13676 Phys. Chem. Chem. Phys., 2012, 14, 13676–13683 This journal is c the Owner Societies 2012

Cite this: Phys. Chem. Chem. Phys., 2012, 14, 13676–13683

Solute–solvent hydrogen-bonding in room temperature ionic liquids

studied by Raman spectroscopyw

Akira Kobayashi, Koji Osawa, Masahide Terazima and Yoshifumi Kimura*z

Received 15th May 2012, Accepted 16th August 2012

DOI: 10.1039/c2cp41567d

The vibrational frequencies of the CQO + CQC band of diphenylcyclopropenone and the NH2

stretching band of p-aminobenzonitrile were determined in various room temperature ionic liquids

(RTILs). The vibrational frequency shifts of the CQO + CQC stretching mode were compared with

Kamlet a values, and frequency shifts of the NH2 stretching mode were compared with Kamlet b values.

A nearly linear relationship was obtained for both parameters, although the solvatochromic parameters

were more sensitive to changes of the cation species. Vibrational frequency calculations of a 1 : 1 cluster

of p-aminobenzonitrile with the RTIL anions using DFT theory reproduced the observed frequency

shifts of the NH2 stretching mode fairly well. The frequency shifts of the CN stretching mode were well

reproduced by the linear combination of dipolarity parameters, the hydrogen-bond donating and

accepting parameters determined by the Raman shift of the solute molecule.

Introduction

Various spectroscopic methods have been used to investigate

how solute molecules in room temperature ionic liquids (RTILs)

are solvated by cation and anion molecules. The most widely

used method is to measure the solvatochromic shift of the

absorption or fluorescence spectrum of a probe molecule using

the sensitivity of electronic transition to the environment.

Solvatochromic shifts in RTILs have been studied extensively.1–13

One of the most widely studied parameters is the ET(30) value

of a Reichart dye. The ET(30) values for typical RTILs such as

1,3-dialkylimidazolium salts are between 0.4 and 0.65, which are

close to the values of dimethylsulfoxide (DMSO) and alcohols.1,2,7

The Kamlet–Taft solvatochromic parameters, including the

dipolarity/polarizability parameter (p*), the hydrogen-bond

acceptor basicity (b), and the hydrogen-bond donor acidity (a),have also been investigated for various classes of RTILs.1,2 It has

been reported that the p* values are generally close to 1,

indicating the large dipolarity/polarizability of RTILs, and that

variations due to the cation and anion species are small. On the

other hand, the a and b values are strongly dependent on the

specific cation and anion species. Welton et al. reported the bvalues of l-butyl-3-methylimidazolium (BMIm) cation-based ionic

liquids,3 and observed that b increases in the following order with

changing anions: [SbF6]�o [PF6]

�o [(CF3SO2)2N]� ([NTf2]�)o

[BF4]� o [CF3SO3]

� ([TfO]�). Spange et al. demonstrated the

anion species dependence of the a and b values using BMIm-based

ionic liquids,12,13 and reported that the a value increases in the

order of Cl�o Br�o [CH3COO]�o [CH3SO3]�o [NO2]

�o[CH3OSO3]

� o [NO3]� o I� o [CF3COO�] o [SCN]� o

[N(CN)2]� o [CF3SO3]

� o [BF4]� o [PF6]

� o [NTf2]�. They

also found that the b value is almost inversely correlated with ain the series of ionic liquids they used.

These solvatochromic parameters have been accumulated

for various conventional RTILs, and are used to evaluate the

polarity or hydrogen-bonding ability of ionic liquids.1 However, it

has recently been shown that the absolute value of the polarity

scale, e.g. the p* value, is strongly dependent on the choice of the

probe molecule, and special attention should be paid when

discussing the solvatochromic parameters.14 Furthermore, the

usefulness of these parameters for interpretation of the local

solvation structure, especially the hydrogen-bonding between

solute and solvent molecules, has not yet been satisfactorily

demonstrated. For example, the vibrational spectrum is more

sensitive to local solute–solvent interactions than the electronic

spectrum, and many vibrational spectroscopy studies have been

performed to investigate solute–solvent interactions in RTILs.15–18

However, a direct comparison between the solvatochromic para-

meters and vibrational spectral shifts in RTILs has not been well

demonstrated until now. Welton et al. investigated the IR spectra

of water dissolved in BMIm-cation based RTILs using ATR-IR

spectroscopy and found that, relative to those in the vapor phase,

Department of Chemistry, Graduate School of Science,Kyoto University, Kyoto 606-8502, Japanw Electronic supplementary information (ESI) available: Chemicalformula and abbreviations of RTILs used in this study, Table for allexperimentally determined parameters (Raman shifts, solvatochromicparameters, solvent parameters determined by the Raman shift,densities and ion concentration) in this study together with thereference data, optimized structure and vibrational frequency deter-mined for 1 : 1 clusters. See DOI: 10.1039/c2cp41567dz Present address: Department of Chemical Science and Technology,Faculty of Bioscience and Applied Chemistry, Hosei University,Koganei, Tokyo, 184-8584, Japan. E-mail: ykimura@hosei.ac.jp;Fax: +81-42-387-7002; Tel: +81-42-387-6138.

PCCP Dynamic Article Links

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both Fermi-splitting OH bands show a significant shift depending

on the RTIL anion species, suggesting the existence of hydrogen-

bonding interactions between the water molecules and RTIL

anions. According to their studies, the hydrogen-bonding

strength between anion and water was as follows: [PF6]� o

[SbF6]� o [BF4]

� o [NTf2]� o [ClO4]

� o [CF3SO3]� o

[NO3]� o [CF3CO2]

�, which is almost the same order of the

b value,3 although quantitative comparison of the vibrational

frequency to solvatochromic parameters has not been per-

formed. Kimura et al. have measured the Raman spectra of

diphenylcyclopropenone (DPCP) in various RTILs.16,17 They

found that the Raman shift of the CQO + CQC stretching

band of DPCP shows a good linear correlation with the

acceptor number (AN) of solvents and used the shift to

evaluate the ANs of various RTILs. They found that the

ANs of typical RTILs are 20 to 30.

In this paper, we present a Raman spectroscopic study on

the solvation of DPCP and p-aminobenzonitrile (ABN) in

various conventional RTILs and attempt to correlate the

results with solvatochromic parameters. The DPCP CQO +

CQC stretching band is a good indicator of hydrogen-bonding

donor acidity of the solvent, and we revisited this molecule’s

solvation properties, covering a wider range of RTILs, and

compared the frequency shift with the solvatochromic para-

meter a. ABN is another molecule that acts as a good Raman

spectroscopic probe to study local solute–solvent interactions.

ABN has two local sites (–CN and –NH2), both of which

strongly interact with solvent molecules. According to previous

studies, the NH2 stretching vibration of ABN or p-nitroaniline

(pNA) is mainly affected by hydrogen-bonding between the

solute and solvent, and the shift may be a good indicator of the

hydrogen-bond accepting ability of the solvent (the b value).19,20

On the other hand, the CN stretching vibration is affected by

several factors and its solvent dependence is rather complicated.21–23

The vibrational frequency is affected by solute–solvent repul-

sive interactions, dipole–dipole interactions, direct hydrogen-

bonding between the –CN site and the solvent molecule, and

indirectly by hydrogen-bonding between the –NH2 site and the

solvent molecule. It is an interesting issue as to whether the

Raman shift of the CN stretching mode is well represented by

a linear combination of solvatochromic parameters represent-

ing the above properties. In the following, we list details of the

experimental procedures. Then we present results of correla-

tions between the Raman shifts with a or b values. Although

the solvatochromic parameters a and b explain the Raman

shifts fairly well, these parameters tend to overestimate the

cation effect in comparison with the Raman shift. The anion

effects on the NH2 stretching mode and the CN stretching

mode are also discussed in terms of a model 1 : 1 cluster

between ABN and anion. Then we introduce new solvato-

chromic parameters to estimate hydrogen-bonding strengths

based on the Raman shifts. The solvent shift of the CN

stretching mode is properly predicted by the solvent para-

meters introduced in this paper.

Experimental

p-Aminobenzonitrile (1, ABN) was purchased fromWako Chemi-

cals, and purified by recrystallization. N,N-Dimethyl-p-nitroaniline

(2, DMPNA) was purchased from Lancaster, N,N-diethyl-

p-nitroaniline (3, DEPNA) from Oakwood Products, Inc.,

p-nitroaniline (4, pNA) from Nacalai Tesque, and Reichert

dye (5) from Aldrich, respectively. pNA was used after recrys-

tallization and others were used without further purification.

Diphenylcyclopropenone (6, DPCP) was purchased from

Nacalai Tesque, and used as received. Methanol (MeOH),

carbon tetrachloride (CCl4), benzene (BZ), ethylacetate

(EtOAc), and acetonitrile (ACN) (all solvents are of spectra

grade) were purchased from Nacalai Tesque or Wako Chemi-

cals and used without further purification. RTILs used in this

study are listed in the table of ESIw together with their

chemical formula and abbreviations of their names.

[P6,6,6,14][NTf2] was purchased from Cytec. [DEMAH][TfO]

from Stella Chemifa was kindly provided by Prof. M. Watanabe

(Yokohama National University). [BMIm][TfO] was synthe-

sized from [BMIm]Cl with the modified procedure of ref. 24,

and washed with water several times. The resulting liquid was

dried under vacuum (less than 10�2 Pa) for several days at

80 1C. All other RTILs were purchased from Kanto Kagaku.

All the solutions, except for [DEMAH][TfO], were dried under

vacuum for a minimum of 2 hours at 60 1C before each

experiment. [DEMAH][TfO] was dried under vacuum for

more than 5 days at 80 1C and the sample solution was

prepared under an argon atmosphere. The water content of

typical RTILs was measured by the Karl-Fisher titration method

(Kyoto Electronics Manufacturing Co. LTD, MKC-610-DT),

and was found to be around 100 ppm.

Raman spectra of ABN and DPCP in various RTILs were

measured with a 901 scattering geometry at the excitation of

532 nm using a green laser (Spectra-Physics, EXLSR-532-150).

A Peltier-cooled CCD camera (Princeton Instrument, Spec-

10:400BRXTE) attached to a spectrometer (Jobin Yvon,

T64000) was used as a detector. The spectrometer was oper-

ated in a single grating mode and an appropriate edge filter

was placed before the slit. The wavelength was calibrated with

neon lamp emission lines, and/or the Raman bands of BZ and

cyclohexane (CHX). The spectral resolution was about 3 cm�1.

The concentration of ABN was adjusted to ca. 100 mmol dm�3,

and that of DPCP was ca. 50 mmol dm�3. The sample solution

was enclosed in a 1 cm path length glass cell, and the

temperature of the sample was controlled at 25 1C by flowing

thermostated water through the cell holder. For some ILs with

high melting points, the measurements were performed at

higher temperatures. The Raman spectrum of the solute was

evaluated by subtracting the Raman band of the solvent from

that of the solution. The absorption spectra were measured

using a UV-Vis spectrometer (Shimadzu UV-2400), and the

densities of several RTILs were measured by a vibrating tube

oscillator at 25 1C (Anton Paar, DMA4500).

Results and discussion

Solvatochromic parameters

Although many data have been accumulated for conven-

tional p*, a, and b values, the values for several RTILs used

in this study have not been reported. In such cases, the

solvatochromic parameters were determined by the following

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13678 Phys. Chem. Chem. Phys., 2012, 14, 13676–13683 This journal is c the Owner Societies 2012

standard equations using the absorption peaks of solvatochromic

probes:25

p*= (n(3)max� n(3, CHX)max)/(n(3, DMSO)max� n(3, CHX)max)

(1)

a = (ET(30)/(kcal mol�1) � 14.6(p* � 0.23) � 30.31)/16.5

(2)

b= (1.035 n(3)max/(1000 cm�1)� n(4)max/(1000 cm

�1)+ 2.64)/2.8

(3)

In previous work, we have defined the polarity scale p�bandusing the absorption band center oa of DMPNA as18

p�band ¼ ðoað2Þ � oað2;CHXÞÞ=ðoað2;DMSOÞ�oað2;CHXÞÞð4Þ

where oa is the first moment of the absorption lineshape

function obtained by integrating the observed absorption line-

shape assuming the log-normal functional shape. Since this

method takes the lineshape variation induced by different

solvent polarities into consideration, it is appropriate to repre-

sent the solvent field strength based on the electronic transition.

The results are summarized in Table 1 together with values

reported in the literature.1–3,8,17–19,25–48 When examining the

literature values, we took care to only quote values determined

through procedures consistent with those used herein. Before

comparing the a and b values with Raman shifts, we mention

here how the value of p�band depends on the solvent species.

Previously, we reported that the ion concentration in RTILs is

a good measure of the RTIL polarity/dipolarity by comparing

the ionic concentration with p�band.18 Since ionic concentration

represents the number of ions in a unit volume, it could be

considered a measure of the Columbic interaction strength

between molecules. Fig. 1 shows the correlation between the

ionic concentration (sum of molarities of cation and anion of

the solvent RTIL; this definition gives a value twice that

reported in ref. 17 and 18) and the p�band value. As seen in the

figure, there is a linear correlation (R = 0.83), although

deviations from the linear correlation are more evident for the

higher concentration ionic liquids. If we exclude the RTILs with

halide anions and a protic ionic liquid, the correlation is much

better (R = 0.90). Due to its small molecular size, the halide

anion has a much stronger interaction with the probe molecule.

CQQQO + CQQQC stretching mode of DPCP

As reported previously, the CQO + CQC stretching mode

around 1620 cm�1 observed for DPCP is a good indicator of

the hydrogen-bond donating ability of the solvent. In previous

work, we compared shifts in conventional liquid solvents with

the ANs.16,17 In the present study, we tested the relation of the

Raman shift with a since there are many reports of a values forRTILs and it has been reported that there is a linear correla-

tion of a with AN.49 Fig. 2 shows typical Raman spectra of

Table 1 Summary of the experimentally-determined solvatochromic parameters and Raman shifts compared with literature values

nCN/cm�1

nNH2/

cm�1 nCO/cm�1 EN

T p* p�band a aR b bR d/g cm�3Ion conc./mol dm�3

[BMIm] [TfO] 2216.4 3362.1 1626.4 0.65 (0.67)8 0.99(1.00)26

0.96 0.63 (0.62)26 0.46 0.47 (0.49)26 0.75 1.3027 8.96

[BMIm]Cl (80 1C) 2209.0 3310.4 1624.817

(50 1C)0.57 (80 1C) 1.171 1.09 0.411 0.55 0.951 1.48 1.0528 (80 1C) 12.05

[AEIm]Br (80 1C) 2208.7 3297.2 1624.6(80 1C)

0.60 (80 1C) 1.12(80 1C)

1.1318 0.41 (80 1C) 0.56 0.71 (801C) 1.67 1.3418 12.35

[BMIm] [Nf2] 2218.3 3392.8 1625.4 0.64 1.02 0.98 0.57 0.52 0.21 0.32 1.36 8.51[EMIm] [Nf2] 2219.1 3393.6 1626.0 0.65 1.03 0.99 0.59 0.48 0.20 0.31 1.5229 10.44[BMIm] [PF6] 2219.0 3413.0 1624.317 0.6925 1.0426 0.9818 0.6326 0.58 0.1926 0.03 1.372 9.65[BMIm] [BF4] 2216.3 3394.1 1624.817 0.673,8 1.0526 0.9918 0.6326 0.55 0.3726 0.30 1.202 10.65[EMIm] [BF4] 2215.1 3390.9 1626.4 0.7130 1.03 1.0018 0.71 0.46 0.35 0.35 1.2818 12.93[AEIm] [BF4] 2216.6 3394.3 1625.6 0.67 1.05 1.0118 0.62 0.51 0.35 0.30 1.2318 10.99[AAIm] [BF4] 2214.8 3391.1 1624.5 0.68 1.05 — 0.63 0.57 0.35 0.34 1.2231 5.17[BMIm] [NTf2] 2218.2 3391.5 1625.917 0.6732 0.96

(0.99)260.9218 0.64 (0.61)26 0.49 0.24 (0.23)26 0.34 1.442 6.87

[EMIm] [NTf2] 2218.6 3389.8 1627.6 0.6933 1.0026 0.93 0.6326 0.39 0.2326 0.36 1.5234 7.76[AAIm] [NTf2] 2221.1 3389.1 1626.3 0.65 0.97 — 0.65 0.46 0.23 0.37 1.4335 3.34[N1,1,1,3] [NTf2] 2219.2 3391.1 1627.817 0.59 0.93 0.8918 0.55 0.38 0.21 0.34 1.4436 7.53[DEME] [NTf2] 2222.0 3391.5 1627.9 0.57 0.94 0.87 0.52 0.37 0.24 0.34 1.4237 6.66[P13] [NTf2] 2219.4 3389.3 1629.2 0.55 0.93 0.87 0.49 0.30 0.25 0.37 1.4538 (20 1C) 7.10[P14] [NTf2] 2217.7 3389.2 1628.6 0.6039 0.8239 0.89 0.6439 0.33 0.1239 0.37 1.4138 (20 1C) 6.68[Pp13] [NTf2] 2218.9 3386.3 1628.0 0.54 0.94 0.8918 0.46 0.37 0.25 0.41 1.4140 6.67[P2,2,2,5] [NTf2] 2219.3 3388.2 1628.4 0.51 0.94 0.90 0.39 0.34 0.25 0.38 1.3241 5.62[P2,2,2,8] [NTf2] 2219.0 3388.8 1628.2 0.50 0.93 0.87 0.39 0.36 0.33 0.38 1.2641 4.93[P6,6,6,14] [NTf2] 2217.1 3389.1 1628.1 0.45 (0.48)31 0.87

(0.83)310.8418 0.33 (0.37)31 0.36 0.54 (0.27)19 0.37 1.0742 2.80

[P4,4,4,1] [NTf2] 2220.5 3393.7 1627.2 0.50 0.98 0.91 0.34 0.41 0.30 0.31 1.2843 5.15[DEMAH] [TfO] 2214.7 3366.9 1627.2 — 0.94 0.90 — 0.41 0.42 0.69 1.2944 10.88[EMIm] [(MeO)(H)PO2] 2208.7 3338.7 1625.2 — 1.05 — — 0.53 0.99 1.08 1.1945 5.78ACN 2218.1 3383.1 1629.2 0.4646 0.6647 0.8018 0.1948 0.30 0.3148 0.46MeOH 2221.4 3362.3 1618.217 0.7646 0.6048 0.7718 0.9348 0.93 0.6248 0.75BZ 2222.7 3401.3 1634.017 0.1146 0.5547 0.5218 0.0048 0.02 0.1048 0.20EtOAc 2219.3 3383.3 1634.717 0.2346 0.4547 0.5118 0.0048 �0.02 0.4548 0.45CCl4 2226.3 3415.1 1634.4 0.0546 0.2147 0.2318 0.0048 0.00 0.0048 0.00

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DPCP in several RTILs. The Raman band assigned to the

CQO + CQC stretching mode shows a solvent dependent

shift. We have determined the peak position of the band by a

multiple Gaussian fit including the bands in the lower Raman

shift region (benzene ring stretching mode).

Fig. 3 shows the relation between the Raman shift of the

CQO + CQC stretching mode of DPCP (nCO+CC) and adetermined by the solvatochromic shift. As is shown in the

figure, a linear correlation is apparent (R = �0.86). However,

it can be noted that the Raman shifts are similar for RTILs

with the same anion, irrespective of the cation species, while

the a value varies as the cation species changes in RTILs with

the same anion. If we consider the Raman shift to be more

sensitive to the local structure than the electronic transition

spectrum, the results suggest that a factor other than solute–

solvent hydrogen-bonding may contribute to the a value

determined by the solvatochromic shift.

NH2 stretching mode of ABN

Fig. 4(a) shows the ABN Raman spectra in the NH2 stretching

region for the same cation ([BMIm]+) and different anions.

Fig. 4(b) shows the Raman spectra in the same anion [NTf2]�.

A strong symmetric stretching vibration band and a weak

asymmetric stretching vibration band at a higher Raman shift

are clearly observed. Both NH2 stretching vibrations showed

large shifts depending on the anion species, while no change

was seen as the cation was varied. The NH2 stretching mode

bandwidth was also dependent on the anion species and

increased with increasing relative shift toward that seen in

the gaseous phase from ca. 20 cm�1 to 40 cm�1 as the FWHM.

In the case of the chloride anion, the Raman band showed an

asymmetric structure with a large broadening of the peak. The

order of the shift was [PF6]�o [BF4]

�o [Nf2]�o [NTf2]

�o[TfO]� o Cl�, which is the same order observed for the OH

stretching mode of water in RTILs.3

Fig. 5 shows the correlation of the NH2 symmetric Raman shift

(nNH2) with the b value determined by the solvatochromic shift.

Similar to the case of the a value, a linear correlation between thesevalues (R = �0.86) is noticeable. However, the b value depends

on the solvent species much more than the Raman shift does, as

was also observed with the a values. For example, the NH2

stretching vibration does not show a meaningful shift when the

cation species is varied and the [NTf2]� anion is fixed, as is shown

in Fig. 4(b), while the value of b is dependent on the cation species.

We now raise the question of which value, the solvatochromic

parameter b or the Raman shift of the NH2 stretching mode

of ABN, is more appropriate to use for estimation of the

Fig. 1 Correlation of p�band with the ion concentration of RTILs.

Fig. 2 Typical DPCP Raman spectra for various ionic liquids.

Fig. 3 Correlation between the Raman shift of the CQO + CQC

stretching mode of DPCP and the conventional a value.

Fig. 4 Typical Raman spectra of ABN in the NH2 stretching region.

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13680 Phys. Chem. Chem. Phys., 2012, 14, 13676–13683 This journal is c the Owner Societies 2012

hydrogen-bond accepting ability of RTILs. In principle, the

Raman shift is determined by various factors, such as the

repulsive interaction between solute and solvent, the dipole–

dipole interaction, and so on.50 However, for the NH2 or OH

stretching modes the hydrogen-bonding effect is more signifi-

cant than other factors, and it is a good approximation to

ascribe the shift to the hydrogen-bonding effect. DFT calcula-

tion results of pNA using the dielectric continuum model for

the solvent also suggest that the shift of the NH2 stretching

mode due to the solvent polarity is not large in comparison

with the shift due to hydrogen-bonding.17,51 Therefore we

consider that the Raman shift of the NH2 stretching mode is

a good indicator of the solvent hydrogen-bond accepting

ability.

Cluster model calculations on the NH2 stretching mode

According to our experimental results, the NH2 stretching

mode Raman shift is strongly affected by the anion. In order

to estimate how direct interactions between the anion and

ABN affect the Raman spectrum, we optimized the structure

of a 1 : 1 cluster of ABN and the anion (Cl�, PF6�, BF4

�,

TfO�, Nf2�, and NTf2

�) and calculated the vibrational fre-

quency using DFT calculations with the B3PW91 functional

and 6-31G+(d,p) or 6-311G++(d,p) basis sets implemented in

Gaussian09.52 For several clusters we also executed ab initio

calculations at the MP2 level using the 6-31G+(d) basis set as

implemented in Gaussian03.53 The optimized 1 : 1 cluster

structures are listed in ESI.w For example, we found two local

minima structures for the BF4�–ABN complex (nearly sym-

metric and asymmetric to the NH2 site), each of which

displayed different binding of the BF4� ion to the NH2 site

of ABN. The vibrational frequency shift of the NH2 stretching

mode was larger in the asymmetric binding case than in the

symmetric binding one. Although we tried to locate the anion

around the CN site, we could not find a local minimum around

the CN site. In most cases, we found several local geometry

minima, and the asymmetric binding to the NH2 site showed

the largest frequency shift effect.

In Table 2, we compare experimental observations and

theoretical calculations of the relative shift of nNH2to the

value of an isolated molecule. The reference for the experi-

mental results is 3423.9 cm�1, which was obtained in the

gaseous phase.20 As is shown in the table, a simple 1 : 1 cluster

model reproduces the experimental results fairly well; i.e. the

anion effect ordering is similar, except for the NTf2� and Nf2

�

anions. Since these two anions have conformers, the surround-

ing cation and anion in a real solution induce conformational

changes to the anion structure, which may result in stronger

interactions between the anion and the ABN molecule.

CN stretching mode of ABN and correlation with the hydrogen-

bonding strength

Fig. 6 shows typical Raman spectra in the ABN CN stretching

region. Apparently, the Raman shift is strongly dependent on

the anion species rather than the cation. As mentioned in the

Introduction, the Raman shift of the CN stretching vibration

of ABN shows a very complicated solvent dependence, reflect-

ing various solute–solvent interactions.20–23 Mainly there are

five factors which affect the vibrational frequency shift relative

to the gaseous phase (DnCN): (i) solute–solvent repulsive

interactions which increase DnCN; (ii) dipole–dipole inter-

actions between CN and the solvent molecules which decreases

DnCN; (iii) direct hydrogen-bonding from the solvent to the

CN site, which increases DnCN; (iv) hydrogen-bonding to the

NH2 site where the solvent molecule acts as a hydrogen-

bonding acceptor, which decreases DnCN; and (v) hydrogen-

bonding to the NH2 site where the solvent molecule acts as a

hydrogen-bonding donor (to the lone pair of NH2), which

increases DnNH2. In the condensed liquid phase, the effect of

the repulsive interaction is not significantly different among

different solvents, and nCN can be modeled by the following

equation,

nCN = n0 � ap* + ba � cb (5)

where n0 denotes the standard frequency, and a, b, and c are

the positive proportional constants. Here p* represents the

effect of (ii), a the effect of (iii) and (v), and b the effect of (iv).

Fig. 5 Correlation between the Raman shift of the NH2 stretching

mode of ABN and the conventional b value.

Table 2 Comparison of the relative NH2 stretching mode Raman shift of ABN between experimental observations and 1 : 1 cluster calculations.The experimental results are for the BMIm-cation based ILs

[PF6]� [BF4]

� [Nf2]� [NTf2]

�[TfO]� Cl�

Exp. �11 �30 �31 �33 �62 �114DFT. Sym. — — �49a(i) — �41a(n) �59(m) �85(e) —

�71a(k)Asym. �92(b) �99(d) �168a(j) �77(g) �90a(o) — �280(f) �669(a)

�190(c) �124a(l) �165(h)MP2 Asym. �6.3 �46.0a Transoid conformation. Alphabetical superscript indicates the cluster structure in ESI. Values are in cm�1.

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We have tested the linear correlation of eqn (5) using the

traditional parameters of p*, a, and b as

nCN = 2228.4 � 8.7818p* + 2.3834a � 10.692b (6)

The correlation between the calculated frequency and the

real frequency is shown in the ESIw figure. The CN stretching

frequency calculated using eqn (6) shows a good linear correl-

ation with the observed CN stretching frequency (R = 0.89).

The sign of each coefficient is consistent with the theoretical

prediction. Therefore, this traditional technique does a good

job in predicting other vibrational properties of the solute

molecules. However, as was discussed in the preceding sec-

tions, the traditional parameters tend to exaggerate the effect

of the cation on the a and b values. Therefore we aimed to

improve the predictions by using the information obtained in

this work. We used p�band for the dipolarity parameter instead

of p, since this method incorporates the lineshape variations

induced by the different solvent polarities. To estimate the

hydrogen-bonding ability of the solvent, we have introduced

the solvent hydrogen-bonding donating and accepting ability

parameters aR and bR using the present DPCP and ABN

Raman shift results, since we consider that the hydrogen-

bonding ability is more properly represented by the Raman shift.

In the case of the aR value, we used two standard solvents

(aR = 0 for CCl4, and aR = 0.93 for methanol (MeOH)

following the conventional values of a), and made a linear

correlation to obtain aR in any solvent as

aR = 0.93(nCO+CC � nCO+CC(CCl4))/(nCO+CC(MeOH)

� nCO+CC(CCl4)) = 93.826 � 0.057407 � (nCO+CC/cm�1)

(7)

In the case of the bR value, we used two standard solvents

(bR = 0.0 for CCl4, and bR = 0.45 for ethylacetate (EtOAc)

following the conventional values of b), and made a linear

correlation to obtain bR in any solvent as

bR = 0.45(nNH2� nNH2

(CCl4))/(nNH2(EtOAc) � nNH2

(CCl4))

= 48.327 � 0.014150 � (nNH2/cm�1) (8)

The calculated values are listed in Table 1.

We then correlated the Raman shift of the CN stretching

mode with the solvatochromic parameters defined above and

obtained the following equation:

nCN ¼ 2229:9� 12:780p�band þ 5:3299aR � 6:0461bR ð9Þ

Fig. 7 shows the correlation between the observed Raman

shift and the shift calculated using eqn (9). As shown in the

figure, the calculated results show a good correlation with the

Fig. 6 Typical Raman spectra for ABN in the CN stretching mode

region.

Fig. 7 Correlation between the experimentally observed vibrational

frequency of the CN stretching mode of ABN and the calculated value

using the linear combination of the solvatochromic parameters.

Table 3 Comparison between experimental observations and 1 : 1 cluster calculations of the relative ABN CN stretching mode Raman shift,together with the decomposition using empirical solvent parameters. The experimental results are for the BMIm-cation based ILs

[PF6]� [BF4]

� [Nf2]� [NTf2]

�[TfO]� Cl�

Exp./cm�1 �10 �13 �11 �11 �13 �20Fit/cm�1 �9 �11 �11 �11 �14 �19p�band 0.98 0.99 0.98 0.92 0.96 1.09

aR 0.58 0.55 0.52 0.49 0.46 0.55bR 0.03 0.30 0.32 0.34 0.75 1.48DFT/cm�1 Sym. — �25a(i) — �25a(n) �26(m) �28(e) —

�20a(k)Asym. �25(b) �28(d) �25a(j) �25(g) �25a(o) — �27(f) �35(a)

�26(c) �20a(l) �25(h)MP2 Asym. �7.9 �8.3 �11.3a Transoid conformation. Alphabetical superscript indicates the cluster structure in ESI.

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13682 Phys. Chem. Chem. Phys., 2012, 14, 13676–13683 This journal is c the Owner Societies 2012

experimental values. The sign of each coefficient is consistent

with the theoretical prediction, and the linear correlation

works well for reproducing the observed Raman shifts. How-

ever, compared with using the traditional parameters, the

quality of the correlation (R = 0.93) was not improved much.

Compared with the case of the fit using the traditional para-

meters, the coefficients related to hydrogen-bonding were

significantly modified as expected, e.g. from 10.692 (b) to

6.0461 (bR), which may be reasonable considering that the

traditional b value tends to exaggerate the effect of the

hydrogen bonding. We expect that in cases where properties

are more sensitive to the hydrogen-bonding environment, the

superiority of the newly introduced parameters may become

apparent.

In order to investigate the anion effect, we compare the cases

of BMIm-cation based ILs with the different anions presented

in Table 3. In this series, the solvent parameters of p�band and

aR are similar to one another. Only the value of bR shows a

large dependence on the anion species. The 1 : 1 cluster

calculation presented in the previous section also reproduced

the experimental results. The theoretical calculations show

larger shifts of the CN frequency for RTILs with larger values

of bR, and the optimized cluster structure generally showed a

hydrogen-bonded structure of the anion to the NH2 of ABN.

The calculation results also indicate that the local solvation

structure is quite important for the Raman spectrum.

Conclusions

In this paper, we have compared the Raman shifts of the

hydrogen-bonding sensitive modes such as CQO+CQC and

NH2 stretching vibrations with traditional solvatochromic

parameters a and b. Although there is an approximately linear

correlation between them, there is a tendency for the solvato-

chromic parameters to be more sensitively dependent on the

solvent species (especially the cation) than the observed

Raman shifts are. We considered that the local solvation

structure related to the hydrogen-bonding is represented by

the Raman shift more properly, and introduced empirical

parameters representing the hydrogen-bond donating and

accepting ability based on the Raman shifts. Using these

parameters, the solvent dependent shift of the CN stretching

mode was well correlated, although improvements to the

traditional parameter approach were subtle. We hope that

aR and bR will be tested for more cases and their usefulness

will be verified. DFT calculations on 1 : 1 cluster models of the

anion and ABN have revealed that direct interaction between

the anion and NH2 site of ABN is the determining factor of the

shift of the NH2 and CN stretching vibrations in most cases,

while contributions from other surrounding molecules are

non-negligible, especially for RTILs with anions that have

conformers.

Acknowledgements

This work is partially supported by the Grant-in-Aid for

Scientific Research (No. 17073012 and No. 23350006) from

MEXT and JSPS, and partially by a Grant from Kyoto

University (Core-Stage Program). We thank Dr Y. Yoshimura

(Kyoto University) for use of the density meter. Theoretical

calculations were partially performed using Research Center

for Computational Science, Okazaki, Japan.

Notes and references

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