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International Journal of Greenhouse Gas Control 20 (2014) 43–48

Contents lists available at ScienceDirect

International Journal of Greenhouse Gas Control

j ourna l h o mepage: www.elsev ier .com/ locate / i jggc

heoretical investigations on the reaction mechanisms ofmine-functionalized ionic liquid [aEMMIM][BF4] and CO2

ang Suna, Xiao-qin Zhoub, Zhimin Xuec, Zheng-yu Zhoub,∗, Tiancheng Muc,∗∗

Department of Chemistry, Zhejiang University, Hangzhou 310027, PR ChinaDepartment of Chemistry, Qufu Normal University, Qufu 273165, PR ChinaDepartment of Chemistry, Renmin University of China, Beijing 100872, PR China

r t i c l e i n f o

rticle history:eceived 7 May 2013eceived in revised form 15 October 2013ccepted 25 October 2013vailable online 22 November 2013

eywords:mine-functionalized ionic liquideaction mechanismsnergy barrieribration frequency

a b s t r a c t

In this study, an amine-functionalized ionic liquid 1,2-dimethyl-(3-aminoethyl) imidazolium tetrafluo-roborate [aEMMIM][BF4] was designed and synthesized for better performance of CO2 capture based onprevious reports. Quantum chemical calculations had been used to investigate the interactions betweenCO2 molecules and the as-synthesized ionic liquids. The molecular structures of the most stable confor-mation of [aEMMIM] cation and [aEMMIM][BF4] were optimized at B3LYP/6-311++G (d, p) level. At thesame time, we had performed CO2 capture by [aEMMIM] cation. Compared with [aEMMIM][BF4], theperformance of [aEMMIM] cation on CO2 capture was not as good as supposed. It was found that theanionic part of [aEMMIM][BF4] played a non-neglectful role in CO2 capture [aEMMIM][BF4] was found tocapture CO2 with a 1:1 mol stoichiometry by quantum calculations due to the steric effect of the methylgroup on C2 position of imidazolium ring. It was consistent with the experimental results. In order tounderstand the reaction mechanisms, we calculated the configuration variations on the reactant, inter-

mediate, transition state and product, as well as energy barrier and vibration frequency changes in gasphase and using the conductor-like polarizable continuum model (CPCM) in water solution. The barrierheight for [aEMMIM] and ([aEMMIM][BF4]) capturing CO2 is 47.25 (38.25) kcal/mol in gas phase and 38.92(20.02) kcal/mol in water solution. These results indicate that the polarity of the solvent has played animportant role on the reaction mechanisms. Frequency analysis indicates that the experimental resultsof the vibration frequencies are in better agreement with the scaled calculated values.

. Introduction

Much attention has been paid to the global warming prob-em caused by increasing carbon dioxide (CO2) in the atmospherehroughout the world. The most important sources of CO2 arenergy related emissions. There are currently about 600 coal-red power plants in operation in the U.S.A. (Energy Informationdministration, 2010). About 25% of the total CO2 emissions in

he world are generated from combustion and nonfuel uses ofossil fuels for electricity generation (Baumert et al., 2005). Addi-ionally, global CO2 energy related emissions are estimated toncrease at a 2.1% rate per year, which is in accordance with the

redicted consumption of fossil fuels for electricity generationEnergy Information Administration, 2008). Whereas renewableources will not be sufficient to supply the essential energy in

∗ Corresponding author.∗∗ Corresponding author. Tel.: +86 10 62514925; fax: +86 10 62516444.

E-mail addresses: zhouzhengyua@sina.com (Z.-y. Zhou),cmu@chem.ruc.edu.cn (T. Mu).

750-5836/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.ijggc.2013.10.025

© 2013 Elsevier Ltd. All rights reserved.

the near future, fossil fuels will maintain the main source forelectricity generation and supply. Therefore, capture of CO2 fromfossil fuel-derived flue gases has become one of the most impor-tant issues for a sustainable society (Xue et al., 2011; MacDowellet al., 2010). A number of processes have been developed, includ-ing membrane separation (Bara et al., 2009), adsorption (Wanget al., 2011), and physical and chemical absorption. By using thesemethods, CO2 may be removed to low levels from mixed gases atlow partial pressure, as in flue gases, but the capturing ability ofthese methods is limited by equilibrium. Thus, new methods arerequired.

In recent years, ionic liquids (ILs) have been widely researchedas potential substitute solvents for CO2 capture, because of theirunique properties, such as minimal volatility, designable proper-ties, thermal stability, and existence as liquid over a broad rangeof temperatures (Mu and Han, 2014). ILs are promising candidateas absorbents for gas separation (Bara et al., 2009; Cheng et al.,

2008, 2009; Wappel et al., 2010; Yunus et al., 2012). ILs provideopportunities to develop new technologies for CO2 capture, sinceCO2 can be dissolved in ILs (Maurer et al., 2003; Jalili et al., 2010;Chen et al., 2013a,b). Brennecke et al. reviewed the solubility of

44 H. Sun et al. / International Journal of Green

Cto2tcta2wto2oraadbaaw2

2

apfg

eomsopRtT

a

Fig. 1. Chemical structure of [aEMMIM][BF4].

O2 in ILs (Anderson et al., 2007), the solubility of CO2 in conven-ional ILs is limited (Fan et al., 2012). To promote the efficiencyf CO2 absorption in ILs, amine-functionalized ILs (Bates et al.,002; Galán Sánchez et al., 2011) were designed by combininghe advantages of the alkanolamine solution and the ILs, whichan absorb CO2 chemically. Higher CO2 absorption capacity withhe 1:1 (CO2: IL) mole ratio could be accomplished by tetheringn amine on both the anion and the cation of ILs (Gurkan et al.,010; Xue et al., 2011). The amine-functionalized ILs are on theay to application as candidates for CO2 capture. Some investiga-

ions have been carried out to understand the reaction mechanismsf amine-functionalized ILs and CO2 (Gurkan et al., 2010; Xue et al.,011), including molecular dynamics simulation on the solvationf non-functionalized IL 1-butyl-3-methylimidazolium hexafluo-ophosphate ([BMIM][PF6]) and CO2 (Shim and Kim, 2010), while

system investigation on the topic by quantum calculations isbsent. An amine-functionalized IL [aEMMIM][BF4] (Fig. 1) wasesigned and synthesized for better performance on CO2 captureased on previous reports, which was designed with a primarymine for a high CO2 absorption capacity (Bates et al., 2002), with

shorter chain length for a low viscosity (Tokuda et al., 2005), andith C2 methylation for a high thermal stability (McEwen et al.,

000).

. Experimental methods

CO2 was supplied by Beijing Analytical Instrument Factory with purity of 99.99%. 1,2-Dimethylimidazolium and NaBF4 were sup-lied by Aldrich. 2-Bromothylamine hydrobromide was obtainedrom Shanghai Nanxiang Reagent Co., Ltd. Other reagents were A. R.rade and were obtained from Beijing Chemical Reagents Company.

[aEMMIM][BF4] was synthesized by Bates et al. method (Batest al., 2002; Xue et al., 2011; Chen et al., 2013b). The reactionf 1,2-dimethyl imidazolium with 2-bromoethylamine hydrobro-ide was first assembled the cation in ethanol. After the solvent and

olid residue were removed, the product salt [aEMMIM][BF4] wasbtained by the ion exchange with NaBF4. The structure and com-osition of the as-synthesized IL were verified by Nuclear Magneticesonance (NMR, Bruker AM 400 MHz) and Electrospray Ioniza-

ion Mass Spectroscopy (ESI-MS, LCMS-2010, Shimadzu, Japan).he product was then dried under vacuum at 80 ◦C for 96 h.

The infrared spectra of [aEMMIM][BF4] was measured on Prestige-21 Fourier transform infrared spectroscopy (FTIR)

Fig. 2. Geometrical parameters of [aEMMIM] cation ca

house Gas Control 20 (2014) 43–48

spectrometer (Shimadzu, Japan), equipped with a DTGS detectorand a ZnSe attenuated total reflection (ATR) cell (ATR-8200H), 40scans were made with a resolution of 4 cm−1 over the range of4600–400 cm−1.

3. Computational methods

Geometries of the reactants, intermediates, transition states andproducts are fully optimized at B3LYP method (Becke, 1993; Hayand Wadt, 1985) using 6-311++G (d, p) basis set. Density functionaltheory (DFT) considers electron correlation in the self-consistentKohn–Sham procedure through the functions of the electron den-sity. It gives a good description for the systems which requiresophisticated treatments of the electron correlation in the con-ventional ab initio approach. So DFT is a cost-effective and reliablemethod. In this work, DFT calculations are carried out with Gaussian03 package (Frisch et al., 2004). The reactants and intermediateshave no imaginary frequency, and transition states have singleimaginary frequency. The frequency values computed at these lev-els contain systematic errors (Sundaraganesan et al., 2005). Toaccount for errors due to the neglecting electron correlation andthe basis set incompleteness, B3LYP systematic errors are con-sidered with a scaling factor of 0.983 up to 1700 cm−1 and 0.958for greater than 1700 cm−1 (Merrick et al., 2007; Karabacak et al.,2009; Sundaraganesan et al., 2005). The assignment of the calcu-lated wave numbers is aided by the animation option of Gaussview3.0 graphical interface for Gaussian programs, which gives a visualpresentation of the shape of the vibrational modes (Denningtonet al., 2003). In some cases, the intrinsic reaction coordinate (IRC)pathways are traced to verify the energy profiles connecting thetransition structure to two desired minima of the proposed mecha-nism. Subsequently, the solvent water for [aEMMIM][BF4] has beentaken into account by the polarizable conductor calculation model(CPCM) (Barone et al., 1998) with united-atom Kohn–Sham (UAKS)radii.

4. Results and discussion

4.1. Mechanism of CO2 capture by [aEMMIM] cation and[aEMMIM][BF4]

[aEMMIM], [aEMMIM][BF4] (R1) and CO2 (R2) have been fullyoptimized at B3LYP/6-311++G (d, p), the geometrical parametersinvolving various species of [aEMMIM] cation and [aEMMIM][BF4]are shown in Figs. 2 and 4. The potential energy surface (PES)profile is depicted in Figs. 3 and 5. Firstly, when CO2 approaches[aEMMIM] cation and [aEMMIM][BF4], O atom in CO2 molecule

forms hydrogen bonding with H2 in [aEMMIM] cation and H4 in[aEMMIM][BF4], the lengths of bonding are 3.48 and 3.32 A, respec-tively. The energies of intermediates (IM and IM1) are lower thanthat of the reactants by 1.44 and 1.92 kcal/mol, respectively. From

pture CO2 optimized at B3LYP/6-311++G (d, p).

H. Sun et al. / International Journal of Greenhouse Gas Control 20 (2014) 43–48 45

0

5

10

15

20

25

30

35

40

45

-5

-10

R1+R2

IM

A

0.00-1.44

6.63

45.81

TS

1.96

40.58

-1.35

E (kcal/mol)

GAS

CPCM

Ficps[[i

dtf(f(1[NoIgt1dT1s0awtnrbwr

lato

Table 1Main vibration spectrum variation of CO2, NH2, IM, TS and A in the process.

Vibrational modeassignment

CO2 capture by [aEMMIM] cation

Frequency (cm−1)a IR intensity

CO2

�s 1349.84 0ı 657.10 33.056�as 2318.77 711.718

NH2

� 745.48 100.308ı 1643.14 30.847�s 3387.30 11.565�as 3465.39 18.840

IM

�s( CO2) 1348.27 3.589ı( CO2) 636.67 58.376ı( CO2) 656.98 30.049�as( CO2) 2318.58 785.434�( NH2) 832.62 120.505ı( NH2) 1641.73 30.752�s( NH2) 3361.65 8.590�as( NH2) 3433.27 14.366

TS

�s( CO2) 1265.80 286.106ı( CO2) 753.44 230.865�as( CO2) 1783.59 734.044�( NH2) 662.41 36.216ı( NH2) 1426.53 67.856�( H) −1685.03 1203.170�( NH) 3332.44 27.978

A

�( OH) 1297.73 466.442�( NH) 1497.47 212.607�(C O)

1720.23 364.719ı( COH)ı( CNH)�( NH) 3475.51 43.808

Fig. 3. Potential energy surface of CO2 capture by [aEMMIM] cation.

igs. 2 and 3, we can also find that the energy of the product A (A1)s higher than IM (IM1) by 6.63 (2.41) kcal/mol. This result indi-ates that IM (IM1) is more stable than the A (A1). Comparing theotential energy surface (PES) in Fig. 3 to that in Fig. 5, it can beeen that the active energy of [aEMMIM][BF4] is lower than that ofaEMMIM] cation, it can be easily found that the active energy ofaEMMIM][BF4] in the solution is lower than that of [aEMMIM][BF4]n the gas phase.

From IRC calculation for transition state (TS), we obtain twoesired minima states of the process. It indicates that TS (TS1) is aransition state, and it has an imaginary frequency (−1685.03 cm−1

or [aEMMIM], and −1678.63 cm−1 for [aEMMIM][BF4]). For TSTS1), the proton H2 of [aEMMIM] (H4 of [aEMMIM][BF4]) trans-ers from N1 to O of CO2, the bond distance of the breaking N1 H2N1 H4) and newly-formed O4 H2 (O3 H4) is 1.28 (1.30) A and.29 (1.28) A, respectively. Because N, H, C and O bonding with H2 inaEMMIM] cation and H4 in [aEMMIM][BF4], form tetra atomic ring,

of [aEMMIM] cation ([aEMMIM][BF4]) will be combined with Cf CO2. It is an important step to form product. In this process fromM (IM1) via TS (TS1) to the product A (A1), N H bond is extendedradually from 1.01 (1.01) A in IM (IM1) to 1.28 (1.30) A in TS (TS1),hen to 2.38 (2.32) A in A (A1). And C O bond is also stretched from.16 (1.15) via 1.27 (1.28) to 1.34 (1.36) A at the same time. So C Oouble bond is stretched to form C O single bond to obtain A (A1).he distance of N C is gradually shortened from 3.17 (2.93) via.59 (1.54) to form the C N bond 1.38 (1.37) A. And O H is alsohortened from 3.48 (3.32) via 1.29 (1.28) to form the O H bond.96 (0.97) A. When [aEMMIM] cation reacts with CO2, H atom ofmino functional group on [aEMMIM] cation forms hydrogen bondith O atom of CO2. Because the hydrogen bond is directional, after

he reaction between [aEMMIM] cation and CO2, [aEMMIM] can-ot form a hydrogen bond with CO2, and [BF4] anion also cannoteact with CO2. So [aEMMIM][BF4] has no further reaction with car-on dioxide. It can be concluded that [aEMMIM][BF4] capture CO2ith a 1:1 mol stoichiometry, which agrees with the experiment

esults.Considering the effects of solvent, we employ the conductor-

ike polarizable continuum model (CPCM) (Dennington et al., 2003)pproximation to estimate the solvent effect. Solvent effect wasaken into account in this single-point energy evaluation by meansf CPCM. The red lines in Figs. 3 and 5 describe the energy diagram

�( OH) 3648.77 84.715

a Frequencies scaled by 0.983 below 1700 cm−1and by 0.958 above 1700 cm−1.

in the gas phase, and the black lines are the energy in the solventwater. It can be seen from Figs. 3 and 5, the energy barrier is 47.25(38.48 for [aEMMIM][BF4]) kcal/mol from the reactants to TS (TS1)for the gas phase, and this step has a high energy level. It is because[aEMMIM] is a cation and its energy is high. In water, the energybarrier is 38.92 (20.02 for [aEMMIM][BF4]) kcal/mol, which is lowerthan that in the gas phase. This indicates that it is easier to getthe product A in the solvent water. As seen from Figs. 3 and 5,we can also find that the energy of the product A (A1) is lowerthan IM (IM1) by 3.31 (4.07) kcal/mol, which indicates that A (A1)is more stable than the IM (IM1). From the energies for the prod-uct A and A1, we can draw a conclusion that the [aEMMIM][BF4]pathway in solution is kinetically and thermodynamically morefavorable than the [aEMMIM] cation pathway (Jinkyu et al., 2011).

Main vibration spectrum variation of CO2, NH2, IM (IM1),TS (TS1) and A (A1) are listed in Table 1 (Table 2). All of theintermediates have been identified for having no imaginary fre-quency, and the transition states have single imaginary frequency.In TS, an imaginary frequency (−1685.03 cm−1 for [aEMMIM]and −1678.63 cm−1 for [aEMMIM][BF4]) appears which indicatesclearly the rupture of the N H bond and the formation of theO H bond. From Tables 1 and 2 we can see that the value ofvibrational modes of the carbon dioxide molecule in the IM (IM1)and TS (TS1) complexes becomes smaller than that of pure car-bon dioxide molecule. Compared IM (IM1) with A (A1), it canbe seen that the vibrational frequency and the mode of somegroups occur some obvious changes in this process. The symmetricstretching vibration in 3361.65 cm−1 and asymmetric stretchingvibration in 3433.27 cm−1 of NH2 in IM disappear when CO2reacts with the [aEMMIM] cation. Compared with IM, we observed

a red-shift in frequency for the stretching vibration �s( NH2)of IM1. The frequency shift should be related to N H F hydro-gen bonding. The symmetric stretching vibration of �s( CO2)

46 H. Sun et al. / International Journal of Green

Table 2Main vibration spectrum variation of CO2, NH2, IM1, TS1 and A1 in the process.

Vibrational modeassignment

CO2 capture by [aEMMIM][BF4]

Frequency (cm−1)a IR intensity

CO2

�s 1349.84 0ı 657.10 33.056�as 2318.77 711.718

NH2

� 883.5 116.189ı 1644.32 24.926�s 3332.61 57.557�as 3424.16 22.272

IM1

�s( CO2) 1344.38 3.386ı( CO2) 612.59 78.555ı( CO2) 658.86 29.698�as( CO2) 2310.62 748.856�( NH2) 924.80 138.729ı( NH2) 1647.02 24.582�s( NH2) 3315.91 69.018�as( NH2) 3401.05 21.753

TS1

�s( CO2) 1270.03 322.336ı( CO2) 712.36 8.143�as( CO2) 1741.55 638.303�( NH2) 964.49 12.335ı( NH2) 1442.09 47.630�( H) −1678.63 1692.97�( NH) 3227.21 261.245

A1

�( OH) 1309.66 274.780�( NH) 1524.42 280.247�(C O)

1712.98 339.415ı( COH)ı( CNH)�( NH) 3295.09 256.406�( OH) 3643.37 63.498

asOto�tvrNst[IItas

a Frequencies scaled by 0.958 below 1700 cm−1and by 0.983 above 1700 cm−1.

nd asymmetric stretching vibration �as( CO2) also have a red-hift in frequency. We may attribute the frequency shift to the

H hydrogen bond. Compared IM1 with A1, it can be seen thathe vibration frequency and mode of some groups occur obvi-us change in this process. The symmetric stretching vibrations( NH2) of IM1 in 3315.91 cm−1 and asymmetric stretching vibra-ion �as( NH2) of IM1 in 3401.05 cm−1 disappear and the stretchingibration of �s( OH) appears in A (3643.37 cm−1) when the CO2eacts with the [aEMMIM][BF4]. It can account for H proton ofH2 transferred to O atom of CO2 and the C O double bond

tretched to C O single bond. The emergence of rocking vibra-ion �( OH) (1297.73 cm−1 for [aEMMIM] and 1309.66 cm−1 foraEMMIM][BF4]) further demonstrates the above proton transfer.n addition to these, we also find bending vibration ı( NH2) of

M in 1641.73 (1647.02) cm−1 and asymmetric stretching vibra-ion �as( CO2) in 2318.58 (2310.62) cm−1 disappear in A (A1). Theppearance of the intense carbonyl stretching vibration �(C O),ymmetric bending vibration ı( CNH) and symmetric bending

Fig. 4. Geometrical parameters of [aEMMIM][BF4] ca

house Gas Control 20 (2014) 43–48

vibration ı( COH) at 1720.23 (1712.98) cm−1 in Tables 1 and 2 is astrong indication that a carbonic acid species is produced from theinteraction between the amino group and CO2 (Jessop et al., 2005).The appearance of the new peak at 3648.77 (3643.37) cm−1 corre-sponding to the �( OH) frequency of COOH is also indicative ofcarbonic acid formation (Jessop et al., 2005).

In a way, there are three main differences between [aEMMIM]cation and [aEMMIM][BF4] capturing CO2. First, the bond distanceis different in two processes. The distance of N C is graduallyshortened from 3.17 via 1.59 to form the C N bond 1.38 A inthe mechanism of CO2 capture by [aEMMIM] cation. However,the distance of N C is gradually shortened from 2.93 via 1.54 toform the C N bond 1.37 A in the mechanism of CO2 capture by[aEMMIM][BF4]. The distances of N H and O H have the samechange. It can be seen that the distance becomes shorter afterjoining the anion [BF4]. When anion [BF4] approaches cation [aEM-MIM], F atom and H atom can form hydrogen bonds. Generally, aC H. . .F hydrogen bond can be identified if the C H. . .F distanceis less than the van der Waals H. . .F distance of 2.70 A and theC H. . .F angle is greater than 90◦ (Bondi, 1964; Fuller et al., 1994).The interaction between [aEMMIM][BF4] and CO2 become stronger.So the distance is little closer. Second, the energy barrier is differ-ent in two processes. From Fig. 5, it can be seen that the energybarrier is little lower (8.77 kcal/mol) than that in Fig. 3 in the gasphase; but in the solvent, it is quite lower (18.90 kcal/mol) thanthat in Fig. 3. The results indicate that the anion [BF4] confers on[aEMMIM][BF4] a favorable CO2 absorption capacity. The reason ofthis phenomenon is that hydrogen bond exists between cation andanion in [aEMMIM][BF4]. It is also shown from Fig. 4 that F atom of[BF4] anion and H atom of [aEMMIM] cation form hydrogen bonds.So the energy barrier is much lower. Third, the charge density of therelated atoms is also different. The charge of N atom becomes lessin [aEMMIM][BF4] than that in [aEMMIM] cation. And the chargeof H atom in N H fragment becomes more in [aEMMIM][BF4] thanthat of H atom in [aEMMIM] cation. F atom and H atom can formhydrogen bonds in [aEMMIM][BF4], the charge of H in N H frag-ment flow to F atom and the charge of N atom flow to H atom, whichresults in a decrease of the charge in N atom and an increase in Hatom.

4.2. Vibrational frequencies

Table 3 lists the wave numbers of the bands observed in the FT-IR spectra of [aEMMIM][BF4] and CO2 capture by [aEMMIM][BF4]. Itgathers the experimental frequencies and the theoretical frequen-cies calculated by B3LYP/6-311++G (d, p). Figs. 6 and 7 show theexperimental and the calculated IR spectrum of [aEMMIM][BF4]

and CO2 capture by [aEMMIM][BF4], respectively. We have usedthe scaling factor values 0.983 up to 1700 cm−1 and 0.958 forgreater than 1700 cm−1 (Merrick et al., 2007; Karabacak et al., 2009;Sundaraganesan et al., 2005), the calculated wave numbers for

pture CO2 optimized at B3LYP/6-311++G (d, p).

H. Sun et al. / International Journal of Greenhouse Gas Control 20 (2014) 43–48 47

Table 3The experimental and calculated frequencies of pure IL [aEMMIM][BF4] and absorption of CO2 at B3LYP/6-311++G (d, p).

Vibrational mode assignment [aEMMIM][BF4] [aEMMIM][BF4] + CO2

Experimental frequency Calculated frequency IR intensity Experimental frequency Calculated frequency IR intensity

�(C5-H) 3130 3149 6.1654 3125 3106 130.316�(C4-H) 3078 3067 253.934 3078 3150 7.912

�(alkyl C H)2947 2961 28.877 2963 2962 3.6072868 2872 42.206 2874 2906 5.7140

�(C C) 1589 1594 14.977 1589 1592 11.840�(C N) 1589 1594 14.977 1589 1592 11.840

ı(alkyl C H)1539 1539 60.833 1539 1537 41.8171418 1427 29.231 1456 1456 8.065

�(alkyl C H)1246 1247 30.362 1246 1278 93.1571020 1032 2.540 1022 1038 5.324955 952 192.812 955 961 33.437

0

5

10

15

20

25

30

35

40

-5

R1+R2

IM1A10.00

-1.92

0.49

TS1

1.98

36.56

-2.09

E (kcal/mol)

GAS

CPCM

22.00

Bopisr�o1w

F

-10

Fig. 5. Potential energy surface of CO2 capture by [aEMMIM][BF4].

3LYP method scale down the calculated harmonic frequencies inrder to improve the agreement with the experiments. The infraredeaks appearing at 3130 cm−1 and 3078 cm−1 in the experiment

s designed to �(C5-H) and �(C4-H) stretching vibration corre-ponding with the calculated values 3149 cm−1 and 3067 cm−1,espectively. The infrared band at 1589 cm−1 refers to �(C C) and

(C N) stretching vibration which is in agreement with the the-ry result 1594 cm−1. The experimental values 1539 cm−1 and418 cm−1 are due to ı( CH) bending vibrations in consistentith 1539 cm−1 and 1427 cm−1. The experimental results are in

4000 350 0 3000 2500 20 00 1500 10 00

[aEMMIM][ BF4]

[aEMMIM][ BF4]+CO

2

wavenu mber / cm-1

ig. 6. Experimental IR spectrum of [aEMMIM][BF4] and [aEMMIM][BF4] + CO2.

Fig. 7. Calculated IR spectrum of [aEMMIM][BF4] and [aEMMIM][BF4] + CO2.

better agreement with the scaled calculated values. The discrep-ancy between the observed and calculated frequencies may be dueto the fact that the calculations have been actually done on a singlemolecule in the gaseous state contrary to the experimental valuesrecorded in the presence of intermolecular interactions. It can beseen that the experiment values of the frequencies are limited bysome experimental conditions, so the calculated frequency valuesare helpful to analysis the vibration modes of the ILs.

Table 3 shows that there are some differences in the vibra-tional frequencies after [aEMMIM][BF4] reacts with CO2. In the caseof CO2 absorption, the vibration frequency �(C5-H) moves to lowfrequency both in the experiment and the calculation. The experi-mental value of �(C4-H) does not change, but its calculated value isblue-shifted from 3067 cm−1 to 3150 cm−1. The distance betweenC5-H and F atom is closer, and the interaction of H F hydrogen bondis stronger. So it is blue-shifted. The vibration frequency �(C C) and�(C N) moves to low frequencies in the calculation, but it staysthe same value in the experiment. It is subject to red-shifted. Boththe experimental value and calculated value �(alkyl C H) becomehigher. These details that cannot be seen in the experiments areshown in Table 3, which reflects the advantages of the theoreticalcalculation.

5. Conclusions

The geometries of the reactants, intermediates, transition states

and products are fully optimized at B3LYP method using 6-311++G(d, p) basis set to understand the reaction mechanism of CO2and amine-functionalized ionic liquid [aEMMIM][BF4]. The DFTcalculations show that a C H. . .F hydrogen bond exists between

4 f Green

[tavcpimctsipp

A

FtRo

R

A

B

B

B

B

B

B

C

C

C

C

D

E

E

F

8 H. Sun et al. / International Journal o

aEMMIM] cation and [BF4] anion. The energy barriers of the reac-ion [aEMMIM] cation and CO2 both in the gas and in the solventre high, indicating that the reaction is thermodynamically unfa-orable. Then, the reaction between [aEMMIM][BF4] and CO2 wasalculated, it demonstrates that the energy barrier reduces com-ared to the reaction between [aEMMIM] cation and CO2, whatever

n the gas or in the liquid phase. The energy barrier decreasesore in the liquid phase than in the gas phase. The [aEMMIM][BF4]

aptures CO2 by forming carbamic acid. In the same time, the vibra-ional frequencies of the [aEMMIM][BF4] and captured CO2 aretudied by B3LYP/6-311++G(d, p), which coincide with the exper-ment results. The difference between the observed and scaledarameters is very small, which indicates that B3LYP/6-311++g (d,) method is feasible.

cknowledgements

This work was supported by the National Natural Scienceoundation of China (21173267), the Natural Science Founda-ion of Shandong Province (ZR2009BM037), and, the Fundamentalesearch Funds for the Central Universities and the Research Fundsf Renmin University of China (12XNLL05).

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