Hydrogen Bonding Interactions between a Representative Pyridinium

J. Phys. Chem. B 2010, 114, 8689–8700

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Hydrogen Bonding Interactions between a Representative Pyridinium-Based Ionic Liquid [BuPy][BF4] and Water/Dimethyl Sulfoxide Nan-Nan Wang,† Qing-Guo Zhang,†,‡ Fu-Gen Wu,† Qing-Zhong Li,§ and Zhi-Wu Yu*,† Key Laboratory of Bioorganic Phosphorous Chemistry and Chemical Biology (Ministry of Education), Department of Chemistry, Tsinghua UniVersity, Beijing 100084, People’s Republic of China, College of Chemistry and Chemical Engineering, Bohai UniVersity, Jinzhou 121000, People’s Republic of China, and The Laboratory of Theoretical and Computational Chemistry, Science and Engineering College of Chemistry and Biology, Yantai UniVersity, Yantai 264005, People’s Republic of China ReceiVed: April 16, 2010; ReVised Manuscript ReceiVed: May 26, 2010

Infrared spectroscopy and density functional theory calculations have been applied to elucidate the hydrogen bonding interactions between water/dimethyl sulfoxide (DMSO) and a representative pyridinium-based ionic liquid, 1-butylpyridinium tetrafluoroborate ([BuPy][BF4]). It has been found that both solvents can interact with the BuPy+ cation through the aromatic C-H. The strength of the H-bonds involving the aromatic C-H in water is similar to that in pure [BuPy][BF4], but is much stronger in DMSO. For DMSO, when it forms H-bonds with the BuPy+ cation through its SdO group, its back-side methyl groups act as electron donors, while the butyl group of the cation acts as an electron acceptor. For water, when it forms the strong anion-HOH-anion complex, it can also form H-bonds with the aromatic C-H on the BuPy+ cation. This is different from the imidazolium-based ionic liquid, where the strong anion-cation interaction and steric hindrance from the alkyls prevent water molecules from H-bonding with the aromatic C-H other than with the anion. 1. Introduction Ionic liquids (ILs) are fused salts and are characterized by a number of physicochemical properties, such as low vapor pressure, nonflammability, chemical stability at high temperatures, and miscibility with polar and nonpolar solvents. Due to these unique properties, ILs are believed to have potential applications in a variety of industrial fields and have thus attracted wide attention from the academic community.1-3 Among the broad range of fundamental studies of ILs, particular efforts have been devoted to understanding the molecular interactions between ILs and cosolvents, as the presence of water or organic cosolvents has a significant effect on the properties of ILs such as density, viscosity, surface tension, electrical conductivity, heat capacity, and solubility.4-9 Hydrogen bonding plays an important role in pure ILs and the interactions between ILs and cosolvents. A body of experimental and theoretical investigations concerning Hbonding interactions have been carried out to understand the properties of ILs and their structural changes induced by cosolvents, especially water.10-28 For instance, Cammarata et al. found by infrared spectroscopy that water molecules absorbed from air (0.2-1.0 mol dm-3) mainly interact with the anions and exist in symmetric anion-HOH-anion H-bonded complexes in imidazolium-based ILs with various anions.12 Porter et al. further confirmed the symmetric 1:2 type interaction in ILs by molecular dynamic (MD) simulations.21 The existence of 1:1 type complexes, where a water molecule doubly H-bonds to one anion or forms a single H-bond with one anion while the other H atom is “quasi-free”, was proposed by Ludwig and * To whom correspondence should be addressed. Phone: (+86) 10 6279 2492. Fax: (+86) 10 6277 1149. E-mail: [email protected]. † Tsinghua University. ‡ Bohai University. § Yantai University.

co-workers.13 Sequential addition of water to pure ILs gradually destroys the three-dimensional network structure of ILs first into ionic clusters, then into ionic pairs surrounded by water molecules, and ultimately into fully hydrated separated ions.10,11,24 MD simulations also demonstrated the existence of a certain turnover point of the structure evolution of ILs during the dilution process with water.20 The complete dissociation of ionic pairs only takes place when the mole fraction of imidazoliumbased ILs is extremely low, for example, 0.015 for 1-butyl-3methylimidazolium tetrafluoroborate ([bmim][BF4]).10 At higher concentrations, for example, in the mole fraction range of 0.5-0.6, ILs form clusters and water molecules interact with the clusters without forming an H-bond network among the water molecules themselves.10 In addition to the ubiquitous solvent water, the aprotic solvent dimethyl sulfoxide (DMSO) has also been investigated due to its excellent solubility to both inorganic and organic compounds and wide applications in various fields.29 Recently, Li and coworkers investigated the interactions between DMSO-d6 and [bmim][BF4], and observed a blue shift in the C-D stretching vibrations of DMSO-d6. They attributed this to the indirect influence of the H-bond formed by the nearby functional group SdO with C2-H of the cation.25 The above-mentioned works have greatly enriched our knowledge on the interactions between ILs and cosolvents. Most of these studies, however, are limited to the imidazolium-based ILs. In addition to imidazolium-based cations, alkylated pyridiniums are another important class of cations in ILs.1-3 The macroscopic properties such as density (excess molar volumes), viscosity, conductivity, and infinite dilution activity coefficients of some pyridinium-based IL-cosolvent systems have been reported.30-35 The microscopic properties, namely the molecular level information concerning the interactions between pyridinium-based ILs and water or other cosolvents, have not been

10.1021/jp103438q  2010 American Chemical Society Published on Web 06/15/2010

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J. Phys. Chem. B, Vol. 114, No. 26, 2010

SCHEME 1: Chemical Structures of 1-Butylpyridinium Tetrafluoroborate ([BuPy][BF4]), H2O, and DMSO

studied in as much detail. Seki et al. found by infrared spectroscopy that the interaction between supercritical CO2 and ILs can be tuned by selecting either [bmim] or 1-butylpyridinium ([BuPy]).36 Very recently, Lauw et al. investigated the effect of water on the surface structure of 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonylimide ([C4mpyr][NTf2]) IL by X-ray reflectometry and MD simulations. They found that a significant amount of water is adsorbed at the surface, with the first layer of the gas-liquid phase boundary occupied mainly by a mixture of cations and water.37 Among various experimental techniques, infrared spectroscopy is a convenient and effective approach for the study of H-bonding interactions at the molecular level.12-18,23-28,38 In this work, by using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and density functional theory (DFT) calculations, the H-bonding interactions between a representative pyridinium-based IL, 1-butylpyridinium tetrafluoroborate ([BuPy][BF4]), and water or DMSO (Scheme 1) have been characterized. Deuterated water (D2O) and deuterated DMSO (DMSO-d6) were used to avoid the overlap of their certain absorption bands and the C-H stretching bands of [BuPy][BF4]. In particular, excess infrared absorption spectroscopy39-43 and two-dimensional (2D) correlation analysis17,24,44,45 have been employed to reveal the details of interaction. Since BF4- anion is a weak base and its H-bonding interaction with water molecules has been reported in a number of studies concerning imidazolium-based ILs,17,22-27 [BuPy][BF4] is selected to elucidate particularly the H-bonding interactions between pyridinium cation and water or DMSO. 2. Experimental Section 2.1. Chemicals. [BuPy][BF4] was synthesized according to the method described elsewhere.46 The product obtained is light yellow in color. After drying for 48 h under vacuum, the ionic liquid was stored in a desiccator. The structure and purity of [BuPy][BF4] were checked by 1H NMR using a JEOL JNMECA 600 NMR spectrometer, δH (600 MHz, D2O): 0.87 (3H, t, C10-H), 1.29 (2H, sextet, C9-H), 1.92 (2H, quintet, C8-H), 4.54 (2H, t, C7-H), 7.99 (2H, t, C3,5-H), 8.46 (1H, t, C4-H), 8.77 (2H, d, C2,6-H). The NMR and IR data are in good agreement with the published work.47 D2O and DMSO-d6 were purchased from Aldrich (99.9 atom % D). 2.2. Sample Preparation. A series of [BuPy][BF4]-D2O and [BuPy][BF4]-DMSO-d6 binary mixtures were prepared by weighing. The mole fractions of D2O in [BuPy][BF4]-D2O mixtures are 0.1032, 0.1997, 0.3023, 0.4140, 0.5065, 0.6051,

Wang et al. 0.7016, 0.8023, and 0.9002. The mole fractions of DMSO-d6 in [BuPy][BF4]-DMSO-d6 mixtures are 0.1000, 0.1978, 0.2980, 0.3984, 0.4972, 0.5965, 0.6970, 0.7974, and 0.8987. 2.3. FTIR Spectroscopy. FTIR spectra over the range from 4000 to 650 cm-1 were collected at room temperature (∼25 °C) using a Nicolet 5700 FTIR spectrometer, equipped with a DTGS detector. A horizontal ATR trough equipped with a ZnSe crystal (refractive index 2.43) with incident angles of 45° and 12 reflections were employed in the experiments. Spectra were recorded with a resolution of 2 cm-1, a zero filling factor of 2, and 16 parallel scans. For each sample, three parallel measurements were carried out. The refractive indexes of mixtures were measured with a refractometer at 25 °C. The formulas suggested by Hansen48 were used to do the ATR corrections. 2.4. Excess Infrared Spectroscopy. The theory of excess infrared spectroscopy has been described in detail elsewhere.39,40 Briefly, an excess infrared spectrum is defined as the difference between the spectrum of a real solution and that of the respective ideal solution under identical conditions. The working equation in calculating the excess infrared spectrum is

εE )

A - (x1ε1* + x2ε2*) d(C1 + C2)

(1)

where A is the absorbance of the mixture, d is the light path length, C1 and C2 are molarities of the two components, x1 and x2 are mole fractions of components 1 and 2, and ε1* and ε2* are molar absorption coefficients of the two components in their pure states, respectively. In addition, when an absorption mode is discussed as a whole, the integral value of excess spectrum within the range of the absorption band is used. The same symbol εE is used in this case to be in line with the general understanding that both the absorbance at a fixed wavenumber and the integral value of an absorption band are represented by the symbol A in infrared spectroscopy. The calculation of the excess infrared spectra was programmed using Matlab 7.0 (Math Works Inc., Natick, MA). The data manipulations, i.e., the subtraction, truncation, baseline correction, and integration of the bands in the excess infrared spectra, were also performed using Matlab 7.0. Deconvolution of the ATR-FTIR and excess infrared spectra was carried out using the curve-fitting routine provided in PeakFit 4.0 software (AISN Software Inc.). The peak type was Gaussian + Lorentz for all deconvolution treatments. 2.5. Two-Dimensional Correlation Analysis. A standard 2D correlation analysis of the FTIR spectra was also performed on Matlab software, based on the algorithm developed by Noda.44 A modified component-normalization method was used to remove the linear contribution to absorbance by concentration and to obtain the information of specific interactions.42,49 The average spectrum of all the normalized spectra over the full concentration range was used as reference spectrum. In the 2D correlation contour map, solid and dashed lines represent positive and negative correlation intensities, respectively. 2.6. Quantum Chemical Calculations. The geometries, interaction energies, harmonic vibrational frequencies, and IR intensities were obtained by DFT at the 6-31++G(d,p) basis set with the Gaussian 03 program.50 DFT has been used extensively to study the interactions between ionic liquids and cosolvents such as water, acetone, and DMSO.24-26,51,52 The 6-31++G** basis set was selected in our calculation following these studies. All the optimized geometries were recognized as local minima with no imaginary frequency. Meanwhile, the basis set superposition error (BSSE) correction was estimated for

H-Bonding between [BuPy][BF4] and Water/DMSO

J. Phys. Chem. B, Vol. 114, No. 26, 2010 8691

Figure 1. ATR-FTIR (upper) and excess infrared (lower) spectra of [BuPy][BF4]-D2O system in the range of the C-H (A and C) and O-D (B and D) stretching vibrations. From top to bottom, the mole fraction of D2O increases from 0 to 1 in (A) and decreases from 1 to 0 in (B) with an increment of about 0.1. The dashed and dashed-dotted lines depict spectra of pure [BuPy][BF4] and D2O. The inset figure in (B) is the enlarged spectra in the 2750-2550 cm-1 region.

obtaining accurate interaction energies. The natural population analysis (NPA) charges were obtained using the natural bond orbital (NBO) approach. 3. Results and Discussion 3.1. Interactionsbetween[BuPy][BF4]andWater.3.1.1. ATRFTIR and Excess Infrared Spectroscopy. The partial ATRFTIR and excess infrared spectra of [BuPy][BF4]-D2O system over the entire concentration range, including those of pure [BuPy][BF4] and D2O, are shown in Figure 1. In the 3200-2800 cm-1 region, the C-H stretching vibrations were observed (Figure 1A). D2O does not have absorptions in this region. For pure [BuPy][BF4], the three bands above 3020 cm-1 are attributed to the coupled pyridinium ring C-H stretches, while those below 3020 cm-1 are attributed to the alkyl C-H stretches.53 With the increase of water content, the absorbance of all these bands that are solely from [BuPy][BF4] decreases monotonically. The C-H stretching vibration is a key band to characterize the H-bonds such as C-H · · · O and C-H · · · F, and it can be used as a probe to reflect the interactions between water and ILs.54 In the ATR-FTIR spectra (Figure 1A), the pyridinium ring C-H stretching bands, centered around 3140, 3097, and 3075 cm-1 in pure [BuPy][BF4], exhibit no evident changes in the spectral profiles upon dilution. The peak position of the band at 3140 cm-1 is slightly red-shifted by 0.5 cm-1 (Figure 2A), while those of the other two bands are nearly fixed, with less than (0.2 cm-1 change in wavenumber. Noticing that H-bonds between the aromatic C-H on the BuPy+ cation and the BF4anion are present in pure [BuPy][BF4],55 these observations suggest that the aromatic C-H on the pyridinium ring experiences very similar H-bonding interactions in water as compared to that in pure [BuPy][BF4]. The alkyl C-H stretching bands,

centered around 2965, 2938, and 2878 cm-1 in pure [BuPy][BF4], also exhibit no evident changes in the spectral profiles upon dilution. However, the peak positions of the three bands show more obvious shifts than that of the ring C-H. They are blue-shifted by 3.5, 2.9, and 2.3 cm-1, respectively (Figure 2B). Excess infrared spectra can reveal the positions of new complexes and the changes in molar absorptivity more clearly than the original IR spectra. In the excess infrared spectra of the C-H stretching vibration (Figure 1C), the most obvious feature is the negative bands from both the pyridinium ring and the alkyl C-H. Besides, the stretching vibration of alkyl C-H has a positive band at the higher wavenumber, located at about 2975 cm-1, which can be attributed to the blue shift of the corresponding bands. Furthermore, there are two small positive bands at the lower wavenumber in the region of the pyridinium ring C-H stretches, located at about 3062 and 3042 cm-1 denoted by arrows in Figure 1C (taking x(D2O) ) 0.5065 as an example). These bands are not distinguished in the ATR-FTIR spectra in Figure 1A. Integral values of the bands in excess infrared spectra are used to represent the extent of molar absorptivity variation of a certain vibrational mode. In the integration process, negative bands yield negative integral values. If positive and negative bands both exist for one vibrational mode, the positive and negative integral values will counteract and a total integral value will be generated. The integral values of the excess infrared bands (εE) for the pyridinium ring and alkyl C-H stretches at different D2O concentrations are given in Figure 2C. Both are negative over the entire concentration range, with that of the pyridinium ring much more negative than the latter. According to the literature,14,24,25 the aromatic C-H-related H-bond is red-shifted caused by an elongation of the C-H bond,

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Figure 2. Wavenumber shift of the pyridinium ring C-H (A) and alkyl C-H (B), and excess molar absorbance of the C-H (C) and O-D (D) stretching vibrations at different mole fractions of D2O in [BuPy][BF4]-D2O system.

while being characterized by a red shift of the respective IR band and an increase of its IR intensity. We note that the slight peak shift and the obvious decrease in the molar absorptivity of pyridinium ring C-H stretches seem to be contrary. As the transition dipole moment of the related vibration may be largely influenced by the presence of cosolvent but not the H-bonding interaction, it is likely that the peak position of the C-H stretching modes is more reliable than the intensity variation in analyzing the H-bonding interaction of ILs, as utilized in most reported literature.12-18,23-28 As discussed above, the C-H bonds on the pyridinium ring have very similar H-bonding interactions in water and in pure [BuPy][BF4], implied by the relatively small peak shift. The two small positive bands of ring C-H stretches at the lower wavenumber in excess spectra, located at about 3062 and 3042 cm-1, indicate that some pyridinium ring C-H bonds have stronger H-bonding interactions upon dilution. The H-bond of alkyl C-H is classified as an “improper blueshifted” H-bond,56 characterized by the shortening of the C-H bond, a blue shift of the corresponding IR band, and a decrease of its IR intensity. In the [BuPy][BF4]-D2O mixtures examined, the blue shift and the decrease in molar absorptivity of alkyl C-H stretches have two possible origins: (1) direct H-bonding interaction between the alkyl C-H and water molecules; (2) indirect effect from the interaction between the pyridinium ring and water molecules. The second is found to dominate as confirmed by the 2D correlation analysis and theoretical calculations, which will be elaborated in the following discussion. In the 2800-1800 cm-1 region, [BuPy][BF4] does not absorb. The strong absorption bands observed are from O-D stretching vibrations of the cosolvent water (Figure 1B). For pure D2O, the broad band around 2473 cm-1 with a shoulder at 2397 cm-1 is due to the cooperative H-bonds in pure water. In the presence of [BuPy][BF4], two or even three new bands appear. The

decrease in the absorbance of the lower wavenumber bands is due to the decrease in the number of strongly H-bonded D2O molecules,13,27 while the bands appearing at the higher wavenumber are due to the appearance of either single or more weakly interacting D2O molecules embedded in the IL environment. In the excess infrared spectra (Figure 1D), a number of positive and negative bands are observed, which reveal the appearance and disappearance of the corresponding water structures in IL, compared to those in pure D2O. The integral values of these excess bands are negative over the entire concentration range (Figure 2D). Since the O-D H-bond belongs to red-shifted H-bond,56 the appearance of the new bands at the higher wavenumber and the decrease of total IR intensity mean a decrease in H-bond strength of liquid water in [BuPy][BF4]. The O-D stretching bands are extensively overlapped. The respective excess infrared spectra show better resolution and can help the performance of deconvolution to separate the overlapped bands and thus to understand the structure evolution of water molecules upon the addition into [BuPy][BF4]. The deconvolution results of the ATR-FTIR spectra at three representative concentrations are shown in Figure 3A (see Figure S1 in the Supporting Information for the deconvolution results over the full concentration range). The assignments of the absorption bands in the O-D stretching vibrational region are listed in Table 1. The two bands at about 2704 and 2604 cm-1 are assigned to the asymmetric and symmetric stretching vibrations, Vas and Vs (V3 and V1 in ref 13), of the single D2O molecules, forming double H-bonds to BF4-, respectively. The intensities of Vas and Vs are similar at x(D2O) ) 0.1032, and consistent with the published values.13,27 There is obviously another absorption band in the 2680-2660 cm-1 region at higher water concentrations. Following the assignment of the terminal



Figure 3. Deconvolution of the ATR-FTIR (A) and excess infrared spectra (B) in the O-D stretching vibrational region at representative concentrations. Solid and dashed lines are experimental and deconvoluted bands. See Table 1 for the assignments.

TABLE 1: Assignments of the Absorption Bands in the O-D Stretching Vibrational Region Based on Published Studies13,24,27 and Our DFT Calculations wavenumber (cm-1) ∼2704 2680-2660 ∼2650 ∼2604 2540-2440 2400-2370

assignment

abbreviation

Vas of D2O doubly H-bonded to BF4terminal OD of self-associated D2O clusters, H-bonded to BF4HDO formed by H/D isotope exchange Vs of D2O doubly H-bonded to BF4self-associated D2O clusters tetrahedrally H-bonded D2O

Vas Vtc Vi Vs Vc Vt

OH of alcohols, it is attributed to the stretching vibration of the terminal OD of the self-associated D2O clusters Vtc. The terminal OD is not completely free and is H-bonded to BF4-. This assignment has also been proved by our DFT calculations (∼2638 cm-1 after correction by a scaling factor of 0.955). The presence of Vas and Vs during the increase of x(D2O) from 0.1 to 0.9 indicates that, within the concentration range examined, single D2O molecules (without self-association) always exist in [BuPy][BF4]. The central band Vi, at about 2650 cm-1, comes from O-D stretching vibration of HDO, which is formed by hydrogen/deuterium (H/D) exchange with trace amounts of residual H2O.13 This band is less obvious at high concentrations of D2O. Along with the increase in water concentration, we can see a growing broad band at 2540-2440 cm-1, which is assigned to the absorption of self-associated D2O clusters Vc. At x(D2O) ) 0.1032, the band Vc is very weak, and the dominant bands are Vas, Vi, and Vs. The results demonstrate that single D2O molecules are isolated from each other, which is in agreement with the conclusions given by both experimental and theoretical investigations of other IL-water systems.12,21,22 Finally, with the increasing water content, there is a band at 2400-2370 cm-1 at the low wavenumber end, which is assigned to the absorption of tetrahedrally H-bonded D2O Vt. Based on the appearance/disappearance and intensity variations of the bands, the structure evolution of water molecules in [BuPy][BF4] can be deduced as follows. First, at low concentrations, single water molecules are isolated from each other and embedded in the IL environment. Then, as the water content increases, the water clusters form. When the size of the clusters is big enough (x(D2O) > 0.4), some tetrahedrally H-bonded D2O molecules appear. Finally, the self-associated D2O clusters but not

tetrahedrally H-bonded D2O become dominant at the highest water concentration we investigated, x(D2O) ) 0.9. At this point, it is noteworthy that single water molecules (without selfassociation) always exist in [BuPy][BF4] over the entire concentration range. By means of the deconvoluted excess infrared spectra (Figure 3B), the difference of water structures in [BuPy][BF4] and in pure D2O is compared. The assignments of the O-D stretching bands in excess infrared spectra are the same as those in the ATR-FTIR spectra. The broad negative bands Vt and Vc are due to the decrease of the self-associated D2O in ILs. The decrease of the tetrahedrally H-bonded D2O is remarkable over the whole concentration range. The band of Vc becomes smaller than Vt at high water concentration, due to the formation of water clusters. In [BuPy][BF4], the single D2O molecules embedded in the IL environment appear and the amount of HDO formed by H/D exchange is larger than that in pure D2O, and they are clearly seen over the whole concentration range. 3.1.2. Two-Dimensional Correlation Analysis. In order to probe the sequential order of interactions in the dilution process of [BuPy][BF4] by water, 2D correlation analysis was performed with a focus on V(pyridinium ring C-H) and V(alkyl C-H) in [BuPy][BF4] with increasing water concentration. The synchronous and asynchronous 2D correlation contour maps of [BuPy][BF4]-D2O mixtures are depicted in Figure 4. There are more correlation bands in the asynchronous spectrum (Figure 4B) than in the synchronous spectrum (Figure 4A), because the asynchronous cross peaks result from the relative dissimilarity of the intensity variation behavior. The two central bands in the pyridinium ring and alkyl C-H stretching regions, at 3097 and 2938 cm-1 respectively (see Figure 1A), are taken as the representative absorptions for 2D correlation analysis. A positive cross peak at (3098, 2930 cm-1) is observed in the synchronous spectrum shown in Figure 4A. This positive peak is due to the same changing direction of the absorption coefficients of V(pyridinium ring C-H) and V(alkyl C-H), as already indicated in the two negative curves of excess molar absorbance shown in Figure 2C. In the asynchronous spectrum shown in Figure 4B, the cross peak at (3096, 2936 cm-1) is also positive. Thus the signs of both synchronous and asynchronous cross peaks of [V(pyridinium ring C-H), V(alkyl C-H)] are the same (other cross peaks have also been tested).

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Figure 4. Synchronous (A) and asynchronous (B) 2D correlation contour maps of C-H stretching vibrations of [BuPy][BF4] in the dilution process with D2O. Solid and dashed lines represent positive and negative correlation intensities, respectively.

According to Noda’s rule,44,45 the absorption coefficient of V(pyridinium ring C-H) varies prior to that of V(alkyl C-H) with the increase of water content. It is suggested that water preferentially interacts with the pyridinium ring, and the spectroscopic variation of the alkyl C-H is mainly due to an indirect effect from the interaction between the pyridinium ring and water molecules. 3.1.3. Theoretical InWestigation of the Interaction between [BuPy][BF4] and Water. Since the structure characteristic of pure [BuPy][BF4] has been reported by Sun et al.55 and the H-bonding interaction between water and the BF4- anion has been well studied,17,22-24 our attention is directed to the H-bonding interaction between water and the pyridinium cation. To characterize the preferred interaction sites between the water molecule and the cation, a set of possible orientations of the complexes consisting of one water molecule and one BuPy+ cation, with or without the BF4- anion, have been selected to perform DFT calculations. The optimized geometries and respective interaction energies obtained are shown in Figure 5. The sum of van der Waals atomic radii of H and O (2.5 Å) and that of H and F atoms (2.45 Å) are used to judge the existence of H-bonds.57 These are denoted by dashed and dotted lines in Figure 5. In the BuPy+-H2O complexes (Figure 5A-C), it is found that H-bonds can exist between C2,6-H, C3,5-H, or C4-H on the pyridinium ring and the O atom of H2O. The interaction energy between C2,6-H and H2O (-41.55 kJ/mol) is larger in absolute value than those of the other two (-34.82 and -34.38 kJ/mol, respectively), indicating that the first conformer is the most stable. The small difference between the interaction energies (∼7 kJ/mol) suggests that all the C-H on the pyridinium ring can form H-bonds with water molecules in [BuPy][BF4]-H2O mixtures. This result is distinct from the interactions between imidazolium ring and water, in which the imidazolium proton N-CH-N (C2-H) is most favorable to form an H-bond with water molecules owing to the electron deficit in the imidazolium ring.14,24,28 The [BuPy][BF4]-H2O complexes (Figure 5D-F) give the interaction modes in the presence of both the BF4- anion and the BuPy+ cation. It was found that H-bonds between H2O and

BF4- are present in all three optimized geometries, consistent with the interaction modes between water and imidazoliumbased ILs where water interacts preferentially with the anions in ILs as concluded in previous studies.12,13,21,22,28 Moreover, H-bonds may also exist between the aromatic even alkyl C-H on the cation and the O atom in H2O as shown in Figure 5D,E.The existence of the H-bonds leads to a larger interaction energy (-383.91 and -381.75 kJ/mol for Figure 5D and 5E, respectively) than that without the H-bond (-375.71 for Figure 5F) in absolute value, suggesting that water can form H-bonds with the anion and the cation of [BuPy][BF4] simultaneously. It is noteworthy that H-bonds between BF4- and BuPy+ always exist in the presence of limited water molecules in the system. The number of H-bonds between the ion pairs, however, has no evident relationship with the interaction energy, indicating that the electrostatic interaction of the cation and the anion is dominant and the H-bonding interaction is incidental in pyridinium-based tetrafluoroborate ILs. Neither the BuPy+-H2O nor the [BuPy][BF4]-H2O complexes can exist alone in a macroscopic system. Nevertheless, the interaction models depicted in Figure 5 provide possible interaction sites between water and BuPy+ cation or [BuPy][BF4]. The favorable interaction sites for water and [BuPy][BF4] are the BF4- anion and C-H on the pyridinium ring, not the alkyl C-H on the side chain. With this in mind, the blue shift and the decrease in molar absorptivity of alkyl C-H stretches are attributed to an indirect or solvent effect of water. To record the representative interaction modes at lower and higher water concentration ranges, the optimized geometries for one water molecule interacting with two [BuPy][BF4] ion pairs and two water molecules interacting with one [BuPy][BF4] ion pair have also been calculated and the results are shown in Figure 6. At low water content, isolated water molecules exist in the IL environment. In the case of [BuPy][BF4], we found that one water molecule forms two H-bonds with two BF4anions via the H atoms, and forms one H-bond with the C2,6-H on the pyridinium ring via the O atom (Figure 6A). Several H-bonds also exist between the BF4- anions and the aromatic



Figure 5. Possible positions and corresponding interaction energies for a water molecule interacting with BuPy+ (A-C) and [BuPy][BF4] (D-F). H-bonds involving water molecules are denoted by dashed lines, those between the anions and cations are denoted by dotted lines, and the corresponding H · · · O and H · · · F distances are labeled in angstroms.

Figure 6. Optimized geometries for one water molecule interacting with two [BuPy][BF4] ionic pairs (A) and two water molecules interacting with one [BuPy][BF4] ionic pair (B). H-bonds involving water molecules are denoted by dashed lines and those between the anions and cations are denoted by dotted lines, and the corresponding H · · · O and H · · · F distances are labeled in angstrom.

even alkyl C-H on the BuPy+ cation. This representative interaction model gives the possibility that the strong anionHOH-anion interaction can be accompanied by the H-bond between the water molecule and the neighboring cations. Previous studies reported that the water molecules mainly interact with the anions and exist in symmetric 1:2 type H-bonded complexes in imidazolium-based ILs at low water content.12,21 There are two reasons why the isolated water molecule in [BuPy][BF4] can act as both donor and acceptor in H-bond: (1) The BF4- anion is a weak base and the H-bonding interaction is incidental in pyridinium-based tetrafluoroborate ILs, so the additional water can easily insert between BF4- and BuPy+. (2) There is only one alkyl chain on the pyridinium ring, so the steric hindrance preventing the water molecule approaching the acidic H atoms on the monoalkylpyridinium cation is limited, different from the case of C2-H on the dialkylimidazolium cation. At higher concentrations, water

molecules can H-bond with themselves in the mixture. Figure 6B provides such an example with the water to [BuPy][BF4] ratio of 2:1. 3.2. Interactionsbetween[BuPy][BF4]andDMSO.3.2.1. ATRFTIR and Excess Infrared Spectroscopy. The partial ATRFTIR and excess infrared spectra of [BuPy][BF4]-DMSO-d6 system over the entire concentration range, including the ATRFTIR spectra of pure [BuPy][BF4] and DMSO-d6, are shown in Figure 7. In the 3200-2800 cm-1 region, C-H stretching vibrations are observed (Figure 7A). DMSO-d6 shows tiny bands in this region due to the residual C-H bonds in the deuterated reagent. The assignments of the bands in pure [BuPy][BF4] have been discussed in section 3.1.1; i.e., the three bands above 3020 cm-1 represent the coupled pyridinium ring C-H stretches, while those below 3020 cm-1 represent the alkyl C-H stretches. With an increase of DMSO content, the absorbances of all the bands from [BuPy][BF4] decrease monotonically. In the ATR-FTIR spectra (Figure 7A), the pyridinium ring C-H stretching bands, centered around 3140, 3097, and 3075 cm-1 in pure [BuPy][BF4], exhibit significant changes in the spectral profiles in the presence of DMSO. They are red-shifted by 5.4, 9.7, and 18.7 cm-1, respectively (Figure 8A). The alkyl C-H stretching bands, centered around 2965, 2938, and 2878 cm-1 in pure [BuPy][BF4], exhibit no evident changes in the spectral profiles in the presence of DMSO. The peak positions of these three bands show less obvious shifts than that of the ring C-H. They are red-shifted merely by 2.7, 1.1, and 1.5 cm-1, respectively (Figure 8B). However, in [BuPy][BF4]-D2O system as discussed in section 3.1.1, the pyridinium ring C-H stretching bands show no evident changes, while the alkyl C-H stretching bands show blue shift upon dilution. The different extents of the shifts show the different influences of water and DMSO on the cation. Thus the aromatic C-H on the pyridinium ring has very similar H-bonding interactions in water as in pure [BuPy][BF4]. By contrast, they have much stronger H-bonding

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Figure 7. ATR-FTIR (upper) and excess infrared (lower) spectra of [BuPy][BF4]-DMSO-d6 system in the range of the C-H (A and C) and C-D stretching vibrations (B and D). From top to bottom, the mole fraction of DMSO-d6 increases from 0 to 1 in (A) and decreases from 1 to 0 in (B) with an increment of about 0.1. The dashed and dashed-dotted lines depict spectra of pure [BuPy][BF4] and DMSO-d6.

Figure 8. Wavenumber shifts of the pyridinium ring C-H (A), alkyl C-H (B), and -CD3 (C), and excess molar absorbance of the C-H and -CD3 stretching vibrations (D) at different concentrations of DMSO-d6 in [BuPy][BF4]-DMSO-d6 system.

interactions in the presence of DMSO as denoted by the marked red shift of the respective absorption bands.

The excess infrared spectra of C-H stretching vibration are shown in Figure 7C. In the region of pyridinium ring C-H



Figure 9. Synchronous (A) and asynchronous (B) 2D correlation contour maps of C-H stretching vibrations of [BuPy][BF4] in the process of increasing concentration of DMSO-d6. Solid and dashed lines represent positive and negative correlation intensities, respectively.

stretches, above 3000 cm-1, there are mainly two small negative bands at higher wavenumbers, located at about 3143 and 3102 cm-1, and two relatively strong positive bands at lower wavenumbers, located at about 3061 and 3038 cm-1. The negative excess bands at higher wavenumbers are due to the red shift of the IR bands. The significant red shift and extremely strong positive excess bands around 3061 and 3038 cm-1 indicate that the pyridinium ring C-H is the direct interaction site of DMSO. Therefore, the two positive excess bands represent the newly arisen H-bonded complexes of pyridinium ring C-H and DMSO. In the region of alkyl C-H stretches, below 3000 cm-1, only small positive bands can be seen. The integral values of excess molar absorbance (εE) for the pyridinium ring and alkyl C-H stretches at different DMSO concentrations are given in Figure 8D. They are positive over the entire concentration range, while that of the former is greater than that of the latter. Now we discuss the origin of these changes. For pyridinium ring C-H, the significant increase in molar absorptivity is in agreement with their marked red-shift property; both are characteristic of the strengthening of the respective H-bonds. In the case of alkyl C-H, the red shift of the bond stretches is less than 3 cm-1, and only small excess bands are observed. In addition, no spectroscopic evidence shows the alkyl C-H acts as preferential interaction sites. All these suggest that the changes in the spectroscopic properties of alkyl C-H are due to an indirect effect from the interaction between the pyridinium ring and DMSO. This has been confirmed by theoretical calculations as will be discussed in the section 3.2.3. In the region 2400-1900 cm-1, C-D stretching vibrations were observed (Figure 7B). [BuPy][BF4] does not absorb in this region. For pure DMSO-d6, the bands at 2249 and 2124 cm-1 arise from the asymmetric and symmetric stretching vibrations of the methyl group of DMSO-d6.58 These two bands are blueshifted by 14.3 and 7.2 cm-1 in the presence of [BuPy][BF4] (Figure 8C). In the excess spectra (Figure 7D), the pattern of positive bands at the higher wavenumbers, located at about 2264 and 2132 cm-1, and the negative bands at lower wavenumbers, located at about 2246 and 2122 cm-1, originates from the blue

shift of the two bands. The integral values of excess molar absorbance (εE) for the methyl C-D stretches are negative in the presence of [BuPy][BF4] as shown in Figure 8D. Since the self-association of DMSO-d6 and the direct interaction of the C-D bond with BF4- anion are not responsible for the blue shift of C-D stretches of DMSO-d6,25 such blue shifts and the decrease in absorptivity of the C-D stretches are explained as an influence from the H-bond between the nearby SdO group and the pyridinium cation. An interesting phenomenon is that the signs of εE for the alkyl group in pyridinium cation and the methyl groups in DMSO are opposite, as shown in Figure 8D. The directions of their band shifts are also opposite. These facts indicate that the roles played by these alkyl groups are different. We will show in section 3.2.3 that the former is an electron acceptor and the latter are electron donors, both making positive contributions to the stability of the complex. 3.2.2. Two-Dimensional Correlation Analysis. Two-dimensional correlation analysis has been performed to determine the sequential order of molecular interactions concerning the pyridinium ring and alkyl C-H of [BuPy][BF4] upon addition of DMSO to the IL. Absorption bands V(pyridinium ring C-H) and V(alkyl C-H), specifically the two representative bands at 3140 and 2965 cm-1, were selected for this study. The synchronous and asynchronous 2D correlation contour maps of [BuPy][BF4]-DMSO-d6 mixtures over the wavenumber region 3300-2800 cm-1 are depicted in Figure 9. According to Noda’s rule,44,45 the negative cross peak at (3145, 2960 cm-1) in the synchronous spectrum (Figure 9A) and the negative cross peak at (3149, 2958 cm-1) in the asynchronous spectrum (Figure 9B) suggest that the absorption coefficient of V(pyridinium ring C-H) varies prior to that of V(alkyl C-H) with increasing DMSO-d6 content. The pyridinium ring C-H is thus suggested to be the preferred site of interactions between the cation and DMSO. 3.2.3. Theoretical InWestigation of the Interaction between [BuPy][BF4] and DMSO. A set of possible conformations of the complexes consisting of one DMSO molecule and the BuPy+ cation, with or without the anion, have been examined by DFT

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Figure 10. Possible positions and corresponding interaction energies for a DMSO molecule interacting with BuPy+ cation (A and B) and [BuPy][BF4] (C and D). H-bonds involving DMSO molecules are denoted by dashed lines, those between the anions and cations are denoted by dotted lines, and the corresponding H · · · O and H · · · F distances are labeled in angstroms.

TABLE 2: Natural Population Analysis Charges (q, in e) of the Methyl and Butyl Groups in the Monomers and Their Changes (∆q) upon Formation of BuPy+-DMSO (Figure 10A,B) and [BuPy][BF4]-DMSO (Figure 10C,D) Complexes

SCHEME 2: Charge Transfer Model of the Alkyl Groups upon H-Bonding between DMSO and BuPy+ Cationa

∆q q(monomer) 2CH3 (DMSO) C4H9 (BuPy+) C4H9 ([BuPy] [BF4])

-0.238 0.354 0.346

Figure 10A

Figure 10B

Figure 10C

Figure 10D

0.091 -0.008

0.071 -0.013

0.068

0.072

-0.011

-0.006

calculations to characterize the interactions between DMSO and the IL. The optimized geometries, interaction energies, and H-bonds obtained are shown in Figure 10. In the BuPy+-DMSO complexes (Figure 10A,B), it is found that H-bonds can exist between the O atom of DMSO and the aromatic C-H or even alkyl C-H on the pyridinium ring. The interaction energy of conformer A (-74.21 kJ/mol) is larger in absolute value than that of conformer B (-60.05 kJ/mol), indicating that the former is more stable. The [BuPy][BF4]-DMSO complexes (Figure 10C,D) give the interaction modes in the presence of both the anion and the cation. H-bonds between the O atom of DMSO and the BuPy+ cation are present in both conformers. Moreover, H-bonds may also exist between the BF4- anion and methyl C-H of DMSO. A common feature of the four conformers is the presence of the H-bond(s) between the O atom of DMSO and the aromatic C-H on BuPy+ cation. We stress here that all of the H atoms on the ring, namely C2,6-H, C3,5-H, and C4-H on BuPy+, are favorable sites for forming H-bonds with DMSO. The NPA charges of the methyl and butyl groups of BuPy+-DMSO and [BuPy][BF4]-DMSO complexes have been calculated. The calculations were performed on the four conformers shown in Figure 10. Table 2 lists the charges (q) of the methyl and butyl groups in the monomers and the changes of the charges (∆q) upon formation of H-bonds in the four conformers. In Table 2, positive (negative) values indicate decrease (increase) in electron density of the respective groups. Clearly, for all four conformers the methyl groups of DMSO have positive ∆q values, while the butyl group of BuPy+ has a negative ∆q value. The negative charge of the DMSO methyl groups in all the complexes is less than that in the DMSO monomer, demonstrating a charge-donating effect of the DMSO methyl groups upon the formation of H-bonds with BuPy+. Zhang et al. have found that the methyl groups of DMSO are

a All the aromatic H atoms on BuPy+ cation are favorable proton donors to DMSO.

electron-donating in the H-bond between SdO and C2-H on the imidazolium ring, but they do not comment on the alkyl groups of imidazolium cation.25 Here, we evaluated the charge population of the butyl group in the BuPy+ cation and found that the positive charge of the butyl groups in all the complexes is less than that in the absence of DMSO, indicating a chargewithdrawing effect of the alkyl group in the pyridinium cation. The charge-donating and charge-withdrawing effects were also not found to be influenced by the presence of H-bonds between the O atom of DMSO and the alkyl C-H on BuPy+ cation (conformer A) and that between the methyl C-H of DMSO and the BF4- anion (conformers C and D). The charge transfer is thus mainly due to the H-bonding interactions between the O atom of DMSO and the aromatic C-H on the BuPy+ cation. A model illustrating the direction of charge transfer of the alkyl groups is given in Scheme 2. The conclusion agrees with our previous results, where the methyl groups in the proton acceptor/donor are found to be electron-donating/electronwithdrawing in the studies of the OH · · · OdS, NH · · · OdC, and CH · · · O H-bonding interactions.39,59,60 In all these cases, both methyl groups make a positive contribution to the formation of H-bonds. 3.3. Comparison of the Pyridinium-Based and Imidazolium-Based ILs. As discussed above, the pyridinium-based (this work) and imidazolium-based (literature studies12-28) ILs are similar in their interactions with water or DMSO. For example, water molecules interact preferentially with the anions rather

H-Bonding between [BuPy][BF4] and Water/DMSO than the cations in both ILs. Also, both water and DMSO interact favorably with the aromatic C-H rather than the alkyl C-H. The band shift and the variation in molar absorptivity of the alkyl C-H stretches are mainly due to the indirect effect from the interactions between the pyridinium ring and water or DMSO. Furthermore, similar models describing the electrondonating effect of the methyl groups of DMSO upon H-bonding with the cations of the two types of ILs have been established (this work and ref 25). However, there are some noticeable differences in the respective interactions. First, as denoted by the theoretical calculation results, all the H-bonds between C2,6-H, C3,5-H, or C4-H on the pyridinium cation and water molecules can probably exist in [BuPy][BF4]-H2O mixtures, while the most favorable site for the imidazolium cation to H-bond to H2O is C2-H.13,14,24,28 A similar example can be found in the case of cosolvent DMSO, where all the aromatic H atoms on the pyridinium cation are favorable proton donors to DMSO, with only C2-H on the imidazolium cation acting as the proton donor to DMSO.25 The reason for such differences is that the electron deficit in the imidazolium ring makes the C2-H much more acidic than other H atoms, while the acidity difference in the H atoms on the pyridinium ring is small. Second, in the presence of water, the H-bond strength involving the aromatic C-H in imidazolium-based tetrafluoroborate ILs is evidently weakened,24,26 while that in pyridiniumbased tetrafluoroborate ILs is not significantly affected. Such a difference is due to the different strengths of the H-bonding interaction in pure imidazolium-based and pyridinium-based ILs. Because of the electron deficit in the imidazolium ring, the C2-H on the imidazolium ring is more acidic than the H atoms on the pyridinium ring, resulting in a stronger H-bond in the former than in the latter. The H-bond in the former, which is also referred to as a charge-assisted H-bond, is also stronger than that between the imidazolium cation and water. Hence, the presence of water will weaken the H-bonds involving the aromatic C-H in imidazolium-based tetrafluoroborate ILs. In the case of the latter, the H-bond between pyridinium cation and BF4- has a strength similar to that between pyridinium cation and water. Thus the addition of water will not significantly affect the H-bond strength involving the pyridinium ring C-H. Third, water molecules form H-bonds mainly with the anions rather than the aromatic C-H on the cations in imidazoliumbased ILs,12,21,22,27 but our calculations in this work suggest that the strong anion-HOH-anion interaction can be accompanied by the H-bond between the water molecule and the pyridinium cation. The reason the isolated water molecule in [BuPy][BF4] can act as both H-bond donor and acceptor at low water content is that the BF4- anion is a weak base, and the additional water can easily insert between BF4- and BuPy+. More importantly, there is only one alkyl chain on the pyridinium cation of [BuPy][BF4], so the steric hindrance preventing the water molecule from approaching the acidic H atoms on the monoalkylpyridinium cation is much smaller compared to the dialkylimidazolium cation. In our recent work concerning the interaction between water and 1-ethyl-3-methylimidazolium ethyl sulfate (EMIES), the water molecule mainly H-bonds to the SO group of the anion via its H atoms and the H-bond between the O atom of H2O and aromatic C-H of the cation is absent at low water content.28 Both the strong cation-anion interaction and the steric hindrance of the alkyl chain are responsible for the absence of the interaction between the O atom of water and the aromatic C-H.

J. Phys. Chem. B, Vol. 114, No. 26, 2010 8699 4. Conclusions In this work, the hydrogen bonding interactions in two binary mixtures, [BuPy][BF4]-water and [BuPy][BF4]-DMSO, have been studied by the combined application of FTIR spectroscopy and DFT calculations. Several conclusions can be drawn. (1) The H-bond strength between BuPy+ cation and BF4- anion is comparable with that between the cation and water. (2) The structure evolution of water molecules upon addition into [BuPy][BF4] was tracked by FTIR: from isolated molecules to clusters with some tetrahedrally H-bonded structures. (3) Water molecules preferentially interact with the BF4- anion, while DMSO can also interact with the anion through weak H-bonds of its methyl groups. Both water and DMSO can interact with the BuPy+ cation, and the favorable sites are the aromatic C-H with slight differences based on their specific positions on the ring. (4) A charge transfer model of the alkyl groups upon H-bonding between DMSO and BuPy+ cation has been proposed. The alkyl groups linked to the H-bond acceptor are electron-donating, while those connected to the H-bond donor are electron-withdrawing, both making positive contributions to the stability of the H-bond. (5) The strong anion-HOH-anion interaction can be accompanied by the H-bond between the O atom of water molecule and the aromatic C-H of the cation in [BuPy][BF4]. The reasons the isolated water molecule in [BuPy][BF4] can act as both donor and acceptor in H-bonding are attributed to the weaker cation-anion interaction and the absence of steric hindrance of alkyl chains. As a comparison, the steric hindrance of the dialkylimidazolium cation prevents the H-bonding interaction between the C2-H on the imidazolium ring and the O atom of the water molecule. To the best of our knowledge, this is the first detailed spectroscopic investigation on the interactions between water/ DMSO and a pyridinium-based ionic liquid. It deepens our understanding of the systems at the molecular level and may shed light on the application of this ionic liquid family. Acknowledgment. This work was supported by the Natural Science Foundation of China (Project Nos. 20633080 and 20973100) and a “973” National Key Basic Research Program of China (Grant 2006CB806203). Supporting Information Available: Deconvolution results of the ATR-FTIR and excess infrared spectra in the O-D stretching vibrational region of [BuPy][BF4]-water binary mixtures over the full concentration range. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ionic Liquids as Green SolVent; Rogers, R. D., Seddon, K. R., Eds.; ACS Symposium Series 856; American Chemical Society: Washington, DC, 2003. (2) Welton, T. Chem. ReV. 1999, 99, 2071–2083. (3) Brennecke, J. F.; Maginn, E. J. AIChE J. 2001, 47, 2384–2389. (4) Seddon, K. R.; Stark, A.; Torres, M. J. Pure Appl. Chem. 2000, 72, 2275–2287. (5) Widegren, J. A.; Saurer, E. M.; Marsh, K. N.; Magee, J. W. J. Chem. Thermodyn. 2005, 37, 569–575. (6) Guan, W.; Wang, H.; Li, L.; Zhang, Q. G.; Yang, J. Z. Thermochim. Acta 2005, 437, 196–197. (7) Guan, W.; Yang, J. Z.; Li, L.; Wang, H.; Zhang, Q. G. Fluid Phase Equilib. 2006, 239, 161–165. (8) Garcia-Miaja, G.; Troncoso, J.; Romani, L. J. Chem. Eng. Data 2007, 52, 2261–2265. (9) Varela, L. M.; Carrete, J.; Turmine, M.; Rilo, E.; Cabeza, O. J. Phys. Chem. B 2009, 113, 12500–12505. (10) Katayanagi, H.; Nishikawa, K.; Shimozaki, H.; Miki, K.; Westh, P.; Koga, Y. J. Phys. Chem. B 2004, 108, 19451–19457.

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JP103438Q

Hydrogen Bonding Interactions between a Representative Pyridinium

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