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Existence of H3O+ ions in glassy aqueous HX solutions (X = Cl and Br)
Volume
107, number
CHEhlICAL
4.5
PHYSICS
LETTERS
8 June 1984
EXISTENCE OF H,O+ IONS IN GLASSY AQUEOUS HX SOLUTIONS (X = Cl AND Br) H. KANNO Department
of Chentist~.
Afeisei Lhzivenit_v. Hino. Tokyo
191, Japan
and J. HIRAISHI National Received
Chemical
Loboratov
for Industry,
I-I H&hi.
Yatabe.
Tsukubn,
lbamki
30.5, Japun
30 March 1984
It is shown that osonium ions are preserved in glassy aqueous HCI and HBr solutions at liquid nitrogen temperature. The vt and ~2 Raman bands of the HsO+ion are more clearly observed in the glassy solution than in the liquid solution at room tempemtue.
1. Introduction After many unsuccessful attempts [l-3] to detect H30+ ions in aqueous solutions of strong mineral acids by means of Raman spectroscopy. Giguire and Guillot [4] have recently observed the v2 band of the HjO+ ion unequivocally in highly concentrated HBr solutions (up to ~55 mol% HBr) by Raman spectroscopy. The major difficulty in observing Raman bands of the H30+ ion in aqueous solution comes from the heavy overlap of Raman bands of the H30+ ion with those of water and from the weakness of the v7 band which is the only band observable without overlap with Raman bands of water. Though there has been some controversy [3-53 about the observability of the Raman bands of the oxonium ion (e.g., some argued that the vrbrational spectrum of oxonium ions is obscured by the high rate of proton transfer among water molecules [3,5]), there stems to be no doubt about the assignment of the weak band at =I210 cm-t to the v2 mode of the H,O+ ion. In view of this success, it is interesting to see if H30+ ions are preserved at low temperatures in aqueous HX solutions (X = Cl and Br). In this letter, we show that oxonium ions in aqueous HX solutions (X = Cl and Br) remain stable in the glassy state and that 438
an aqueous solution the characterization solution.
in the glassy state is suitable for of weak Raman bands in the
2. Experimental Aqueous HX solutions form glasses in the concentration ranges of >LO wt% HCl and 234 wt% HBr, respectively (the upper limits of the glass-forming composition ranges have not been well determined) [6]. As the highest concentration of HBr solution commercially available is R = 5.06 (-7 wt% HBr; R = moles of water/moles of HX), we used here aqueous solutions of R = 5.1 for both HCI and HBr. Vitrification of a sample solution was achieved by immersing an aliquot of the solution in a 4-5 mm inner diameter Pyrex glass Raman cell in liquid nitrogen. The overall cooling rate was about 4 X IO2 K/min. The Raman measurements were essentially the same as previously reported [7]. The signal-to-noise ratio was enhanced by multi-scan averaging.
3. Results and discussion As shown in fig. 1, the v2 band of the HxO+ ion is 0 009-2614/84/S 03.00 0 Elsevier Science Publishers (North-Holland Phvsics Publishine Division)
B.V.
Volume
107. number
CHEMICAL
4.5
r\
GLASSY
PHYSICS
STATE
1600 R A W A li
Fig. 1. Raman
spectra
clearly observed in both Actually the observation
of aqueous
8 June 19S-l
LETTERS
HCI and HBr solutions
glassy HCl and HBr solutions. of the ~2 band of the H,O+
ion in aqueous HCI solution at ordinary temperatures (supersaturated with HCl up to 35 mol%) was reported by Ocks et al. [8] as early as 1940. However, they erroneously concluded that the band could not be due to the H,O+ ion because its relative intensity remains unchanged at all concentrations, when compared with
the water band at 1640 cm-l. For infrared spectra, it is reported [9,10] that the u2 band is already visible in dilute HX solutions (2-3 mollo). In contrast, it is scarcely discernible in the Raman spectrum of a HBr solution (=20 mol%) at room temperature (fig. I). An interesting point of the results is that the intensity of the LJ? band of the H,O+ ion appears to be larger in glassy solution than in liquid solution at room
S H 1 F T
/
1WC
!2c!O
! I’J
cc-
(R = 5.1)inthe frequency
range
from
1000
to 1800
strong acid such as HCI. HBr and HNO; water at ordinary temperatures [ 111.
cm-’
_
is very high in
This apparent contradiction can be reconciled by the difference in the lifetime ofa H,Oi ion in rhe liquid solution and the glassy solution. In an aqueous HX solution at ordinary temperatures, the proton moves around very quickly among the water molecules so that each H,O+ ion is short-lived scarcely enough to vibrate as a distinct species. In fact, the high mobility of the proton has often been cited as the main cause for the difficulty of observing the H,O+ ion in water by Raman spectroscopy [3.5]. On the other hand, translational movements of the proton are severely suppressed in a glassy HX :olurion and the lifetime of each HsO+ ion should be much longer than in the liquid state at room temperature_ relaxation time for the exchange
Ln other words. the of a proton between
temperature when the intensities are compared using the y2 band of water at al630 cm-l as a reference intensity standard. This indicates that the concentration of HxO+ions may be larger in the glassy solution
water molecules is much shorter in the liquid solution at room temperature than in the glassy solution, resulting in a broadening of the Raman bands of the H,O+
than in the solution
ion in the liquid
at room
temperature.
However,
acid dissociation usually decreases while lowering the temperature. Furthermore, the dissociation rate of a
solution.
explanation is also subsrantiated by the changes in the OH stretching Rarnan spectrum going from the This
439
CHEMICAL PHYSICS LETTERS
Volume 107. number 45
I
I
AQUEOUS
HCL
SOLUTION
LlOUlD GLASS
3000
3800 RAMAN
St!IFT
-
2600 /CM-’
Fig. 2. Raman spectra of an aqueous HCYsolution (R = 5.1) in the frequency range from 2200 to 3800 cm-‘. The - - curve shows the approximate spectrum of the v1 band for the H30+ion.
in fig. 2, the OH stretching Raman spectrum has a continuous scattering down to about 2200 cm-J_ The intensity of this continuum rises with increasing acid concentration [ 121. Comparison of the Raman spectra of the liquid HCl solution and the glassy one indicates that the continuum decreases considerably in intensity for the glassy solution and the glassy one indicates that the continuum decreases considerably in intensity for the glassy for by the ceasing of proton transfers. We can see a very weak Raman band at =2800 cm-l in the Raman spectrum of the glassy HCl solution. This band should be ascribed to the v1 band of the HsO+ ion.
liquid to the glassy state. As shown
Another cause for the continuum comes from the strong hydrogen bonds between oxonium ions and water molecules. Diffraction studies of aqueous HCI solutions [13,14] showed that the average H30+-OH2 inter-oxygen distance is 2.52 A in aqueous HCl solution, much shorter than that (2.76 A) in liquid water and ice. The existence of various combinations of hydrogen bonds between oxonium ions and water mole440
8
June 1984
culesgives rise to the spread in the OH stretching vibrational Raman band to the lower frequency side. There remains a continuum to some extent even in the glassy solution as a reflection of the complex combinations of strong hydrogen bonds and the resulting continuous distribution of vibrational frequencies. As is seen in fig. 1, the apparent intensity of the v2 band appears to be larger for the HCI solution than for the HBr solution when we compare the intensity with that of the u2 band of water in the solution. This is mainly due to the difference in intensity of the v2 bands of water in the HCI and HBr solutions. Horne and co-workers 1151 found that the intensities of the v1 and v2 bands of water in aqueous alkali halide solutions increase on going from a chloride, bromide to a iodide solution. These intensity enhancements are primarily ascribed to preresonance Raman effects arising from the charge transfer states of the hydrogen bonds between water molecules and halide ions [7,16]. The fact that the peak height of the xl230 cm-J band becomes significantly larger for the glassy solution than for the liquid solution at room temperature provides strong evidence that the origin of the band is really due to the oxonium ion in the solution. As the v2 band of the H,O+ ion is very broad and weak, it is difficult to locate the peak frequency precisely. Our Raman results gave a frequency of 1240 f 10 cm-l for the HCI solution (R = 5.1) and of 1230 f 10 cm-l for the HBr solution (I? = 5.1). These values are essentially the same as the one reported by Giguere and Guillot for aqueous HBr solution [4]. Vitrification seems to induce a small frequency shift for the ~2 band. However, the frequency decreases with increasing HX concentration (1210 cm-1 for a HCl solution of R = 3.8), indicating that the hydrogen bonds between H30+ ions and water molecules become weaker with increasing HX concentration. In the Raman spectrum of a H30+-SBCI; solution in CHZCIZ, Huong and Desbat [ 171 assigned the four bands at 3560,3510,16OO and 1095 cm-l to the symmetric stretching mode ZQ, the asymmetric stretching mode ~3, the asymmetric bending mode u4 and the al symmetric mode v2 of a pyramidal structure for the H30+ ion. The high frequency of the v2 band in aqueous HX solutions implies that the hydrogen
bonds between H30+ ions and water molecules are strong.
Volume
107, number
4.5
CHEAIICAL
PHYSICS
It is to be noted here that there has been some controversy about the vibrating unit responsible for the ~1230 cm-l band [3-5, 12. 18,191. Zundel and coworkers [ 121 favored the H,Os ion over the H30f ion as the vibrating species for the Raman bands at ~1220.1730 and 2900 cm-t. There have been several X-ray and neutron diffraction studies on the structure of hydrated osonium ions in aqueous HCI solution [ 13,141. Lee et al. [ 141 examined three models: (a) H30+ (OH2)4 regular tetrahedra, (b) H,O+(OH,), , planar triangle and (c) H30+(0H2)s-(HOH) trigonal pyramid, for the structure of hydrated oxonium ions, and concluded that best is the (c) model in which three water molecules combine with a H,O+ ion through rather strong hydrogen bonds and one water molecule lies at the apex of the OH;(OH2)2 pyramid. Model calculations by O’Ferrall et al. [3] for the frequencies of the vl, u2, v3 and v4 modes of the oxonium ion also support configuration (c) above. Therefore, we consider that the average configuration around a H,O+ ion is represented by model (c) with varying configurations associated with rapid proton transfers and complex liquid structure_
References [ 11 R. Fonteyne, [2] R. Heinzinger
Nature 138 (1936) 886. and R.E. Weston Jr., J. Phys. Chem.68
(1964) 744. R.A.ht. O’Fenal, G.W. Koeppland A-l. liresge. J. Am. Chem. Sot. 93 (1971) 1. [4] P.A. Giguere and J.G. Guillot, J. Phys. Chem. 86 (1982) 3231. [3]
8 June
LEl-fERS [S] T. Ackermann, 253;41 (1964)
2. Physik. 113.
Chem.
(Frankfurt)
1984
27 (1961)
161 H. Kanno and K. Satoh, unpubiished data. [7]
H. Kanno and J. Hiraishi, Chem. Phys. Letters 62 (1974) 82; 72 (1980) 541; J. Phys. Chem. 87 (1983) 3664. [S] L. Ocks, J. Cucron and hl. Magat, J. Phys. Radium (1940) 85. [9] hl. FaIk and P-A. Ciguere, Can. J. Chem. 35 (1957) 1195: P.A. Giguere and S. Turrell. Can. J. Chem. 53 (1976) 3477. [lo]
P. Rhine, D. Williams, G.M. Hale and 3l.R. Qurrry, J. Phys. Chem 78 (1974) 238,140s. [ 111 Fh. Co!ton and G. Wilkinson, Advanced inorganic. 3rd Ed. chemistry (Interscience. New York, 1978) ch. 5. [l?J G. Zundeland H. Metzger, Z. Physik. Chcm. (Frankfurt) 58 (1968) 225; I. Pernoll. U. Maier, R. Janoschek and G. Zundcl, J. Chem. Sot. Trans. Faraday II 71 (1975) 201. [13] R. Trio10 and AH. Narten, J. Chem. Phys. 63 (1975) 3624. [ 141 H.-G. Lee, Y. Matsumoto. T. Yamaguchi and H. Ohtaki. Bull. Chem. Sot. Japan 56 (1983) 443. [15 1 W-R. Busing and D.F. Horn&, J. Phys. Chrm. 65 (1961) 284: J.W. Schultz and D.F. Hornig. J. Phys. Chrm. 65 (1961) 2131. [16] N. Xbeand .\f. Ito. J. Raman Spectry. 7 (1978) 161. 117] P.V. Huong and B. Desbat. J. Rzman Spectry. 2 t 1974) 373; B. Desbat and P-V. Huong, Spectrochim. .+~a 31 .A (1975) 1109. [ 181 AS. Gilbert and N. Shappard, J. Chem. Sot. Trans. Faraday II 69 (1973) 1628. [ 191 J. Roziere and J. Potier, J. Inorg. NucL Chem. 35 (1973) 1179.
441
107, number
CHEhlICAL
4.5
PHYSICS
LETTERS
8 June 1984
EXISTENCE OF H,O+ IONS IN GLASSY AQUEOUS HX SOLUTIONS (X = Cl AND Br) H. KANNO Department
of Chentist~.
Afeisei Lhzivenit_v. Hino. Tokyo
191, Japan
and J. HIRAISHI National Received
Chemical
Loboratov
for Industry,
I-I H&hi.
Yatabe.
Tsukubn,
lbamki
30.5, Japun
30 March 1984
It is shown that osonium ions are preserved in glassy aqueous HCI and HBr solutions at liquid nitrogen temperature. The vt and ~2 Raman bands of the HsO+ion are more clearly observed in the glassy solution than in the liquid solution at room tempemtue.
1. Introduction After many unsuccessful attempts [l-3] to detect H30+ ions in aqueous solutions of strong mineral acids by means of Raman spectroscopy. Giguire and Guillot [4] have recently observed the v2 band of the HjO+ ion unequivocally in highly concentrated HBr solutions (up to ~55 mol% HBr) by Raman spectroscopy. The major difficulty in observing Raman bands of the H30+ ion in aqueous solution comes from the heavy overlap of Raman bands of the H30+ ion with those of water and from the weakness of the v7 band which is the only band observable without overlap with Raman bands of water. Though there has been some controversy [3-53 about the observability of the Raman bands of the oxonium ion (e.g., some argued that the vrbrational spectrum of oxonium ions is obscured by the high rate of proton transfer among water molecules [3,5]), there stems to be no doubt about the assignment of the weak band at =I210 cm-t to the v2 mode of the H,O+ ion. In view of this success, it is interesting to see if H30+ ions are preserved at low temperatures in aqueous HX solutions (X = Cl and Br). In this letter, we show that oxonium ions in aqueous HX solutions (X = Cl and Br) remain stable in the glassy state and that 438
an aqueous solution the characterization solution.
in the glassy state is suitable for of weak Raman bands in the
2. Experimental Aqueous HX solutions form glasses in the concentration ranges of >LO wt% HCl and 234 wt% HBr, respectively (the upper limits of the glass-forming composition ranges have not been well determined) [6]. As the highest concentration of HBr solution commercially available is R = 5.06 (-7 wt% HBr; R = moles of water/moles of HX), we used here aqueous solutions of R = 5.1 for both HCI and HBr. Vitrification of a sample solution was achieved by immersing an aliquot of the solution in a 4-5 mm inner diameter Pyrex glass Raman cell in liquid nitrogen. The overall cooling rate was about 4 X IO2 K/min. The Raman measurements were essentially the same as previously reported [7]. The signal-to-noise ratio was enhanced by multi-scan averaging.
3. Results and discussion As shown in fig. 1, the v2 band of the HxO+ ion is 0 009-2614/84/S 03.00 0 Elsevier Science Publishers (North-Holland Phvsics Publishine Division)
B.V.
Volume
107. number
CHEMICAL
4.5
r\
GLASSY
PHYSICS
STATE
1600 R A W A li
Fig. 1. Raman
spectra
clearly observed in both Actually the observation
of aqueous
8 June 19S-l
LETTERS
HCI and HBr solutions
glassy HCl and HBr solutions. of the ~2 band of the H,O+
ion in aqueous HCI solution at ordinary temperatures (supersaturated with HCl up to 35 mol%) was reported by Ocks et al. [8] as early as 1940. However, they erroneously concluded that the band could not be due to the H,O+ ion because its relative intensity remains unchanged at all concentrations, when compared with
the water band at 1640 cm-l. For infrared spectra, it is reported [9,10] that the u2 band is already visible in dilute HX solutions (2-3 mollo). In contrast, it is scarcely discernible in the Raman spectrum of a HBr solution (=20 mol%) at room temperature (fig. I). An interesting point of the results is that the intensity of the LJ? band of the H,O+ ion appears to be larger in glassy solution than in liquid solution at room
S H 1 F T
/
1WC
!2c!O
! I’J
cc-
(R = 5.1)inthe frequency
range
from
1000
to 1800
strong acid such as HCI. HBr and HNO; water at ordinary temperatures [ 111.
cm-’
_
is very high in
This apparent contradiction can be reconciled by the difference in the lifetime ofa H,Oi ion in rhe liquid solution and the glassy solution. In an aqueous HX solution at ordinary temperatures, the proton moves around very quickly among the water molecules so that each H,O+ ion is short-lived scarcely enough to vibrate as a distinct species. In fact, the high mobility of the proton has often been cited as the main cause for the difficulty of observing the H,O+ ion in water by Raman spectroscopy [3.5]. On the other hand, translational movements of the proton are severely suppressed in a glassy HX :olurion and the lifetime of each HsO+ ion should be much longer than in the liquid state at room temperature_ relaxation time for the exchange
Ln other words. the of a proton between
temperature when the intensities are compared using the y2 band of water at al630 cm-l as a reference intensity standard. This indicates that the concentration of HxO+ions may be larger in the glassy solution
water molecules is much shorter in the liquid solution at room temperature than in the glassy solution, resulting in a broadening of the Raman bands of the H,O+
than in the solution
ion in the liquid
at room
temperature.
However,
acid dissociation usually decreases while lowering the temperature. Furthermore, the dissociation rate of a
solution.
explanation is also subsrantiated by the changes in the OH stretching Rarnan spectrum going from the This
439
CHEMICAL PHYSICS LETTERS
Volume 107. number 45
I
I
AQUEOUS
HCL
SOLUTION
LlOUlD GLASS
3000
3800 RAMAN
St!IFT
-
2600 /CM-’
Fig. 2. Raman spectra of an aqueous HCYsolution (R = 5.1) in the frequency range from 2200 to 3800 cm-‘. The - - curve shows the approximate spectrum of the v1 band for the H30+ion.
in fig. 2, the OH stretching Raman spectrum has a continuous scattering down to about 2200 cm-J_ The intensity of this continuum rises with increasing acid concentration [ 121. Comparison of the Raman spectra of the liquid HCl solution and the glassy one indicates that the continuum decreases considerably in intensity for the glassy solution and the glassy one indicates that the continuum decreases considerably in intensity for the glassy for by the ceasing of proton transfers. We can see a very weak Raman band at =2800 cm-l in the Raman spectrum of the glassy HCl solution. This band should be ascribed to the v1 band of the HsO+ ion.
liquid to the glassy state. As shown
Another cause for the continuum comes from the strong hydrogen bonds between oxonium ions and water molecules. Diffraction studies of aqueous HCI solutions [13,14] showed that the average H30+-OH2 inter-oxygen distance is 2.52 A in aqueous HCl solution, much shorter than that (2.76 A) in liquid water and ice. The existence of various combinations of hydrogen bonds between oxonium ions and water mole440
8
June 1984
culesgives rise to the spread in the OH stretching vibrational Raman band to the lower frequency side. There remains a continuum to some extent even in the glassy solution as a reflection of the complex combinations of strong hydrogen bonds and the resulting continuous distribution of vibrational frequencies. As is seen in fig. 1, the apparent intensity of the v2 band appears to be larger for the HCI solution than for the HBr solution when we compare the intensity with that of the u2 band of water in the solution. This is mainly due to the difference in intensity of the v2 bands of water in the HCI and HBr solutions. Horne and co-workers 1151 found that the intensities of the v1 and v2 bands of water in aqueous alkali halide solutions increase on going from a chloride, bromide to a iodide solution. These intensity enhancements are primarily ascribed to preresonance Raman effects arising from the charge transfer states of the hydrogen bonds between water molecules and halide ions [7,16]. The fact that the peak height of the xl230 cm-J band becomes significantly larger for the glassy solution than for the liquid solution at room temperature provides strong evidence that the origin of the band is really due to the oxonium ion in the solution. As the v2 band of the H,O+ ion is very broad and weak, it is difficult to locate the peak frequency precisely. Our Raman results gave a frequency of 1240 f 10 cm-l for the HCI solution (R = 5.1) and of 1230 f 10 cm-l for the HBr solution (I? = 5.1). These values are essentially the same as the one reported by Giguere and Guillot for aqueous HBr solution [4]. Vitrification seems to induce a small frequency shift for the ~2 band. However, the frequency decreases with increasing HX concentration (1210 cm-1 for a HCl solution of R = 3.8), indicating that the hydrogen bonds between H30+ ions and water molecules become weaker with increasing HX concentration. In the Raman spectrum of a H30+-SBCI; solution in CHZCIZ, Huong and Desbat [ 171 assigned the four bands at 3560,3510,16OO and 1095 cm-l to the symmetric stretching mode ZQ, the asymmetric stretching mode ~3, the asymmetric bending mode u4 and the al symmetric mode v2 of a pyramidal structure for the H30+ ion. The high frequency of the v2 band in aqueous HX solutions implies that the hydrogen
bonds between H30+ ions and water molecules are strong.
Volume
107, number
4.5
CHEAIICAL
PHYSICS
It is to be noted here that there has been some controversy about the vibrating unit responsible for the ~1230 cm-l band [3-5, 12. 18,191. Zundel and coworkers [ 121 favored the H,Os ion over the H30f ion as the vibrating species for the Raman bands at ~1220.1730 and 2900 cm-t. There have been several X-ray and neutron diffraction studies on the structure of hydrated osonium ions in aqueous HCI solution [ 13,141. Lee et al. [ 141 examined three models: (a) H30+ (OH2)4 regular tetrahedra, (b) H,O+(OH,), , planar triangle and (c) H30+(0H2)s-(HOH) trigonal pyramid, for the structure of hydrated oxonium ions, and concluded that best is the (c) model in which three water molecules combine with a H,O+ ion through rather strong hydrogen bonds and one water molecule lies at the apex of the OH;(OH2)2 pyramid. Model calculations by O’Ferrall et al. [3] for the frequencies of the vl, u2, v3 and v4 modes of the oxonium ion also support configuration (c) above. Therefore, we consider that the average configuration around a H,O+ ion is represented by model (c) with varying configurations associated with rapid proton transfers and complex liquid structure_
References [ 11 R. Fonteyne, [2] R. Heinzinger
Nature 138 (1936) 886. and R.E. Weston Jr., J. Phys. Chem.68
(1964) 744. R.A.ht. O’Fenal, G.W. Koeppland A-l. liresge. J. Am. Chem. Sot. 93 (1971) 1. [4] P.A. Giguere and J.G. Guillot, J. Phys. Chem. 86 (1982) 3231. [3]
8 June
LEl-fERS [S] T. Ackermann, 253;41 (1964)
2. Physik. 113.
Chem.
(Frankfurt)
1984
27 (1961)
161 H. Kanno and K. Satoh, unpubiished data. [7]
H. Kanno and J. Hiraishi, Chem. Phys. Letters 62 (1974) 82; 72 (1980) 541; J. Phys. Chem. 87 (1983) 3664. [S] L. Ocks, J. Cucron and hl. Magat, J. Phys. Radium (1940) 85. [9] hl. FaIk and P-A. Ciguere, Can. J. Chem. 35 (1957) 1195: P.A. Giguere and S. Turrell. Can. J. Chem. 53 (1976) 3477. [lo]
P. Rhine, D. Williams, G.M. Hale and 3l.R. Qurrry, J. Phys. Chem 78 (1974) 238,140s. [ 111 Fh. Co!ton and G. Wilkinson, Advanced inorganic. 3rd Ed. chemistry (Interscience. New York, 1978) ch. 5. [l?J G. Zundeland H. Metzger, Z. Physik. Chcm. (Frankfurt) 58 (1968) 225; I. Pernoll. U. Maier, R. Janoschek and G. Zundcl, J. Chem. Sot. Trans. Faraday II 71 (1975) 201. [13] R. Trio10 and AH. Narten, J. Chem. Phys. 63 (1975) 3624. [ 141 H.-G. Lee, Y. Matsumoto. T. Yamaguchi and H. Ohtaki. Bull. Chem. Sot. Japan 56 (1983) 443. [15 1 W-R. Busing and D.F. Horn&, J. Phys. Chrm. 65 (1961) 284: J.W. Schultz and D.F. Hornig. J. Phys. Chrm. 65 (1961) 2131. [16] N. Xbeand .\f. Ito. J. Raman Spectry. 7 (1978) 161. 117] P.V. Huong and B. Desbat. J. Rzman Spectry. 2 t 1974) 373; B. Desbat and P-V. Huong, Spectrochim. .+~a 31 .A (1975) 1109. [ 181 AS. Gilbert and N. Shappard, J. Chem. Sot. Trans. Faraday II 69 (1973) 1628. [ 191 J. Roziere and J. Potier, J. Inorg. NucL Chem. 35 (1973) 1179.
441