T. R. White and W.
2942
U. SCHINDEWOLF. Yes; but of course it needs more corrections because, e.g., the effect of temperature is much larger than the thermal expansion would indicate. Our CNOE corrections are only the zeroth order approximation. J. BELLONI.How were the solvated electrons formed?
P. KREBS. The solvated electrons were formed by uv photolysis of added methyl alcohol and potassium hydroxide which also act as scavengers for unwanted reactive photolysis products. So we actually made use of the efficient technique introduced by Mme. Belloni some years ago.
M. J. SIENKO.It would be interesting to look a t +2 salts and find structure in the liquid.
U. SCHINDEWOLF.So far we studied only alkali metal halides. Nitrites, nitrates, sulfates, and others are being reduced at the cathode, so no electrons are injected. Studies with alkali earth salts will be taken up. Discussion on paper by J. B. Weinstein and R. F. Firestone, J . Phys. Chern., 79,1322 (1975).
M. SILVER.Your conclusion regarding the effect of preexisting sites and the correlation is not surprising. When one considers so simple a system as helium fluid, near where the localized electronic state as a function of density is becoming stable, the mobility of the electron is completely determined by preexisting density fluctuations. In this critical region, the mobility due to these preexisting “traps” can drop to 1/10 to 1/100 with only a small change in
S.Glaunsinger
density. Above the critical density, after trapping in the preexisting traps, one does get a distortion which further lowers the energy.
R. HOLROYD.Your suggestion of reaction of the electron with preexisting aggregate rather than by reaction with an alcohol monomer, followed by aggregation (Mozumder model), is in agreement with our electron rate constant study in alkanes since we find that the rate of reaction of the electron with the methanol monomer is slow ( k 5 lo8 M-’sec-l). G. R. FREEMAN. Your work is a further indication that very long range polarization interactions are relatively unimportant in determining the observed properties of electrons in these liquids. Furthermore, you would not expect to observe spectral changes in the room temperature 5050 mixed solvents after a microsecond. Molecular species diffuse hundreds of lngstroms in that time. J. C. THOMPSON. Do concentration variations of AE match those of AG?
R. FIRESTONE. There is no systematic relationship between the variation of AG vs. composition and that of AH or of A S except within a single class of solutions over limited concentration ranges; e.g., alcohols in alkanes. A E values are not presently available. It seems unlikely that a single form for AG vs. c or AE vs. c exists, except perhaps within single classes of solutions in limited c ranges. No systematic dependence exists, incidentally, between .iEE~max,c and either AH or ASE nor among AGC, AH, and ASE a t constant c .
Laser-Raman Investigation of Dilute Metal-Ammonia Solutions T. R. White and W.S. Glaunsinger. Department of Chemistry, Arizona State Universlty, Tempe, Arizona 8528 1 (Received July 18, 1975)
The temperature dependence of the Raman spectra of ammonia and 6 X M lithium- and 3 X M calcium-ammonia solutions has been studied between 195 and 300 K. A careful study has been made of the low-frequency region, using an iodine filter below 50 cm-l, but no solvated-electron band was observed. Band positions, widths, and depolarization ratios have also been measured in the N-H stretching region (3100-3500 cm-l). All band maxima increase linearly with temperature, and the solution bands occur at lower frequencies than those of ammonia. Uncoupled line widths have been determined using the coupleddamped-oscillator model. The uncoupled line widths decrease linearly with temperature, and the line width in the solutions is less than that in ammonia. The band shift and line width data are interpreted in terms of hydrogen bonding and nonreorientational relaxation processes, respectively.
Introduction
The nature of the solvated electron in dilute metalammonia solutions has been the subject of several experimental and theoretical inve~tigations.l-~ In very dilute solutions the solvated cations and electrons are unassociated, so that they may be treated independently. Copeland et al.4 have proposed a configuration-coordinate model for the solvated electron, in which the electron is situated in a cavity surrounded by preferentially oriented ammonia molecules. They predict a totally symmetric, Raman-active viThe Journal of Physical Chemistry, Vol. 79, No. 26, 1975
bration in the range 25-60 cm-l. Applying the treatment of Klick and Schulman6 to metal-ammonia solutions, Rusch6 has’suggested that the symmetric vibration may be in the 400-700-~m-~region. However, Raman studies of dilute sod i ~ m - and ~ , ~potassium-ammonia7 solutions have failed to detect the predicted solvated-electron band. Possible reasons for the failure to observe the predicted band could be (1)inapplicability of present theoretical models,4s6 (2) large band width, (3) insufficient concentration of solvated electrons, and (4)interference from intense Rayleigh scattering below 50 cm-l.
Laser-Raman Study of Metal-Ammonia Solutions
2943
TUBING
GRADEDSEAL
-
111 i 1
Double
j Monochrmator
Scramblsr
-
4e
u
+t
Polarizer Collimator Lena
I> .8
Dove + sample Collector Lens
Beam Splltier
Figure 2.
Figure 1. Sample-preparationassembly. Dimensions are in cm.
In the present study, an iodine filter has been employed to attenuate the Rayleigh-scattered light.s!9 In order to check the possibility that the concentration of solvated electrons may be too low, dilute lithium- and calciumammonia solutions were employed, and a careful search conducted for symmetric metal-nitrogen stretching bands near 250 cm-l.l0 If the solvated-cation bands can be detected, then there would be reason to believe that the Raman spectrometer has sufficient sensitivity to detect a solvated-electron band, if it exists and is not too broad, at the concentration used. In addition, a careful study has been made of the band positions, widths, and depolarization ratios in the N-H stretching region in ammonia and dilute lithium- and calcium-ammonia solutions. An infrared study of lithiumand potassium-ammonia solutions indicates that the band positions shift to lower frequency with increasing concentration,ll whereas Raman spectra of ~ o d i u m - and ~ , ~ potassium-ammonia7 solutions, which are of higher resolution than the infrared spectra, fail to show a shift in band positions. However, the Raman spectra of lithium- and calcium-ammonia solutions may exhibit meapurable shifts due to the greater cationic influence on the solvent. Experimental Section
Sample Preparation. 6 X IO-* M lithium- and 3 X M calcium-ammonia solutions were prepared by distilling a measured quantity of dry ammonia (Matheson, anhydrous, 99.99%) from a sodium-ammonia solution into the central tube of a special sample-preparation assembly, shown in Figure 1, containing the metal, which had been previously cut and weighed in an argon-filled glovebox, in amount sufficient to obtain the desired concentration. The entire assembly was then immersed in a dry ice-ethanol slush and the metal-ammonia solution poured into a specially cleaned12 quartz sample cell, which consists of three quartz optical flats (0.5 in. dia X l/16 in. thick) sealed at 90° to one another, and subsequently sealed. The accuracy of the sample concentration is about 10%. The cell design al-
Schematic diagram of the Raman spectrometer.
lows quantitative polarization measurements and spectrophotometric determination of concentration. Apparatus and Measurement Procedure. A schematic diagram of the Raman spectrometer, which uses the conventional 90° geometry, is shown in Figure 2. A Spectra Physics Model 165 argon ion laser operating at 5145 A was the source of the incident light. The scattered light was analyzed by a Spex 1402 double monochromator followed by an SSRI photon-counting detection system. The photomultiplier tube had a dark count of 20 cps at room temperature. Prior to entering the monochromator, the scattered light was passed through a Dove prism to rotate the image goo, which makes the image compatible with the vertical entrance slit of the monochromator, and a scrambler to remove the polarization dependence of the monochromator. In general, powers of about 100 mW were focused into the sample cell. Unpolarized, polarized, and depolarized spectra were recorded with a constant slit width (usually 200 /.L) and scanned typically at 5 cm-l/min. The scanning drive of the spectrometer was calibrated using indene and the resolution checked with carbon tetrachloride. All frequencies are reported accurate to f 2 cm-l. The 225-, 312-, and 459cm-l bands of carbon tetrachloride were used to determine the accuracy of the polarization measurements reported in this work. Depolarization ratios for these bands were in excellent agreement with predicted and previous experimental ~alues.l3-'~ In order to attenuate the Rayleigh-scattered light, the laser was single moded by incorporation of an intra-cavity air-spaced etalon and an iodine filter installed before the monochromator (see Figure 2). The laser can be tuned to the iodine rotational line lying under the gain curve of the 5145-A line of the argon-ion laser by the following procedure: (1) adjust the etalon for maximum power output causing the iodine cell to fluoresce; (2) tilt the etalon slightly, which causes the power to drop slowly and the fluorescence to disappear; and (3) continue tilting the etalon until the fluorescence reappears and the power drops suddenly. The laser is now tuned to the iodine rotational line and maximum absorption of the Rayleigh-scattered light will occur. As shown in Figure 2, two iodine cells were employed in our setup. The first cell was 50 cm long, 2.3 cm i.d., and The Journal of Physical Chemktry, Vol. 79, No. 26, 1975
T. R. White and W. S.Glaunsinger
2944
TABLE I: Observed Band Maxima (cm-') and Depolarization Ratios in Liquid Ammonia at 298 K v2 v, VI 2v, v 3 3301 (0.08) 3303 3301 (=0.01) 3298 (0.03) so
II I!! WITH I* CELL
-
WITHWT I2 CELL
/
r( 0
100
200
300
A9
400
lcm-'l
Flgure 3. Typical Raman spectrum in the low-frequency region recorded with and without the iodine filter.
had a mirror attached at the far end. This cell was maintained at room temperature and used to indicate precise tuning of the laser to the iodine rotational line. The second cell was 5 cm long and 2.8 cm i.d. It was heated to 350 K and used to absorb the Rayleigh-scattered light. The iodine cell was most effective in attenuating the Rayleigh-scattered light in the range 5-50 cm-l. Temperatures between 195 and 300 K were obtained by flowing chilled nitrogen gas over the sample. Temperatures were measured with a copper-constantan thermocouple attached to the outside of the cell. The temperature gradient across the sample dimension with the laser off was about 1 K. The spectral position and widths were independent of laser power up to 100 mW, which indicates that the samples were not being heated appreciably by the laser a t the powers employed in these experiments. The temperature was stabilized to within 0.5 K during the measurements.
Results and Discussion This section is divided into two subdivisions: the first is concerned with a careful search for the solvated-electron band in dilute metal-ammonia solutions, and the second with the temperature-dependent Raman behavior in the N-H stretching region in ammonia and dilute lithium- and calcium-ammonia solutions. Solvated-Electron Band. A careful search was conducted for the solvated-electron band in the range 5-700 cm-l in the lithium- and calcium-ammonia solutions using the iodine filter in the low-frequency region to attenuate the Rayleigh-scattered light. A typical Raman spectrum in the low-frequency region is shown in Figure 3. The attenuation of the Rayleigh-scattered light is particularly evident in the range 5-50 cm-'. We were unable to detect a solvated-electron band between 195 and 300 K in any of our studies. In addition, the solvated-cation bands detected in salt solutions a t about 240 cm-' (width about 35 cm-') for lithium salts and about 265 cm-1 (width about 70 cm-') for calciThe Journal of Physical Chemistry, Vol. 79, No. 26, 1975
3384 (0.4) 3386 3380 (0.5) 3386 (1.0)
Ref 18 19 6
This work
um salts1' were not observed. Hence the solvated-electron band would have to be very narrow to be detected. Our failure to detect solvated-cation and electron bands indicates that the concentration of scatterers is too low to be observed by conventional laser-Raman techniques. Due to absorption problems, increasing the metallic concentration is not expected to result in a significant increase in the intensity of the Raman-scattered light.fi N-H Stretching Region. Five bands are observed in the Raman spectrum of liquid ammonia. At 298 K, the band maxima occur near 1045, 1640, 3215, 3300, and 3385 cm-' and are assigned to the symmetric bending mode (4, asymmetric bend ( u 4 ) , symmetric stretch (ul), first overtone of the asymmetric bend (2uq), and asymmetric stretch (us), respectively.1°J6 The assignlhent of the 3215- and 3300cm-l bands to ul and 2V4, respectively, is based on a recent Raman study16 of liquid ammonia using the coupleddamped-oscillator model to analyze quantitatively the Fermi resonance between u1 and 2 ~ 4 If . Fermi resonance is neglected, then it is possible to resolve the Raman spectrum in the N-H stretching region into four bands.1° In addition to the u1, 2u4, and u3 bands already mentioned, a very broad fourth band centered a t 3270 cm-l is found and assigned to the symmetric stretch of an ammonia molecule associated through one of its hydrogens. However, neglecting the Fermi-resonance interaction in resolving the Raman spectrum into four bands in the N-H stretching region is clearly not justified. Unfortunately, after correcting a sign error in the original coupled-damped-oscillator calculation,16 we find that both the coupled-damped-oscillator and four-band approaches provide excellent fits to the Raman spectrum of ammonia in the N-H stretching region. Hence it appears that nothing will be gained by introducing a fourth band into the coupled-damped-oscillator model. Although isotope studies provide convincing evidence for the existence of a fourth band,17 inclusion of the Fermi-resonance interaction between u l and 2v4 into the resolution calculation will lead to a significant reduction in its intensity. Hereafter, we adopt the coupled-damped-oscillator model and neglect the presence of the weak fourth band. The observed band maxima and depolarization ratios in liquid ammonia at 298 K are summarized in Table I. The Dove prism was removed and the cell rotated 90' for the depolarization measurements. For comparison, data from recent Raman studies of liquid ammonia are also shown in Table I. The band maxima are in fair agreement with previous reports, but the depolarization ratios differ markedly from those reported previously. Due to the cell design, we believe that our depolarization measurements are the most accurate to date. The small depolarization ratios for the v i and 2v4 bands means that only isotropic nonreorientational processes, such as vibrational and rotational mechanisms, contribute to the line width. In addition, the total depolarization of the v3 band and nonnegligible depolarization of the u2 and u4 bands indicate that both reorientational and nonreorientational processes contribute to the line width. Now we concentrate on comparing the Raman behavior M lithium- and 3 of liquid ammonia to that of the 6 X
Laser-Raman Study of Metal-Ammonia Solutions
2945
D
"3 LI
- NH,,
6X
IO"M
A Ca-NH3,3X10-*M
c
3350
Ca-NH,
32x,
3 X IO"M 3200
-
e
n 1
-
,
.
-
n
-
i 190
. 210
*
230
.
A 250 T(K)
270
290
310
Flgure 5. Band maxima A ~ vs. J temperature for the u1 (bottom),2V4 (middle),and u3 (top) bands in ammonla (O),6 X # lithiumammonia (0),and 3 X # calcium-ammonia (A)solutions. The frequencies are accurate to f 2 cm-'. The lines represent leastsquares fits to the data.
TABLE 11: Slopes of the Band Maxima vs. Temperature Lines in Figure 5 Solution
M calcium-ammonia solutions in the N-H stretch-
ing region (3100-3500 cm-'). The metallic concentrations chosen were low enough to avoid appreciable absorption at 5145 A6 and, since the solvated-electron concentration is the same in both solutions, permit the influence of the lithium and calcium cations on the stretching bands of ammonia to be investigated. Raman spectra recorded a t two widely separated temperatures in ammonia and the calcium-ammonia solution are shown in Figure 4.It is evident that the addition of calcium results in a narrowing of the v1 and 2v4 bands and a shift of all band maxima to lower frequencies. Similar, although less pronounced, behavior is observed in the lithium-ammonia solution. The observation of significant differences between the Raman spectra of ammonia and these dilute solutions is in sharp contrast to the behavior found in previous Raman studies of dilute sodium3 and potassiumammonia' solutions. Our results indicate that Li+, and to a larger extent Ca2+, have a greater effect on ammonia than Na* and K+. The larger interaction of Li+ and Ca2+ with ammonia is reasonable in view of their higher charge densities. The temperature dependence of the band maxima in ammonia and the solutions is shown in Figure 5. The important features of Figure 5 can be summarized as follows: (1)
dv,/dT, cm-'/100 K
6k2
4 i 2
l0k 2
3+2
6 + 2 8-c2
Li-NH, 3 x 10-4 M Ca-NH,
7 + 2
3 i 2
1+-2
6X
x
d(2v,)ldT, cm-'/100 K
M
"3
Flgure 4. Raman spectra recorded at two widely separated temperM calcium-ammonia solution. The atures In ammonia and 3 X low-frequency band is v l , the intermedlate-frequency band 2v4, and the high-frequency band u3.
dv,/dT, cm-'/100 K
all bands shift to higher frequencies linearly with increasing temperature, (2) the solution bands occur at lower frequencies than those of ammonia at all temperatures, and (3) the calcium-ammonia bands occur at lower frequencies than those of the lithium-ammonia solution at all temperatures. The slopes of the band maxima vs. temperature lines in Figure 5 are summarized in Table 11. Since there is less thermal energy available to break hydrogen bonds at lower temperatures, it is reasonable to expect the extent of hydrogen bonding in these systems to decrease with an increase in temperature. In general, in hydrogen-bonded systems one finds that the frequency of stretching vibrations increases as the amount of hydrogen bonding decreases, whereas opposite, although less pronounced, behavior is observed for bending vibrations.20 Similar behavior is found for overtones. Using these general rules, the increase in the frequencies of the ul (symmetric stretch) and u3 (asymmetric stretch) bands with temperature is expected, but the similar behavior observed for the 2 ~ (asymmetric 4 bend) band is perhaps unexpected. However, the increase in frequency of the 2v4 band with temperature is smaller than the increase for the and u3 bands (see Table 11). In addition, the fact that u1 and 2u4 are in Fermi resonance means that the V I and 2 ~ 4modes are mixed, so that the 2u4 mode acquires some symmetricstretching character. In view of the Fermi resonance beThe Journal of Physical Chemistry, Vol. 79, No. 26, 1975
2948
T. R. White and W. S. Glaunsinger
40
i
L30
IS0
210
230
250 T (K1
270
290
Figure 6. Uncoupled line widths andT2vs. temperature for the vl (lower)and 2v4 (upper)bands in ammonia ( 0 )and 3 X M calcium-ammonia solution (A).The line widths are accurate to f 2 cm-'. The lines represent least-squares fits to the data.
tween v 1 and 2u4, it is not surprising that the temperature dependence of the frequency of the 2v4 band does not exhibit the characteristic behavior observed for some bending vibrations. In fact, if the Fermi resonance between v1 and 2 ~ is 4 analyzed on the basis of the coupled-damped-oscillator model,16then it is found that the uncoupled band maxima ( V I ) ; and (2Ua)u do indeed show the expected behavior, with ( V I ) " increasing with temperature more rapidly than (2v4),, decreases with temperature. The results are in good agreement with a previous report,16 which should be consulted for further details. The fact that the lithium- and calcium-ammonia bands occur at lower frequencies than those of ammonia indicates that the N-H bond is weaker in the solutions. The weaker N-H bond arises from the coordination of the lone electron pair of an ammonia molecule to a cation. Furthermore, the weakening of the N-H bond makes the proton more available for hydrogen bonding, which in turn should result in stronger, and probably additional, hydrogen bonds in the solutions. As discussed previously, increased hydrogen bonding also causes the band maxima to shift to lower frequencies. The occurrence of the calcium-ammonia bands a t lower frequencies than those of the lithium-ammonia solution simply reflects the greater ability of Ca2+ to attract the lone electron pair of an ammonia molecule due to its higher charge density. It is interesting to compare our results with those of a recent infrared study'l of dilute lithium-ammonia solutions a t 203 K. Estimating the infrared shifts in a 6 X M lithium-ammonia solution by interpolation, we find about 0, -5, and -10 cm-' for the shifts of the vl, 2 V 4 , and v3 bands relative to those in ammonia, which should be compared to the Raman shifts of about -4, -2, and -4 cm-l. The Journal of Physical Chemistry, Vol. 79, No. 26, 1975
We believe that the Raman shifts are more reliable due to the much higher resolution attained in the Raman spectra. Uncoupled line widths of the v l and 2 V q bands, which we denote by I'l and rz, respectively, have been obtained from the experimental Raman spectra using the coupleddamped-oscillator rnodel.l6 The temperature dependence of rl and r2 in ammonia and the calcium-ammonia solution is shown in Figure 6. The important features of Figure 6 can be summarized as follows: (1) both I'l and r2 decrease linearly with temperature in ammonia and in the calcium-ammonia solution, (2) the rate of decrease of the line width with temperature is the same in ammonia and in the calcium-ammonia solution, and (3) the line width in the calcium-ammonia solution is less than that in ammonia. I'l and r2 in the lithium-ammonia solution were between the ammonia and calcium-ammonia data and exhibited a similar temperature dependence, but for clarity the lithium-ammonia data have been omitted from Figure 6. In ammonia, the 2'l data are in quantitative agreement and the rl data are in qualitative agreement with a previous report.l6 Before proceeding, it should be remembered that only nonreorientational processes contribute to I'l and r2.The nonreorientational processes considered to be the most important are vibrational relaxation, translational diffusion, collisional line broadening, inhomogeneous polymerization, and relaxation via low-frequency hydrogen-bond vibrations and solvent deformations; hereafter referred to as mechanisms 1-5, respectively.16 Mechanisms 1-3 result in an increase in line width with temperature, which is opposite to the observed behavior. In contrast, mechanisms 4 and 5 predict that the line width should decrease with temperature, as observed. Hence the decrease in the r's with temperature in ammonia and the solutions is attributed to mechanisms 4 and 5; however, the dominant mechanism cannot be identified solely from the temperature dependence of the line width. The similar temperature dependence of and I'z in ammonia and in the solutions suggests that the same linebroadening mechanism is operative in both systems. In order to estimate the activation energy E , for the process or processes causing the observed decrease in line width with temperature, the temperature dependence of rl and r2have been fit to the Arrhenius equation
r = rOeEJRT
(1)
Within experimental error, E , = 0.2 kcal/mol for both rl and l'2. It is significant that the experimental activation energy is much less than that for the hydrogen-bondbreaking process (5-10 kcal/mol). If one mechanism makes the dominant contribution to the line width, then this resuIt suggests that the mechanism causing rl and r2 to decrease with temperature does not involve hydrogen-bond breaking, but rather perhaps a subtle structural change. However, several competing mechanisms may contribute to the line width, in which case the activation energy would have little quantitative significance. ' The line narrowing observed when lithium or calcium is added to ammonia suggests that several competing mechanisms contribute to the line width. Dissolution of a metal in ammonia results in weaker N-W bonds, increased hydrogen bonding, less anharmonicity (causes vibrational relaxation), slower translational diffusion, and less-frequent collisions. Mechanisms 1, 4, and 5 predict, incorrectly, that the line should broaden upon the addition of metal. Further-
Laser-Raman Study of Metal-Ammonia Solutions more, mechanisms 2 and 3 broaden the bands as the temperature is increased, but these mechanisms are consistent with the observed decrease in line width in the solutions. Hence it appears that (at least) mechanisms 2-5 are important in these systems. It is now possible to understand the small activation energy found above. Mechanisms 4 and 5 cause the bands to narrow as the temperature is increased, whereas mechanisms 2 and 3 cause the opposite behavior. The net result is a line width that decreases weakly temperature, indicating that mechanisms 4 and 5 are slightly more important, and a small activation energy. Finally, we comment upon the observation of significant differences between the Raman spectra of ammonia and the solutions, when, on the basis of concentration, one might expect the differences to be too small to be observable. We offer two possible reasons for the large cation effects observed in the Raman spectra of the solutions. First, the cation can influence a great number of ammonia molecules because the solvent is extensively polymerized," and second, the cation increases the polarizability of an ammonia molecule parallel to its principle axis all and hence increases the intensity of the Raman-scattered light, which is proportional to all2. Acknowledgment. We wish to thank Dr. C. T. Walker for
making the light-scattering facilities in his laboratory available to us for this study, Mr. M. Anderson and Drs. W. Love and J. Potts for helpful conversations, and Ms.Lorna Glaunsinger for her drafting expertise. We gratefully acknowledge support of this research by Arizona State University and the Research Corporation. References and Notes (1) "Metal-Ammonia Solutions", Proceedings of Colloque Weyl I, G. Lepoutre and M. J. Sienko, Ed., W. A. Benjamin, New York, N.Y.. 1964. (2) "Metal-Ammonia Solutions", Proceedings of Colloque Weyl II, J. J. Lagowski and M. J. Sienko, Ed., Butterworths, London, 1970. (3) "Electrons In Fluids", Proceedings of Colloque Weyl Ill, J. Jortner and N. R. Kestner, Ed., Springer-Verlag,New York, N.Y., 1973. (4) D. A. Copeland. N. R. Kestner, and J. Jortner, J. Chem. fhys., 53, 1189 (1970). ( 5 ) C. C. Kiick and J. H. Schulman, SolM State fhys., 5, 97 (1957).
2947
B. I. Smlth and W. H. Koehler, J. fhys. Chem., 77, 1753 (1973). Reference 3, pp 161-166. G. Hibler, J. Llppert, and W. L. Petlcolas, Spex Speaker, 16, 10 (1971). G. E. Devlin. J. L. Davis, L. Chase, and S. Gschwind, Appl. fhys. Lett., 19, 138 (1971). K. R. Plowman and J. J. Lagowski, J. fhys. Chem., 78, 143 (1974). P. F. Ruschand J. J. Lagowskl, J. Phys. Chem., 77, 210(1973). S. Nalditch and J. E. Werde. J. Vacuum Sci. Techno/., 5, 54 (1968). A. E. Douglas and D. H. Hank, J. Opt. SOC.Am., 38, 281 (1948). A. F. Slombs, C. D. Hinman, and E. H. Siegler, Proceedings of the Conference on Analytical Chemistry and Applied Spectroscopy, 1965. W. F. Murphy, M. V. Evans, and P. Bender, J. Chem. Phys., 47, 1836 (1967). M. Schwartz and C. H. Wang, J. Chem. Phys., 59,5258 (1973). A. T. Lemley, J. H. Roberts, K. R. Plowman, and J. J. Lagowskl, J. fhys. Chem., 18, 2185 (1973). 0.Seiller. M. Ceccaldl. and J. P. Leicknam. Method. Phys. Anal., 4, 388 (1968). 8 . Bettlgnies and F. Wallart, C. R. Acad. Sci., 271, 640 (1970). G. Pimentel and A. McClellan. "The Hydrogen Bond", W. H. Freeman, San Francisco, Calif., 1960.
Discussion K. PLOWMAN, Over the concentration range a t which the spectra were taken no cation-solvation mode is expected to be observed. The minimum concentration a t which we were able to observe the band was 0.2 M . In addition we saw no metal-solvation effect to concentration two orders of magnitude greater than that of your experiments. The calcium work is interesting but perhaps it should be repeated for confirmation.
T. WHITE. We were aware of the concentration problem in detecting these bands. However, when operating at the 5145-A exciting line, where one can utilize the iodine filter, the method requires such concentrations to avoid absorption problems. We too were surprised to see these effects from such a small amount of metal. We chose these metals for their small size and high charge density. We felt that if there was an effect on the spectra of the solvent, it would be greatest with these metals. I agree that the study should be repeated for confirmation, but I must add that frequency shifts reported are about the same as those we estimated from previous infrared data [P. F. Rusch and J. J. Lagowski, J. Phys. Chem., 77,210 (1973)]. Also note that (1)the ammonia data were reproducible and agree well with previous studies ["Electrons in Fluids, The Nature of Metal-Ammonia Solutions", Colloque Weyl 111, W. A. Benjamin, New York, N.Y., 1973, p 145; G. Sellier, M. Ceccaldi, and J. P. Leicknam, Method Phys. Anal., 4, 388 (1968); M. Schwartz and C. H. Wang, J . Chem. Phys., 59, 5258 (1973)], and (2) although the lithium data are close to those for ammonia, the calcium data are well outside experimental error.
The Journal of Physical Chemistry, Vol. 79, No,26, 1975
