2654
L. M. Toth and G . E. Boyd
Experimental Section Synthesis of Bis(tripheny1phosphine)iminium Salts. The bis(tripheny1phosphine)iminium salts were prepared by combination of 5 : l molar ratios of the sodium or potassium oxocarbon salt and triphenylphosphineiminium chloride. respectively. These were dissolved separately in boiling water and combined. The product precipitated immediately upon mixing of the two solutions, and was recrystallized from CHzClz and dried under vacuum. (It was necessary in the case of the rhodizonate anion to carry out the reaction and filtration quickly to prevent oxidative decarboxylation to croconate anion.) 2[(c&5)3P]zA~+ C4042-.2Hzo. Electronic spectrum , , ,A 275 nm (6 2.8 X lo4),268 nm (2.9 X lo4). (CHzCl.2): Anal. Calcd for [(Ph3P)zX]zC404.2HzO: C, 74.50; H, 5.26; P, 10.11; S , 2.29; 0, 7.38. Found: C, 74.37; H, 5.11; P, 10.30; N, 2.42; 0, 7.58. 2[(C&5)3P]zN* C5052-. Electronic spectrum 373 nm ( t 3.5 x 104), 275 nm (1.0 x IO4), (CH2C12): A,,, 268 nm (1.3 x lo4), 262 nm (1.1 x 104). Anal. Calcd: C, 75.8; H, 5.0; P, 10.8; N, 2.3. Found: C, 75.8; H , 5.1; P, 10.3; N, 2.6. 2[(C6H5)3P]&+ C S O ~ ~ - . Electronic spectrum 487 ( e 4.0 X 1 O 4 ) , 275 nm (1.0 X lo4), 268 (CHzC12):,,,A, nm (1.3 X lo4): 262 nm (1.1 x 104). Anal. Calcd: C, 75.2; H. 4.9; P, 9.9; K, 2.2. Found: C, 75.0; H, 5.0;P,9.9; N, 2.4. Electrolytic Oxidation and Reduction. Solutions in dichloromethane were made up to be approximately 0.001 M in sample and 0.01 M in tetra-n-butylammonium perchlorate, used as a supporting electrolyte. The electrolytic cells, sample preparation. and apparatus have been described elsewhere.12s13 Electrolysis was performed at the lowest potential necessary for passage of current through
the cell. On oxidation the C,O,.- radicals appeared a t a potential of about 5 V with a current of 0.3 PA. The signals were stable to -60", but slowly decreased in intensity at higher temperatures. g values were measured with a dual cavity using peroxglamine disulfonate anion (g = 2.00550 f 0.0001) as a reference.14 Acknowledgment. This work was partially supported by a grant from the Kational Science Foundation. We are indebted to Professor John Ruff for suggesting the preparation of bis(tripheny1phoshine)iminium salts of the oxocarbon anions. References and Notes (1) Previous paper in this series: E. Patton and R. West, J. Phys. Chem.. 74, 2512 (1970) (2) For reviews, see R. West and J, Niu in "Non-Benzenoid Aromatic Compounds," Vol: I , J. Snyder, Ed., Academic Press, New York, N, Y . , 1969. p 312; R . West and J. Niu in "The Chemistry of the Carbonyl Group," Vol. / / , J. Zabicky, Ed., Interscience, New York. N. Y., 1970. pp 241-275. (3) R. West and H. Y. Niu, J. Amer. Chem. SOC.,84, 1324 (1962). (4) W. Buchner and E. A. C. Lucken, Helv Chim Acta, 47, 2113 (1 964) (5) R . Appel and A. Huass. 2. Anorg. Alig. Chem.. 311. 290 (1961); J. K . Ruff, lnorg. Chem., 6, 2080 (1967). (6) M.Karplusand G. K. Fraenkel, J. Chem. Phys,. 35,1312 (1961). (7) M. Broze and Z . L u 2 . J . Chem. Phys.. 51, 749 (1969). (8) E. V. Patton, Ph.D. Thesis. University of Wisconsin. 1971. These calculations used ho = 0.3, k c o = 1.4. Maclachlan calculations were carried out with the same ho and k c o parameters, and an adjustable parameter. A, of 1.0. For earlier Huckel M O studies see N. C. Baenziger and J , J. Hegenbarth, J. Amer. Chem. Soc.. 86. 3250 (1964); R. West and D. L. Powell, i b i d , 85, 2577 (1963). (9) K . Sakamoto and Y . 1 . I'Haya, Bull. Chem. SOC J a p , 44, 1201 (1971). (IO) A. J. Stone, Moi. Phys., 6, 505 (1963). (11) B. G . Segal. M. Kaplan, and G. K . Fraenkel, J. Chem. Phys.. 43. 4171 (1965). (12) E . Carberry. R . West. and G . E. Glass. J. Amer. Chem. Soc., 91 5446 (1969). (131 G. E. Glassand R . West, lnorg. Chem.. 11, 2847 (1972) (14) M. K. Carter and G. Vincow, J. Chem Phys.. 47, 292 (1967).
Raman Spectra of Thorium(lV) Fluoride Complex Ions in Fluoride Melts' L. M. TothX and G. E. Boyd Oak Ridge National Laboratory. Oak Ridge, Tennessee 37830 (ReceivedJuly 11, 7973) Publication costs assisted by the Oak Ridge National Laboratory
Raman spectra of LiF-NaF-ThF4 mixtures a t 650" have been examined to establish the coordination behavior of Th(1V) in molten fluorides. Eight coordinated Th(1V) has been identified in melts with excess fluoride ion by comparing their spectra with that of crystalline K5ThFg. Seven coordinated Th(IV), present in fluoride ion deficient melts, was identified by shifts in the frequency of V I with melt composition changes. These results are compared with previous coordination studies of U(1V) and Zr(1V).
Introduction The presence of either alkaline earth or transition metal complex ions in molten salts frequently has been inferred from Raman spectral measurements. However, there has been little attempt to compare the spectra of various catThe Journal of Physical Chemistry, Vol. 77, No. 22, 1973
ions which are expected to have similar coordination geometries in the liquid state. The ions, Th(IV), U(IV), and Zr(IV), have been regarded as structurally similar in molten salts because they form many analogous crystalline compounds. A consequence of considering these ions as similar in molten salt solutions is that their thermody-
2655
Raman Spectra of Thorium(1V) Fluoride Complex Ions in Fluoride Melts namic activity coefficients are also presumed to be the same, principally because there are no data by which to distinguish these ions. Therefore an indication of structural differences in the complex ions which these three cations form would exemplify the limitations in the above thermodynamic approximations. We have recently reported2 that the coordination behavior of Zr(1V) in molten alkali-metal fluorides varies from eight to less than six. In contrast, we have shown by absorption spectroscopy3 that U(1V) exists as either sevenor eight-coordinated complexes in molten LiF-BeF2 solutions depending on the free fluoride ion concentration of the system. These two different sets of measurements suggest that the coordination behavior of U(1V) and Zr(1V) are not similar as had been anticipated or that the two experimental methods were not corroborative. The absorption spectral measurements were most ideally suited for U(1V) solutions which absorb in the visible while the transparent solutions of Zr(1V) were more suited to Raman spectral measurements. Th(1V) in molten fluoride was studied in this research in an attempt t o resolve the apparent difference in the coordination chemistry of U(1V) and Zr(1V). Raman spectroscopy was employed because the solutions of Th(1V) in fluoride melts are transparent. The coordination numbers of the Th(1V) species present in the molten state were identified by comparison with crystalline spectra when it was possible to legitimately do so. However, when this method failed, it was necessary to turn to identification based on frequency shifts in the spectra with changes in solvent composition. The results of these measurements are compared with the previous data for U(1V) and Zr(1V). Experimental Section The experimental procedure used here has been described previously in detai1.2x4 In brief, the molten fluorides were contained in a nickel windowless cell sealed in a quartz capsule, and their spectra were measured with a Jarrell-Ash 25-300 Raman spectrometer using the 4880-A line of a Coherent Radiation Model 52B argon-ion laser operating a t 1.0 W output power for excitation. The fluoride salts were prepared from laboratory reagents which were purified by sparging the molten mixtures with HF + Hz gas to remove oxide impurities. Subsequent handling of the purified salts was performed in a helium-filled glove box of less than 1 ppm 0 2 and H 2 0 content. Ternary mixtures of LiF-NaF-ThF4 were employed because they melted a t lower temperatures throughout the range of ThF4 concentrations studied than did simple binary mixtures of alkali-metal fluoride and ThF4.5 Raman spectra of LiF-NaF-ThF4 melts were measured a t constant temperature as a function of the ThF4 concentration. The ThF4 composition was varied from 14 t o 40 mol % while the LiF/KaF mole ratio was held fixed a t 0.87. This method of varying the melt composition may be visualized by considering a ternary phase diagram illustrated as an equilateral triangle with each corner occupied by a pure component. The composition change is then described as moving along a line drawn from the ThF4 corner of the triangle t o a point on the opposite edge corresponding to the LiF/NaF ratio chosen.5 A constant LiF/ NaF ratio was maintained to minimize the complications in interpreting the melt spectra which arise from changes in the alkali-metal ion.
I1
1
A , MOLTEN
L I F - No F - T h F4
40-46-14mole% 65O"C, s l i t 1Ocm-l 5 x 103 cps B, POLYCRYSTAI-LINE
L-' ' 1 800
700
600
500 400 300 FREQUENCY icm-';
200
100
Figure 1. Raman spectra of ( A ) molten LiF-NaF-ThF4 (40-46-
1 4 mol %) at 650' under perpendicular and parallel p:larizattons of incident beam and (B) polycrystalline KsThFg at 25
Results a n d Discussion The Raman spectrum of 14 mol 70 ThF4 in LiF-NaF (40-46 mol %) a t 650" is shown in Figure 1, curve A, where the polarization of the incident beam is indicated as perpendicular or parallel to the plane defined by the incident beam and the direction of the scattered light. Two broad bands are evident, a strong polarized band a t 474 cm-I assigned to the symmetric stretch of ThF,4-r and a depolarized band centered a t 250 cm-l. Under parallel polarization and resulting decrease in the Raleigh wing, two components at ca. 270 and 225 cm-I are indicated within the broad depolarized band profile. Furthermore, if the spectrum in Figure 1 is compared with that of 14 mol % ZrF4 in LiF-NaF (40-46 mol %) shown in Figure 2 of ref 2, a remarkable similarity in the two spectra can be noted. By analogy with the ZrF4 melt spectrum, an additional band would be suggested in the ThF4 spectrum on the high-frequency side of the 250-cm-l band. The band was observed in the ZrF4 melt because it disappeared when the melt composition was changed. However, in the case of the ThF4 melts, increasing the ThF4 content did not have as pronounced an effect. As shown in Figure 2, the depolarized band for various concentrations of Th(1V) in LiF-NaF mixtures only decreased in intensity and shifted from 250 (curve A) to 280 cm-1 (curve D). Increases in the ThF4 concentration produced limited shifts in the frequency of the polarized band a t 474 cm--I (which is also in contrast to that seen for the ZrFb system2). The position of the band remained fixed for composition changes from 14 to 20 mol %, increased to 478 cm-l in going from 20 to 25 mol 70,and then showed no change for subsequent additions of ThF4 (see Figure 3). These experimental observations are compared in Table I with those for similar ZrF4 melts in both the polarized and the depolarized regions of the ThF4 and ZrF4 spectra. The comparison is intended to suggest that up to a point, namely, 25 mol % ThF4 and ZrF4, the behavior of the two is the same. The changes in the ThF4 and ZrF4 melt spectra for this composition range are interpreted as due to a transition from an eight- to a seven-coordinated complex in solution. The melt composition a t which the polarized band frequency begins to increase (uiz.,20 mol %) supports the identification that the higher coordinated species is eight. Furthermore, the spectrum of polycrystalThe Journal of Physical Chemistry, Voi. 77, No. 22, 1973
L. M. Toth and G. E. Boyd
2656 TABLE I:
Comparison between ThF4 and ZrF4 in LiF-NaF Melts at 650"where LiF-NaF Mole Ratio is Constant at Q.87/1.0a
Frequency position of
Interpretation ( s u g gested presence of)
Polarized region
Depolarized region
( l a ) At47.4cm-1 for 14mol 9/0ThFi ( 1 b) Does not shift for 14-20
( 1 ) Broad band seen at 250 cm-I for 14-20 mol % T h F 4 . By comparison with spectrum of ZrF4, a shoulder at ca. 300
ThFB4
(2) Increasing ThF4 content from 20 to 40
ThF73
-
__.__._
T h F 4 melt
mol % T h F a
cm - ' is suggested
( 2 ) Shifts to 478 cm mol YOT h F 4
ZrF4
melt
-
for 25
mol YO only produces a gradual decrease in intensity
(3) Does not change for 25-40 moi YOThF4 ( l a ) A t 555 c m - ' for 1 4 mol % ZrF4 (1b ) Does not shift for 14-20 mol YOZ r F 4
Fluoride bridging (1)
Broad band seen at 250 c m - ' for 14-20 mol O h Z r F a . By comparison with 33 mol % ZrF4 spectrum, a shoulder at 322 cm- is also present
ZrFB4
( 2 ) Shifts to 568 cm - ' for 25 mol % ZrFa
zr~,3
(3a) Shifts to 577 cm- ' for 33
Band at 322 cm-' disappears Spectrum matches that of Li2ZrF6 (3) Further increases u p to 40 mol % cause appearance of band at 165 cm '
ZrF6*
(2)
mol YOZrF4
(3b) Shifts continuously to higher values for 25-40 mol YOZrF4
ZrF5 o r Z r F 4
~
* ZrFd melt data taken from ref 2 1 ' 1 L i F - N a F F T h F q o + 650°C 40 - 4 6 - 14 m o l e % 35 - 40 - 2 5 m o l e % 31 - 36 - 33 m o l e % 28 - 3 2 - 40 %ole %
'
1
'
- 474
MELTS AT 653°C WiTh CONPOSiTiOYS: A (40-46-14 mole % ) 6 (37-43-20noIe % i
C (35-40-25 mole % )
700
600
500
400
FREQUENCY ( c m - ' ) 500
400
zoo
1-J
FREQUEYCY (cm-')
Figure 2. Raman spectra of 100-400-cm-' region at 650" of LiF-NaF-ThF4 melts with the above indicated compositions (slit = 10 cm-', 5 x I O 3 counts per second, cps) line KjThFg ( c f . , curve E, Figure 1). which contains discrete ThFg4- groups (the ninth fluorine atom displaced 4.53 A from the nearest thorium atom6), also supports the identification of the ThFs4- complex in the melts with 14-20 mol 70 ThF4. The bands at approximately 270 and 225 cm-1 in the melt can be identified with corresponding bands in KsThFs at 280 and 235 cm-1, and the shoulder in the melt spectrum at ca. 300 cm-1 is identified with the additional weaker bands in the spectrum of KsThFg a t 310 and 338 cm-l. The displacement of the strong band a t 474 c m - l in the melt relative to that in KsThFg a t 467 cm-I is attributed to the difference in the alkalimetal cations which alter the strengths of the repulsive The J o u i n a i of Physica! Chem/stry, Yo/. 77. No. 22, 7973
Figure 3. Raman spectra band profiles in the region 400-700 cm-' of LiF-NaF-ThF4 melts at 650" with the compositions as indicated (slit = 10 c m - l , 5 X I O 3 cps).
forces between nonbonded fluoride ions ( i . ~ .fluoride , ions on neighboring ThFX4-x groups). This arises because the smaller alkali-meta! ion, Li+, allows closer approach of the neighboring ThFX4--Xgroups. Despite these expected differences, the close similarity in the spectra of the polycrystalline K5ThFs and the 14 mol 70ThF4 melt supports the identification of ThFs4-- ion as the highest coordinated species in the molten state. When the ThF4 content in the melt is increased, the position of the symmetric stretching frequency increases the same as in the ZrFc melts; but for ThF4 melts, the upward shift stops a t 478 cm-1 for 25 mol % and shifts no more for further increases in the ThF4 content. The spectrum a t 33 mol % ThF4 is not appreciably different from that at 25 mol %. I t is not similar to the 33 mol % ZrF4
Vibrational Transitions in Anharmonic Oscillators melt spectrum which was identified as arising from a ZrFG2- species. Therefore. the lowest coordination number in the ThF4 melts is established as seven because the shift in the polarized band stops a t 25 mol % ThF4 and the entire spectrum never does correspond to one which can be identified with ThF& (by analogy to the ZrF& melt spectrum). If the minimum coordination number in ThF4 melts is seven, then melts of ThF4 composition greater than 25 mol 70must share fluoride ions ( i e . , form bridging links between neighboring thorium ions) to maintain that coordination number. However no spectral evidence for fluoride bridging is observed. Bridging is expected to cause a progressive decrease in the symmetric stretching frequency position arising from the decreases in the bond order and the consequent decrease in the force field associated with a particular bond. Still, the degree of bridging must be greatly limited even at 40 mol 70ThF4 ( e . g . , from the melt stoichiometry, an average of three bonds are bridged if the minimum coordination number is seven) and perhaps for this reason is not observed. The same is found in the ZrF4 melt case; there is no spectral evidence for the bridging which must occur in melts of high ZrF4 composition if the proposed coordination complexes are present. As a result of these measurements, it is now possible to explain the previous experiments and the dissimilarities of the three ions of Th(IV), Zr(IV), and U(IV), in molten fluoride solutiom. The Raman experiments with ThF4 melts have proved to be consistent with those of the visible-uv spectral results for U(IV), i.e., there is an equilibrium between seven- and eight-coordinated species which is dependent upon the melt composition. Six coordination, or lower, does not occur for either Th(1V) or U(1V) in fluo-
2657
ride melts. These results show that it was not the different experimental approaches which were responsible for the difference in the results for the three cations, but rather, the nature of the ions themselves. Zirconium behaves much like Th(1V) and U(1V) in melts with excess fluoride ion concentration because it, too, manifests a seven-eight coordination equilibrium in the same melt composition limits. However, Zr(1V) is unique in that coordination numbers lower than seven also occur, especially the octahedral six coordination for which there is strong evidence. Lower coordination numbers are possible for Zr(1V) melts undoubtedly because Zr(1V) has a much smaller ionic radius and therefore can accomodate fewer fluoride ions with little decrease in the stability of the complex ion. An analogous behavior occurs in crystals; no six-coordinated Th(1V) or U(1V) inorganic crystalline fluoride salts are known, whereas six-coordinated Zr(1V) fluorides are readily ~ b t a i n a b l e . ~
References and Notes (1) Research sponsored by the Oak Ridge National Laboratory operated by the Union Carbide Corporation. (2) L. M . Toth, A . S. Quist, and G . E. Boyd, J . Phys. Chem.. 77, 1384 (1973). (3) L. M. Toth, J. Phys Chem., 75, 631 (1971). (4) A . S. Quist. J. B. Bates, and G. E. Boyd, J . Chem. Phys.. 54, 4896 (1971). (5) The phase diagram for the LiF-NaF-ThF system has never been reported. However the anaiogous LiF-NaF-UF4 system was used from R. E. Thoma, H. insley, B. S. Landau, H. A. Friedman, and W . R. Grimes, J. Amer. Ceram. Soc., 42, 22 (1959). (6) R. R . Ryan and R. A. Penneman, Acta Crystaliogr., Sect. E , 27,829 (1971). (7) R. A. Penneman and R. R. Ryan, "Structural Systematics in Actinide Fluoride Complexes," in "Structure and Bonding," Vol. 13, SpringerVerlag, New York. N. Y., 1973.
Vibrational Transitions in Anharmonic Oscillatorsla Hyung Kyii Shin Deparfmenf of Chemistry. l b Universify of Nevada. Reno. Nevada 89507
(Received July 5. 1973)
Pub/icafion costs assisted by the U . S. A / r Force Office of Scienfific Research
The 0 1 vibrational transition probability of an anharmonic oscillator, POI,has been formulated using the potential function which is a sum of quadratic and cubic terms in the vibrational coordinate. The potential fits the Morse function closely in the region of interatomic distance where the first two energy levels of the anharmonic oscillator lie. For small energy transfer ( t
