J. Phys. Chem. B 1997, 101, 5607-5610
5607
Excess Proton Hydrate Structures with Large Proton Polarizability, Screened by Tris(2-ethylhexyl) Phosphate Bogumil Brzezinski,† Franz Bartl,‡ and Georg Zundel* Faculty of Chemistry, Adam Mickiewicz UniVersity, Grunwaldzka 6, 60780 Poznan, Poland, Institute of Medical Physics and Biophysics, UniVersita¨ ts klinikum Charite, Humboldt UniVersity Berlin, Ziegelstrasse 5/9, D-10098 Berlin, and Institute of Physical Chemistry, UniVersity of Munich, Theresienstrasse 41, D-80333 Munich, Germany ReceiVed: January 22, 1997; In Final Form: April 7, 1997
Complexes of tris(2-ethylhexyl) phosphate (EHPO) with HAuCl4 and its hydrates with various amounts of water were studied by FT-IR spectroscopy. In the water free solution of HAuCl4 in EHPO homoconjugated PdO‚‚‚H+‚‚‚OdP hydrogen bonds with proton polarizability are formed. We demonstrate that in the 1:1 and 1:2 mixture with water only the H5O2+ species is present and no H3O+ species is formed. In the 1:1 mixture with water in addition to the H5O2+ species, 50% homoconjugated hydrogen bonds of protonated EHPO are still present. This homoconjugated hydrogen bond, as well as the central bond in H5O2+ shows large proton polarizability, as indicated by IR continua. In the 1:3 mixture of the homoconjugated complex with water a cyclic hydrogen-bonded system with a three-minima proton potential is present. The proton polarizability of this system is the largest, as indicated by a particularly intense IR continuum. In the case of the 1:4 mixture, a cyclic hydrogen-bonded arrangement with a four-minima proton potential is present, in which the barriers between the minima are larger than in the three-minima potential.
1. Introduction
SCHEME 1
It has been shown already that, in strong acidic polyelectrolytes and in aqueous solutions of strong acids, the H5O2+ species1-8 (not the H3O+ species) is of main significance. The hydrogen bonds within the H5O2+ groups cause intense IR continua in the region below 3000 cm-1, indicating that these hydrogen bonds show large proton polarizability due to proton fluctuation in a double-minimum proton potential.9-12 Recently Stoianov et al.13-15 have shown that the excess proton hydrate structures can be studied under well-defined conditions if these structures are embedded in micelles of tensides. In this paper, we studied by FT-IR spectroscopy proton hydrates in micelles formed by tris(2-ethylhexyl) phosphate. 2. Experimental Section Tris(2-ethylhexyl) phosphate (EHPO) was purchased from Fluka and used without any purification. EHPO was carefully dried with a 3 Å molecular sieve. The water free HAuCl4 was prepared from gold, following the well-known procedure. The complex of EHPO with HAuCl4 was prepared by dissolving water free HAuCl4 in EHPO, such that the concentration of HAuCl4 was always 0.5 mol dm-3. The hydrates of the complex of EHPO with HAuCl4 were prepared by adding to the complex various amounts of water: 0.5 mol dm-3 (mixture 1:1), 1.0 mol dm-3 (mixture 1:2), 1.5 mol dm-3 (mixture 1:3), and 2.0 mol dm-3 (mixture 1:4). Mixtures with larger amounts of water were not formed. All preparations and handling of the compounds were performed in a carefully dried glove box. A cell with Si windows with a wedge-shaped layer was used to avoid interferences (mean layer thickness 0.025 mm). The IR spectra were taken with a FT-IR spectrophotometer IFS 113v * To whom correspondence should be addressed. Mailing address: Prof. Dr. Georg Zundel, Bruno-Walter-Strasse 2, A-5020 Salzburg, Austria. † Adam Mickiewicz University. ‡ Humboldt University Berlin. § University of Munich. X Abstract published in AdVance ACS Abstracts, June, 1; 1997.
S1089-5647(97)00276-9 CCC: $14.00
from Bruker (Karlsruhe, Germany) using a DTGS detector (125 scans, resolution 1 cm-1). 3. Results and Discussion In Scheme 1 the structure of tris(2-ethylhexyl) phosphate (EHPO) is shown. The FT-IR spectrum of EHPO is given in Figure 1 as a dashed line and also in the other figures for comparison. The spectrum drawn with a solid line in Figure 1 is the spectrum of 0.5 mol dm-3 of H2O in EHPO. It shows that a very small amount of free water molecules is present (very weak band at 3650 cm-1). The broad band complex in the region 3600-3300 cm-1 is caused by water molecules bound to the PdO groups. The shoulder at 3550 cm-1 is νs, and the band with maximum at 3470 cm-1 is νas of these water molecules.16,17 The scissor vibration of the water molecules is found at 1632 cm-1. The comparison of the spectra given in Figure 1 shows that the intensity of the ν(PdO) band at 1263 cm-1 decreases © 1997 American Chemical Society
5608 J. Phys. Chem. B, Vol. 101, No. 28, 1997
Brzezinski et al.
Figure 1. FT-IR spectra of (s) 0.5 mol dm-3 H2O in EHPO and (- -) EHPO.
Figure 2. FT-IR spectra of (s) 0.5 mol dm-3 water free HAuCl4 in EHPO and for comparison (- -) EHPO.
Figure 4. FT-IR spectra of 0.5 mol dm-3 water free HAuCl4 in EHPO plus (a) (- ‚ -) 0.5 mol dm-3 water (calculated spectrum: 1:1 mixture minus spectrum of the homoconjugated complex times 0.5 plus spectrum of EHPO times 0.5), (s) 1.0 mol dm-3 water (mixture 1:2), and for comparison (- -) EHPO and (b) (s) 1.0 mol dm-3 water (mixture 1:2), (- ‚ -) 1.5 mol dm-3 water (mixture 1:3), and (- -) 2.0 mol dm-3 water (mixture 1:4).
SCHEME 3
Figure 3. FT-IR spectra of 0.5 mol dm-3 water free HAuCl4 in EHPO plus (- ‚ -) 0.5 mol dm-3 water (mixture 1:1), 1.0 mol dm-3 water (mixture 1:2), and for comparison (- -) EHPO.
SCHEME 2
due to the addition of water. Furthermore, this band broadens strongly toward smaller wavenumbers (see also Figure 5b, dashed-dotted spectrum). In Figure 2 (solid line) the FT-IR spectrum of a 0.5 mol dm-3 solution of HAuCl4 in EHPO is shown. In this spectrum a continuum is observed, which is particularly intense in the region 1700-700 cm-1. Such a continuum is characteristic for strong homoconjugated hydrogen bonds with a broad flat single minimum proton potential, showing large proton polarizability.18-20 The band at 1200 cm-1, which is not observed in the spectrum of pure EHPO, is the bending vibration of the central proton in the homoconjugated hydrogen bonds. Hence, the structure, shown in Scheme 2, is present in the solution. Figure 3 shows the spectra of the EHPO solutions of 1:1 (dashed-dotted line) and 1:2 (solid line) mixtures of the homoconjugated complex with water. In both cases, a con-
tinuum in the region 3000-1600 is observed which is twice as intense in the 1:2 mixture as that in the 1:1 mixture. In the region 1700-700 cm-1 the continuum is much more intense in the 1:1 mixture than in the 1:2 mixture, suggesting that in the 1:1 mixture the above discussed homoconjugated hydrogen bonds are still present. To demonstrate that in the 1:1 mixture only H5O2+ species is present we calculate from the spectrum of the 1:1 mixture given in Figure 3 (dashed-dotted line) a new spectrum in the following way: we subtracted from this spectrum the spectrum of the homoconjugated complex (Figure 2, solid line), multiplied by a factor 0.5, and added, with the same factor, the spectrum of EHPO (Figure 2, dashed line). In Figure 4a this calculated spectrum is compared with the spectrum of the 1:2 mixture (solid line). This comparison shows that they are essentially the same spectra, only the intensity of the continuum in the spectrum of the 1:2 mixture is doubled. This finding demonstrates that in the 1:1 mixture the H3O+ species, shown in Scheme 3, is not formed. In this mixture the homoconjugated hydrogen bonds and H5O2+ are present in a ratio 1:1. The same observation, that with increasing degree of hydration only H5O2+ (never H3O+) is formed, was earlier made with polystyrene sulfonic acid.1,2,4 Also in aqueous solutions of strong acids the species H3O+ is observed only under very
Excess Proton Hydrate Structures
J. Phys. Chem. B, Vol. 101, No. 28, 1997 5609 SCHEME 5
SCHEME 6
Figure 5. FT-IR spectra in the region 1300-1150 cm-1 (a) of (- -) EHPO, (- ‚ -) 0.5 mol dm-3 water free HAuCl4 in EHPO, (s) 0.5 mol dm-3 water free HAuCl4 in EHPO plus 0.5 mol dm-3 water (mixture 1:1), (- ‚‚ -) 0.5 mol dm-3 water free HAuCl4 in EHPO plus 1.0 mol dm-3 water (mixture 1:2) and (b) of (- -) EHPO, (- ‚ -) 0.5 mol dm-3 H2O in EHPO, (s) 0.5 mol dm-3 water free HAuCl4 in EHPO plus 1.5 mol dm-3 water (mixture 1:3), and (- ‚‚ -) 0.5 mol dm-3 water free HAuCl4 in EHPO plus 2.0 mol dm-3 water (mixture 1:4).
SCHEME 4
special conditions.7,8 Thus, in the 1:1 mixture only the homoconjugated hydrogen bond and the structure shown in Scheme 4, containing H5O2+, is present. The continua observed in the spectra of the 1:1 and 1:2 mixtures are caused by the hydrogen bonds with large proton polarizability in H5O2+, with the homocojugated hydrogen bonds also contributing to the continuum in the 1:1 mixture. The relatively weak band at 1684 cm-1 is the scissor vibration of the water molecules of the H5O2+ cations. At 1200 cm-1 the bending vibration of the central proton in these cations is observed (see also Figure 5a, dashed-dotted line), and at 2220 cm-1 the overtone of this vibration is found (Figure 3, solid
line). Such a bending vibration was extensively studied in aqueous HCl solutions in ref 21. In aqueous HAuCl4 solutions this band is observed as a very broad band.8 Figure 5a shows that the intensity of the ν(PdO) band at 1285 cm-1 decreases with increasing water content. This result demonstrates that the water molecules of H5O2+ (Scheme 4) interact very strongly with the O atoms of the phosphate group. The remaining band is caused by the EHPO which is not involved in the complex. Figure 4b shows the spectra with more water molecules added to the homoconjugated complex. The spectrum of the 1:3 mixture is drawn with a dashed-dotted line, and that of the 1:4 mixture with a dashed line. In the case of the 1:3 mixture the continuum is much more intense than the continuum caused by the hydrogen bond in the H5O2+ groups. The broad intense band with a maximum at about 1650 cm-1 is the scissor vibration caused by the water molecules, interacting with the excess proton and the PdO groups. The ν(PdO) band at 1285 cm-1 is still observed as a less pronounced shoulder. All these results taken together, especially the very pronounced continuum, suggest that the proton fluctuates in a threeminima proton potential. These minima are indicated as dots in Scheme 5. The hydrogen-bonded system must be represented by three proton-limiting structures in which the proton is present at one of these dots. The dashed line in Figure 4b shows the spectrum of the 1:4 mixture of the homoconjugated complex with water. The intensity of the continuum below 2600 cm-1 has strongly decreased, whereas a very intense absorbance is observed in the region 3400-2600 cm-1. The H2O scissor vibration is found with a maximum at 1713 cm-1. Figure 5b (dasheddotted line) shows that the ν(PdO) of uncomplexed EHPO has completely vanished. The ν(PdO) band of the PdO groups, which are involved in the hydrogen bonds formed by the water
5610 J. Phys. Chem. B, Vol. 101, No. 28, 1997 molecules of the excess proton solvate structure, are observed as a very broad, intense band in the region 1290-1175 cm-1. The features of the continuum observed with these mixtures are very similar to those observed with the previously studied monoprotonated benzo-18-crown-6 ether.22 From these results we conclude that in the 1:4 mixture the species shown in Scheme 6 is formed. The proton is fluctuating in a four-minima proton potential. The hydrogen-bonded system in this complex must be represented by four proton-limiting structures. In each of them the proton is located at one of the four dots. The barriers between the minima are, however, much higher, compared with those in the structure shown in Scheme 5. This follows from the results that the intensity of the continuum is shifted toward higher wavenumbers and that ν(PdO) is observed as band which should, of course, be very broad, as observed. 4. Conclusions In the water free solution of HAuCl4 in EHPO, a strong homoconjugated PdO‚‚‚H+‚‚‚OdP hydrogen bond with a broad flat single-minimum proton potential is present (Scheme 2). This hydrogen bond shows proton polarizability. If water is added to this homoconjugated species, depending on its concentration, various complexes are formed with hydrogen bonds showing large proton polarizability due to proton fluctuation. In the 1:1 mixture, half of the homoconjugated bonds remain and H5O2+ is formed. No H3O+ species is observed. In the 1:2 mixture only H5O2+ is found (Scheme 4). The central hydrogen bond in H5O2+ shows large proton polarizability. These hydrogen bonds cause an IR continuum in the region below 3000 cm-1. In the 1:3 mixture of the homoconjugated species and water, a cyclic hydrogen-bonded structure with a three-minima proton potential is present (Scheme 5). This structure shows very large proton polarizability as demonstrated by the particularly large intensity of the infrared continuum. In the 1:4 mixture of the homoconjugated species and water, a new structure is formed (Scheme 6), indicated by the change of intensity and shape of the continuum and by the feature of the ν(PdO) band. In this case the proton is fluctuating in a four-minima proton potential, in which the barriers between these minima are higher than in the three-minima potential.
Brzezinski et al. Acknowledgement. We thank the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie, as well as the Polish Committee for Scientific Research (KBN), Grant 3T09A 084 09, for their support of this work. References and Notes (1) Zundel, G.; Metzger, H. Z. Physik. Chem. (Frankfurt) 1968, 58, 225. (2) Zundel, G. Hydration and Intermolecular Interaction; Academic Press: New York, 1969 (Mir: Moscow, 1972). (3) Schio¨berg, D.; Zundel, G. Z. Physik. Chem. (Frankfurt) 1976, 102, 162. (4) Zundel, G. The Hydrogen Bond Recent DeVelopments in Theory and Experiments; Schuster, P., Zundel, G., Sandorfy, C., Eds.; North Holland: Amsterdam, 1976; Vol. 2, Chapter 15. (5) Maiorov, V. D.; Librovich, N. B.; Vinnik, M. I. IzV. Akad. Nauk. SSSR, Ser. Khim. 1979, 282. (6) Zarakani, N. G.; Maiorov, V. D.; Librovich, N. B. Zh. Strukt. Khim. 1973, 14, 370. (7) Zundel, G. Chemical Physics of SolVation; Dogonadzˇe, R. R., Ka´lma´n, E., Kornyshev, A. A., Ulstrup, J., Eds.; Elsevier: Amsterdam, 1986; Vol. 2. (8) Leuchs, M.; Zundel, G. Canad. J. Chem. 1980, 58, 311. (9) Weidemann, E. G.; Zundel, G. Z. Naturforsch. 1970, A25, 627. (10) Janoschek, R.; Weidemann, E. G.; Pfeiffer, H.; Zundel, G. J. Am. Chem. Soc. 1972, 94, 2387. (11) Janoschek, R.; Weidemann, E. G.; Zundel, G. J. Chem. Soc., Faraday Trans. 2 1973, 69, 505. (12) Hayd, A.; Weidemann, G.; Zundel, G. J. Chem. Phys. 1979, 70, 86. (13) Stoianov, E. S. Zh. Strukt. Khim. 1993, 34, 72. (14) Stoianov, E. S.; Lastovka, L. V. Itoyi Nauk; Tekh., Ser.: Neorg. Khim. 1981, 26, 744. (15) Stoianov, E. S. Boreskov Institut of Katalysis, Novosibirsk, Russia. Personal Communication, 1995. (16) Greinacher, E.; Lu¨ttke, W.; Mecke, R. Z. Elektrochem. 1955, 59, 23. (17) Schio¨berg, D.; Luck, W. A. P., J. Chem. Soc., Faraday Trans. 1 1979, 75, 762. (18) Bo¨hner, U.; Zundel, G. J. Phys. Chem. 1986, 90, 964. (19) Brzezinski, B.; Schroeder, G.; Zundel, G.; Keil, Th., J. Chem. Soc., Perkin Trans. 2 1992, 819. (20) Brzezinski, B.; Brycki, B.; Zundel, G.; Keil, Th., J. Phys. Chem. 1991, 95, 8598. (21) Buanam-Om, C.; Luck, W. A. P.; Schio¨berg, D. Z. Phys. Chem. (Frankfurt) 1979, 117, 19. (22) Brzezinski, B.; Schroeder, G.; Rabold, A.; Zundel, G. J. Phys. Chem. 1995, 99, 8519.
