6758
J. Phys. Chem. B 1997, 101, 6758-6762
Solvation of Al3+, Fe3+, and Cr3+ Cations in Water-Acetonitrile Mixtures Studied by IR Spectroscopy Dorota Jamro´ z and Marek Wo´ jcik* Faculty of Chemistry, Jagiellonian UniVersity, 30-060 Cracow, Ingardena 3, Poland
Jan Lindgren Institute of Chemistry, UniVersity of Uppsala, Box 531, S-751 21 Uppsala, Sweden
Janusz Stangret Department of Physical Chemistry, Technical UniVersity of Gdan´ sk, 80-952 Gdan´ sk, Poland ReceiVed: March 28, 1997X
Mixtures of water and acetonitrile with trivalent cationssAl3+, Fe3+, and Cr3+sin the full concentration range have been studied by infrared spectroscopy. CN stretching vibrations of the CD3CN molecules have been used as probes of the structural environments. The CN band in each case is a superposition of four subbands, which may be attributed to the CD3CN bonded in the first, second, and third coordination shells of the cation and to the free acetonitrile. The changes of the integral intensities of the subbands with the H2O:Me3+ molar ratio show that water tends to dislodge acetonitrile from the coordination spheres of the cation. The composition of the first coordination sphere is determined only by the H2O:Me3+ molar ratio, whereas the composition of the second and third spheres depends on the concentration of both solvents.
Introduction Dissolving a salt in acetonitrile causes a considerable change of its IR spectrum, within the CN stretching region. This region, in the case of pure acetonitrile, consists of two absorption bands; one of them, situated at 2254 cm-1, originates from the CN stretching mode (ν2), and the other, shifted by ca. 40 cm-1 toward higher frequencies, is a combination band ν3 + ν4 (CH3 bending and C-C stretching modes) of enhanced intensity, due to Fermi resonance. In the presence of a salt, a new component appears in this region. Its position depends entirely on the cation present in the solution, and its intensity grows with an increase of salt concentration at the expense of the pure acetonitrile CN band.1-7 These observations allowed to assign the new component to the acetonitrile molecules bonded directly to the cation. The value of the splitting is, in the case of the III group cations, very close to the splitting between the two components present in the pure acetonitrile spectrum, which makes the assignment of the components ambiguous. For that reason applying deuterated acetonitrile (CD3CN) was suggested,3 as in this case Fermi resonance does not occur and only one band, located at 2263 cm-1, appears in the CN stretching region. The shift of the cation-coordinated acetonitrile subband toward higher frequencies seems to be rather surprising. One may expect that the electrostatic interaction between the cation and the nitrogen atom of acetonitrile would weaken the C-N bond and, in consequence, lower the frequency of its stretching vibration. To explain the observed shift one assumes that the cation attracts mostly the lone electron pair localized at the N atom, which has a partly antibonding character. This leads to strengthening of the C-N bond and thus to increasing its force constant. This conception was supported by the results of theoretical calculations performed for complexes of acetonitrile with various cations.8,9 In solvents whose molecules are hydrogen bonded the solvation sheath of an ion extends to greater distances and X
Abstract published in AdVance ACS Abstracts, July 15, 1997.
S1089-5647(97)01099-7 CCC: $14.00
consists of more than one coordination sphere. Solvent molecules coordinated directly to the ion constitute the first solvation sphere. Next spheres are formed as a result of bonding of solvent molecules by those already present in the solvation sheath of an ion. In the case of mixed solvents, the composition of the first coordination sphere of cations depends on various factors, the most important of which seems to be the preference of the cation to bond the particular solvent. The examination of Raman spectra of the solutions of AlCl3 in a mixture of water and acetonitrile allowed the conclusion that at the H2O:Al3+ molar ratios smaller than 6 both solvents are present in the first coordination shell of Al3+.10 At higher water concentrations only the Al[(H2O)6]3+ complex ions were found in the solution. Similar results were obtained from NMR study of Al(ClO4)3 solutions in the water-acetonitrile mixture.11 This paper presents results of the investigation of solvation of Al3+, Fe3+, and Cr3+ cations in deuteroacetonitrile-water mixtures using IR spectroscopy. An analysis of changes within the CN stretching band of CD3CN allows us to find a correlation between the composition of the inner coordination sphere of the cation and the composition of the solvent. Experimental Section The perchlorates of Al3+, Fe3+, and Cr3+ cations were commercial products, additionally dehydrated under vacuum, in the presence of concentrated H2SO4 as a drying agent. The exact degree of hydration of the above salts was calculated on the basis of titrimetric determination of the metal content and was found to be 5.8, 5.7, and 5.0 water molecules per 1 Al3+, Fe3+, and Cr3+ cation, respectively. Deuteroacetonitrile was a Merck product (NMR spectroscopic grade, 99.9% of deuterium) used without any additional treatments. For each of the cations, two sets of spectra were recorded. In each set, the molar ratio CD3CN:Me3+ was kept constant at 12 and 25 (25 and 50 in the case of chromium due to lower solubility of Cr(ClO4)3 in acetonitrile) and the H2O:Me3+ molar © 1997 American Chemical Society
Solvation of Al3+, Fe3+, and Cr3+ Cations
J. Phys. Chem. B, Vol. 101, No. 34, 1997 6759
TABLE 1: Parameters of the Component Bands for the Systems of the CD3CN:Al3+ Molar Ratio Equal to 12 H2O/Al3+ 5.8
8 10 20 42
center (cm-1)
height (abs. units)
half-width (cm-1)
L/G ratio
H2O/Fe3+
2328.5 2281.6 2270.2 2262.1 2280.9 2269.6 2262.4 2280.4 2268.8 2262.5 2278.9 2268.6 2262.9 2268.4 2263.1
0.118 1.562 0.166 0.442 1.135 0.416 0.328 0.860 0.581 0.282 0.258 0.872 0.226 0.822 0.114
13.39 15.68 16.50 5.94 15.40 18.57 5.94 15.35 17.50 5.94 15.19 14.15 5.94 11.96 5.94
0 0.57 1 a 0.48 1 a 0.42 1 a 0.35 1 a 1 a
5.6
a For the pure CD CN component band, only parameters for the first 3 of the three subcomponents (see the text) are listed. The other two are shifted with respect to the first one by fixed values of -4.21 and -8.74 cm-1. Their heights and widths are related to those of the first curve by the fixed ratios of 0.317, 0.120 (heights) and 1.302, and 2.175 (widths).
TABLE 2: Parameters of the Component Bands for the Systems of the CD3CN:Al3+ Molar Ratio Equal to 25 H2O/Al3+ 5.6
8 10 20 42 a
TABLE 3: Parameters of the Component Bands for the Systems of the CD3CN:Fe3+ Molar Ratio Equal to 12
center (cm-1)
height (abs. units)
half-width (cm-1)
L/G ratio
2327.6 2281.1 2271.1 2262.6 2280.9 2271.1 2262.5 2280.5 2268.9 2262.6 2278.5 2268.0 2262.7 2267.7 2262.8
0.039 0.961 0.124 0.854 0.609 0.243 0.716 0.433 0.390 0.584 0.141 0.660 0.447 0.822 0.233
11.48 14.34 15.36 5.94 13.76 15.87 5.94 13.97 18.66 5.94 15.91 12.93 5.94 12.46 5.94
0 0.83 1 a 0.68 1 a 0.94 1 a 0.22 1 a 1 a
See footnote a in Table 1.
ratio varied in the range of ca. 6 to ca. 40. The spectra of solutions containing chromium cation were recorded after a few hours after the preparation of the solutions due to a very low rate of the ligand exchange process taking place in those systems. The spectra were registered on a Digilab FTS-45 FTIR spectrometer, with a resolution of 1 cm-1. The sample cell was equipped with CaF2 windows separated with a Teflon spacer. The optical path length, determined interferometrically, was equal to 0.0178 cm. All solutions were thermostated at 20 °C. Band Shape Analysis In order to find a correlation between the composition of the solutions and the amount of CD3CN bonded in each coordination sphere, a quantitative analysis of the spectra was performed using a curve-fitting procedure. The calculations were carried out under the following assumptions: (1) The number of subbands for each spectrum was determined by the number of maxima of the second derivative. A linear base line was additionally refined. (2) The components assigned to CD3CN in the coordination spheres were assumed to be a mixture of a Gaussian and a Lorentzian curves. Such shapes of the components were chosen
8
10
20 30 a
center (cm-1)
height (abs. units)
half-width (cm-1)
L/G ratio
2307.4 2279.8 2270.1 2262.4 2306.8 2279.8 2270.1 2262.5 2306.8 2279.6 2270.1 2262.7 2279.4 2269.1 2262.7 2268.0
0.758 0.988 0.086 0.454 0.371 0.811 0.392 0.334 0.177 0.702 0.554 0.350 0.167 0.959 0.185 1.059
16.30 21.44 17.09 5.94 16.95 18.65 20.60 5.94 15.50 16.93 15.44 5.94 12.70 14.76 5.94 15.36
0 1 1 a 0 1 1 a 0 0.91 1 a 1 1 a 1
See footnote a in Table 1.
TABLE 4: Parameters of the Component Bands for the Systems of the CD3CN:Fe3+ Molar Ratio Equal to 25 H2O/Fe3+ 5.6
8
10
20 30 a
center (cm-1)
height (abs. units)
half-width (cm-1)
L/G ratio
2306.9 2279.6 2270.1 2262.5 2306.0 2279.1 2270.1 2262.7 2306.3 2279.4 2270.1 2262.7 2278.1 2267.4 2262.6 2267.8 2262.7
0.362 0.650 0.142 0.899 0.115 0.444 0.443 0.786 0.105 0.448 0.466 0.796 0.159 0.812 0.431 1.031 0.381
16.70 18.65 15.40 5.94 16.74 16.76 15.36 5.94 15.56 15.71 15.40 5.94 18.24 15.84 5.94 13.60 5.94
0 1 1 a 0 1 1 a 0 1 1 a 0.18 1 a 1 a
See footnote a in Table 1.
on the basis of the results of preliminary refinements to secure the best reproduction of the experimental spectra. (3) The free CD3CN component was considered, according to our previous results,12 to be a sum of three Lorentzian curves, whose positions, widths, and relative heights were treated as fixed parameters. The refinement process was carried out in a few steps. In the first step the positions of the components were fixed at the values indicated by the second derivative. On the basis of the results of these calculations average widths of the components were estimated for each cation. Those data were subsequently used as fixed parameters in the next step of the refinements. Parameters obtained in the second stage were used as input data for the final step, in which all parameters, except the width of the free CD3CN subband, were refined. Such a multistep procedure was chosen to secure the physically most credible resolution of the experimental spectra. The calculations were performed using a slightly modified curve-fitting procedure included in the program Spectra Calc. The optimized parameters of the component bands for different mixtures and CD3CN:Me3+ molar ratios are listed in Tables 1-6. Results and Discussion The CN stretching band for the series of Al(ClO4)3 + H2O + CD3CN mixtures of the CD3CN:Al3+ molar ratio equal to
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Jamro´z et al.
TABLE 5: Parameters of the Component Bands for the Systems of the CD3CN:Cr3+ Molar Ratio Equal to 25 H2O/Cr3+ 5
10
20
a
center (cm-1)
height (abs. units)
half-width (cm-1)
L/G ratio
2320.5 2281.5 2271.1 2262.5 2320.2 2280.2 2268.7 2262.7 2320.4 2279.9 2268.4 2262.9
0.377 0.581 0.169 0.783 0.160 0.272 0.552 0.458 0.440 0.096 0.680 0.411
14.75 14.39 16.13 5.94 13.28 14.46 16.94 5.94 11.68 14.36 13.89 5.94
0.30 0.80 1 a 0.30 0.80 1 a 0.30 0.80 1 a
See footnote a in Table 1.
TABLE 6: Parameters of the Component Bands for the Systems of the CD3CN:Cr3+ Molar Ratio Equal to 50 H2O/Cr3+ 5
10
20
a
center (cm-1)
height (abs. units)
half-width (cm-1)
L/G ratio
2320.5 2281.8 2271.1 2262.5 2320.4 2280.4 2268.7 2262.8 2320.6 2280.6 2268.4 2262.9
0.200 0.290 0.108 0.912 0.083 0.155 0.421 0.683 0.035 0.760 0.505 0.658
14.61 14.36 16.94 5.94 13.11 14.36 16.94 5.94 12.49 14.36 13.89 5.94
0.30 0.80 1 a 0.30 0.80 1 a 0.30 0.80 1 a
Figure 1. CN stretching region (A) and its second derivative (B) of the Al3+ + H2O + CD3CN system at CD3CN:Al3+ molar ratio equal to 25 and the H2O:Al3+ molar ratio equal to (a) 5.8, (b) 8, (c) 10, (d) 20, and (e) 42.
See footnote a in Table 1.
25 is shown in Figure 1A. The band is split into several components, whose number and relative intensities depend on the composition of the solution. In order to determine the exact number of the components in each spectrum, the second derivatives were calculated (Figure 1B). One of the components is present in nearly every spectrum, except for the solution of the composition Al3+:H2O:CD3CN ) 1:42:12. Its position (2262 cm-1) corresponds exactly to the position of the single CN peak of pure CD3CN. We attribute this component to the CD3CN molecules that do not interact either with water or the cations. The three other components, shifted toward higher frequencies, are assigned to the CD3CN bonded in solvation shells of the cations. One of these subbands, whose shift with respect to the CN peak of pure CD3CN has the greatest value (ca. 65 cm-1), originates from the CD3CN molecules that undergo extremely strong intermolecular interactionssmuch stronger than the interactions with water molecules. This suggests that this peak should be assigned to the CD3CN molecules bonded directly to the cations, i.e. CD3CN in the first coordination shell. This conclusion is further supported by the fact that this component is present only at the lowest H2O:Al3+ molar ratio (5.8) where the probability of acetonitrile penetrating the first solvation shell of the cation is considerably higher than in the other cases. The two remaining subbands are located at ca. 2281 and 2270 cm-1. The intensity of the former decreases considerably with an increase of the water content, and at a H2O:Al3+ molar ratio equal 42 this subband vanishes completely. The later is almost invisible at H2O:Al3+ ) 5.8, but its intensity grows as the H2O: Al3+ molar ratio increases. At the same time its maximum shifts, reaching for the H2O:Al3+ ) 42 the position very close to that of the CN subband present in the spectra of waterdeuteroacetonitrile mixtures, which was found to originate from
Figure 2. CN stretching region (A) and its second derivative (B) of the Fe3+ + H2O + CD3CN system at the CD3CN:Fe3+ molar ratio equal to 25 and the H2O:Fe3+ molar ratio equal to (a) 5.7, (b) 8, (c) 10, (d) 20, and (e) 42.
CD3CN molecules bonded to water.12 These intensity changes suggest that the component at 2281 cm-1 may be attributed to the CD3CN molecules present in the second solvation sphere
Solvation of Al3+, Fe3+, and Cr3+ Cations
J. Phys. Chem. B, Vol. 101, No. 34, 1997 6761
Figure 5. Integral intensities of the CN stretching band components for the Al3+ + H2O + CD3CN system (the CD3CN:Al3+ molar ratio equals 25) as a function of the H2O:Al3+ molar ratio.
Figure 3. CN stretching region (A) and its second derivative (B) of the Cr3+ + H2O + CD3CN system at the CD3CN:Cr3+ molar ratio equal to 25 and the H2O:Cr3+ molar ratio equal to (a) 5.0, (b) 10, and (c) 20. Figure 6. Integral intensities of the CN stretching band components for the Fe3+ + H2O + CD3CN system (the CD3CN:Fe3+ molar ratio equals 25) as a function of the H2O:Fe3+ molar ratio.
Figure 4. CN stretching band of the Me3+ + H2O + CD3CN systems (the Me3+:H2O:CD3CN molar ratio equals 1:10:25) resolved into components: (A) Me3+ ) Al3+, (B) Me3+ ) Fe3+, (C) Me3+ ) Cr3+. The dotted line marks the sum of all the components and the base line.
of Al3+ and the component at 2270 cm-1 may be considered as resulting from overlapping of two near absorption bands, coming
from CD3CN molecules bonded to water in the second coordination sphere and free water molecules. The absorption spectra in the CN stretching region of the systems containing Fe3+ and Cr3+ cations (Figures 2 and 3) show similar properties as in the case of Al3+. The CN band is a superposition of four subbands, whichson the basis of the above presented argumentsmay be attributed to the CD3CN bonded in the first, second, and third coordination shells of the cations and to the free deuteroacetonitrile. One feature, however, clearly differentiates the spectra for the three cations: this is the position of the component attributed to the CD3CN in the first coordination shell, which equals 2327.5, 2320.5, and 2307.5 cm-1 for Al3+, Cr3+, and Fe3+, respectively. Another difference, observed between the spectra of the system containing Al3+ cation and the Fe3+ and Cr3+ containing systems, consists of conspicuously weaker dependence between the relative intensities of the components and the water contents, occurring in the later systems. As opposed to the case of Al3+, in the systems containing Fe3+ or Cr3+, the first component retains considerable intensity over a wide range of H2O:Me3+ molar ratios and vanishes only at a H2O:Me3+ ratio above 20. The results of the curve-fitting analysis for the exemplary chosen systems of the molar ratio Me3+:H2O:CD3CN ) 1:10: 25 are shown in Figure 4. By resolving the CN band into components, we were able to calculate their integral intensities. Figures 5-7 show the variation of the intensities (as calculated per 1 mol of the cation) with the H2O:Me3+ molar ratio. The qualitative character of the changes of the CD3CN concentration in the consecutive coordination spheres, which accompany the varying H2O:Me3+ molar ratios, is very similar for all the three cations. An increase of the water concentration
6762 J. Phys. Chem. B, Vol. 101, No. 34, 1997
Jamro´z et al. by the H2O:Me3+ molar ratio, whereas the composition of the second and third spheres depends on the concentration of both solvents. A comparison of the rate of vanishing of the first subband with increasing H2O:Me3+ molar ratio allows us to state that the tendency of the cations to bond water diminishes in the sequence Al3+ . Fe3+ > Cr3+, which corresponds to the decrease of the ionic radii and, in consequence, the increase of polarization power. Acknowledgment. This work has been supported by grants from the Swedish Natural Science Research Council (NFR) and the Polish Committee on Research (No.3 T09A 035 13), which are hereby gratefully acknowledged.
Figure 7. Integral intensities of the CN stretching band components for the Cr3+ + H2O + CD3CN system (the CD3CN:Cr3+ molar ratio equals 25) as a function of the H2O:Cr3+ molar ratio.
in the system is reflected in a considerable decrease of the intensities of the first and second components and a simultaneous increase of the intensity of the third one. This result clearly shows that water tends to dislodge acetonitrile from the coordination spheres of the cation, a tendency that is especially well pronounced in the case of the Al3+ cation. The differences in the intensity of the first component for the systems of the same H2O:Me3+ molar ratio and different CD3CN:Me3+ ratios turn out to be insignificant for all the three cations, whereas the intensities of the second and third subbands depend on the CD3CN content and are greater for the higher CD3CN:Me3+ ratio. This observation leads to the conclusion that the composition of the first coordination sphere is determined only
References and Notes (1) Zundel, G.; Fritsch, J. Interactions in and Structures of Ionic Solutions and Polyelectrolites. Infrared Results. In The Chemical Physics of SolVation Elsevier: Amsterdam, 1986; Vol. 2. (2) Ke¸ cki, Z.; Witanowski, J. Roczn. Chem. 1964, 38, 691. (3) Ke¸ cki, Z.; Wojtczak, J. Roczn. Chem. 1970, 44, 847. (4) Izdebska, B.; Ke¸ cki, Z. Pol. J. Chem. 1978, 52, 1531. (5) Izdebska, B.; Buslayeva, M. N. Roczn. Chem. 1977, 51, 1005. (6) Bugaeva, L. N.; Kriukov, A. N. Zhur. Neorg. Khim. 1978, 23, 1288. (7) Perelygin, I. S.; Klimchuk, M. A.; Beloborodova, N. N. Zh. Neorg. Khim. 1980, 54, 2968. (8) Ke¸ cki, Z.; Gołszewska, J. Roczn. Chem. 1967, 41, 1817. (9) Sadlej, J.; Ke¸ cki, Z. Roczn. Chem. 1969, 43, 2131. (10) Emons, H.-H.; Janneck, E.; Kabisch, G.; Pollmer, K. Z. Anorg. Allg. Chem. 1984, 511, 148. (11) Supran, L. D.; Sheppard, N. Chem. Commun. 1967, 832. Ruben, Y.; Reuben, J. J. Phys. Chem 1976, 80, 2394. Akitt, J. W.; Elders, J. M.; Howarth, O. W. J. Chem. Soc., Faraday Trans. 1 1989, 85, 2035. (12) Jamro´z, D.; Stangret, J.; Lindgren, J. J. Am. Chem. Soc. 1993, 115, 6165.
