D. J. Gardiner, R. B. Girling, arid R. E. Hester
640
of [Co(NH3)s]J+ and [Co(NH3)5Hz0I3+ with S 0 2 - are dependent upon ,12 They become quite large in dilute solutions, but their changes at high ionic strengths are much less. The equilibrium quotient for [Co(NH3)5Hz0]3+ with SO42- is 10.1 at 1.1 = 1.05 and 25”, and the value for [Co(NI13)6I3+ is similar. Evans and Nancollas reported that the equilibrium quotients for the outer sphere complexes between [Co(en)#+ and halides a t 1.1 = 0.30 are of the order of 10-20, and would be smaller a t higher ionic strengths. Our measurements were limited to solutions of moderately high concentrations. It was difficult to keep the ionic strength constant because of limited solubility for perchlorate in the presence of [C0(en)3l3+, and the use of other salts for the adjustment of ionic strengths are not suitable. Posey and Taube showed that a t higher concentrations the change of the equilibrium quotient with pf/2 is not large. Moreover, in our results the effect of chloride on the 59Ca chemical shift is reflected by the difference i~i&’ in eq 5 , and 6,’ for 31P is not affected by the concentration of tetramethylammonium
phosphate. In fact, the reasonable agreement between the experimental results and the data calculated from a single equilibrium quotient for both the 59C0 and resonance a t different concentrations indicates the validity of our assumptions. The variation of the equilibrium quotient with the ionic strength studied is likely to be within the error limits13 allowed for the calculated value, 12.0 2.0 M - l , and the value of 12.0 can be regarded as a median one at 1.1 around unity. Tbe fact that K = 12 M - l implies that the outer sphere complex between C0(en)3~+and Po43-is a weak one a t moderate ionic strengths. The interaction is probably electrostatic in nature but may also involve hydrogen bonding between the N H proton and oxygen. The pH dependence of the S9C0 shift and the much smaller effect of C1- compared to an equal amount of Po43are consistent with both kinds of interaction.
Acknowledgment. This work was supported by the National Science Foundation.
Raman Studies of Molten Salt Hydrates. The Magnesium Chlorate-Water System . J- Gardiner7*R. 6 . Girling, and R . E. Hester Departinent of Chemistry, University of York, York, England (Received May 24, 7972)
The magnesium chlorate-water system has been investigated by Raman spectroscopy. Spectra of solids from the anhydrous salt through to the crystalline hexahydrate are compared with spectra obtained from simple aqueous solutions of Mg(C103)~over the concentration range from dilute to saturated solution and through a, series of molten salt hydrates to the composition Mg(C103)2-2.8H20. The solution and melt data have been analyzed in three main composition regions: dilute solution to hexahydrate; hexahydrate to tetrahydrate; and the region of lower hydration numbers. Perturbations of the simple C3“ C103ion spectrum are interpreted as arising from hydrogen bonding with water in the first solution region; from formation of Mg2+-C103- contact ion pairs in the intermediate region, with continuing C103--Hz0 interactions; and, in part, from more extensive quasilattice interactions in the final region of lowest water content.
Introduction It is well established that Raman spectroscopic studies can provide significant structural information on aqueous electrolyte solutions and on molten sa1ts.l In particular, much use has been q a d e of the characteristics of oxyanion spectira and their sensitivity to changing environment in the liquid state for the purpose of probing structural relationships that exist in concentrated aqueous solutions and molten salts. Spectroscopic criteria for distinguishing between the formation of direct, or contact, ion pairs and solvent-separated ion pairs have been established with metal nitrate solution^,^^^ and it was one of the purposes of the present work to determine the extent to which similar effects could be observed with c103- as the structural “probe.” Previous work by Oliver and Jam4 has shown the ClO3ion spectrum to be sensitive to the nature of ion hydration ,and interionic interactions in the LiC103-HzO system. The Journal of Physical Chernistry, Vol. 77, No. 5, 1973
The suitability of the magnesium salt for the purpose a t hand was suggested by the recent detailed study by Peleg5 of the corresponding nitrate system where formation of contact ion pairs was indicated only on reduction of the water content to below that of the hexahydrate, and by the high aqueous solubility of magnesium chlorate. Both HzO and the Mg2+ ion are known to interact more strongly with 0 than with 61 atoms, so that it could be antici. pated that any such asymmetric interactions with C103would lead to loss of degeneracy of the E type vibrational modes of this ion, thus providing a sensitive indication of local structure in the liquid. (1) R . E. Hester, Annu. Rep. Chem. S o c . 6, 79 (1970). (2) D. E. Irish and A . R. Davis, Can. J . Chem.. 46, 943 (1968) (3) D. E. Irish, A . R. Davis, and R. A . Plane, J Chem. Phys.. 5 0 , 2262 (1969). (4.) 6 .G.Oliver and G . J. Janz. J , Phys. Chem , 75,2948 (1971). (5) M. Peleg,J. Phys Chem.. 76, 1019 (1972).
Raman Studies of Molten Salt Hydrates
6411
x !00 1
IO0 60
--
30
.----
9 4
-'
I / / \ \ \'---
-
id-iil_ld--l--l-/lj 1100
1000
900
MOO
Cm.l
700
600
500
400
100
I_____._____
Figure 1. Raman spectra of molten hydrates and solutions of composition MgC103~xH20using vertically polarized light. Experimental Section Raman spe:tra were recorded on an instrument comprising a Hilger and Watts D330/331 double monochromator and a Brookdeal ac amplification/phase-sensitive detection system tuned to 800 Hz, the frequency at which a Coherent Radiation Model 52A argon ion laser beam (used a t 5145 A) was chopped. Right angle scattering was used throughout The system was calibrated with hexane and the curve-resolved band positioiis are believed to be accurate to *5 cm-l, The sample holder and furnace have been described elsewhere.6 The temperatures required to melt the range of hydrates studied varied from ambient for the higher hydrrltes and solutions to 120" for Mg(C103)z. 2.8HzCI. This composition was found to be the limit of the stable melt range; a t lower water contents the salts decomposed on heating without melting. Reagent grcde Mg(c103)2*6Hzo (BDH) was used without further purificatron. Anhydrous Mg( C10& was prepared from this by beating in a vacuum oven at 100" for 7 2 hr. The various hydrates were then prepared by addition of calculated amounts of water to the anhydrous salt. Magnesium chlorate was always handled in a drybag as it is extremely hygroscopic. Compositions were checked by analysis for M g 2 &(ElDTA), C103- (iodimetry), and HzO (nmr and elemental analysis on a Perkin-Elmer Model 240 GHN analy sea.). Raman spectra of the solid hydrates were obtained by allowing an anhydrous sample to pick up water slowly from the atmosphere spectra being obtained a t the beginning and ai intervals during the hydration to the hexahydrate. Considerable band overlap was found to be a characteristic of all the spectra obtained, so that it was necessary to adopt stantlard curve-resolving procedures to facilitate analysis of the spectra. A Du Pont Model 310 curve resolver was used for this purpose, with care being taken to avoid generating more bands than were needed to give a close fit to the experimental profiles. A 50/50 Gaussian/ Lorentzian pr3duct function gave a fit with a physically
Figure 2. High-frequency region of the Raman spectra of molten hydrates and solutions of composition MgC103.xH20 using horizontally polarized light. reasonable number of bands, the residuals in all cases being less than 10% of the total band area. Greatest difficulty was encountered in curve fitting in the wings of sharp bands, and the procedure was adopted of accumulating the errors in curve fitting to a single residual as shown in Figures 3 and 4 by a broken line. No physical significance is to be attached to these residuals. In order to test the ability of a simple polarization perturbation model to predict changes in the CBO3- ion spectrum, due to association with Mg2+ or HzO, calculations were made of the effects of adjusting C1-0 bond force constants such that as one decreased the other two increased so as to maintain a constant sum of the three. This polarization simulation was performed in a manner analogous to earlier work with other oxyanions.7.8
Results Raman spectra of the molten hydrates and aqueous solutions of Mg(C103)2, obtained with parallel plane polarized laser light, are shown in Figure 1. Figure 2 shows the high-frequency region using perpendicular plane polarized light which, due to the polarized nature of the lowest frequency component, renders the multiplet structure more readily resolvable. Typical results of the curve resolution procedure are shown in Figures 3 and 4, taken from three different composition regions. The experimentally observed variation of' band frequencies with composition is shown graphically in Figure 5 , water content of the system being expressed as percentage by weight in order to show detail in the low water content region. The measured frequencies are listed in Table I. (6) J. H. R. Clarke and R. E. Hester, d. Chem. Phys.. 50, 3106 (1969). (7) H. Brintzinger and R. E. Hester, Inorg. Chem.. 5, 980 (1966). (8) R . E. Hester and W. E. L. Grossman, Inorg. Chem.. 5, 1308 (1966). The Journalof Physical Chemistry, Vol. 77, No. 5. 1973
D. J. Gardiner, R. B. Girling, and R. E. Hester
642 I100
IO00
900
WC
A 7oc
bOC
5oc
4oc
"k HO , 30c I100
1000
000
100 ce
700
LOO
SO0
400
SO0
0
90
a0
70
bo
50
40
10
SO
10
0
__I___
Figure 3. Typical curve resolution results on three Raman spectra of Figure 1 taken from three different composition regions, MgCI03.xH20.
Figure 5. T h e experimentally observed variation of band frequencies with percentage by weight of water content of MgC103.xH20. The three composition regions A, B, and C are indicated. TABLE I: Observed Raman Frequencies of Molten Mg(C103)~ xH2O
Frequencies, C M - '
X
200 100 50 20 9.8 8.1 6.0 4.6 4.4 4.0 3.8 3.2 2.8 4.6
2.8
In a separate series of experiments with several of the systems of different composition the effects of varying temperature over the range 20-120" were examined. Within the limits of uncertainty involved in the curve resolution procedure, the temperature effects were found to be insignificant throughout. The series of Raman spectra of solids, from anhydrous Mg(C103)2 through to Mg(C103)2.6H20, are shown in Figure 6. Table I1 lists the observed frequencies of the anhydrous solid and the hexahydrate. The Journal of PhysicaY Chemistry, Vol. 77, No. 5, 1973
467 467 474 474 473 474
484 487 495 493 489 496
608 606 608 607 605 607 604 605 608 616 614 603 612
930 929 930 930 930 932 928 930 932 946 941 947 949
961 963 965 962 968 964 965 967 986 1009 995 999 7015
996
1000 998 994 997 1002 1012 1017 1022 1044
1039 1041 1052
Figure 7 shows the results of the C1Q- ion polarization calculations described in the previous section, with the various band frequencies being plotted as a function of the polarization distortion of the C1-0 bond force constants, AF.
Discussion The unperturbed C103- ion belongs to the C3" symmetry point group and has a vibrational representation r = 2A1 2E, giving rise to four fundamental modes, all of which are active in both Raman and infrared spectra. Our previous works established the full vibrational assignment and force constants for C103-. It is convenient to consider the data obtained in the following three composition regions: (A) dilute Mg( ClO& solution in water to the molten salt hydrate of composition Mg(C103)~.6HzO; (B) liquids Mg(C103)2.6H20 t o Mg(C103)2,4H20; (C) liquids Mg(ClOs)~-4H20 to Mg(C103)2. 2.8H20. Although the full liquid composition range examined was attainable only by varying temperature over the range from room temperature to 120", it has been established that the spectra under discussion are essentially invariant over this range. The trends in the data presented in Table I and Figure 5 therefore reflect the changing structural features of the liquids through the dif-
+
Figure 4. Typical curve resolution results on three Raman spectra of Figure 2 taken from three different composition regions, MgC103.xH20
475 474 476 477 474 476 473
Raman Studies of Malten Salt Hydrates ,
“
“
“
“
“
“
“
643
I
‘lOOPm-’
1000
800
700
600
5 00 i
i\\
/\ 400
300 00
0.4
0-8
1.2
1.6
2.0
plot of calculated frequencies against polarization distortion AF for the ClO3- ion.
Figure 7. A
1100
1000
900
800
cm., 7 0 0
600
600
400
Swiss of Raman spectra of solids from anhydrous MgC103 to MgCIQ~*SRI~O.
Figure 6.
TABLE I I : Observed Reman Frequencies of Solid Anhydrous Mg(CIQ3)z and lblg(CI Frequencies, cm-’ _ l _ l l _ _ l l _ l l _
Mg (C103)2.6kl2O
Mg(C103)a
470
494
48 5
600 660
935 949(sh) 966
970 1054
ferent composition regions. Although no sharp divisions exist between the regions A, B, and C, there are sufficient differences in the overall patterns of the spectra to justify separate consideration of these regions. Throughout region A the Raman band frequencies remain essentially constant and may be assigned to internal modes of the C103- ion as follows9 475 cm-1 605 cm-l 930 cm-l 965 cm-l 997 cm-l
v.+(E) asymmetric va(A1) symmetric VI(&) symmetric v$(E:) asymmetric vaD(E)asymmetric
0-C1-0 deformation 0-C1-0 deformation C1-0 stretch C1-0 stretch C1-0 stretch
This assignment is based on the C3” model, but is modified in recognition of the observed v3 band splitting to cover the evidsnt lo^+^ of degeneracy from this E mode. By analogy with the nitrate analyses of Devlin and coworkers,IOJ1 this v 3 splitting might be attributed to LO-TO phonons in a iiquid quasilattice, although this interpretation might :more appropriately be applied to the most concentrated systems. Alternatively, a perturbation of c103arising from LL specific interaction with water molecules may be invoked. Throughout composition region A the chlorate ions may be assumed to have a water environment, and the invariance of the spectra through this region implies :haC the c103- ion probe is insufficiently
sensitive to distinguish “free” water environment from the cation hydration water environment which must exist at, the upper concentration limit of this region. That this v3(E) band splitting is the most sensitive spectroscopic feature available is apparent from the results presented here, and also is consistent with expectations based on ion polarization force constant calculations (Figure 7 ) of the type which have proved useful in analyses of nitrate spectra.7.8 In region A it is not proposed that Mg2+ ions are effective in polarizing ClOa-, but that the dipolar HzO molecule, either electrostatically or through hydrogen bonding, could have a similar, but weaker, effect. Our composition region B corresponds with what Braunstein12 bas called “hydrate melts,” and it is useful to examine spectral data for this region for evidence of the coexistence of various hydrated magnesium ions, ranging from Mg(H20)62+ to Mg(H20)42+-,the C103.- ions occupying outersphere sites around the metal ions. In this region we note (Table I and Figure 5) the onset of a distinct upward trend in the frequencies of the split v3(E) band components, while z q remains essentially constant and 1 4 also splits into two components. This latter splitting is substantial, reaching ca. 20 em-], necessitating more than 2 mdyn k 1change in the relative values of the C1-0 bonds if the C103- ion,polarization model is to explain the effect (see Figure 7 ) . This change is unreasonably large, implying a corresponding ca. 50% change in the C1-0 bond order. The model predictions also fail to reproduce the behavior of v1 (compare Figures 5 and 7 ) . Analogous splitting of the v4 deformation band of N o s - in concentrated aqueous solutions has heen interpreted as indicative of formation of contact ion pairs.2 , 3 However, although the loss of H2O from Mg(HzO)$+ does leave in(9) D. J. Gardiner. R. B. Girling, and R. E. blester, J. Moi s t r u c t 13, 105 (1972). (10) J. P. Devlin, D. W . James, and R. Frech, J. C h e v . Phys.. 53, 4394 (1970). (11) J. P. Devlin, P. C. Li, and G . Pollara, J Chem Phys.. 52, 2267 (1970). (12) J. Braunstein, ionic interactions. 1 , 179 (1971). The Journal of Physical Chemistry, Vol. 77, No. 5, 1973
D. J. Gardiner, R . B. Girling, and R. E. Wester
644
nersphere coordination sites available for C103- ion occupancy, it seems likely that interactions between HzO and C103- simultaneously coordinated to Mg2+ are responsible for differences observed between nitrate and chlorate systems. In composition region B we propose the formation of mixed aquo-chlorate complexes with ligand interactions, which .nay be represented in the following schematic way, to account for the observed spectra.
Plots were made of trends in intensities and half-widths of bands in region B. However, these were largely unsuccessful due to the accummulation of errors in the curve resolution procedures and no structural conclusions were made on the b a s s of these data. In the Concentration region C, extending from Mg(C103)2-4HzC) to Mg(C103)2-2.8K20, the several changes in the spectral features recorded for region B are seen to be further extended (Table I and Figure 5). The splitting of the two components of v3(E) does not vary significantly from 40 cm-I throughout the region, but the upward trend in the fTequencies of both components is more marked. Unless a n increased C1-0 bond order is proposed, this upward trend implies an external restriction on C1-0 bond stretching, which also is reflected in the upward trend of the vl(A1) frequency through this region C. In the molecular deformation region, however, the changes are smaller. v z ( A 1 ) remains virtually constant, while the components of vq(E) remain separated by ca. 20 cm-l, both shifting to slightly higher frequencies than observed for region B. This behavior Is in general accord with the predictions of the polarization calculations (Figure 7), with the notable exception of the q ( A J frequency. More extensive interactions between Mg2+-Hz0-C103 mixed ligand complexes are probable in this region, the order and cohesion of the liquid quasilattice so formed being due in part to intermolecular I-I bonding between mixed H2OC103- complexes of magnesium, uiz. H
I
Mg _-__ .-O--K
0
I ___0-C1-0
l l _ _ l
_____Mg
The general broadening of the bands in the low water region C (Figures 3 arid 4) could be interpreted in terms of disorder in the extended quasilattice. In this situation Devlin’s TO-LO component interpretation of the 4 E ) mode splitting may be applicable. Alternatively, a n interpretation in terms of a multispecies equilibrium, involving species of which the ion pair is merely one simple form,
The Journal of Physicai Chemistry, Vol. 77, No. 5, 1973
might be entertained. Attributing the band a t 930 cm-1 to the essentially “free” C103- species, the broadening and shift to higher frequencies observed with increasing salt concentration may be attributed to the growth of a band (bands) at ca. 945 cm-l characteristic of ion-pair or ioncluster species. The solid-state spectra, given in Figure 6, may profitably be compared with those obtained from the liquids of corresponding composition in regions I3 and C (molten salt hydrates). There are clearly marked differences between the two states throughout the composition range, perhaps the most significant being the apparent lack of splitting of the C103- v3(E) band, which occurs a t 970 cm-1 in the solid hexahydrate. The corresponding band in the anhydrous salt occurs at 1054 em-2. This lack of splitting may be used as evidence of symmelric c103- in the anhydrous solid lattice, but the splitting of v 4 ( g ) in the solid hexahydrate excludes this conclusion in this case. The high-frequency shifts noted in the liquid spectra as water was removed also are apparent in the solid spectra, though in the solids v z ( A 1 ) also is strongly affected by dehydration. Spectra of solids of intermediate composition appear in the main to be weighted superpositions of the hexahydrate and anhydrous salt spectra, though additional features in the regions 900-930 and ca 630 c m - l are apparent in some of the traces and perhaps arise from distinct hydrate species containing fewer than six HzO. However, the situation is still more complex for the liquids. The shoulder a t 949 cm-1 in the spectrum of the anhydrous solid may be assigned to a 37C1 isotope component, by comparison with Kc103 spectra,13 though the absence of this feature from anhydrous LiC103 spectral4 suggests an alternative assignment, perhaps in terms of factor group splittings which are absent from the LiC103 case. In conclusion, these results favor the following structural features for the Mg(ClO&H20 systems. With excess water, the Mg2+ ions are fully hydrated and exert little influence on the spectrum (molecular symmetry and electronic distribution) of c103--.C103- is, turbed by interaction with HzO, either in bulk water or in the Mg2+ hydration sheaths. At water levels less than the hexahydrate C103- occupies innersphere, or contact, coordination sites around Mg2+, thereby undergoing polarization distortion in addition to local interactions with hydrate water molecules. There is no evidence for discrete Mg(H20),2+ species with n < 6, but extended lattice-like interactions appear likely in melts of low water content.
Acknowledgment. We thank Dr. A. Covington of Newcastle University for allowing us use of his Du Pont curve resolver. (13) J. B. Bates, J. Chem. Phys 55, 494 (1971) (14) We thank a reviewer for this datum,
