Vibrational Spectra of M+N03- Ion Pairs
2265
Vibrational Spectra of M+N03- Ion Pairs Variably Hydrated or Ammoniated in an Argon Matrix Gary Rltrhaupt and J. Paul Devlin' Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74074 (Received May 2 1, 1975) Publication costs assisted by the National Science Foundation
Studies of the vibrational spectra of M+N03- ion pairs have been extended by matrix isolation of the vapors of the molten salts in the 4O0-45O0C range in mixed matrices of argon-water and argon-ammonia at 10 K. These vapors are known to be composed of largely monomer ion pairs which in the mixed matrices are observed to associate strongly with the HzO and NH3 molecules. The dominant spectroscopic effect of this association is a progressive reduction in the cation distortion of NO3- as measured by the splitting, Av3, of the asymmetric stretching mode. This splitting, which, for example, decreases from 260 cm-l for Li+N03- in argon to 65 cm-l in a pure H20 matrix, is observed to decrease sharply to 168 cm-l for single hydration of the cation and, then, gradually to the pure water matrix value as the percent water in the matrix is increased. This behavior, as well as that for Li+N03- in mixed argon-ammonia matrices and for K+N03 in argoq matrices mixed with H20 and "3, is firm evidence that the overall reduction in Av3 is primarily the result of cation coordination with the H2O ("3) molecules with a resultant diffusion of the cation charge density and thus of its polarizing power as reflected in the Av3 values. The new data support previous conclusions, based on residual cation effects in the pure H2O and NH3 matrices, that the M+N03- ion pair retains a contact character for all matrix compositions. Thus the ion pair data for glassy H20 and NH3 matrices can be used with considerable confidence to identify contact ion pair features in aqueous and ammonia liquid solution vibrational spectra of nitrates. The matrix data are consistent with recent interpretations of ammonia solution spectra but suggest a new approach to the existing aqueous solution results.
Introduction
The use of vibrational spectra to characterize ionic solutions by identifying the nature and relative amounts of species present and, in some instances, to yield insights regarding the structure of the particular species has been a practice for many years. With the ease and accuracy of laser Raman methods that have evolved in the last decade this practice has increased and been considerably refined.l Nevertheless, there are severe problems associated with such studies the most formidable being that, in general, there are several different but related species present in such systems and the individual species spectra and structures for the particular solution environment have not all been determined in independent studies. Thus, for example, if one considers an aqueous solution containing a moderate concentration (-5 M ) of LiN03 it is usually assumed that the nitrate spectrum is a composit of that of the completely solvated NOS- ion and the solvated contact ion pair (Li+N03-),la while a t higher concentrations the nitrate ion environment progressively approaches that of the molten salt. The spectra for the two extreme cases are accessible through studies of highly dilute sohtions2 and the molten salt,3 but independent data for the individual contact ion pairs have not been available. Recent matrix isolation studies that have confirmed that the dominant species in the vapors of many heated salt crystals and molten salts are the monomeric ion pairs, such as Li+N03-, have presented the possibility for such independent studies of the ion pairs. Thus, extensive data have been reported for nitrate, chlorate, and perchlorate alkali metal ion pairs in argon mat rice^.^-^ These data have emphasized the extreme degree of distortion that an anion ex-
periences when polarized by a contacting cation in an inert medium. Thus the 260-cm-l splitting of the nitrate v3 degenerate stretching mode translates into a bonding scheme wherein one N-0 bond has roughly one-third the strength of the other two N-0 bonds.4 This represents unique quantitative information about the nitrate ion for a well-defined, noncrystalline case. The relationship of this contact ion pair spectra for a solid argon medium to spectra for nitrate aqueous solutions was not immediately apparent, but that relationship has since been displayed by substituting water for argon as the matrix m e d i ~ mSince . ~ recent studies have shown that water deposited at 10 K forms a glassy film, with X-ray diffraction patterns similar to those for liquid water,8 it can be assumed that the ion-pair environment in the water matrices closely resembles that for the aqueous solutions and should thus permit identification of the contact ion-pair components of the spectra for aqueous solutions. Similar data and reasoning have been presented for ammonia solutions as well. The glassy water and ammonia data have both been marked by evidence for a dramatic reduction in the anion polarization by the contacting ~ a t i o n For . ~ example, the 170-cm-' v3 splitting for K+N03- (argon) is reduced to 20 cm-' in "3. Such reductions have been related to an effective neutralization (or diffusion) of the cation charge densities through donation of lone pair solvent electrons to the cation during the solvation process. However, the magnitude of the reduction of the anion distortion has raised the possibility that solvent molecules are actually inserting themselves between the ion-pair components as the matrices form. Arguments, such as the one based on the sizable cation effect still evident in the glassy water and ammonia matrices, have been presented to discount this possibility7 The Journal of Physical Chemistry, Vol. 79, No. 2 1, 1975
2266
Gary Ritzhaupt and J. Paul Deviin
but additional evidence was sought to confirm the contact nature of the HzO(NH3) matrix ion pairs before proceeding to apply the new data to the interpretation of solution spectra. If the reduction in anion distortion in a HzO(NH3) matrix occurs through diffusion of the cation charge, the stepwise removal of water (ammonia) with substitution of argon as solvent should result in a gradual increase in the anion distortion. However, the insertion of a solvent molecule between the cation and anion would seem to occur in a single step so that a sharp, rather than gradual variation in anion distortion would occur during the stepwise replacement of water (ammonia) molecules by argon atoms. Thus, following a suggestion by Hestery we have measured the vibrational spectra for Li+N03- and K+N03- deposited at high dilution in matrices that vary in composition from pure argon to pure water (ammonia). In this paper we describe the new data obtained from such mixed matrices and consider the implications for the interpretation of liquid solution spectra.
Experimental Section The volatilization of the metal nitrates was from the melts in resistance heated Pyrex glass Knudsen cells at temperatures in the 400-500°C range. The matrix gases, mixtures of argon with NH3 and H2O vapors, were metered through a Fisher-Porter YlG-in. glass flowmeter and cocondensed with the nitrate molecular beam within a standard low-temperature infrared cell fitted to an Air Products CS-202 closed-cycle helium refrigerator. Matrix to sample ratios were chosen to give complete isolation of the nitrate species by setting the Knudsen cell temperatures and gas flow rates at values previously found to give dominantly monomer ion-pair spectra in argon mat rice^.^,^ The substrate deposition temperature was 10 K which suggests that the water-rich films were glassy in naturea8 The infrared spectra were recorded using a Beckman IR-7 infrared spectrometer. Spectra were routinely scanned from 600 to 4000 cm-l to monitor the degree of isolation of the water (ammonia) in the argon a t the various waterargon compositions. However, the nitrate data were restricted to the 800- and 1 4 0 0 - ~ m -regions ~ corresponding to the NO3- v2 and v3 regions, respectively. Results and Discussion and U3b infrared curves The complete set of observed for matrix compositions ranging from pure argon to pure H20, or pure “3, are presented in Figures 1-4 for LiN03 and KN03. The top and bottom curves of each figure have previously been published but are presented here for comparison with the mixed matrix results. In addition to the large magnitude of Av3 (Av3 = usa - V3b) of 170 cm-’ for KNOBand 260 cm-l for LiN03, a noteworthy aspect of the pure argon matrix data is the corresponding values of 85 and -150 cm-1 for the weak features of dimeric (M+NO3-)2 species. As already noted the Av3 values for the pure glassy water and pure NH3 matrices (top curves Figures 1-4) are drastically reduced relative to the argon matrix data. The data for Li+N03- in the mixed HZO-argon matrices are most easily discussed since, after an initial relatively sharp reduction in the splitting (6% cases), the Av3 value decreases in a gradual uniform manner throughout the composition range. For example, Figure 1shows that in the range 0-6% H20the Au3 value for Li+N03- reduces, in esThe Journal of Physical Chemistry, Vol. 79, No. 21, 1975
Figure 1. Infrared bands for the u3 nitrate mode for Li+N03- ion pairs isolated in matrices with composition varying from pure argon to pure H20. The numbers indicate the percent water in the mixed water-argon matrices.
Z
0 v) u,
Z
dc
MOO
14 00
1900
cm-1 Flgure 2. Infrared bands for the v3 nitrate mode for K+NO3- ion pairs isolated in matrices with composition varying from pure argon to pure water. The numbers indicate the percent water in the mixed water-argon matrices.
sentially a single step, from 260 to 168 cm-l. As justified in the next paragraph, we relate this reduction to the coordination of one HzO molecule with each Li+ cation. Addition of further water to the matrix reduces Av3 to 115 cm-’ (12%), 90 cm-l (25%), 70 cm-l (50%),and finally 65 cm-l for the pure HzO case. The definite peaking of the v3 components at different values for the various percent compositions, plus the realization that a cation associated with
2267
Vibrational Spectra of M+N03- ion Pairs
25 I
3
3 I-
Figure 5. Infrared spectra of the v3 nitrate mode for Li+N03- ion pairs isolated in water-argon mixed matrices with low percent H20 content as indicated by the numbers.
mrl Figure 3. infrared bands for the v3 nitrate mode for Li+N03- ion pairs isolated in matrices with composition varying from pure argon
to pure NH3. The numbers indicate the percent ammonia in the mixed ammonia-argon matrices.
__
moo
~
-
-
1400
1300
Em-’
Flgure 4. Infrared bands for the v3 nitrate mode for K+N03- ion pairs isolated In matrices with compositlon varying from pure argon
to pure ammonia. The numbers indicate the percent ammonia in the mixed ammonia-argon matrices.
NOS- is unlikely to coordinate with more than four or five solvent molecules, tempts one to relate the Li(H20)2+. NO3- species to the 120-cm-’ splitting and the 90-cm-l splitting to the Li(H20)3+-N03- molecules. Unfortunately, the mobility of the water molecules in the fluid surface of the matrix as it forms permits the H2O molecules to seek
out the cation to a limited degree, and thus prevents any rigorous use of the known Hz0:argon ratios in assigning the Av3 values. For example, at 6% H2O the statistically most probable environment for the Li+ ion in a Li+N03- ion pair would be with “n” near neighbor argon ions (where “n” is the coordination number for Li+ reduced to account for the contact paired NOS- ion). However, the -100-cm-I reduction in Av3 a t this matrix composition very strongly suggests that, on the average, one H2O has entered the cation inner solvent shell. Figure 5 compares the pure argon Li+N03- spectrum to that for 3 and 6% H2O-argon matrices and thereby shows the growth of the singly hydrated ion pair bands to dominance. In the 3%case the components that dominate in the 6% H2O case (AVQ = 168 cm-I) are observable, but there are also (a) bands that are nearly superimposable on the pure argon matrix features (Avg = 260 cm-I) and (b) a pair of bands at 1290 and 1495 cm-l with only -50 cm-l reduction for Avg. The latter bands are believed to reflect the influence of outer shell coordinated water molecules but could result from water-NO3- interaction a t the anion end of the Li+N03- contact ion pairs or from different matrix sites such as are responsible for multiplet structure in the pure argon matrices. An equivalent discussion could be given for the KfN03species in “3-argon mixed matrices based on the data of Figure 4. The major point regarding both of these cases is that the behavior of the Av3 splitting is precisely what one would predict provided the cation-anion contact is retained throughout: namely, the first associating solvent molecule to enter the cation inner coordination sphere has a sizeable effect on the cation polarizing power as reflected in a sharp drop in Av3. Subsequent inner shell solvent molecules have a progressively smaller effect so the Av3 value tends to approach the pure HzO (or “3) value in a gradual monotonic manner. This behavior is demonstrated in the plots of Av3 vs. percent HzO(NH3) in Figure 6. The spectra of Figure 2, the data of Table I, and the corresponding plots of Figure 6 suggest, at first glance, a considerably different behavior for K+N03- in H2O. A naive consideration of the data might prompt the conclusion that the Av3 value produced by anion distortion by the K+ ion is The Journal of Physical Chemistry, Vol. 79, No. 27, 1975
Gary Ritzhaupt and J. Paul Devlin
2268
TABLE I: Frequencies in cm-I for the Dominant v 3 Band Components, as well as u2 and Au3 Values, for Li+NOa- and K+N03- Ion Pairs Isolated in Mixed Argon-Water Matrices 6%
12%
25%
50%
100%
Argon
H20
H,O
H2O
H@
HzO
817 1264 1524 260
820 1302 1470 168
823 1342 1432 90
1350 1420 70
827 1347 1412 65
830 1291 1462 171
826 1337 1416 79
1347 1401 54
1350 1400 50
1348 1498 50
LiNO, v2
vSa
vQb Av,
820 1335 1450 115 KNO,
v2
vga vQb Av,
Figure 6. Plots of the splitting, Lug, in cm-I for the dominant infrared features as a function of matrix composition.
reduced in two large steps from 170 cm-' to the pure H2O matrix value of -50 ern-'. This view, however, ignores the fact that the Nos- ion, in dilute aqueous solutions, has a Av3 value of -50 cm-l such that, regardless of the magnitude of the cation distortion of the anion, the u3 splitting in a pure glassy HzO matrix cannot be reduced below 50 cm-l. With this in mind, it becomes reasonable to propose a much closer analogy for the solvation of the K+N03- in "3-argon and K+N03- in mixed HzO-argon matrices than is suggested by comparison of Figures 2 and 4 alone. That is, in the absence of the H20-NO3- interaction splitting of 50 cm-', it is easy to imagine that the series of Au3 values for K+N03- in HaO-argon mixed matrices would go as 79 cm-' (6%), -60 cm-' (12%), -40 cm-l (25%), -30 cm-' (50%),and -20 cm-l (100%) as for the "3-argon case. It is clear from a comparison of Figures 2 and 4, as well as the plots of Figure 6, that the first two Hz0 molecules to enter the K+ inner shell have nearly the identical effect on Au3 as do the first two NH3 molecules. At higher HzO(NH3) ratios spectral differences arise primarily because the Hz0 distortion of the NO3- ion prevents Au3 from dropping below 50 cm-'. Thus the difference between Figures 2 and 4 arise from the much larger distortion of NO3- (Au = 50 cm-l) by H20 than by NH3 (A4 5 20 cm-l), rather than any strong difference in the ability of NH3 and HzO to diffuse the cation charge. The foregoing is affirmed to a large extent by comparison of Figures 1 and 3 and Tables I and 11. The Li+N03- in NH3-argon matrix u3 spectra (Figure 3) compare closely to the corresponding BzO-argon cases. Since the limiting pure H20 and pure NH3 Av3 valhes, 65 and 47 cm-', are both significantly greater than for the K+N03- cases, it is obvious that the Au3 values for Li+N03- for all matrix compositions are determined primarily by the lithium distortion of the anion. For these cases, where the direct solvent-anion distortion is of secondary importance, the HzO and NH3 coordination with Li+ are seen to have basically the same effect on cation charge density, just as presumed for the K+N03- discussion in the preceding paragraph. The behavior of the vz mode has been found to be predictable, and of little interest throughout these studies but The Journal of PhysicalChem/stry,Voi. 79, No. 21, 1975
828 1345 1405 60
TABLE 11:Frequencies in cm-I for the Dominant u3 Band Components, as well as u2 and A YValues, ~ for Li+NO3- and K+N03- Iop Pairs Isolated in Mixed Argon-Ammonia Matrices
LiNO,
v2 uSa
vSb Au,
817 1264 1524 260
826 1300 1478 178
830 1291 1462 171
830 1335 1412
827" 827 (1335)O 1338 (1450)" 1405 (125)" 67
828 1343 1399 56
828 1345 1392 47
830 1347 1374 27
830 1350 1370 20
KNO,
v2 uga
uQb Au3 a
77
830 1345 1400 55
830 1346 1380 34
Estimated from 8% ND3 in argon spectrum.
frequencies are tabulated for completeness in Tables I and 11. Thus v2 retains a value of 830 cm-l for all K+N03- in HzO-argon mixtures while shifting slowly from the pure argon matrix value of 817 cm-' to the pure H2O matrix value of 827 for Li+NOs-. The Li+N03- value similarly shifts from 817 to 828 cm-' through the series pure argon to pure "3. Conclusions The major conclusion of this study has already been indicat,ed namely, that the data are consistent with and confirm the view that the contact ion pairs, M+N03-, are the dominant species regardless of matrix composition and that the reduction previously reported for the Au3 value, in changing from an inert to associating matrices, is primarily the result of decreased cation polarizing powers produced by coordination of the associating matrix molecules with the cation. However, with these facts established there are several additional significant conclusions to be drawn. For example it is clear that the effect of a HzO molecule in the cation inner coordination shell on the cation polarizing power is quite similar to that of an NH3 molecule. Further, a comparison of the (H20)M+N03- u3 values with those noted earlier for the dimers, (M+N03-)2, suggests that a
Vibrational Spectra
of M'N03- Ion Pairs
second NO3- ion in the M+ coordination shell also has a similar attenuating effect on the M+ polarizing power. Thus the Av3 values for the three cases, where M+ is K+, are seen t,o be 79 (HzO), 77 (NH3), and 85 (NO3-) whereas for M+ = Li+ the Av3 values are 168 (HzO), 178 (NH3), and -150 (NO3-). In view of the observed effect on the M+ polarizing power from additional inner shell HzO(NH3) molecules, an extension of this comparison to molten or glassy LiN03, where each Li+ is coordinated to several NO3species, makes it difficult to accept the Av3 value for that case (110 cm-I)lo as resulting from distortion of the anion by the cation alone. In fact, dilution of molten LiN03 with LiC103 has shown that only -70 cm-l of that splitting is from anion distortion,1° a value that is close to the Av3 values for the Li+N03- ion pair in pure glassy HzO and "3. Thus, a consistent picture is beginning to emerge. A similar dilution study for glassy KN03 in KC103, via codeposition methods, indicates that Av3 for K+ polarization is 33 cm-l, also reasonably close to the value for K+N03- in pure glassy NH3ll (but not in pure HzO where the H-bond solvent effect is dominant). The principal conclusion, cited above, also strongly suggests that published vsa and V3b values for Li+N03- contact ion pairs in aqueous solutions (1389 and 1464 cm-l),la as estimated by curve resolution techniques, are in serious error. This observation in turn challenges the validity of the procedure (i.e., curve resolution) by which these values were estimated. The 1464-cm-l value is 42 cm-l above the Li+N03- value of 1412 cm-l for the glassy HzO matrix. This difference seems to be too great to be attributed to any basic differences between the glassy Hz0 and liquid HzO environments, particularly considering that the corresponding band positions for NH3 solutions, determined without the need for curve resolution, agree within 5 cm-l (1392 cm-l from Teble I1 and -1396 cm-l from ref l b and IC). It seems apparent that curve resolution is not a viable procedure where cooperative effects contribute significantly to a spectrum as is apparently the case for intensely infrared active modes of molten salts and concentrated salt solutions.le,l0 On the other hand, the vsa and V3b values reported for LiN03 in NH3 for the 6:l mole ratio concentration range have been correctly assigned to the contact ion pair.lbPc Reduction of this splitting to values less than 50 cm-l (the glassy NH3 matrix ion pair value) for more dilute solutions apparently is the result of overlap of the ion pair doublet with the v3 band of the completely solvated NO3-. On the other hand, reduction of the "3-nitrate mole ratio below 4 1 causes Av3 to approach the 110-cm-l molten salt value as cooperative effects based on dipole-dipole coupling take over.lbJ0 As indicated, we agree with the basic NH3 solution assignments of Lemly and Lagowskilc but one relevant inconsistency in their arguments should be noted. They report that for a 16:l LiN03 mole ratio Av3 is 33 cm-', which thus becomes an upper limit on the NO3distortion splitting via H bonding with "3. However, they subsequently assign a 49-cm-l splitting for concentrated NH4N03 in NH3 to just such a distortion splitting; a clear
2269
inconsistency. Our results show that, in fact, the NH3 Hbonding distortion splitting must be less than or equal to 20 cm-l, the K+N03- in glassy NH3 value. We have also observed Av3 for NDd+N03- contact ion pairs in pure glassy ND3 and found Av3 to be 23 cm-l. The origin of the 49cm-l splitting for NH4N03 in NH31c may be a combination of distortion splitting of the NO3- by -23 cm-l (via either NH4+ or NH3 association, or both) together with dipoledipole coupling. Other conclusions suggested by the new data include one based on the recognition that Av3 produced by cation distortion for K+NO3- ion pairs in an HzO solution is less than the Av3 splitting resulting from NO3- association with H20 molecules. Thus, the Av3 value is the same for contact ion paired and solvent separated NO3- ions so that the v3 spectra are not distinguishable. Although, the emphasis of this paper has been on cation-anion interaction, clearly the matrix-anion interactions are large and must not be ignored. This situation is further emphasized by the observation that NO3- is dynamically, as well as statically, coupled to the matrixes. Thus, although it has not been pointed out in our presentation of results, the absolute values of v3, and V3b increase -5 cm-l when DzO (ND3) is substituted for H20 ("3). Since the direction of this shift is opposite to that expected on a mass basis, it is likely a result of resonant interaction with the nearby matrix molecule bending modes which are above v3 for the protonated cases but below v3 for the deuterated solvents. Finally, from this study vibrational frequencies are now available for the species (HzO)M+N03and (NH3)M+N03- (Le., 6% columns of Tables I and 11).Such data are of particular interest since they provide a potential test of ab initio quantum chemical calculations such as have recently been reported for hydrated ions.12 Acknowledgment. This research has been supported by the National Science Foundation under Grant No. MPS 74-17689.
References and Notes See, for example: (a) D. E. Irish, D. L. Nelson, and M. H. Brooker, J. Chem. Phys., 54, 654 (1971); (b) D. J. Gardiner. R. E. Hester, and W. E. L. Grossman, ibM., 59, 175 (1973); (c) A. T. Lemly and J. J. Lagowski, J. Phys. Chem., 78, 708 (1974); (d) J. D. Rlddell. D. J. Lockwood. and D. E. Irish, Can. J. Chem.. 50, 2951 (1972): and (e) D. J. Gardiner. R. B. Girling, and R. E. Hester, J. Phys. Chem.. 77, 640 (1973). D. E. Irish and A. R. Davis, Can. J. Chem., 48, 943 (1968). J. P. Devlin, P. C. Li, and G. Pollard, J. Chem. Phys., 52, 2267 (1970). D. Smith, D. W. James, and J. P. Devlin, J. Chem. Phys., 54, 4437 (1971). N. Smyrl and J. P. Devlin, J. Chem. Phys.. 80, 2540 (1974). G. Ritzhaupt and d. P. Devlin. J. Chem. Phys., in press. N. Smyrl and J. P. Devlin, J. Phys. Chem.. 77, 3067 (1973). C. G. Venkatesh, S. A. Rice, and A. H. Narten, Science, 188, 928 (1974). R. E. Hester. private communication. J. P. Devlin, 0. Ritzhaupt, and T. Hudson, J. Chem. Phys., 58, 817 (1973). G. Ritzhaupt and J. P. Devlin, unpublisheddata. H. Kistenmacher, H. Popkie, and E. Clemente, J. Chem. Phys., 59, 5842 (1973).
The Journalof Physical Chemistry, Vol. 79, No. 21, 1975
