liquids. J. Chem. Soc., Chem. Commun. 1986, 0, 1753–1754. (79) Samanta, A. Dynamic Stokes Shift and Excitation Wavelength Dependent Fluorescence of Dipolar Molecules in Room Temperature Ionic Liquids. J. Phys. Chem. B 2006, 110, 13704–13716. (80) Paul, A.; Mandal, P. K.; Samanta, A. How Transparent Are the Imidazolium Ionic Liquids? A Case Study with 1-Methyl-3-Butylimidazolium Hexafluorophosphate, [Bmim][PF6]. Chem. Phys. Lett. 2005, 402, 375–379. (81) Mandal, P. K.; Paul, A.; Samanta, A. Excitation Wavelength Dependent Fluorescence Behavior of the Room Temperature Ionic Liquids and Dissolved Dipolar Solutes. J. Photochem. Photobiol., A 2006, 182, 113-120. (82) Guirgis, G. A.; Klaboe, P.; Shen, S.; Powell, D. L.; Gruodis, A.; Aleksa, V.; Nielsen, C.; Tao, J.; Zheng, C.; Durig, J. R. Spectra and Structure of Silicon-containing Compounds. XXXVI—Raman and



Infrared Spectra, Conformational Stability, Ab Initio Calculations and Vibrational Assignment of Ethyldibromosilane. J. Raman Spectrosc. 2003, 34, 322–336. (83) Polavarapu, P. Ab Initio Vibrational Raman and Raman Optical Activity Spectra. J. Phys. Chem. 1990, 94, 8106–8112. (84) Katsyuba, S. A.; Vener, M. V.; Zvereva, E. E.; Fei, Z.; Scopelliti, R.; Laurenczy, G.; Yan, N.; Paunescu, E.; Dyson, P. J. How Strong Is Hydrogen Bonding in Ionic Liquids? Combined X-Ray Crystallographic, Infrared/Raman Spectroscopic, and Density Functional Theory Study. J. Phys. Chem. B 2013, 117, 9094–9105.


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TOC Graphic


Impact of Size and Electronegativity of Halide Anions on Hydrogen

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