A. S. QUIST,J. B. BATES,AND G. E. BOYD
78 viously much larger than the standard deviations listed for E,, thus emphasizing the often-made point that linearity of the Arrhenius plot in itself is no guarantee that the data are accurate. The use of these standard deviations as a measure of the reliability of E , is valid only if one is certain that systematic errors have been eliminated. The entropy of activation, as usual, shows the highest sensitivity to systematic errors. An error of f 15% in the enthalpy of activation AH = E , - RT 15 kcal/mol would result in an error in the entropy of k 7 . 5 eu. The differences in entropy of activation
-
found in the study of the cytosine derivatives fall in this range. The present study has reemphasized that TLS analysis of exchanging systems should not be extended much above the coalescence temperature, where uncertainties in the determination of Av provide the principal limitation on the accurate calculation of k. It has furthermore been demonstrated that IC is subject to appreciable error if line widths in the absence of exchange are not well known. This condition is likely to exist for moderate size molecules if the line width variation with temperature is ignored.
Raman Spectra of Tetrafluoroberyllate Ion in Molten Sodium Fluoride
and Lithium Fluoride to 6860 by Arvin S. Qui&* John B. Bates, and George E. Boyd Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
(Received J u l y 29, 1971)
Publication costs assisted by O a k Ridge S a t i o n a l Laboratory
The expected four normal vibrational modes of the tetrahedral BeFd2- ion have been observed in the Ramaii spectra of molten LizBeF4, n'azBeF4, and 17 mol % BeFz in NaF-LiF (53:30 mol %, respectively) to 686'. Raman spectra of aqueous (NH4)zBeFh and of solid 6LizBeF4and 7Li2BeF4 also were measured, and the infrared spectra of the latter two compounds were recorded. Force constants for the BeFd2- ion were calculated from the observed vibrational frequencies. The symmetric stretching force constants, FIT, for the isoelectronic series BeFd2-, BFd-, and CF4 were correlated with the anion charge and the M-F bond distance (M = Be, B, C).
Introduction The predominantly covalent chemistry of beryllium, in contrast to the ionic behavior of the other group I1 elements, makes it unique in its chemical behavior as compared with the other members of this series. Beryllium-fluorine compounds are of particular interest because in them the smallest divalent cation is combined with the most electronegative anion. Although beryllium-fluorine compounds are often isomorphous with silicon-oxygen compounds, some tetrafluoroberyllates are more similar to sulfates in their crystal chemistry and solubility behavior. Among the alkali metal tetrafluoroberyllates, LizBeF4 is of importance because it is the solventj for fissionable and fertile components of homogeneous molten salt thermal breeder nuclear reactors. The BeFd2-ion is of interest also because it is the first member of the isoelectronic series BeF42-, BF4-, and CF4, in which the valence shell of the central atom is limited to eight electrons, and consequently no double bond formation is possible. T h e Journal of Physical Chemistry, Val. 76, 1Vo. 1 , 19'72
The vibrational frequencies of BeFd2- in melts have not been reported, although infrared and Raman specand aqueous5 tetrafluoroberyllates have tra of been measured. Raman spectral studies are particularly well-suited for investigations on this anion as all of the four normal modes of vibration of the tetrahedral BeFd2- anion are Raman active whereas only two modes are infrared active. Most of our measurements have been with molten LizBeFQbecause of the local interest in this salt. Molten NazBeF4 was also studied to obtain information on the effect of the cation on the vibrational spectrum of BeF42-. Molten Na2BeF4 and LizBeF4 contain (1) R . D. Peacock and D. W. A. Sharp, J . Chem. Soc., 2762 (1959). (2) J. LeComte, C. Duval, and C. Wadier, C. R. Acad. Sei., 149, 1991 (1959). (3) A. I. Grigorev, Y. V. Orlova, V. A . Sipachev, and A. V. Novoselova, Dok2. A k a d . N a u k U S S R , 152, 762 (1963) (Eng. transl.).
(4) E. Funck, Be?. Bunsengas. P h y s . Chem., 68, 617 (1964). (5) R. E. Mesmer and C. F. Baes, Jr., Inorg. Chem., 8 , 618 (1969).
RAMAN SPECTRA OF BeF4,-
IN
MOLTEN NaF AND LiF
only the minimum fluoride ion concentration necessary for complete four-coordination of all beryllium to form BeF4+; hence, it was of interest to measure the spectrum of BeF42- in an excess of F- ion where the liklihood o f dimer formation (BezFYa-) is minimized. Accordingly, a solution of 17 mol % Bel?, in KaF-LiF (53:30 mol yo, respectively) also was studied. Additional measurements were made with aqueous (NH4),BeF4solutions at 25" to obtain the spectrum of BeF42- ion under conditions of reduced interionic interactions. Solid LizBeF4 was of interest, and the vibrational spectra of the isotopically substituted 6LizBeF4and 7LizBeF4 forms are included in this report.
Experimental Section Flu0 ride melts are generally corrosive towards the usual optical window materials; therefore, special techniques have been developed to contain and study these systems. A modification of the captive-liquid or "windowless" nickel cell, previously employed to measure the visible absorption spectra of fluoride melts,6 was developed for use with laser light sources to measure the Raman spectra of the BeF42--containing melts. This cell has been de~cribed,~ as has the furnace used n i t h it.* The ?LiZBeF4examined in these studies was purified in an all-nickel apparatus by treatment with a mixture of HF and H, at 600" and filtered through fritted nickel under Hz. The 6LizBeF4was prepared from the reaction of 6LiF (99.6yo 6Li) with vitreous BeF2. The NazBeFr and the mixture containing 53, 30, and 17 mol % NaF, LiF, and BeFz, respectively, were prepared from Harsham single crystal NaF and LiF (Harshaw Chemical Co., Cleveland, Ohio) and from a clear BeF2 glass which had been vacuum distilled in a nicks1 apparatus. The windowless cells were filled in a vacuum drybox in which the water content was maintained at less than 3 ppm Subsequent transfers of the sample-containing cells also were carried out in this drybox. The aqueous solution of 2.5 M (NH&BPF~in 2 111 NH4F was prepared from reagent grade materials and filtered through an F-porosity Pyrex filter. A complete description of the experimental procedures for loading and manipulating the cells is given e l ~ e w h e r e . ~ -Briefly, ~ the windowless cells are loaded, placed in a quartz tube, heated to 150-200" under vacuum for about 1 hr, and then the sample is melted under an atmosphere of dry helium[. The cells are cooled, transferred to another quartz tube having an optical flat fused to one end, again heated under vacuum, and finally sealed in this latter quartz tube under a helium pressure of about 0.3 atm. The initial melting process ensures that the cell will be filled with an appropriate quantity of melt. The final arrangement whereby the cell is sealed in a quartz tube takes advantage of the desirable optical features of a quartz cell and also completely
79 encloses thc sample in an incrt atmosphere. Sample containment is of particular importaricc bccausc of the toxicity of beryllium. It should bc rioted that the molten fluoride docs not come into contact with thc quartz tub^.^,^ Raman spectra m r e recorded with a Jarrcll-Ash Model 25-300 spcctrophotorytcr (Jarrell-Ash Co:, Waltham, Mass.) using 4SSO-A radiation from a Spectra-Physics Model 141 argon ion laser (Spectra-Physics, Mountain View, Calif.) to excite the spectra. The optical system for focusing and collecting the light has been The infrared spectrum of LizBeI?4at 25" was measured with a Perkin-Elmer 621 spectrophotometer. A small crystalline sample was ground onto the surface of a AgBr plate which was placed at the focus of a 4 X beam condenser. By using a cover plate of AgBr and sealing the edges between the two plates, the powdered sample of LizBeF4was prevented from escaping into the air.
Results Raman spectra of molten LizBeFl (melting point 459O"J) were obtained from 487 to 640". A typical spectrum measured at 533" is shown in Figure 1. Three of the four bands are visible at frequencies near 390, 550, and 800 cm-I. The expected fourth band is located near 260 crn-', but it is somewhat obscured in Figure 1 by the steeply rising background; this band may be observed more readily in other spectra, such as in Figure 2 for molten nTazBeF4at 616". The Raman and infrared spectra of solid Li2BeF4at 25" are presented in Figures 4 and 5, respectively. Table I summarizes the vibrational frequencies observed for molten LizBeF4 at several temperatures. The results for solid ?Li2BeF4and 6LiPBeF4,for molten NazBeF4(mp 595"11), for a melt containing 17 mol yo BeFz in NaFLiF, and for aqueous BeF42- in excess F- also are reported.
Frequency Assignments
*
The polarization and intensity of the band at 550 3 cm-l in the Raman spectra of molten tetrafluoroberyllates (Figures 1-3) make it logical to assign it to the totally symmetric stretching mode, vl(al), of the tetrahedrally coordinated anion. Its frequency does not vary significantly (within experimental error) with temperature or when the cation is changed from lithium to sodium. The vl(al) frequency (6) J. P. Young, A n a l . Chem., 36, 390 (1964). (7) A . S. Quist, A p p l . Spectrosc., 25, 80 (1971) (8) A. S. Quist, ibid., 2 5 , 82 (1971). (9) A . S. Quist, J. B. Bates, and G. E. Boyd, J . Chem. Phys., 54, 4896 (1971). (10) K. A. Romberger and J. Braunstein, Reactor Chemistry Division Annual Progress Report, ORNL-4586, July 1970, p 3. (11) D . M. Roy, R . Roy, and E. F. Osburn, J . A m e r . Ceram. SOC., 36, 185 (1953).
The Journal of Physical Chemistry, Vol. 76, N o . 1, 1972
80
A. S. QUIST,J. B. BATES,AND G. 8. BOYD
Li2Be F4( 2 )
5339c
9.0crn-'Slit 5 Y io2 c/s
I
I
I
I
(000
1
900
I
I
800
700
I
I
500
600
I
400
I
I
200
300
cm-'
Figure 1. Rainan spectrum of molten LizBeF4 at 533'.
Table I : Vibrational Frequencies Observed in the Raman Spectra of BeF4*- in Melts, in Solid LizBeF4, and in Aqueous Solution (Frequencies in cm-1)
,-----LizBeFa
Molten NazBeFi
645, 686'
r------LinBeFi(s) ----Raman--7Li
6Li
7Li
8Li
... -1
at 25°-------7 Ir-----
487'
5339
640'
at 616"
255
240
260
265
255
...
257 295
257 297
... ...
385
390
390
385
380
380
348 377
354 377
333 360 372
402
...
405
440 475
...
435 463 500
486 520
563 775 795 850
563
...
...
... ... ...
775 805 860
862
547 800 a
melts at--
NaFLiF-BeFs 2.5 M (53:30: 17) (NH4)2BeFd mol %) at in 2 M NH4F
550 800
545 800
550 800
at 2 5 O
552 800
548 795
...
I:zq
Assignment
va (e1
vdfz)
380
a( 9 1
v,(al) v.3 (f2
)
Vibrations from Li-F stretching modes in crystalline LizBeF,.
is essentially the same in aqueous solution as in the melts; only in solid LizBeF4does it occur a t a different (higher) frequency. The observed frequency and assignment are consistent with previously reported values for the Raman spectra of solid alkali metal tetrafluoroberyllates* and for aqueous solution^.^ Although the position of the vl band in the melt does not vary greatly with temperature or cation, its half-bandwidth decreases considerably when lithium is replaced by sodium. Thus, in molten LizBeFd at 582', the VI halfbandwidth is about 100 cm-', whereas in molten NazBeF4 at 616' this value is ca. 50 cm-l. The triply degenerate v3(f2) and v4(f2) vibrational T h e Journal of Physical Chemistry, VoE. 76, No. 1, 1973
modes of tet'rahedral ions are both Raman and infrared active. Infrared studies with solid alkali metal fluoroberyllates have resulted in the assignment of bands near 800 cm-I to the v3 ~ i b r a t i o n l -and ~ of the bands near 380 cm-l t o ~ ~ . 2 The - ~ Raman spectrum of solid LizBeF4 exhibits three bands in each of these regions, consistent with a lowering of the T , symmetry of the BeFhz- ions which occupy sites of C1 symmetry in the hexagonal unit cell lattice.12 In molten and aqueous tetrafluoroberyllates, only single bands a t 800 =t 10 and 385 =t 5 em-' are observed for the v3 and v4 vibra(12)
J. 11. Burns and E. K. Gordon, Acta
C~yst.,20, 135 (1966).
81
RAXANSPECTRA OF BeFd2-IN MOLTEN NaF AND LiF
i
No2BeF4 616°C
io cm-'siit 1 103 cps
I
I
I
I
i
900
700
500
300
100
FREQUENCY. cm-'
Figure 2. Raman spect,rum of molten NazBeFh a t 616": (a) incident light polarized perpendicular to plane containing slit and laser beam; (b) parallel polarization.
talline Li2BeF4 (Table I) is supported by the obscrvcd isotopic shifts. Similar bands observed in the 500- to 400-cm-1 region of Li2C03also were found to shift to ca. 17 to 34 cm-' higher frequencies when 'jLi was substituted for 7Li, while the modes due to C032- showed either a very small ( E a > K > R b > Cs. The explanation advanced for the change of 6(vl) in molten nitrates1’ may be applicable to the case of the change in 6 observed for the V I mode of BeFa2-. An increase in the potential barrier for hindered rotation of BeF42- ions in molten Li2BeF4 compared with the barrier height in molten Na2BeF4 results in an increase in 6 for vl. Because of their comparatively smaller size, the Li+ ions are able to get closer to the center of the BeFd2- anions than are the N a + ions; the closer proximity of Li+ to BeF42- produces a larger barrier to anion libration. On this basis, the change in B(v1) for molten K, Rb, and Cs fluoroberyllates may be expected to follow the trends observed in molten nitrates. Additional experiments are required, however, to test these assumptions. The correlation between bond force constants and anion charge for a number of isoelectronic tetrahedral species was discussed previously by Woodward.18 The trends exhibited by the series BeF42-, BF4-, and CF4 (Figure 6) follow the expected general pattern of increasing fd and F11 and decreasing 14-F bond length as the anion charge varies from - 2 to 0. Both the covalent 11-F bond strength and the coulombic attraction between the 11 and F atoms increase in the series from BeF42- to CF4.l8 An interesting feature shown in Figure 6 is that, while the graphs of bond lengths, XI--F, and symmetrized force constants, 8’11, exhibit the largest rate of increase from BeF4*- to BF4-, the plot of fd us. anion charge shows the greatest rate of increase from BF4- to CF,. The potential constants, f d , are related to the restoring force for a single M-F bond, while the 8’11 constants are related to the restoring force for simultaneous, symmetric stretching of all four AI-F bonds. Thus, it seems reasonable that the varia-
83 tion of KI with anion charge should be similar t o that exhibited by the graph of bond length us. anion charge while, for the single bond M-F force constants, similar reasoning does not hold. Both the equilibrium bond lengths and the resistance to stretching of four AT-F bonds simultaneously and with equal displacements (symmetric stretching mode) are influenced by the mutual interactions of the 11-F bonds. The effects of bond-bond interaction have been removed (within the approximation of the calculations described above) in the computation of f d . The Raman and infrared spectra of crystalline LizBeF4 (Figures 4 and 5 and Table I) were surprisingly simple in view of the number of formula weights (18) of Li2BeF4in the primitive unit cell. All atoms occupy sites of general symmetry;12 hence, a total of 18 Raman-active and 18 infrared-active internal modes (derived from motions of the BeF42- ions) are allowed. The observed spectra appear t o be those of noninteracting BeF42- tetrahedra located on sites of low symmetry in which degeneracy is completely removed in the e and f2 modes by the static field. Correlation field effects were only weakly apparent from the small differences observed between the frequencies of the infrared and Raman components of a given internal mode (Table I).
Conclusions The BeF42- anion appears to retain its tetrahedral symmetry in aqueous solution and in molten Li2BeF4 and Ka2BeF4. Both the frequencies and force constants of “free” BeF42- were well characterized from these studies. A comparison of the frequencies and half-bandwidths for v1 in molten Li2BeF4and in molten Na2BeF4 showed that cation-anion interactions may be important, while effects of anion-anion interactions were not apparent. Further studies of the effects of cation size and charge on the band shape of v1 would be helpful in determining the nature of the cation-anion interaction.
Acknowledgments. The authors would like to thank E(. Romberger and B. Hitch of the ORYL Reactor
Chemistry Division for supplying high-purity samples of Li2BeF4and BeF2, and D. E. LaValle of the Analytical Chemistry Division for preparing the samples of 6Li2BeF4. This research was sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporation. (17) N. A. Ponyatenko and I. V. Radchenlro, Opt. Spectrosc., 2 6 , 353 (1969). (18) L. A . Woodward, T r a n s . Faradag SOC.,54, 1271 (1958).
T h e Journal of Physical Chemistry, Vol. 76, hTo. 1, 1072
