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FT-Raman spectroscopic evidences for the preferential solvation of sodium tetrafluorobrate in acetonitrile-based mixed solvents
Spectrochimica Acta Part A 62 (2005) 500–505
FT-Raman spectroscopic evidences for the preferential solvation of sodium tetrafluorobrate in acetonitrile-based mixed solvents Xiaopeng Xuan, Jianji Wang ∗ , Yang Zhao, Hucheng Zhang Department of Chemistry, Henan Normal University, Xinxiang, Henan 453002, P.R. China Received 19 November 2004; accepted 7 February 2005
Abstract Solutions of sodium tetrafluorobrate in acetonitrile–dimethylformamide, and acetonitrile–dimethylsulfoxide mixtures have been studied by FT-Raman spectroscopy for three solvent compositions, respectively. New bands due to solvent molecules in the first solvation shell of Na+ were detected in the region of the O C N deformation and CH3 rocking mode for amide and of the S O and C S stretching modes for sulfoxide. The individual solvation numbers of sodium cation in different environment were deduced. In all the cases, it is found that the sodium ion was preferentially solvated by DMF or DMSO in respective binary solvents. This result was further supported by ab initio calculations. © 2005 Elsevier B.V. All rights reserved. Keywords: Sodium tetrafluorobrate; Dimethylformamide; Acetonitrile; Dimethylsulfoxide; Raman spectrum; Preferential solvation
1. Introduction Vibrational spectroscopy has been widely used to study the structures and dynamics of electrolyte solutions [1–8]. Much attention has been paid to the ion solvation and association in acetonitrile (AN) [5–7] and AN–water binary solvents [9–12] in the past three decades. The main reason for this interest is its widespread use as a significantly common solvent such as a solvent-phase in liquid chromatography [13]. The C N stretch mode was frequently used as a probe of the structural changes to determine the composition of solvation shell. Its unusually large shift has been a specific focus of spectroscopic study [5–7]. For example, Fawcett and co-workers [5,6] used FT-IR spectroscopy to analyze the effect of the electrolyte concentrations on the vibrations of C N and C C stretches. They also studied the ion pair for Mg(ClO4 )2 in AN solutions [5]. Effects of other metal cations such as sodium [5], lithium [5,6], and silver [7] are also discussed. To well understand the interaction between metal cation and AN in different environment, solvation of metal ions ∗
Corresponding author. Tel.: +86 373 332 6544; fax: +86 373 332 6544. E-mail address: [email protected] (J. Wang).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.02.003
in binary mixtures has also been studied, especially in acetonitrile–water mixtures. Akopyan and Solov’eva [9] carried out a spectroscopic investigation of aqueous acetonitrile solutions in the presence of sodium or lithium perchlorate. They observed a shoulder on the higher wavenumber side of C N stretch band of AN in NaClO4 /AN solution, and found that this shoulder disappeared when water was added. The explanation was that the addition of water resulted in water molecules replacing AN molecules in the solvation shell. Evans and Lo [10] studied the Raman and infrared spectra of ZnCl2 /AN–H2 O solutions and showed that the site of attachment to Zn2+ is through the nitrogen lone-pair electrons. Raman studies of silver nitrate in water–acetonitrile mixtures carried out by Oliver and Janz [11] determined the coordination number of Ag+ ion as four. Recently, the structures of aqueous acetonitrile containing trivalent cations were also examined by means of infrared spectroscopy [12]. A similar work has also been performed in the aqueous deuterated acetonitrile solutions [13,14]. However, very few of the vibrational spectroscopic studies have attempted to report the solvation of metal ions in the mixtures of two organic solvents. One of the few that did is the work of Sajeevkumar and Singh [15]. They studied the
X. Xuan et al. / Spectrochimica Acta Part A 62 (2005) 500–505
solvation of lithium ion in binary mixtures of acetonitrile and dimethylformamide (DMF). The large frequency shifts of the C N stretch and O C N deformation were used to investigate the composition of solvation shell. It is reported that the lithium ion is preferentially solvated by DMF molecules. The magnesium ions in this mixture also showed a similar behavior [16]. In the present paper, we extend the study to solvation of sodium ion in AN binary solvents mixed with DMF and dimethylsulfoxide (DMSO), respectively. These solvents were chosen partly because of the practical importance, and partly because of the varying electron-donating ability. We analyzed the interaction between sodium cations and the solvent molecules, calculated the solvation number of cation, assessing the selective solvation by evaluating bands of the CH3 rock of DMF, C S stretch of DMSO. Both of them are very sensitive to their surroundings and can give a fairly indication of ion–molecule interactions. The preferential solvation is well explained by the difference in donor number of solvent and ab initio calculations.
2. Experimental NaBF4 (Shanghai SSS Reagent, CP) was re-crystallized twice from conductivity water, followed by drying in vacuum at increasing temperatures up to 110 ◦ C. DMSO (Beijing Reagent Factory, AR) was refluxed over CaH2 for 1 day, then distilled in vacuum and dried over 4A molecular sieves. DMF (Tianjin Reagent Co., AR) was purified as described in the literature [3,4]. Both the solute and the solvents were kept in a desiccator under vacuum. Preparation of the solutions was performed in a dry room and precautions were taken to minimize the contamination by water. No significant absorption in the OH stretching of water region could be detected in the IR spectra, indicating that the water content in the solutions was negligible. The concentration of solutions was expressed as molalities (mol kg−1 ). The AN mixtures of DMF or DMSO are fixed at different molar ratio of 3:1, 1:1 and 1:3, respectively. Concentrations of the dissolved salt are up to 2.00, 2.05, 0.99, 1.00, 1.48 and 0.78 mol kg−1 for DMF–AN(3:1), DMF–AN(1:1), DMF–AN(1:3), DMSO–AN(3:1), DMSO–AN(1:1), and DMSO–AN(1:3) mixtures, respectively. Fourier transform (FT)-Raman spectra were recorded at 1064 nm excitation by an Nd:YAG laser on a Raman module of a Nicolet Nexus spectrometer with a resolution of 2 cm−1. The laser power was set to 300–700 mW and 1000–2000 scans were accumulated to achieve a good signal-to-noise ratio. The liquid samples were sealed in NMR tubes and examined at room temperature (ca. 22 ◦ C). The band intensity of the C N C symmetric stretching of DMF at 865 cm−1 was used as an internal standard for normalization in the solutions involving DMF, and that of CH3 rock for DMSO at 1420 cm−1 for the DMSO–AN solutions. Original Raman data were collected with Omnic 6.0 software and further pro-
501
cessed with Win-IR software (version 4.0). This package fits the peaks using mixed Gaussian–Lorentzian linear combinations and a linear baseline. Ab initio calculations were carried out with the Gaussian 98 package [17] at the B3LYP/6-311++G(d,p) level. The molecular geometries of AN, DMF and DMSO and their solvation structures were fully optimized with no geometrical constraints. The vibrational frequencies were calculated at the converged geometry to confirm the stability of structure. For estimation of the Gibbs free energies of the complexes, the zero-point energy corrections were included.
3. Results and discussion There have been numerous studies concerning the infrared and Raman spectra of AN [5,6,18], DMF [19,20] and DMSO [21,22], and the detailed assignments of these bands have been made [18,23–25]. In general, the effect of anions with large size on these bands is minima and negligible. For the systems studied here, Raman spectra of the solutions with different compositions show similar trends against the salt concentration. As examples, therefore, only the salt concentration dependences of spectroscopic behavior for NaBF4 /DMF–AN(1:1) and NaBF4 /DMSO–AN(1:1) system are shown in the following discussions. Fig. 1 shows the Raman spectra of NaBF4 /DMF–AN(1:1) solutions over different salt concentrations in four selected regions: O C N deformation, CH3 rocking and C O stretch of DMF, and C N stretch of AN. It is interesting to note that all the spectra related to DMF change with increasing salt concentration, but the situation is not true for the C N stretch of AN. The O C N deformation splits into two components: the original at 655 cm−1 and the new at 668 cm−1 . The intensity of the new component increased at the expense of the original as shown in Fig. 1a. Similar changes have also been observed in NaBF4 /DMF [4], LiCF3 SO3 /DMF [8], LiClO4 /DMF [19], LiNO3 /DMF [20], and NaClO4 /DMF [26] solutions. The CH3 rocking mode of DMF (at 1093 cm−1 ) in Fig. 1b is also sensitive to the interaction between Na+ and DMF. With the addition of NaBF4 , its shape gradually broadens and shifts towards 1104 cm−1 , demonstrating the effect of increased salt concentrations. The C O group of DMF (1670 cm−1 ), which is very intense and has a complicated contour, is changed considerably by the dissolution of salt. This band shifts towards the higher frequency and decreases in intensity strongly, giving rise to a doublet with a shoulder at 1680 and 1660 cm−1 in the 2.05 mol kg−1 NaBF4 /DMF–AN(1:1) solution in Fig. 1c. These observed changes of C O stretch of DMF are due to the interaction with lithium cation, probably by through the oxygen atom of carbonyl group. A similar result has also been observed in NaBF4 /DMF[4] and LiClO4 /DMF [8] solutions. No obvious change in the C N stretch region of AN is detected as presented in Fig. 1d. Many studies [5–7] shown that the C N stretch is very sensitive to the inter-
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Fig. 2. Raman spectra for DMSO or AN in NaBF4 /DMSO–AN(1:1) solutions as a function of the salt concentrations. For top to bottom for each component, concentrations of NaBF4 are 0, 0.50, 1.00 and 1.48 mol kg−1 , respectively. Fig. 1. Raman spectra for DMF or AN in NaBF4 /DMF–AN(1:1) solutions as a function of the salt concentrations. From bottom to top for each component, concentrations of NaBF4 are 0, 0.51, 1.01, 1.53 and 2.05 mol kg−1 , respectively.
action between cation and AN, and a large shift is often caused. As can be seen in Fig. 1d, only in the most concentrated solutions, the C N stretch shows a slight change on the side of the higher frequency. This qualitatively indicates that the interaction between Na+ and AN is very weak, and Na+ is preferentially solvated by DMF in the DMF–AN(1:1) mixtures. The Raman changes of NaBF4 /DMSO–AN(1:1) illustrated in Fig. 2 show a similarity with those of NaBF4 /DMF–AN(1:1) solutions. The two bands at 667 and 699 cm−1 in Fig. 2a can be ascribed to the C S stretches of DMSO. A considerable shift and decrease in intensity for the band at 667 cm−1 and a split for band at 699 cm−1 were observed. A new band at higher frequency side arises from the DMSO molecules solvating the sodium ion. The S O stretch of DMSO in Fig. 2b is very complicated, and some conflicts are found in literature. According to our experiment, the assignment by Fawcett and Kloss [21] is more reasonable. They
proposed a two-band model to explain the complexity of their IR results: the band at 1058 cm−1 is ascribed to monomers and that at 1044 cm−1 to cyclic dimers, respectively. In our work, curve fitting analysis of the Raman band in the range of 620–740 cm−1 produces three bands at 1031, 1044 and 1057 cm−1 , respectively (Fig. 3). The relative intensity of the component at 1031 cm−1 arising from vibration of the methyl group is found to be constant in the concentration range studied as shown in Fig. 4. The remaining two bands, at 1057 and 1044 cm−1 , are assigned to the monomers and cyclic dimers of DMSO [21]. With increasing salt concentrations, the band at higher frequency (at 1057 cm−1 ) increases its intensity at the expense of the 1044 cm−1 component (Fig. 4). This suggests that cation modified the dimerization equilibrium of 2DMSO ⇔ (DMSO)2 , and promoted the formation of monomeric DMSO molecules, which can solvate the sodium ion more effectively. From the analysis by the same way, we can believe that the Raman spectra of C N stretch of AN are not modified by sodium ion (Fig. 2c). This is similar to the situation in NaBF4 /DMF–AN(1:1) solutions. The above manifestation of interactions in the solutions studied leads to the conclusion that there is strong interaction
X. Xuan et al. / Spectrochimica Acta Part A 62 (2005) 500–505
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decreases but that at 1057 cm−1 increases in intensity (not shown). This reveals that the addition of AN promotes the formation of mononic DMSO. This result is supported by thermodynamic result [28]. However, it must be pointed out that the effect of AN is less than that of Na+ . In order to get more quantitative information on solvation, we calculated the solvation number of sodium in DMF/AN and DMSO–AN solvents. As described previously, the interactions between Na+ and DMF or DMSO cause some bands split. The new bands are ascribed to the solvated solvent molecules. Since the Raman intensity is linear to the concentration of species, the procedure developed by Deng and Irish [29,30] is suitable for the calculation of solvation number. According to this approach, when the total integrated intensity It of the normalized band is plotted against the integrated intensity Is of the solvating species, the experimental data points must be fitted to the expression Jf Is + Jf mT It = 1 − (1) Js Fig. 3. Curve fitting for the S O stretch region of DMSO in 0.50 mol kg−1 NaBF4 /DMSO–AN(1:1) solution.
between Na+ and DMF or DMSO molecules, and that sodium ion is preferentially solvated by DMF or DMF in mixed solvents, even in the case of mixtures with high AN content, such as NaBF4 /DMF–AN(1:3) and NaBF4 /DMSO–AN(1:3) solutions. Additionally, no obvious difference is found for the Raman spectrum between pure DMF and DMF in DMF–AN solvents. Therefore, no strong interactions exist between AN and DMF molecules. This is in agreement with the dielectric relaxation measurement [27]. The situation is not true for DMSO–AN binary mixtures, especially for the S O stretch region. With increasing AN contents, the band at 1044 cm−1
Fig. 4. Variation of the normalized intensities of the bands at 1031, 1044, 1057 cm−1 and their sum with the molality of NaBF4 .
where Jf and Js stand for the corresponding molar scattering coefficients of the free and solvating solvents, and mT is the molar concentration of the solvent. For the systems containing DMF, the CH3 rocking mode is usually used to calculate the solvation number [4,8]. Curve fitting of the rocking mode of DMF in 1.53 mol kg−1 NaBF4 /DMF-AN(1:1) solutions is shown in Fig. 5. The result gives rise to three components: 1063, and 1093 and 1104 cm−1 . Plotting the normalized intensities of the band at 1093 and 1104 cm−1 and theirs sums (It ) against the salt concentration, Fig. 6 was obtained. It can be seen that the intensity of band at 1093 cm−1 decreases and that at 1104 cm−1 increases with increasing salt concentration. The total intensity It increases slightly, which means the molar scattering coefficient of the band at 1093 cm−1 is lower than that of the band at 1104 cm−1 . As a result, the
Fig. 5. Curve fitting for the CH3 rock region of DMF in 1.53 mol kg−1 NaBF4 /DMF–AN(1:1) solution.
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Table 2 Calculated Gibbs energy change for the solvation process of Na+ at B3LPY/6-311++G** level Reaction Na+
+ AN → Na(AN)+
Na+ + DMSO → Na(DMSO)+ Na+ + DMF → Na(DMF)+ Na(AN)+ + AN → Na(AN·AN)+ Na(AN·AN)+ + DMF → Na(DMF·AN)+ + AN Na(AN·AN)+ + DMSO → Na(DMSO·AN)+ + AN
Fig. 6. Variation of the normalized intensities of the bands at 1093 and 1104 cm−1 and their sums against the molality of NaBF4 .
linear fitting can be expressed as It = 0.27731Is + 0.4431 with a linear relation coefficient of 0.90. Thus, the molar scattering coefficients are found to be Jf = 0.0505 and Js = 0.0699, respectively. The solvation numbers of sodium ion, ns , is defined as: ns =
Is (mNa+ Js )
(2)
where mNa+ is the concentration of free sodium ions. To the first approximation, we used the concentration of NaBF4 , which is higher than that of Na+ because of ion associations, to calculate the solvation number, and a mean solvation number of 3.0 is obtained. The solvation number of sodium in other binary solvent with different compositions are also calculated and the value is tabulated in Table 1, and mean solvation number of 3.2 and 2.9 in DMF–AN(3:1) and DMF–AN(1:3) mixtures are suggested, respectively. It must be pointed out that the obtained solvation number is only the number of DMF in the first solvated shell, not the total solvation number. It is known that Na+ is generally regarded to have primary solvation shell containing four solvent molecules in a variety of organic solvents [4,11,18,31], such as AN [31] and DMF [4]. The calculated solvation number demonstrates a strong preferential solvation of Na+ by DMF. The situation is similar to that of lithium ion in DMF–AN mixtures [15]. Table 1 Raman scattering coefficients for the free and solvated solvents System
Jf
Js
R
ns
DMF–AN (3:1) DMF–AN (1:1) DMF–AN (1:3) DMSO–AN (3:1) DMSO–AN (1:1) DMSO–AN (1:3)
0.03242 0.05054 0.09092 0.09550 0.08653 0.06543
0.03975 0.06987 0.1039 0.07688 0.1280 0.1564
0.92 0.90 0.97 0.92 0.84 0.77
3.2 3.0 2.9 6.0 3.7 3.5
Jf and Js , linear relation coefficient, R, and individual solvation number, ns , in mixed systems.
G (kJ mol−1 ) −147.26 −159.37 −191.43 −84.18 −12.46 −38.17
For DMSO–AN systems, the calculated numbers of DMSO in the first solvated shell range from 3.5 to 6.0 (Table 1) based on the analysis for C S stretches of DMSO. This is very close to the result in literature [32,33]. Molecular dynamics simulations [32] shown that the first solvation shell of the sodium ion composes of six DMSO solvent molecules located at the apexes of a distorted octahedron by their oxygen atoms. Therefore, the conclusion that Na+ is exclusively solvated by DMSO can be obtained. The preferential solvation can well be explained by the donor–acceptor approach. The magnitude of the donor number (DN) of a solvent is a measure of the solvation power, and is very useful in determining the solvent environment around cation in mixed solutions. The donor number values of the solvents investigated in the present study are 26.6 for DMF, 29.8 for DMSO, and 14.1 for AN [34], respectively. It can be seen that the DN values of DMF and DMSO are considerably higher than that of AN. Thus, preferential solvation of Na+ by DMF or DMSO in their respective mixtures with AN is expected. Additionally, the preferential solvation can also be verified by the ab initio calculations. The changes of Gibbs energy (G) may be obtained by the equation: G(Na(S)+ ) = G(Na(S)+ ) − G(Na+ ) − G(S)
(3)
where S is the solvent. The calculated values of G for the different reaction systems are collected in Table 2. It can be seen that values of G(Na(AN)+ ), G(Na(DMSO)+ ) and G(Na(AN)+ ) are negative, indicating favorable interactions between Na+ and DMF, DMSO or AN. When one AN molecule in Na(AN·AN)+ is replaced by one DMF, the G for this process is negative, suggesting a preferential solvation process of Na+ with DMF. This is also true for DMSO molecules. Although these calculations are based on gas phase, the result well explained our spectroscopic observations. In summary, the interactions between Na+ and DMF or DMSO is stronger than that between Na+ and AN, and sodium ion was preferentially solvated by DMF or DMSO in respective binary solvents.
Acknowledgements Financial support from the National Natural Science Foundation of China (29973009) and Youth Science Foundation of
X. Xuan et al. / Spectrochimica Acta Part A 62 (2005) 500–505
Henan Normal University (2004007) are gratefully acknowledged.
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FT-Raman spectroscopic evidences for the preferential solvation of sodium tetrafluorobrate in acetonitrile-based mixed solvents Xiaopeng Xuan, Jianji Wang ∗ , Yang Zhao, Hucheng Zhang Department of Chemistry, Henan Normal University, Xinxiang, Henan 453002, P.R. China Received 19 November 2004; accepted 7 February 2005
Abstract Solutions of sodium tetrafluorobrate in acetonitrile–dimethylformamide, and acetonitrile–dimethylsulfoxide mixtures have been studied by FT-Raman spectroscopy for three solvent compositions, respectively. New bands due to solvent molecules in the first solvation shell of Na+ were detected in the region of the O C N deformation and CH3 rocking mode for amide and of the S O and C S stretching modes for sulfoxide. The individual solvation numbers of sodium cation in different environment were deduced. In all the cases, it is found that the sodium ion was preferentially solvated by DMF or DMSO in respective binary solvents. This result was further supported by ab initio calculations. © 2005 Elsevier B.V. All rights reserved. Keywords: Sodium tetrafluorobrate; Dimethylformamide; Acetonitrile; Dimethylsulfoxide; Raman spectrum; Preferential solvation
1. Introduction Vibrational spectroscopy has been widely used to study the structures and dynamics of electrolyte solutions [1–8]. Much attention has been paid to the ion solvation and association in acetonitrile (AN) [5–7] and AN–water binary solvents [9–12] in the past three decades. The main reason for this interest is its widespread use as a significantly common solvent such as a solvent-phase in liquid chromatography [13]. The C N stretch mode was frequently used as a probe of the structural changes to determine the composition of solvation shell. Its unusually large shift has been a specific focus of spectroscopic study [5–7]. For example, Fawcett and co-workers [5,6] used FT-IR spectroscopy to analyze the effect of the electrolyte concentrations on the vibrations of C N and C C stretches. They also studied the ion pair for Mg(ClO4 )2 in AN solutions [5]. Effects of other metal cations such as sodium [5], lithium [5,6], and silver [7] are also discussed. To well understand the interaction between metal cation and AN in different environment, solvation of metal ions ∗
Corresponding author. Tel.: +86 373 332 6544; fax: +86 373 332 6544. E-mail address: [email protected] (J. Wang).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.02.003
in binary mixtures has also been studied, especially in acetonitrile–water mixtures. Akopyan and Solov’eva [9] carried out a spectroscopic investigation of aqueous acetonitrile solutions in the presence of sodium or lithium perchlorate. They observed a shoulder on the higher wavenumber side of C N stretch band of AN in NaClO4 /AN solution, and found that this shoulder disappeared when water was added. The explanation was that the addition of water resulted in water molecules replacing AN molecules in the solvation shell. Evans and Lo [10] studied the Raman and infrared spectra of ZnCl2 /AN–H2 O solutions and showed that the site of attachment to Zn2+ is through the nitrogen lone-pair electrons. Raman studies of silver nitrate in water–acetonitrile mixtures carried out by Oliver and Janz [11] determined the coordination number of Ag+ ion as four. Recently, the structures of aqueous acetonitrile containing trivalent cations were also examined by means of infrared spectroscopy [12]. A similar work has also been performed in the aqueous deuterated acetonitrile solutions [13,14]. However, very few of the vibrational spectroscopic studies have attempted to report the solvation of metal ions in the mixtures of two organic solvents. One of the few that did is the work of Sajeevkumar and Singh [15]. They studied the
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solvation of lithium ion in binary mixtures of acetonitrile and dimethylformamide (DMF). The large frequency shifts of the C N stretch and O C N deformation were used to investigate the composition of solvation shell. It is reported that the lithium ion is preferentially solvated by DMF molecules. The magnesium ions in this mixture also showed a similar behavior [16]. In the present paper, we extend the study to solvation of sodium ion in AN binary solvents mixed with DMF and dimethylsulfoxide (DMSO), respectively. These solvents were chosen partly because of the practical importance, and partly because of the varying electron-donating ability. We analyzed the interaction between sodium cations and the solvent molecules, calculated the solvation number of cation, assessing the selective solvation by evaluating bands of the CH3 rock of DMF, C S stretch of DMSO. Both of them are very sensitive to their surroundings and can give a fairly indication of ion–molecule interactions. The preferential solvation is well explained by the difference in donor number of solvent and ab initio calculations.
2. Experimental NaBF4 (Shanghai SSS Reagent, CP) was re-crystallized twice from conductivity water, followed by drying in vacuum at increasing temperatures up to 110 ◦ C. DMSO (Beijing Reagent Factory, AR) was refluxed over CaH2 for 1 day, then distilled in vacuum and dried over 4A molecular sieves. DMF (Tianjin Reagent Co., AR) was purified as described in the literature [3,4]. Both the solute and the solvents were kept in a desiccator under vacuum. Preparation of the solutions was performed in a dry room and precautions were taken to minimize the contamination by water. No significant absorption in the OH stretching of water region could be detected in the IR spectra, indicating that the water content in the solutions was negligible. The concentration of solutions was expressed as molalities (mol kg−1 ). The AN mixtures of DMF or DMSO are fixed at different molar ratio of 3:1, 1:1 and 1:3, respectively. Concentrations of the dissolved salt are up to 2.00, 2.05, 0.99, 1.00, 1.48 and 0.78 mol kg−1 for DMF–AN(3:1), DMF–AN(1:1), DMF–AN(1:3), DMSO–AN(3:1), DMSO–AN(1:1), and DMSO–AN(1:3) mixtures, respectively. Fourier transform (FT)-Raman spectra were recorded at 1064 nm excitation by an Nd:YAG laser on a Raman module of a Nicolet Nexus spectrometer with a resolution of 2 cm−1. The laser power was set to 300–700 mW and 1000–2000 scans were accumulated to achieve a good signal-to-noise ratio. The liquid samples were sealed in NMR tubes and examined at room temperature (ca. 22 ◦ C). The band intensity of the C N C symmetric stretching of DMF at 865 cm−1 was used as an internal standard for normalization in the solutions involving DMF, and that of CH3 rock for DMSO at 1420 cm−1 for the DMSO–AN solutions. Original Raman data were collected with Omnic 6.0 software and further pro-
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cessed with Win-IR software (version 4.0). This package fits the peaks using mixed Gaussian–Lorentzian linear combinations and a linear baseline. Ab initio calculations were carried out with the Gaussian 98 package [17] at the B3LYP/6-311++G(d,p) level. The molecular geometries of AN, DMF and DMSO and their solvation structures were fully optimized with no geometrical constraints. The vibrational frequencies were calculated at the converged geometry to confirm the stability of structure. For estimation of the Gibbs free energies of the complexes, the zero-point energy corrections were included.
3. Results and discussion There have been numerous studies concerning the infrared and Raman spectra of AN [5,6,18], DMF [19,20] and DMSO [21,22], and the detailed assignments of these bands have been made [18,23–25]. In general, the effect of anions with large size on these bands is minima and negligible. For the systems studied here, Raman spectra of the solutions with different compositions show similar trends against the salt concentration. As examples, therefore, only the salt concentration dependences of spectroscopic behavior for NaBF4 /DMF–AN(1:1) and NaBF4 /DMSO–AN(1:1) system are shown in the following discussions. Fig. 1 shows the Raman spectra of NaBF4 /DMF–AN(1:1) solutions over different salt concentrations in four selected regions: O C N deformation, CH3 rocking and C O stretch of DMF, and C N stretch of AN. It is interesting to note that all the spectra related to DMF change with increasing salt concentration, but the situation is not true for the C N stretch of AN. The O C N deformation splits into two components: the original at 655 cm−1 and the new at 668 cm−1 . The intensity of the new component increased at the expense of the original as shown in Fig. 1a. Similar changes have also been observed in NaBF4 /DMF [4], LiCF3 SO3 /DMF [8], LiClO4 /DMF [19], LiNO3 /DMF [20], and NaClO4 /DMF [26] solutions. The CH3 rocking mode of DMF (at 1093 cm−1 ) in Fig. 1b is also sensitive to the interaction between Na+ and DMF. With the addition of NaBF4 , its shape gradually broadens and shifts towards 1104 cm−1 , demonstrating the effect of increased salt concentrations. The C O group of DMF (1670 cm−1 ), which is very intense and has a complicated contour, is changed considerably by the dissolution of salt. This band shifts towards the higher frequency and decreases in intensity strongly, giving rise to a doublet with a shoulder at 1680 and 1660 cm−1 in the 2.05 mol kg−1 NaBF4 /DMF–AN(1:1) solution in Fig. 1c. These observed changes of C O stretch of DMF are due to the interaction with lithium cation, probably by through the oxygen atom of carbonyl group. A similar result has also been observed in NaBF4 /DMF[4] and LiClO4 /DMF [8] solutions. No obvious change in the C N stretch region of AN is detected as presented in Fig. 1d. Many studies [5–7] shown that the C N stretch is very sensitive to the inter-
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Fig. 2. Raman spectra for DMSO or AN in NaBF4 /DMSO–AN(1:1) solutions as a function of the salt concentrations. For top to bottom for each component, concentrations of NaBF4 are 0, 0.50, 1.00 and 1.48 mol kg−1 , respectively. Fig. 1. Raman spectra for DMF or AN in NaBF4 /DMF–AN(1:1) solutions as a function of the salt concentrations. From bottom to top for each component, concentrations of NaBF4 are 0, 0.51, 1.01, 1.53 and 2.05 mol kg−1 , respectively.
action between cation and AN, and a large shift is often caused. As can be seen in Fig. 1d, only in the most concentrated solutions, the C N stretch shows a slight change on the side of the higher frequency. This qualitatively indicates that the interaction between Na+ and AN is very weak, and Na+ is preferentially solvated by DMF in the DMF–AN(1:1) mixtures. The Raman changes of NaBF4 /DMSO–AN(1:1) illustrated in Fig. 2 show a similarity with those of NaBF4 /DMF–AN(1:1) solutions. The two bands at 667 and 699 cm−1 in Fig. 2a can be ascribed to the C S stretches of DMSO. A considerable shift and decrease in intensity for the band at 667 cm−1 and a split for band at 699 cm−1 were observed. A new band at higher frequency side arises from the DMSO molecules solvating the sodium ion. The S O stretch of DMSO in Fig. 2b is very complicated, and some conflicts are found in literature. According to our experiment, the assignment by Fawcett and Kloss [21] is more reasonable. They
proposed a two-band model to explain the complexity of their IR results: the band at 1058 cm−1 is ascribed to monomers and that at 1044 cm−1 to cyclic dimers, respectively. In our work, curve fitting analysis of the Raman band in the range of 620–740 cm−1 produces three bands at 1031, 1044 and 1057 cm−1 , respectively (Fig. 3). The relative intensity of the component at 1031 cm−1 arising from vibration of the methyl group is found to be constant in the concentration range studied as shown in Fig. 4. The remaining two bands, at 1057 and 1044 cm−1 , are assigned to the monomers and cyclic dimers of DMSO [21]. With increasing salt concentrations, the band at higher frequency (at 1057 cm−1 ) increases its intensity at the expense of the 1044 cm−1 component (Fig. 4). This suggests that cation modified the dimerization equilibrium of 2DMSO ⇔ (DMSO)2 , and promoted the formation of monomeric DMSO molecules, which can solvate the sodium ion more effectively. From the analysis by the same way, we can believe that the Raman spectra of C N stretch of AN are not modified by sodium ion (Fig. 2c). This is similar to the situation in NaBF4 /DMF–AN(1:1) solutions. The above manifestation of interactions in the solutions studied leads to the conclusion that there is strong interaction
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decreases but that at 1057 cm−1 increases in intensity (not shown). This reveals that the addition of AN promotes the formation of mononic DMSO. This result is supported by thermodynamic result [28]. However, it must be pointed out that the effect of AN is less than that of Na+ . In order to get more quantitative information on solvation, we calculated the solvation number of sodium in DMF/AN and DMSO–AN solvents. As described previously, the interactions between Na+ and DMF or DMSO cause some bands split. The new bands are ascribed to the solvated solvent molecules. Since the Raman intensity is linear to the concentration of species, the procedure developed by Deng and Irish [29,30] is suitable for the calculation of solvation number. According to this approach, when the total integrated intensity It of the normalized band is plotted against the integrated intensity Is of the solvating species, the experimental data points must be fitted to the expression Jf Is + Jf mT It = 1 − (1) Js Fig. 3. Curve fitting for the S O stretch region of DMSO in 0.50 mol kg−1 NaBF4 /DMSO–AN(1:1) solution.
between Na+ and DMF or DMSO molecules, and that sodium ion is preferentially solvated by DMF or DMF in mixed solvents, even in the case of mixtures with high AN content, such as NaBF4 /DMF–AN(1:3) and NaBF4 /DMSO–AN(1:3) solutions. Additionally, no obvious difference is found for the Raman spectrum between pure DMF and DMF in DMF–AN solvents. Therefore, no strong interactions exist between AN and DMF molecules. This is in agreement with the dielectric relaxation measurement [27]. The situation is not true for DMSO–AN binary mixtures, especially for the S O stretch region. With increasing AN contents, the band at 1044 cm−1
Fig. 4. Variation of the normalized intensities of the bands at 1031, 1044, 1057 cm−1 and their sum with the molality of NaBF4 .
where Jf and Js stand for the corresponding molar scattering coefficients of the free and solvating solvents, and mT is the molar concentration of the solvent. For the systems containing DMF, the CH3 rocking mode is usually used to calculate the solvation number [4,8]. Curve fitting of the rocking mode of DMF in 1.53 mol kg−1 NaBF4 /DMF-AN(1:1) solutions is shown in Fig. 5. The result gives rise to three components: 1063, and 1093 and 1104 cm−1 . Plotting the normalized intensities of the band at 1093 and 1104 cm−1 and theirs sums (It ) against the salt concentration, Fig. 6 was obtained. It can be seen that the intensity of band at 1093 cm−1 decreases and that at 1104 cm−1 increases with increasing salt concentration. The total intensity It increases slightly, which means the molar scattering coefficient of the band at 1093 cm−1 is lower than that of the band at 1104 cm−1 . As a result, the
Fig. 5. Curve fitting for the CH3 rock region of DMF in 1.53 mol kg−1 NaBF4 /DMF–AN(1:1) solution.
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Table 2 Calculated Gibbs energy change for the solvation process of Na+ at B3LPY/6-311++G** level Reaction Na+
+ AN → Na(AN)+
Na+ + DMSO → Na(DMSO)+ Na+ + DMF → Na(DMF)+ Na(AN)+ + AN → Na(AN·AN)+ Na(AN·AN)+ + DMF → Na(DMF·AN)+ + AN Na(AN·AN)+ + DMSO → Na(DMSO·AN)+ + AN
Fig. 6. Variation of the normalized intensities of the bands at 1093 and 1104 cm−1 and their sums against the molality of NaBF4 .
linear fitting can be expressed as It = 0.27731Is + 0.4431 with a linear relation coefficient of 0.90. Thus, the molar scattering coefficients are found to be Jf = 0.0505 and Js = 0.0699, respectively. The solvation numbers of sodium ion, ns , is defined as: ns =
Is (mNa+ Js )
(2)
where mNa+ is the concentration of free sodium ions. To the first approximation, we used the concentration of NaBF4 , which is higher than that of Na+ because of ion associations, to calculate the solvation number, and a mean solvation number of 3.0 is obtained. The solvation number of sodium in other binary solvent with different compositions are also calculated and the value is tabulated in Table 1, and mean solvation number of 3.2 and 2.9 in DMF–AN(3:1) and DMF–AN(1:3) mixtures are suggested, respectively. It must be pointed out that the obtained solvation number is only the number of DMF in the first solvated shell, not the total solvation number. It is known that Na+ is generally regarded to have primary solvation shell containing four solvent molecules in a variety of organic solvents [4,11,18,31], such as AN [31] and DMF [4]. The calculated solvation number demonstrates a strong preferential solvation of Na+ by DMF. The situation is similar to that of lithium ion in DMF–AN mixtures [15]. Table 1 Raman scattering coefficients for the free and solvated solvents System
Jf
Js
R
ns
DMF–AN (3:1) DMF–AN (1:1) DMF–AN (1:3) DMSO–AN (3:1) DMSO–AN (1:1) DMSO–AN (1:3)
0.03242 0.05054 0.09092 0.09550 0.08653 0.06543
0.03975 0.06987 0.1039 0.07688 0.1280 0.1564
0.92 0.90 0.97 0.92 0.84 0.77
3.2 3.0 2.9 6.0 3.7 3.5
Jf and Js , linear relation coefficient, R, and individual solvation number, ns , in mixed systems.
G (kJ mol−1 ) −147.26 −159.37 −191.43 −84.18 −12.46 −38.17
For DMSO–AN systems, the calculated numbers of DMSO in the first solvated shell range from 3.5 to 6.0 (Table 1) based on the analysis for C S stretches of DMSO. This is very close to the result in literature [32,33]. Molecular dynamics simulations [32] shown that the first solvation shell of the sodium ion composes of six DMSO solvent molecules located at the apexes of a distorted octahedron by their oxygen atoms. Therefore, the conclusion that Na+ is exclusively solvated by DMSO can be obtained. The preferential solvation can well be explained by the donor–acceptor approach. The magnitude of the donor number (DN) of a solvent is a measure of the solvation power, and is very useful in determining the solvent environment around cation in mixed solutions. The donor number values of the solvents investigated in the present study are 26.6 for DMF, 29.8 for DMSO, and 14.1 for AN [34], respectively. It can be seen that the DN values of DMF and DMSO are considerably higher than that of AN. Thus, preferential solvation of Na+ by DMF or DMSO in their respective mixtures with AN is expected. Additionally, the preferential solvation can also be verified by the ab initio calculations. The changes of Gibbs energy (G) may be obtained by the equation: G(Na(S)+ ) = G(Na(S)+ ) − G(Na+ ) − G(S)
(3)
where S is the solvent. The calculated values of G for the different reaction systems are collected in Table 2. It can be seen that values of G(Na(AN)+ ), G(Na(DMSO)+ ) and G(Na(AN)+ ) are negative, indicating favorable interactions between Na+ and DMF, DMSO or AN. When one AN molecule in Na(AN·AN)+ is replaced by one DMF, the G for this process is negative, suggesting a preferential solvation process of Na+ with DMF. This is also true for DMSO molecules. Although these calculations are based on gas phase, the result well explained our spectroscopic observations. In summary, the interactions between Na+ and DMF or DMSO is stronger than that between Na+ and AN, and sodium ion was preferentially solvated by DMF or DMSO in respective binary solvents.
Acknowledgements Financial support from the National Natural Science Foundation of China (29973009) and Youth Science Foundation of
X. Xuan et al. / Spectrochimica Acta Part A 62 (2005) 500–505
Henan Normal University (2004007) are gratefully acknowledged.
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