RAMANSPECTRA OF’MgX2-KX MELTS
155
I _ -
Table IV: Catalysis ~ l the ’ Hydration and Iodination of Pyruvic Acid and the Alkyl Pyruvates c-------
Pyruvio acid ~
H (~MO -1
sec-1)”
6.0
k ~ ~ (M 0 -~1 sec-1) kacoz-/lcazo (M-’ WC-’) kHao+ (M-1 sec-1)
x
10-4
d d
1.73
Hydrationa**--methyl pyruvate
5.95
x
10-4
8.8 X
150 0.78
--7
Ethyl pyruvate
4.66
x
10-8
7.2 X 150
0.68
r
Pyruvic add
2.05
x
10-8
a
Enolization-----. methyl. pyriivate
3.4
x 10-8
3.1
x io-*
5700
1.68 x 10-4 5500
I
i
1.94
d
1.8
x 10-8 x 10-4
Ethyl pyruvate
’
Reference 1.5. Reference 20. ~ C H ~ O= k0/55.5 mol 1.-1. It should, however, be pointed out that the respective transition states may contain more than one molecule of water. Analysis of the general catalysis of both the hydration and enolization of pyruvic acid by formate buffers is difficultsince the inherent acidity of pyruvic acid causes pH changes as the concentrations of NCOaH and HC02- are simultaneously varied. e Reference 19. Too small to be detected. I
carbon-hydrogen bond k already aided to such an extent by the strongly electron withdrawing effect of the
adjacent carboalkoxy group that neither inter- nor intramolecular acid catalysis is required.
Structural Studies of Magnesium Halide-Potassium Halide Melts
by Raman Spectroscopy by V. A. Maroni,* E. J. Hathaway, and E. J. Cairns Chemical Engineering Division, Argonne National Laboratory, Argonne, Illbwis 60439 (Received‘August 6 , 1970) Publication costs assisted by the CJ. S. Atomic Energy Commission
Ramal? spectra of MgXT-KX melts (X = C1, Br, and I) have been obtained over a range of X-/Mg2+ mole ratios. Results for the MgClz-KCl and MgBr2-KBr systems aye interpreted in terms of a complex equilibrium between a residual ionic lattice, [MgX,],, similar in structure to solid MgClz and solid MgBr2, and a complex anion of the form MgX,(2-n), The concentration of MgXn(2-n)units increases as the X-/Mg2+ ratio is increased from 2.5 to 4.0. Raman spectra of MgIZ--KImelts with I-/Mg2+ = 3.0 and 3.5 indicate the presence of a single, highly symmetric species. The observation of two depolarized, primarily bondbending modes for this species is most consistent with the existence of tetrahedral MgId2-. This indicates that three large I- ions do not effectively shield the field of the Mgz+ ions. Extrapolation strongly suggests that with C1- and Br-, the lower coordination, MgX3-, is even less likely.
Introduction Raman studies of divalent metal halides in the molten state have been directed primarily toward (1) investigating the existence of complex ions in these melts and (2) determining the structures of the complex ions. The chlorides of Mg(II), Zn(II), Cd(II), Hg(II), Sn(II), and Pb(PI), the bromides of Mg(II), Zn(II), and Bg(II), and the iodide of Hg(I1) have been subjected to Raman investigation. Comprehensive discussions of these investigations with references to many of the original papers have appeared in several review articles;’-* consequently only a brief summary will be presented here.
I n studies of the chloride melts of Mg(II),lbv4Zn(II),G-7Cd(II),7,8and Sn(II),g evidence was found for
(1)
(a) V. A. Maroni and E. J. Cairns, “Molten Salts: Characterization and Analysis,” G. Mamantov, Ed., Marcel Dekker, New York, N. Y., 1969, pp 231-251; (b) pp 263-280; (0)pp 256-260. (2) D. W. James in “Fused Salt Chemistry,” M. Blander, Ed., Interscience, New York, N. Y., 1964. (3) G. J. Janz and S. C. Wait, Quart. Rev., Chern. Soc., 17, 225 (1963). (4) K. Balasubrahmanyam, J . Chem. Phys., 44, 3270 (1966). (5) D. E. Irish and T. F. Young, Jr., {bid., 43, 1765 (1965). (6) R. B. Ellis, J . Electrochem. Soc., 113, 485 (1966). (7) W. Bues, 2.Anorg. Allg. Chem., 279, 104 (1955). (8) M. Tanaka, K. Balasubrahmanyam and J. 0%. Bockriu, EZeelrochim. Acta, 8 , 621 (1963). The Journal of Physical Chemistry, Vol. Y 6 , No. 1, lS7l
156) the existence of a polymeric species [MCl2], with a structure resembling that of the solid in each case. On increasing the amount of excess halide in these melts, usually by addition of an alkali metal halide, smaller discrete unii,s of the type MC1,(2-") were found to separate from the polymeric structure. The species MCL, h!IC13-, and MC12- have all been reported to exist in the chloride melts of ZII(II)~-' and Hg(II).'O MC1,- has been reported in Cd(II)'r8 and Sn(II)g chloride melts, and l\iIC1d2- has been reported in Pb(11) chloride mei5s.l' There is some controversy concerning the existence of NIX3- in molten binary salt systerns.l2 In this regard, Moyer, Evans, and L013reinvestigated the Raman spectra of ZnCl2-KCl melts and found it possible to explain the data without invoking ZnC18-. I n other cases, notably Sior Cd(I1) and Sn(II), the available ~ l a t a ~are - ~incomplete in terms of evidence for either MC1,- or MClt2- and do not allow a conclusive assignment to be madc. From a Raman study of the MgClz-KC1 system, Bala~ubrahmangam~ concluded that the layer structure of solid i\ilgC12 was at least partly broken up in pure liquid hilgClzto form octahedral MgClo4-units. When the relative amourt of chloride ion was increased by addition of KC1, MgC13- and possibly MgC1d2- were said to replace RilgCh4- as the principal species. A subsequent Raman study of the MgC12-IICl system, carried out in our laboratory,Ib gave somewhat different results for the number of bands, the peak frequencies, and the states of polarization than were obtained by Balasirbrahmanyam; results analogous to those for the l!dgC1:rKC1 system were also reported for the MgBrz-KBr system. Since the publication of these results, we have performed additional studies of the MgC12-KC1 and MgBr,-KBr systems and have also recorded Raman spectra of Mg12-KI melts. In light of these new data, we now wish to revise several of our earlier conclusions regarding the IvlgC12-KC1 and MgBrz-KBr systems. A discussion of the most plausible species involved in each magnesium(I1)-halide complex equilibrium IS also presented.
Experimental Section The Raman spectrophotometer used in this investigation consisted of a Spectra-Physics Model 125 oHelium-Neon Laser (nominal power 50 mW at 6328 A) in tandem with a 8pex Industries Model 1400 double spectrometer. The exciting radiation was chopped at 400 cps and the scattered light was detected with a Namaniatsu TV Go., Ltd., R-136 photomultiplier tube (cryostated at - 50') coupled to an ac lock-in amplifier. The amplified signal was measured with a phase-angle voltmeter and plotted by a strip-chart recorder. A typical Raman cell consisted of a quartz tube with an optically flat window at each end. Each cell was provided with a side arm through which the sample could The Journal of Physical Chemistry, VoL 76,No. 1, 1971
V. A. MARONI,E. J. HATHAWAY, AND E. J. CAIRNS
~
i____l____l
300
250
200
150
u , cm"
Figure 1. Raman spectra of MgCl2-KC1 melts: T = mol of Cl-/mol of Mg2+; time constant = 50 see; scan speed = 2 cm-'/min; spectral band pass = 10 cm-1.
be loaded. After loading, the side arm was sealed, and the cell was placed in a split-tube furnace with its axis vertical. The laser beam was passed through the upper optical flat collinear with the axis of the quartz tube. Scattered radiation emerging perpendicular to the tube axis through a slot in the side of the furnace was collected with a biconvex lens and focused into the spectrometer slit. The apparatus and general technique for these experiments have been described and diagramed in greater detail elsewhere.IC Samples were prepared using commercially available analytical grades of KC1, KBr, and K I which were heated a t 200" under vacuum for several days prior t o use. Anhydrous nilgC12 (Anderson Physics Laboratory), 98+% MgBr2, and 98+% hfgL (Research Organic/Inorganic Chemicals) were sublimed under vacuum prior to use. All subsequent handling of the anhydrous salts was carried out in a helium atmosphere having a moisture content of less than I ppm. Salt mixtures of the required composition were equilibrated and then filtered through fine quartz frits under helium pressure. Care was taken to ensure that all of the liquid material passed the frit. Chemical analyses of the filtered samples were in excellent agreement with the compositions determined from the weights of material used to form the prefiltered mixtures. The filtered samples were ground to a fine powder, and por(9) J. H. R. Clarke and C. Solomons, J . Chem. P h ~ s . ,47, 1823 (1967). (10) G. J. Janz and D. W. James, ibid., 38, 905 (1963). (11) K. Balasubrahmanyam and L. Nanis, ibid., 40, 2657 (1964). (12) (a) See reference lb, pp 55-80; (b) M. A. Bredig, Electrochim. Acta, 5 , 299 (1961). (13) J. R. Moyer, J. C. Evans, and G. Y-S. Lo, J . Electrochem. Soc., 113, 158 (1966).
RAMANSPEC~TRA OF MgX2-KX MELTS
157
tions were loaded into the Raman cells for the spectral measurements.
Results and 1)iscussion A . The MgC12-KCZ System. Raman spectra of R’IgC1z-Iigatedhere, the sit)uation is somewhat different in the bending region. TWQ well-resolved, depolarized bands are observecl. Although the depolarized primarily stretching mode is not observed, the spectra are nonetheless consistent only with tetrahedral MgI.?-, the only structure m Table I characterized by two depolarized bending modes. The extension of the concluftions for the Mg12-KI system to the chloride and bromide systems appears to be reasonable. If indeed four iodide ions are required to effectively shield an Mg2+ion, Mg(1I) must $ P expected to coordinate a t least four chloride ions or faar bromide ions. Therefore, the results presen4,ed here are considered to be in closest agreement with the conclusions of Kleppa and McCartylG and of Brediglz that, MgGI42is the predominant complex in ~ ~ melts. g The predominance of PliIgBr42-in MgBrz-also implied.
Acknowledgments. The authors are indebted to %'I. A. Bredig for helpful discussions pertaining to the conclusions of this investigation. The support and encouragement of Drs. R. C. Vogel and A. D. Tevebaugh are gratefully acknowledged. This work was performed under the auspices of the U. 8. Atomic Energy Commission. (22) This generalization is based on a review of extended compilations of vibrational data for metal-halogen complexes. See, for example, K. Nakamoto, "Spectra of Inorganic and Coordination Compounds," Wiley, New York, N. Y . , 1963, pp 86, 106, and 114. (23) We have recently obtained spectra of MgClz-KCI melts with Cl-l/Mg;+ = 4.0 using the 5145-A line of an argon ion laser. These spectra show a broad depolarized band centered near 331 em-' which is probably due to the antisymmetric stretching vibration, Also, because of the greater exciting power of the argon ion laser, we have been able to significantly reduce our slit widths and our noise level SO that we can now resolve the band in the 75-175-cm-' region into two depolarized components. Our studies using the the argon ion laser will be extended to the MgBrz-KBr system and, if possible, to the M g I a K I system. We plan to publish these results as soon as possible.
The Journal of Physical Chentistry, Val. 76,No. Z, 1971
~
