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The Journal of Physical Chemistry, Voi. 83, No. 25, 1979
Koehler et al.
(6) W. E. Wallace in "Hydrides for Energy Storage", A. F. Andersen and A. J. Maeland, Ed., Pergamon Press, New York, 1978, p 33. (7) A. Elattar, W. E. Wallace, and R. S. Craig, Adv. Chem. Ser., No. 176, 7 (1979). (8) V. T. Coon, Ph.D. Thesis, University of Pittsburgh, Aug 1977. (9) C. A. Luengo, A. L. Cabrera, H. 8. MacKay, and M. 9.Maple, J. Catai., 47, 1 (1977).
(10) G. 9.Atkinson and L. J. Nicks, J . Catal., 46, 417 (1977). (1 1) M. A. Vannice, J. Catal., 37, 449, 462 (1975); 40, 129 (1975); 44, 152 (1976); 50, 228 (1977); 58, 236 (1979). (12) H. Imamura and W. E. Wallace, J . Phys. Chem., 83, 2009 (1979). (13) F. M. Nelsen and F. T. Eggertsen, Anal. Chem., 30, 1387 (1958). (14) H. L. Gruber, Anal. Chem., 34, 1828 (1962).
Solvent and Phase Dependence of Fermi Resonance In Ammonia Wllllam H. Koehler,' James W. Lundeen, Ahmad Moradl-Araghl, Berth de Bettlgnles, Llnda D. Schultr, Department of Chemistty, Texas Christian University, Fort Worth, Texas 76 129
and Martin Schwartz Department of Chemlstry, North Texas State University, Denton, Texas 76203 (Received May 25, 1979) Publication costs assisted by the Robert A. Welch Foundation
The 3300-cm-' region of the Raman spectrum of ammonia in the solvents benzene, carbon tetrachloride, and pentane and in the vapor phase has been analyzed by using the damped coupled oscillator model for Fermi resonance. The experimental spectra in both phases were fit quite well by using only four adjustable parameters in the model. It was found that the Fermi resonance interaction constant, the uncoupled frequencies, and the line widths varied smoothly between the values of the neat liquid and the gas phase upon dilution, demonstrating the same trends in all three solvents studied. These results indicate that ammonia-ammonia interactions are absent at sufficiently low concentration and, further, that there appears to be little, if any, residual hydrogen bonding of ammonia in any of the systems studied.
Introduction A number of investigations of the Raman spectrum of liquid ammonia have been performed with the goal of elucidation of the hydrogen-bonding and other structural properties of this complex Analysis of the N-H stretching region of the spectrum (3300 cm-l) is complicated by the presence of a third, intense peak. While it i s generally accepted that this extra peak arises from the overtone band 2v4, enhanced by Fermi resonance with vl, the symmetric stretching mode, there has been considerable controversy over which of the two polarized bands should be assigned to the fundamental vibration and which to the overtone. Most recently, the damped coupled oscillator model, adapted by Schwartz and Wang6 for the analysis of Fermi resonance, was utilized by them and by Lundeen and Koehler5 to successfully interpret the temperature dependence observed in the N-H stretching region of the liquid ammonia spectrum. The results of these studies appear to offer a statisfactory resolution of the assignment problem of the vibrational modes. However, an alternative model, assuming two, spectroscopically distinct, molecular species and invoking the existence of an additional peak in the region between the two observed bands, has also been used to interpret the spectrum in the neat liquid7and, more recently, in solutions of ammonia in carbon tetrachlorides and benzene-d6.' In an effort to gain a better understanding of the hydrogen-bonding interactions of this molecule in solution, we have applied the coupled oscillator model to the spectrum of ammonia as a function of concentration in the solvents carbon tetrachloride, benzene, and pentane. The Kaman spectrum of the vapor phase has also been acquired and analyzed, and these results are compared to those obtained in solution. OO22-3654/79/2083-3264$01 .OO/O
Experimental Section Spectral-quality benzene (Matheson Coleman and Bell) and spectral-quality pentane (MCB) were dried with sodium wire and stored over molecular sieves. Spectral-grade carbon tetrachloride (MCB) was dried over molecular sieves. All solvents were degassed via repetitive freezeevacuate-thaw cycles. Anhydrous ammonia (Matheson, 99.99%) was twice distilled from sodium metal. All solution samples were prepared by distilling the appropriate solvent into a calibrated tube and measuring the volume at room temperature. The solvent was then transferred into the Raman cell (g-mm, medium-wall tubing) by distillation. Ammonia was measured by using a calibrated gas bulb and a manometer. The ammonia gas was quantitatively condensed into the Raman cell. Samples were sealed off under vacuum and examined for phase separation upon warming to room temperature. Mole fractions were calculated by using the volume and density of the solvent and the ideal gas law for ammonia. Gas phase spectra (ca. 12-atm pressure) were obtained for ammonia vapor in equilibrium with liquid ammonia in a sealed Raman cell at room temperature. Isotropic spectra of solid ammonia could not be obtained due to the scrambling of polarization of the spectrum in this phase. Raman spectra were obtained by using an instrument which has been described previously.6 The instrument was operated under computer control in a mode in which there is no time constant distortion. All spectra were deconvoluted to remove the effects of slit distortion. The isotropic Raman spectrum was calculated by the relation
at each frequency. The spectra were analyzed with a Fortran language nonlinear least-squares regression pro@ 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 25, 1979 3265
Fermi Resonance in Ammonia
l-oo
1
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J Flgwe 1. Resolution of the isotropic Raman spectrum of ammonia vapor at 12 atm and 22 O C . The observed and calculated spectra and the residual (observed minus calculated) are shown. The intensity scale is normalized.
Figure 3. Resolution of the isotropic Raman spectrum of 7.7 mol % ammonia in carbon te!rachloride at 22 O C .
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Flgure 2. Resolution of the isotropic Raman spectrum of liquid ammonia at 22 OC.
gram employing the corrected5 damped coupled oscillator model on an IBM-360/50 computer. The mathematical expression for the isotropic intensity due to two coupled vibrational modes has been presented previously5v6and will not be repeated here. The parameters in the model are the unperturbed peak frequencies (Q1and Q,) and line widths (PI and r2),the resonance interaction constant b, and the relative scattering strengths of the two modes (C, and C2). However, it has been shown5g6that Q1 and Q, may be calculated directly from the value of b and the experimentally observed band maxima (wl and w2). Furthermore, it is an excellent approximation to assume that the uncoupled intensity of the overtone in the Raman spectrum is negligible (i.e., C2 = 0). Therefore, it is necessary to vary only the four parameters rl, rZ,b, and C1 to apply the coupled oscillator model to the observed spectra.
.I9
3150
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Figure 4. Resolution of the isotropic Raman spectrum of 28.6 mol % ammonia in benzene at 22 O C . 1.00
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Results The isotropic Raman spectrum of ammonia in the vapor phase at a pressure of 12 atm is shown in Figure 1. This spectrum may be compared to the Raman spectrum of pure liquid ammonia (Figure 2). It may be noted that, in contrast to the liquid, where both polarized peaks are comparable in intensity, the lower frequency band is far weaker in the gas phase, indicating that it is due to the overtone 2u4. The figures show superimposedobserved and calculated isotropic Raman spectra, and the residual intensities are shown at the bottom of the figures. The low intensities of these residual curves indicate that the damped coupled oscillator model fits the observed bands quite well. The parameters obtained from these fits are listed in Table I. A comparison of the calculated uncoupled frequencies listed in Table I reveals that the value of Q1 for the vapor is far higher (by -60 cm-l) than that of pure liquid am-
3250 3350 WAVENUMB E R S
3250
3350
WAVENUMBE RS
+---+b-
Flgure 5. Resolution of the isotropic Raman spectrum of 26.8 mol % ammonia in pentane at 22 OC.
monia, whereas Q2 is shifted in the opposite direction. The line width r2also has a much lower value in the vapor phase. Finally, it should be noted that the Fermi resonance coupling parameter b, indicating the degree of anharmonicity, is significantly diminished in the gas phase compared to the value obtained in the neat liquid. Representative plots of the calculated and observed isotropic spectra of ammonia in the solvents carbon tetrachloride, benzene, and pentane are shown in Figures 3-5, respectively. Interference from vibrational modes below 3200 cm-l in benzene solutions was eliminated via a curve-fit subtraction procedure. The limited solubility of
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The Journal of Physical Chemistry, Vol. 83, No. 25, 1979
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TABLE I : Best Fit Parameters for Ammonia in Solution Using the Damped Coupled Oscillator Model solvent NH, vapor (12 atm) CCI,
CJ12
NH, liquid
r,,
rz,
lo-",
mol%of "3
cm-'
cm-'
cm-'
w , , cm-I
4.3 7.7 13.4 21.5 31.1 56.5 75.9 10.0 17.2 28.6 46.5 59.0 79.6 14.2 20.6 26.8 30.8 100.0
7.0 7.8 8.1 8.7 6.5 8.4 13.6 17.9 8.9 9.0 9.6 11.2 11.7 13.7 9.0 7.0 9.3 8.7 18.6
11.6 9.7 12.6 17.8 29.0 28.6 32.4 33.5 10.8 14.6 17.2 21.7 25.4 31.8 18.5 20.7 24.1 26.7 42.0
2.29 2.25 2.40 2.59 2.79 2.79 2.84 2.76 2.38 2.54 2.64 2.65 2.75 2.77 2.49 2.64 2.66 2.71 2.77
3334.3 3310.6 3309.9 3306.8 3305.7 3304.2 3301.0 3297.6 3302.0 3300.7 3299.5 3297.4 3299.6 3298.0 3307.9 3308.0 3307.7 3307.8 3295.3
ammonia in pentane prohibited measurements above approximately 30 mol % concentration. Restriction of spectral interpretation to the isotropic intensities (required by the coupled oscillator model), in addition to eliminating interference in this region from the depolarized u3 mode, removes all reorientational contributions to the line widths. This latter breadth may be respondible for the residual intensities between the two polarized peaks for spectra consisting of the sum of polarized and depolarized scattering components and attributed to the presence of an additional banda7 Examination of the figures shows that a very satisfactory fit of the observed spectra of ammonia in solution, as well as in the neat l i q ~ i dis, ~obtained ~~ by the use of the coupled oscillator model without inclusion of an extra band. The best fit parameters for the various concentrations of ammonia in the three solvents investigated are included in Table I. The smooth variation of these parameters with concentration, coupled with the good fits to the experimental spectra (as indicated by the low intensities of the residual curves below each figure), indicates that the coupled oscillator model provides a consistent description of Fermi resonance in ammonia. It may be seen from the table that, as reported in earlier s t ~ d i e s ,there ~ , ~ is an observable, although very slight, shift in the observed peak frequencies w1 and w2 with concentration in carbon tetrachloride and in benzene. flowever, it is in the uncoupled frequencies Ql and Q2 that a far more pronounced dependence upon solution composition is observed. The line widths rl and r2are also noted to increase in value with increasing concentration. This effect is particularly striking in the case of r2,the line width of the overtone band. Finally, inspection of Table I reveals that, in all three solvents studied, the magnitude of b, the Fermi resonance coupling parameter, increases smoothly from the gas-phase value and, with increasing ammonia concentration, asymptotically approaches the value obtained for the neat liquid. Discussion In an investigation of gaseous ammonia utilizing submillimeter wave spectroscopy,1°it has been estimated that only a small fraction of the molecules exist as dimers at moderate pressures. Therefore, it may be reasonably assumed that the gas phase spectra obtained herein repre-
w,,
cm-'
3221.6 3205.4 3205.1 3204.9 3204.4 3205.5 3207.0 3208.0 3204.8 3203.3 3203.2 3203.0 3206.4 3207.4 3202.4 3202.2 3205.0 3205.5 3208.1
a , , cm-'
a,, cm-'
3322.4
3233.9
3297.8 3294.9 3288.1 3282.3 3279.5 3271.9 3267.5 3285.4 3281.4 3277.4 3274.1 3272.8 3268.6 3291.7 3289.3 3287.9 3286.6 3261.1
3218.5 3220.5 3223.6 3228.4 3230.7 3236.7 3238.6 3221.7 3223.1 3225.8 3226.8 3233.8 3237.5 3219.0 3221.4 3225.3 3227.3 3242.8
sent, almost exclusively, scattering from isolated, nonbonded ammonia molecules. The observed decrease in Ql with the concomitant increase in Q2 upon condensation to the neat liquid is the trend anticipated for stretching and bending vibrations, respectively, upon hydrogen-bond formation. These results are consistent with assignment of the higher frequency peak observed in the room-temperature Raman spectrum of liquid ammonia to u l , the stretching f ~ n d a m e n t a l . ~ The increase in b, the interaction parameter, upon liquifaction is of particular interest. This is an indication that the vibrational anharmonicity in ammonia is enhanced by hydrogen-bond formation and verifies the results reported earlier from this laboratory5 for the neat liquid, in which the value of b was observed to increase at lower temperatures. Three types of interactions are possible in the liquid mixtures investigated herein: solvent-solvent, solutesolvent (ammonia-solvent), and solute-solute (ammoniaammonia). Earlier reports8v9suggest that no significant changes occur in the spectrum of either benzene or carbon tetrachloride upon admixture with ammonia. Therefore, it may be reasonably concluded that solvent-solvent interactions do not vary significantly in these systems. Accordingly, it was not deemed necessary to monitor solvent-solvent interactions in pentane solutions. As the relative concentration of ammonia in a solution is lowered, the strong hydrogen bonds present in the neat liquid would be expected to be replaced by interactions (if present) with the various solvents. While such interactions are anticipated to be weak in the ammonia-pentane system, as evidenced by the relatively low solubility of ammonia in this solvent, there has been some evidence presented for an NH3-CCl4 interaction through the lonepair electrons on chlorine,ll and benzene may be expected to interact with ammonia through the P system.12 HOWever, as discussed below, such interactions appear to be relatively weak or nonexistent in binary systems of ammonia. First (vide supra), it is observed that the magnitude of the coupling constant b decreases smoothly in dilute ammonia solution toward the value obtained in the gas phase. Indeed, as shown in Figure 6, the concentration dependence in all three solvents may be reasonably represented by a single curve. In contrast, any significant degree of hydrogen bonding with the solvent should be manifested
The Journal of Physical Chemistty, Uol. 83,
Fermi Resonance in Ammonia
No. 25, 1979 3207
7 7 5.00 40'0
0 X
1
2,401 2.20
A
2.00
I
I
1
0.0 20.0 40.0 60.0 80.0 100.0
% "3
Flgure 0. Variation of the coupling constant b with ammonia concentration for the solutions: (@) carbon tetrachloride, (A)benzene, (0) pentane, (B) liquid ammonia, (0)ammonia'vapor at 12 atm.
L
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J
200 400 600 800 1000
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Flgure 8. Variation of line widths and I?, with ammonia COnCentratii for the solutions: (@ and 0 )carbon tetrachloride, (A and A)benzene, (0and I)pentane (m and @) liquid ammonia, (0)ammonia vapor at 12 atm, respectively. 'E
expected to be more sensitive to increases in the vibrational anharmonicity,13which is one of the principal causes of line broadening in hydrogen-bonded systems.
I
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320000 2 0 0 4 0 0 600 800 1000 % NHJ
Figure 7. Variation of the uncoupled frequencies 0 , and 0, with ammonia concentration for the solutions: (0 and 0 ) carbon tetrachloride, (A and A) benzene, (0and). pentane, (B and @) liquid ammonia, (0)ammonia vapor at 12 atm, respectively.
by an extrapolated value for the coupling constant greater than that obtained for the isolated ammonia molecule. The results obtained for the uncoupled frequencies and line widths also indicate a monotonically decreasing interaction at lower ammonia concentrations. Figure 7 shows that O1 increases steadily, with a decrease in Rz, as the ammonia concentration is lowered. This observation is consistent with a gradual breakup of hydrogen-bonded structures. As noted in the above section, variation of the uncoupled frequencies is significantly greater than that observed for the apparent peak maxima (wl and wz). The relative insensitivity of the observed peak positions is easily explained by the fact that, as the hydrogen bonding is increased, the uncoupled frequencies of the stretching and bending modes approach each other (Figure 7). However, this is counteracted by an increased repulsion between the two frequencies due to increased Fermi resonance, as indicated by the concurrent increase in the calculated values of b, resulting in observed peak frequencies which are almost concentration independent. Finally, as shown in Figure 8, both vibrational line widths rl and rz show an increase in value from that of the gas phase with increasing mole fraction of ammonia. This trend is more evident in the case of rZ,increasing by as much as a factor of 3 over the concentration range studied. A possible explanation of this observation is the fact that the band parameters due to the overtone are
Summary and Conclusions In this investigation, the damped coupled oscillator model was employed to analyze the Fermi resonance region of the Raman spectrum of ammonia in the gas phase and as a function of concentration in the solvents carbon tetrachloride, benzene, and pentane. It was found that the model, utilizing only four adjustable parameters, auccessfully fit the experimentalisotropic spectra in all phases, without the necessity of introducing an additional band in this region. The value of the Fermi resonance coupling parameter b was lower in the vapor phase than in the liquid, indicating that vibrational anharmonicity is diminished in the absence of hydrogen bonding. The resonance interaction was also observed to decrease smoothly toward the gasphase value at lower concentrations in all of the solutions studied, indicating that the hydrogen-bonded structures present in the neat liquid are broken apart upon dilution and that any residual ammonia-solvent interactions are quite weak. The uncoupled frequencies and line widths showed the same trends in solution. The uncoupled frequencies, in particular, displayed a significantly greater variation with concentration than did the observed peak maxima positions, which remained almost constant. In conclusion, it is believed that Fermi resonance, which has long been considered a complicating factor in the interpretation of the vibrational spectrum of ammonia and many other systems, may be treated successfully with the damped coupled oscillator model. When such an analysis is performed, the nonperturbed band line widths and peak maxima may be obtained, in addition to the values of the resonance coupling parameter, which are important added pieces of information that may be used to probe the nature of interactions in solution. Acknowledgment. The investigators at Texas Christian University gratefully acknowledge the financial support of the Robert A. Welch Foundation. The investigator a t North Texas State University acknowledges the assistance
3268
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83,No. 25, 1979
of the North Texas State University Faculty Research Fund. References a n d Notes (1) C. A. Plint, R. M. B. Small, and H.L. Welsh, Can. J. Phys ., 32, 653 (1954). (2) T. Birchall and I.Drummond, J . Chem. SOC. A , 1859 (1970). (3) D. J. Gardiner, R. E. Hester, and W. E. L. Grossman, J. Chem. Phys., 59, 175 (1973). (4) J. H. Roberts, A. T. Lemley, and J. J. Lagowski, Spectrosc. Lett., 5, 271 (1972).
Irish et al. (5) J. W. Lundeen and W. H.Koehler, J. Phys. Chem., 79,2957 (1975). (6) M. Schwartz and C. H. Wang, J. Chem. Phys., 5Q, 5258 (1973). (7) A. T. Lemley, J. H. Roberts, K. R. Plowman, and J. J. Lagowski, J . Phys. Chem., 77, 2185 (1973). (8) D. J. Gardiner, R. E. Hester, and W. E. L. Grossman, J. Raman Spectrosc., 1, 87 (1973). (9) J. H. Roberts and B. de Bettignies, J. Phys. Chem., 78, 2106 (1974). (IO) G. G. Simmestad, G. W. F. Pardoe, and H. A. Gebble, J . Ouant. Spectrosc. Radiat. Transfer, 12, 559 (1972). (11) P. Datta and G. M. Barrow, J. Am. Chem. SOC.,87, 3053 (1965). (12) W. B. Smlth and A. M. Ihrig, J. Phys. Chem., 75, 497 (1971). (13) I. L. Babich, I. I.Kondilenko, P. A. Korotkov, and V. E. Pogorelov, Opt. Specfrosc. (USSR), 29, 537 (1970).
Raman, Infrared, and Ultrasonic Relaxation Studies of Some Sodium and Lithium Salts in Dimethylacetamide' D. E. Irish," S.-Y. Tang, Guelph-Waterloo Centre for Graduate Work in Chemistry, Waterloo Campus, University of waterloo, Waterloo, Ontario, Canada N2L 3 0 1
H. Talts, and S. Petruccl Department of Chemistry, Polytechnic Institute of New York, Brooklyn, New York 11201 (Received April 26, 1979) Publication costs assisted by the Natural Sciences and Engineering Research Council Canada
Raman spectra, some infrared spectra, and ultrasonic relaxation spectra of solutions of LiSCN, NaSCN, LiN03, and NaN03in N,N-dimethylacetamide (DMA) have been obtained and correlated. Vibrational bands of free and ion-paired anions have been identified. Relative integrated intensities have been measured and used to obtain stability constants for the principal equilibria. Ultrasonic relaxation spectra, in the frequency range 3-350 MHz, show a single Debye relaxation. These kinetic data have been interpreted in terms of the Eigen theory, utilizing the conclusions from the Raman study where applicable.
Introduction Raman and infrared spectral studies of electrolyte solutions provide structural information; evidence for contact ion-pair formation, the populations of species in the major equilibria, and the values of contact ion-pair formation constants are frequently 0btainable.~8Ultrasonic relaxation techniques provide information about the kinetics of the formation of the ~pecies.~ Vibrational spectra provide information about species with a lifetime as short as s and ultrasonic methods are currently used for transformations relaxing in a time scale range of lo4 to lo4 s. The motivation for the present study was to assess the possibility of directly correlating and unifying the conclusions from these two lines of experimentation. By making measurements in comparable concentration ranges we believed that we might obtain direct confirmation of the species invoked in the mechanism and that the data might be coupled with advantage. To these authors' knowledge such a direct correlation has never been attempted before. N,N-Dimethylacetamide (DMA) has a dielectric constant of 37.8 a t 25 "C, a dipole moment of 3.81 D, and a wide liquid range;5 the solubility of many electrolytes is high in this solvent. We have chosen for examination of the ion-ion interactions the salts NaSCN, LiSCN, NaN03, and LiN03in DMA. Ultrasonic results are also presented for KSCN and NaN02. In order to establish the molar intensity of the "free" (i.e., solvated) SCN- ion, we have prepared solutions of ammonium thiocyanate containing tetrabutylammonium perchlorate and measured the Ra0022-365417912083-3268501.OOlO
man intensity of the v(C-N) stretching mode. Experimental Section N,N-Dimethylacetamide (Aldrich Chemicals, 99+ %) was redistilled twice in vacuo in all-glass apparatus with no grease in the ground joints. The column was a 3 f t Vigreux line. LiN03 (Fisher Certified), NaSCN (Baker Analyzed), KSCN (Baker Analyzed), NaN03 (Fisher Certified), NH4SCN (Fisher Certified), NaN02 (Fisher Certified), and NH4N03(Baker Analyzed) were dried to constant weight in an oven at 120 "C and atmospheric pressure. LiSCN.xHzO (Alfa Inorganics) was dried in vacuo at room temperature to the approximate composition LiSCN.H,O; it was then converted into the anhydrous form with constant weight by slowly raising the temperature over a several-day period to 110 "C. Shortening the process led to decomposition of the salt and the appearance of a yellow-green color. NaC104-H20and LiC104 (G. F. Smith Reagent) were dried in vacuo a t 60-70 "C. Anhydrous tetrabutylammonium perchlorate and NaC104 (G. F. Smith Reagent) were employed without further purification. Solutions were prepared by weight, filtered through Millipore FHL PO 13OO,0.5-pm filters directly into Pyrex glass capillaries, and sealed for Raman study. Spectra were excited with the 514.5-nm line of a Spectra-Physics argon ion laser operating at 0.8 W and were recorded on a Jarrell-Ash Raman system.6 Solutions contained both the electrolyte of interest and a second salt to provide an in@ 1979 American Chemical Society
