J. Phys. Chem. 1984, 88. 5851-5855
5851
onance, but the expected resolution into a pair of doublets was not obtained. Comparing the widths of the CH2 and H-3 resonances, we can estimate a broadening of ATYI = 21 rad s-l. With a chemical shift difference of 0.5 ppm, this broadening would correspond to T , , ~= 3 X s. Sullivan and WilliamsI2 used INDO calculations to estimate the barrier to inversion in the diquat dication at about 25 kJ mol-'. Thus at room temperature, we expect T ~ zi ~IO6- s-l,~ consistent with the further prediction of Sullivan's INDO calculations that the rate of inversion is slower in the dication than in the cation radical.
120
100 e
1
2 4 80
Acknowledgment. The experimental assistance of James Van Epp, several timely suggestions by R. G. Lawler, and the advice and encouragement of J. 0.Edwards are gratefully acknowledged. 60
Figure 3. Predicted broadening of the diquat CH2 resonance as a function of the conformationallifetime, T , ~ ,in D@. ( T ~ D= 2 X s, T ~ D = 7 x 10-3 s).
Registry No. MV2', 4685-14-7; MV'., 25239-55-8; MV2' chloride salt, 1910-42-5;BVZt, 13096-46-3;BV+., 49765-27-7; BV2' chloride salt, 1102-19-8;DQ2', 2764-72-9;DQ'., 63406-50-8;DQ2+,85-00-7;Zn(Hg), 11146-96-6; Me2S0, 67-68-5; methanol, 67-56-1; sodium methoxide, 124-41-4.
argued for a rate in the range 5 X lo6 to 5 X lo8 s-l on the basis of ESR line widths. The N M R spectrum of DQz+ at 189 K (in the absence of radical cation) showed considerable broadening of the CH2 res-
Supplementary Material Available: Tables 1s-4s containing NMR line width data and Figures 1s-5s containing experimental and simulated ESR spectra (9 pages). Ordering information is given on any current masthead page.
5
7
6 -log
8
9
Tap
Temperature Study of Rayleigh and Raman Line Shapes in Liquid Carbonyl Sulfide B. Hegemann and J. Jonas* Department of Chemistry, School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801 (Received: April 10, 1984; In Final Form: July 13, 1984) The Raman u2 and u3 and depolarized Rayleigh line shapes of neat liquid OCS under vapor pressure are investigated over the temperature range T = 183-303 K. Results are compared when possible to previous results on the ul mode of OCS and the depolarized Rayleigh line shapes of CS2. The relationships observed are discussed and compared to theoretical predictions. I. Introduction In the course of our recent investigations of the u1 mode line shapes of liquid OCS under vapor pressure1 and the depolarized Rayleigh scattering (DRS) line shapes of liquid CS, as a function of temperature and p r e s s ~ r e ,we ~ , ~realized that it is of interest to compare the Raman u2, u3 and DRS line shapes of liquid OCS under vapor pressure with the results of our previous investigations. We have therefore measured the above line shapes over the temperature range T = 183-303 K. In this study we have the following specific goals: (1) compare the appearance and behavior with temperature/density of the OCS DRS line shape with that of CS,; (2) compare rotational corre, from different Raman vibralation function times, T ~obtained tional modes of OCS; and (3) compare and contrast the possible collision-induced contributions to the line shapes of the three Raman modes and DRS of OCS.
11. Experimental Section The OCS preparation is the same as that given in ref 1. A description of the Raman scattering apparatus and details of the data acquisition and analysis methods have been previously p~blished.~.'The 4880-A exciting line of an argon-ion laser was used for all line shapes. A slit width of -0.6 cm-' was used for the u, and u, line shapes
(1) Hegemann, B.; Jonas, J. J . Chem. Phys. 1983, 79, 4683. (2) Hegemann, B.; Baker, K.; Jonas, J. J . Chem. Phys. 1984, 80, 570. (3) Hegemann, B.; Jonas, J. J . Chem. Phys., submitted for publication. (4) Schroeder, J.; Schiemann, V. H.; Sharko, P. T.; Jonas, J. J . Chem. Phys. 1977,66, 3215. (5) Perry, S.; Zerda, T. W.; Jonas, J. J . Chem. Phys. 1981, 75, 4214.
0022-365418412088-585 1$01SO10
for which deconvolution was found to be unnecessary. Both VV and VH polarization geometry were recorded for the Raman bands. A larger slit width ( 1.2 cm-') was used for the DRS line shapes in order to obtain as large a dynamic range of intensities as possible. The VH polarization geometry was used in recording all DRS line shapes. It is important to note that the use of wide slits will produce a significant amount of isotropic Rayleigh-Brillouin scattering leakage 2s well as stray scattered laser light at frequencies less than -6 cm-' from the exciting line, rendering the line shape unreliable in this region. The detailed balance correction, I ( w ) = l/JmeaS(u)[l exp(-@hw)], and X4 scattering factor correction have been applied to all DRS spectra.
-
+
111. Results and Discussion
A . Depolarized Rayleigh Line Shape. A typical DRS line shape is shown in Figure 1. As found previously for CS2,293there are four distinct regions evident in the line shape. In the CS, study, based upon the discussion of Madden and c o - w o r k e r ~ ,region ~,~ I was attributed to the symmetry-allowed collective reorientational DRS line shape, and regions 11-IV were attributed to collisioninduced contributions.2 This separations rests upon the condition that the molecule of interest reorients slowly enough to allow the local environment to adjust instantaneously to the rotational motion.* In our previous study of OCS' it was suggested that this may be the case for OCS as well as CS2. Therefore, because of the similarity in the CS, and OCS DRS line shapes, we attribute region I of the OCS DRS line shape to be the symmetry-allowed (6) Cox, T. I.; Battaglia, M. R.; Madden, P. A. Mol. Phys. 1979, 38, 1539.
(7) Tildesley, D. J.; Madden, P. A. Mol. Phys. 1983, 48, 129. (8) Frenkel, D.; McTague, J. P. J . Chem. Phys. 1980, 72, 2801.
0 1984 American Chemical Society
Hegemann and Jonas
5852 The Journal of Physical Chemistry, Vol. 88, No. 24, 1984 TABLE I: DRS Line Shape Analysis Parameters for Neat Liquid OCS under Its Own Vapor Pressure temp, K density, a g A,,, cm-' 4 1 1 , cm-' A,,, em-' M(4)/M(2), cm-l 27 5280 303 0.956 19.6 19.8 27 5360 273 1.049 20.9 20.0 26 5600 243 1.130 22.8 20.3 21 5720 213 1.199 25.7 20.2 26 5930 183 1.253 29.8 19.8
P,,,~~O~I(W), cm-' ~,,,~,w~I(w), em-' 40.1 42.0 44.9 50.6 55.4
77.1 77.6 80.9 78.9 81.3
From ref 28. I
T=183K
I
I
166;
t
w
I
I
I
I
50
100
150
I 200
1
250
FREQUENCY (cm")
Figure 1. Four observed regions of the DRS line shape for neat liquid OCS under its own vapor pressure (T = 183 K, p = 1.233 g em-').
collective reorientational contribution and regions 11-IV to be the collision-induced contributions. Due to the isotropic Rayleigh-Brillioun leakage problem for region I, quantitative analysis of this region is not possible. Nevertheless, we do observe a narrowing of the line shape in this region with decreasing temperature/increasing density, as expected for rotational motion in the diffusional limit. (That the OCS rotational motion is near diffusional under these conditions was previously inferred in our earlier study.' In liquid CS2, using the theoretical framework of Madden's DRS theory: region I1 has been attributed to solidlike oscillatory motions, while region I11 is attributed to gaslike translational motions, with the origin of both regions arising from the diRegion IV was pole-induced dipole (DID) suggested to arise from a higher order multipole mechanism, e.g., quadrupole-induced dipole, with the behavior also governed by gaslike translational motions?,3 We wish to see if these ideas will be valid for OCS as well. We have characterized regions 11-IV by the exponential fit parameter A ( I ( , ) = e"/*). The exact boundaries between the regions are, of course, not well-defined. The actual frequency range of each region changes slightly with temperature/density as well. However, frequency ranges could be found which corresponded clearly to a portion of each exponential 11-IV region for all measured line shapes. It was these ranges which were used for the A fits (region 11: 40-60 cm-I; region 111: 80-1 10 cm-I; region IV: 180-220 cm-I). We caution that region I1 is not particularly well characterized by an exponential, as it always retains a slight curvature in a log plot. Nevertheless, in the absence of a more suitable analytic function to characterize this region, we will use the A value as a convenient, approximate means to monitor the behavior of this region. Care was taken to find the proper line shape base line which was then set equal to zero in order to ensure a correct A fit. The A values obtained are given in Table I. It is found that region I1 is essentially indistinguishable from region I11 at T = 303 K, but as temperature decreases/density increases AI, increases dramatically, whereas AI,, and A,, show no clear trend. The correct explanation of the region I1 and the behavior of our approximative A,, with density can be explained in terms of the theoretical model proposed by Guillot, Bratos, and Birnbaum." Their theoretical calculation of scattered light from rare-gas fluids (9) Madden, P. A. Mol. Phys. 1978, 36, 365. (10) Madden, P. A.; Cox, T. 1. Mol. Phys. 1981, 43, 287.
is based on the Zwanzig-Mori theory of Brownian motion and the lattice-gas model. In their calculation of theoretical DRS intensities they predict appearance of a shoulder in the 50-cm-' region with increased density due to enhanced oscillatory mode associated with oscillations around a given site in a fluid. Therefore, the approximative A,, changes so much with density. As we mentioned above, we use All only to indicate general trends in DRS line shape with density. These results can be interpreted in light of our previous results for CS2 in which the separate temperature and density effects were determined.' In the liquid CS2study, All was found to increase substantially with increasing density, but to be largely independent of temperature. The A111 and A,, values were found to increase moderately with increasing temperature and density, with AIv > AIll. It is now clear that our previous interpretation of the CS2 data is consistent with the observed behavior of OCS as well. The large increase in AI1 found in OCS is a result of the effect of increasing density, while the behavior of AII1 and Arv results from a competition of temperature and density effects. Furthermore, AIv > Alll as in CS2, which is consistent with a higher order multipole mechanism giving rise to region IV.Io These results lend considerable support to the concept of the same fundamental dynamic processes giving rise to the collision-induced DRS line shape properties of CS2 and OCS. In addition to looking directly at line shape behavior, moment analysis has also been used extensively to analyze line shapes. The difficulties involved in obtaining accurate values of the spectral moments of DRS line shapes has been well documented. This arises principally from the large uncertainties of the area of the measured line shape near the exciting line, a problem which is particularly severe in our case due to the large slit widths used. In fact, we were unable to determine reliably any absolute spectral moments. Nevertheless, due to the large dynamic range of the measured line shape, we were able to measure reliable values for J"omw2Z(w) dw and Jomw4Z(w) dw, and therefore the spectral moment ratio M(4)/M(2), as the portion of these integrals arising from frequencies less than 6 cm-l from the exciting line is negligible. The ratios determined are also shown in Table I. The physical meaning of M(4)/M(2) is not clear, as the presence of collision-induced scattering precludes any meaningful evaluation of mean square torques from this quantity.I2 However, we feel this quantity will provide a useful criteria of comparison with possible future molecular dynamics calculations along the lines of those of ref 8 and 13. For completeness we have also tabulated v,,, for the functions w2Z(w) and w4Z(w) in Table I. B. u2 Mode Line Shape. This mode is of II symmetry and is predicted to be depolarized, Le., Zvv(w) = 4/3ZvH(w). Ignoring possible collision-induced contributions, the Fourier transform of a depolarized line shape yields the product of the vibrational and rotational correlation functions, CYib(t)Crot(t)(in the absence of vibration-rotation correlation). In the case of rotational diffusion, generalized Hubbard relations exist which can be used to relate 70 = J"Crot(t) dt between modes of differing syymetry for linear rnolec~les.'~For OCS T~~ = 6/57e2t where T~~ is obtained from the v, or u3 mode line shape. The preliminary half-width-half-maximum (hwhm) data for the u2 mode are presented in Table 11. Two unexpected features (11) Guillot, B.; Bratos, S.;Birnbaum, G. Phys. Rev.A 1980, 22, 2230. (12) DeSantis, A.; Moretti, E.; Sampoli, M. Mol. Phys. 1982, 46, 1271. (13) Ladanyi, B. M. J. Chem. Phys. 1983, 78, 2189. (14) Levi, G.; Marsault, J. P.; Marsault-Herail, F.; McClung, R. E. D. J . Chem. Phys. 1980, 73, 2443.
The Journal of Physical Chemistry, Vol. 88, No. 24, 1984 5853
Temperature Study of Rayleigh and Raman Line Shapes
TABLE II: Half-Width-Half-Maximum (Hwhm) Data for VV and VH v2 Lines Shapes of Neat Liquid OCS under Its Own Vapor Pressure
-
temp, K
density,' g cm-3
center, cm-I
low6
hwhm-VV, cm-I highC
avd
lowb
hwhm-VH, cm-' highc
avd
303 273 243 213 183
0.956 1.049 1.130 1.199 1.253
518.6 518.3 517.8 517.4 517.1
13.9 10.0 7.7 6.0
14.2 10.6 8.6 6.4
14.0 10.3 8.2 6.2
12.8 9.7 7.8 5.7 4.8
13.7 10.1 8.1 6.4 5.2
13.2 9.9 8.0 6.0 5.0
'From ref 28. bLow-frequency-sidehwhm. cHigh-frequency-sidehwhm. "Average of low hwhm and high hwhm.
'"0
3 0.25
0.5
0.75
1.0
1.25
1.5
FREOUENCY (cm-'1
TIME ( p s e c l
Figure 2. Typical examples of correlation function obtained from v2
Raman depolarized line shape of neat liquid OCS under its own vapor pressure (T = 303 K 0;T = 213 K: 0). are immediately apparent: the slightly broader VV spectra compared to the VH line shape; and the slight asymmetry of both line shapes, being somewhat broader on the high-frequency side. The former effect arises from a weak but nonzero isotropic component of the VV line shape (ZIso(w) = Zvv(w) - 4/3Z~H(w)).The isotropic line shape cannot be at all functionally characterized because of its low intensity and high noise, but it is found to be quite broad. It is apparently a real spectral feature and is not due to any possible hidden optical artifact of the experiment, as we did find the u3 band to be depolarized at all frequencies (see section IIIC). As the isotropic component is symmetry forbidden for the v2 mode for an isolated molecule, the observed isotropic spectrum must arise from a collision-induced effect, Le., from a molecular interaction induced nonzero trace polarizability. The presence of an observable collision-induced isotropic component for an otherwise symmetry-depolarized Raman band has been previously proposed on theoretical grounds.15 The broad features of the observed isotropic line shape are consistent with the expected short time scale of a collision-induced process. The origin of the asymmetric nature of the v2 depolarized line shape is not clear. Hot band and isotope peaks would broaden the low-frequency side of the profile, and vibration-rotation coupling due to change in the moment of inertia with vibrational state, which would produce an asymmetric profile, has been calculated to be negligible.16 It is possible the asymmetry arises from the vibrational relaxation contribution to the line shape.",'* However, in the absence of a symmetry-allowed isotropic line shape, whose Fourier transform would yield Cvib(t) separately, such a suggestion is a matter of speculation. For further analysis of the v2 depolarized line shape, we will use only the VH profile as it lacks any isotropic contributions. Since the asymmetry of the u2 depolarized line shape is slight (Table 11), we will as an approximation consider only the real correlation function obtained from the Fourier transform of the asymmetric profile. Furthermore, in the absence of a symmetry-allowed isotropic line shape, we will consider CYib(t)Crot(t) = crOt(t), in other words, assume the decay of vibrational cor(15) Bancewicz, T.; Kielich, S.J . Chem. Phys. 1981, 75, 107. (1 6) Dreyfus, C.; Berreby, L.; Dayan, E.; Vincent-Geisse, J. J . Chem. Phys. 1978,68, 2630. (17) Bratos, S.; Marechal, E. Phys. Rev. A 1971, 4, 1078. (18) Knapp, E. W.; Fischer, S. F. J . Chem. Phys. 1981, 74, 89.
Figure 3. Typical examples of v2 Raman depolarized line shape of neat liquid OCS under its own vapor pressure (T = 273 K: A; T = 213 K: 0).
TABLE III: Comparison of Values Obtained from u2 and v , Raman Modes of Neat Liauid OCS under Its Own Vawr Pressure temp, K density,' g re" from v2, ps reZt from v l r bps 303 273 243 213 183
0.956 1.049 1.130 1.199 1.253
0.44 0.57 0.73 0.92 1.1
0.48 0.57 0.74 1.07 1.7
"From ref 28. bFrom ref 1. relation is much slower than the reorientational motion. On the basis of our previous results for the v1 mode of OCS,' Cvib(t) will most likely decay more slowly than Crot(t),but the effect of Cyjb(t) cannot be expected to be negligible. We will therefore keep in mind the possible effects of ignoring Cyjb(t) in our future discussion. Typical correlation functions obtained from the depolarized v2 line shape are shown in Figure 2. They exhibit "free-rotor-like" behavior at short times, and exponential diffusional behavior at long times. However, a log plot of the line shapes (Figure 3) shows the exponential wings characteristic of collision-induced scattering, with a central Lorentzian which becomes more pronounced with decreasing temperature/increasing density. This is the same behavior that was seen previously for the vl mode of 0CS.l In that study, the separability of symmetry-allowed and collisioninduced contributions to the line shape was assumed, with the increasing prominence of the central Lorentzian with increasing density related to a decrease in the collision-induced intensity relative to the symmetry-allowed intensity. Furthermore, the free-rotor-like behavior at short times of Crot(t)was inferred to be at least partially a result of the collision-induced effects. Various methods of calculating T~ were evaluated, and it was concluded that T~ calculated from exponential fit to the long-time portion of CrOt(t)was the most reliable, as the rotational motion was found to be near diffusional. Because of the similar behavior of the depolarized v2 line shape and correlation function with that of the v, mode, we use the same method for calculating T # in this study. The 7#values are presented in Table I11 and compared with those previously obtained from the v1 band. The predicted ren = 6/57Bztis not found, as rOn= T~~~ at the highest temper+ lowest temperaatures/lowest densities and T~~ < T ~ at~ the tures/highest densities. However, we are inclined to believe the lack of agreement with theory arises from the neglect of Cvib(t)
Hegemann and Jonas
5854 The Journal of Physical Chemistry, Vol. 88, No, 24, 1984 1.25
TABLE I V Comparison of A Values Obtained from u1 and u1 Raman Modes of Neat Liauid OCS under Its Own Vapor Pressure A,... -"L, cm-' temp, K density," g cm-' lowb highc AVl,dcm-I 303 273 243 213
0.956 1.049 1.130 1.199
18.8 21.3 22.2 25.3
16.9 18.3 19.9 22.0
n
t
0.75
18.7 19.6 21.2 23.4
T 1243 K
From ref 28. A measured from low-frequency-side exponential wings of line shape. e A measured from high-frequency-side exponential wings of line shape. dFrom ref 1. (I
,
,
,
,
I
,
I
I
,
T=213K
L
-0.25 1900
f
5
I
I 2000
I
FREOUENCY (
I
I
2100
I
2200
ern-' )
Figure 5. Typical example of u3 Raman depolarized line shape of neat liquid OCS under its own vapor pressure ( T = 243 K, p = 1 . 1 30 g cm-'). u3 Mode Raman Line Shape Analysis Parameters for Neat Liquid OCS under Its Own Vapor Pressure hwhm, cm-I density," g temp, K cm-' center, cm-I lowb high' A,,, cm-I
TABLE V
10-3
303 273 243 213 183
4 20 40 60 80 100 120
0
FREQUENCY (crn-'~
Figure 4. Comparison of u2 Raman depolarized line shape (lower curve) with DRS line shape (upper curve)e for neat liquid OCS under its own vapor pressure ( T = 213 K, p = 1.199 g cm-')). in determining .on from v2, which has the effect of T~~~~~ < 7flrue. On the basis of the v l mode behavior of Cvib(t),' this effect would most likely be greatest at the lowest temperatures/highest densities, which is where the disagreement with theory for 7gnmeas is the greatest. We have characterized the collision-induced exponential wings with the fit parameter A for both the high- and low-frequency sides of the v2 depolarized line shape. As before, great care was taken to find the correct base line which was then set equal to zero, in order to ensure a correct A fit. From the results in Table IV it is apparent that the same symmetry is present in the A values as was found in the hwhm. In previous work in our laboratory on the v l anisotropic line shape of OCS we measured the highand low-frequency A values for the depolarized line shape before correction for hot bands, isotope peaks, and vibrational re1axati0n.I~ It was found that these A values did not differ significantly from each other or the corrected A value, implying in our case that the high-frequency A values are largely insensitive to any hot band/isotope, vibrational relaxation, or possible detailed balance correction to the line shape. Therefore, the origin of the asymmetry remains a matter of speculation. In the DRS theory of Madden: as applied to collision-induced Raman line shapes,10-20 it was shown that for high-frequency wings where translational coordinates dominate the observed dynamics, all vibrational modes which are symmetry allowed in Raman will have collision-induced A values which are equal to each other and to the DRS region I11 value, as in all these cases the collisioninduced contribution arises from gaslike translational motion through the DID mechanism. From Table IV it is seen that the A values obtained previously from the v, mode lie between the high- and low-frequency-side A values of the v2 mode. However, comparison with the DRS A values of Table I shows that the A value behavior of the v 1 and v2 modes with temperature/density is more similar to AI1than to AIII,but with AII> Avl, A,,,. (That AII > Avl,Au2rather than A,, = Av2:Av3cannot be a result of having applied a detailed balance correction only to the DRS line shape, as this correction decreases the A value from the uncorrected value.) In Figure 4, a direct comparison of the depolarized vz and (19) Hegemann, B.; Jonas, J., unpublished data. (20) Cox, T. I.; Madden, P. A. Mol. Phys. 1980,39, 1487.
0.956 1.049 1.130 1.199 1.253
2044.7 2042.2 2039.8 2037.2 2033.1
22.1 19.3 16.7 14.2 10.7
29.7 29.4 30.0 29.5 29.0
18.3
19.1 19.0 19.3 19.1
From ref 28. bSee footnote b of Table 11. eSee footnote c of Table IT
11.
DRS line shapes shows subtle qualitative differences in behavior between the line shapes as well. The v2 line shape is similar to the v l line shapes previously reported' with the central Lorentzian and clearly exponential wing with no apparent change in A to 100 cm-' where the signal is lost to noise. On the other hand, the DRS line shape region I1 is not truly exponential, and there is a clear change between regions I1 and 111 at -70 cm-'. Therefore, the predicted similarity of high-frequency, collisioninduced behavior between Raman and DRS line shapes may be too simplistic in the case of OCS. C. v3 Mode Line Shapes. The v3 mode is of 2+symmetry and therefore both isotropic and anisotropic Raman spectra are symmetry allowed. Upon recording both VV and VH spectra for the v3 mode, we find the Raman scattering to be depolarized at all frequencies, Le., Zvv(o)- 4 / 3 Z v H ( ~ > . = 0. This must be due to unfavorable intensity factors for the isotropic spectrum. We also find the line shape to be strongly asymmetric, being considerably broader on the high-frequency side (Figure 5). In previous IR studies of the v3 mode of OCS in neat gas phase,21,22diluted in noble or in s o l ~ t i o nonly ~ ~ *broadening ~~ of the lowfrequency side due to hot bands and isotope peaks was seen. In an IR spectral study of OCS in liquid 0 2 , 2 7 a Fermi-resonanceenhanced 4v2 band at 2103 cm-l was resolved near the v3 fundamental at 2053 cm-'. It is possible that the large asymmetry in our case may arise from an unresolved Fermi-resonance-enhanced 4v2 band on the high-frequency side of the v 3 band, although the possibility of Cvib(t) as the origin of the asymmetry cannot be discounted. In particular, similar asymmetry is also seen in the CS2 u3 line shape3.l0 where it has been attributed to
-
(21) van Thanh, N.; Bovanich, J. P.; Rossi, I.; Strapelias, H. Can. J. Phys. 1981,59,1563. (22) Cattani, M.; van Thanh, N.; Rossi, I, Can. J . Phys. 1974,52, 2313. (23) Clermontel, D.; Vu, H.; Vodar, B. J . Quant. Specfrosc. Rodiat. Transfer 1976,16, 695. (24) Dkyfus, C.; Dayan, E.; Vincent-Geisse, J. Mol. Phys. 1975,30, 1453. (25) Iogansen, A. V.; Rassadin, B. V.; Romantsova, G. I.; Grushina, N. M.Opt. Spekfrosk 1978,44, 1104. (26) Bize, A. M.; Soussen-Jacob, J.; Vincent-Geisse, J.; Legay, D.; Perchard, J. P. Can. J . Chem. 1972,50, 217. (27) Bertsev, V. V.; Bulanin, M. 0.; Kolomiitsova, T. D. Opt. Spekrrosk. 1973,35, 277. (28) Partington, J. R.; Neville, H. H. J . Phys. Colloid. Chem. 1951,55, 1550.
J. Phys. Chem. 1984,88, 5855-5857
5855
of the unusual nature of the v3 line shape. The actual v3 vibrational motion of the OCS molecule is quite similar to the vibrational motion of the forbidden v3 mode of a symmetric linear triatomic, e.g., CS2, and the line shape appearance of the OCS v3 band is similar to that of the CS2 band.3*10Therefore, the OCS v3 line shape may contain significant forbidden transition line shape characteristics even though from strict symmetry arguments it is an allowed transition. This may have serious consequences in any attempt to compare the OCS v3 collision-induced wings with those of the other symmetry-allowed transition line shapes.
IV. Conclusion
10-2,
OCS ; 20
40
60
80
IO0
FREQUENCY (~6')
Figure 6. Log plot of representative v3 Raman depolarized line shape of neat liquid OCS under its own vapor pressure (T = 213 K, p = 1.199 g cm-)).
a transition dipole-transition dipole resonant energy exchange mechanism, and therefore this mechanism may also be responsible for the line shape asymmetry in the present study of the OCS v3 line shape. The hwhm data for the depolarized v3 line shape is given in Table V. While the low-frequency side narrows with decreasing temperature/increasing density, as expected for rotational diffusion, the high-frequency-side hwhm is remarkably constant. Furthermore, the hwhm are considerably larger than those of the v2 and v1 depolarized line shapes. Because of the unusual shape and behavior of the central region of this line shape, the origin of which is uncertain, we shall not subject this region to further rotational analysis. The high-frequency side of the v3depolarized line shape is shown in Figure 6 . While the low-frequency behavior is markedly different from a Lorentzian, the high-frequency behavior exhibits the familiar exponential characteristic of collision-induced scattering. (The low-frequency side of the v3 mode is not shown as the presence of partially resolved hot bands and isotope peaks obscures the functional nature of the line shape.) We have again characterized the high-frequency behavior by the exponential fit parameter A (Table V) after setting the base line equal to zero as before. Comparison with Tables I-IV shows that unlike the v 1 and v2 modes, Av3does appear to exhibit behavior similar to that of AII1of the DRS line shape as predicted by the Madden theory. This result must be treated with caution, however, because
We have found the DRS line shape appearance of to the strikingly similar to that previously obtained for CS1. The behavior of the collision-induced DRS line shape with temperature/density is also consistent for these two linear triatomic molecules. From this it is apparent that the presence of a permanent dipole moment has little effect on the overall appearance and behavior of the DRS line shape. The depolarized v2 line shape exhibits the same functional appearance and behavior as the u1 mode previously measured. While the presence of a vibrational relaxation contribution to the v2 line shapes precludes any firm conclusions, it appears that the T~ behavior obtained from the v2 mode is not inconsistent with that obtained from the v 1 mode, and that the origin and dynamics responsible for the behavior of the collision-induced contribution to the v1 and v2 modes are the same, but may differ from that of the DRS line shape. A collision-induced isotropic component has also been observed for the v2 mode. The v3 mode is depolarized and highly asymmetric. The origin of the asymmetry is uncertain. This feature precludes any firm analysis of the v3 line shape, although the presence of a collision-induced component can be ascertained. It is at present not apparent why effects of collision-induced scattering contributions to different symmetry-allowed line shapes should not be the same in OCS as was found in the case of CS2.3*10 An explanation of this behavior must await further theoretical work on collision-induced scattering in dense fluids.
Acknowledgment. This research was supported in part by the National Science Foundation under grant N S F C H E 81-1 1176. We express our thanks to Professor S. Bratos for his valuable comments . Registry No. OCS, 463-58-1.
Role of Oils and Other Factors in Microemulsion Design B. W. Ninham, Department of Applied Mathematics, Research School of Physical Sciences, Australian National University, Canberra, 2600, Australia
S. J. Chen, and D. Fennel1 Evans* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (Received: April 17, 1984; In Final Form: June 28, 1984)
The role of oils in prescribing microemulsion formation and structure is illustrated by a comparison of three-component microemulsions formed from water, the double-chained surfactant didodecyldimethylammonium bromide, and alkanes or alkenes. The alkenes (1-hexene through 1-tetradecene) form water-continuous microemulsionsat a considerably lower water content (-3% for hexene) than the corresponding alkanes.
In several recent papers'J we report the existence and some of the properties of a three-component ionic microemulsion system.
The microemulsions are formed by using the double-chained surfactant didodecyldimethylammonium bromide. Since this
0022-3654/84/2088-5855$01.50/00 1984 American Chemical Society
