J. Phys. Chem. 1995, 99, 15421-15427
15421
Confinement Effects on the Dynamics of Liquid Carbon Disulfide S. L. Wallen, L. Nikiel, J. Yi, and J. Jonas* Department of Chemistry, University of Illinois, Urbana, Illinois 61801 Received: February 28, 1995; In Final Form: June 28, 1 9 9 p
The effects of confinement on the dynamics of liquid CS2 were studied by spontaneous Raman spectroscopy. The Raman bands of the fundamental symmetric stretching mode, V I (-655 cm-I), and the first overtone, 2v2 (-800 cm-I), of liquid CS2 confined to porous sol-gel silica glasses with pore radii in the range from 12 to 50 A were investigated at 20 "C. In contrast to earlier literature reports, the confinement affects the dynamics of the confined liquid CS:! in a major way. The experimental data allowed us to determine the effects of confinement on the vibrational and reorientational correlation functions, the Fermi resonance strength S, and the anharmonic force constant K122. The decreasing pore size leads to a decrease in the vibrational dephasing time and an increase in the reorientational correlation time. It is interesting to find that the correlation times scale with the pore size as 1/R ( R is the pore radius). The similarities in the confinement effects and the effects produced by high pressure on the dynamics of bulk liquid CS2 are discussed in a phenomenological way. Both the decrease of S and K122 with decreasing pore size (scaling as 1/R) can be qualitatively interpreted in terms of increased orientational order of the confined CS2.
I. Introduction In studying the effect of confining a molecule to spaces the size of several molecular diameters, it has been conclusively established that changes in the molecular dynamics occur for both wetting and nonwetting confined liquids.'-4 Molecular dynamics simulation and Raman scattering experiments by Zerda and co-workers have demonstrated that the geometric confinement alters the molecular dynamics of the nonwetting liquids sulfur hexafluoride and c y ~ l o h e x a n e . ~However, -~ two other studies concerning the vibrational relaxation and reorientational motion of liquid CS2 indicated that confinement to silica glasses had no influence on the fluid dynamic^.^^^ The first study by Wamock et aL8 used a picosecond induced optical birefringence- technique to probe the reorientational dynamics of nitrobenzene and CS2. Within the framework of a two-state fast-exchange model, their results indicated that the strongly interacting nitrobenzene showed a marked increase in the reorientational correlation times of the surface layer while the liquid not in contact or near the surface had a reorientational correlation time equal to that of the bulk liquid. When the surface silanol groups were modified with ethanol, the reorientational correlation time was equivalent to the value in the bulk liquid. For the nonwetting liquid, CS2, the reorientational correlation time observed in the porous silica glasses was equal to the bulk liquid value within the experimental error. In a second study performed by Nikiel et al.? spontaneous Raman experiments on confined CS:! failed to discem a difference in the vibrational or reorientational correlation times relative to the values observed in the bulk liquid. In consideration of the conflicting results concerning the behavior of CS:! and other nonwetting liquids,'-3 it is the goal of the present investigation to characterize the vibrational and reorientational relaxation processes in liquid CS2 when confined to porous silica glasses. The choice of CS2 allows us to investigate not only the vibrational and reorientational relaxation dynamics but also Fermi resonance coupling,'0 which is an intramolecular process, as well as the recently observed noncoincidence effect'' for this confined system. @
Abstract published in Aduunce ACS Abstracts, October 1, 1995.
0022-3654/95/2099-15421$09.00/0
When using Raman spectroscopy to probe the reorientational motions of molecules, there are two important points that must be considered. First, a separation of the intrinsic vibrational line shape and other nonreorientational contributions from the reorientational broadening must be p ~ s s i b l e . ' ~ -A ' ~difference in the time scales of the orientational and nonorientational processes is necessary to ensure that the broadening mechanisms are separable. For CS2 the reorientational and vibrational relaxation processes occur on the order of 1 and 20 ps, respectively. This indicates that to a first approximation the vibration-rotation coupling can be neglected. Cox et a1.I6 considered the effect of the interaction induced processes on the reorientationalbroadening and concluded that the time scales of the interaction induced broadening process and the molecular reorientation process were separable for the liquid range of CS:!. Another consideration, when investigating the reorientational motion by Raman spectroscopy, is whether the measured reorientational correlation time is truly the single particle reorientational correlation time and does not reflect collective effects. This issue is resolved by comparing the single particle reorientation times from NMR, NQR, and dilution depolarized Rayleigh scattering to the reorientation times determined by Raman spectroscopy. Results obtained by Whittenburg and Wang,I7 by Cox et a1.,I6 and in our laboratory" indicate that the single particle reorientational correlation times agree well with the Raman reorientational correlation times for CS:!. This verifies that the latter reflect the motions of single molecules and are not influenced by collective molecular interactions. Fermi resonance is another phenomenon frequently studied in the bulk CS2 as the symmetric stretch, V I , is accidently degenerate in energy with the overtone, 2v2, of the bending mode. Due to the anhannonic force constant, K122, a mixing of the wave functions of these two levels results in new, disturbed energy levels v+ and v-. An exchange in the intensity distribution appears in the observed Raman lines, designated as I+ and I-. Consequently, the Fermi resonance interaction can be characterized from the experimental values of the observed energy difference, 6 = Iv+ - v-1, and the intensity ratio, R = I+lI-, and by the calculated quantities of the unperturbed energy difference, A = Iv1 - 2191, and the 0 1995 American Chemical Society
15422 J. Phys. Chem., Vol. 99, No. 42, 1995
Wallen et al.
anharmonic force constant, Kl22. The Fermi resonance constant, W122, is defined aslo W,,, = (1/2)(d2 - A’)”’
(1)
where A can be defined as
A = d [ l - 4R/(R -t 1)*]”’
(2)
The strength of the Fermi resonance, S, is described as
(3) and the anharmonic force constant, definition:
K,*? = [d(2R)”’]/(R
Kl22,
has the following
+ 1)
(4)
Ikawa and Whalleyl* investigated the Fermi resonance coupling in liquid CS2 in the pressure range from 0.001 to 10.0 kbar at a constant temperature of 22 “C. They reported a value of the Fermi constant, W122 = 28.5 cm-I, which agreed with the value for the gas phase (W1.2 = 28.6 cm-’) determined from the data of S ~ z u k i . ’Their ~ conclusion was that the value of WI22 was independent of molecular interactions and pressure, which is contrary to results recently obtained in our laboratory.’0 Later, the value of Wl22 reported by Ikawa and Whalley for liquid CS? was recalculated by Bertran and La Serna using several different methods.?] The paper of Bertran and La Serna demonstrates that, in order to compare values of W I Z ?for different thermodynamic states, the method of calculating W ,22 must be the same. The results, calculated according to the methodlo used in the present study, showed that the Fermi constant, Wl22, the anharmonic force constant, Kl22, and the strength of the Fermi resonance, S, increase as density was increased at constant temperature. Two published reports concerning the effect of confinement on Fermi resonance have been published by Zerda and co-w~rkers.~.’?In these studies of confined acetonitrile and ammonia, both fairly strong, surfaceinteracting liquids, the Fermi resonance coupling was shown to increase for monolayer surface coverage due to perturbation by surface interactions. One goal of the present investigation is to examine the effect of confinement on the Fermi resonance coupling in a nonpolar, noninteracting liquid such as CS?. Spontaneous Raman spectroscopy allows one to investigate a number of important molecular properties including the noncoincidence of anisotropic and isotropic peak frequencies. The anisotropic component of a vibrational mode reflects the effects due to angularly dependent intermolecular and intramolecular forces while the isotropic component reflects the spherically symmetric average of the intermolecular and intramolecular forces. Due to these differences in the angular dependence of the molecular potentials, the anisotropic and isotropic components differ in their band shapes and, often, in their peak frequencies. This difference in the anisotropic, V A X I S O ,and isotropic, vIsO, peak frequencies is the phenomenon known as the noncoincidence effect: = V A N S O - ‘IS0
(5)
Our earlier indicated that surface silanol groups could significantly perturb the isotropic and anisotropic peak frequencies, resulting in the reduction of the noncoincidence value in polar liquids such as acetone and dimethylformamide capable of hydrogen bonding and interacting strongly with the surface. In this study our final goal is to characterize the effect of
geometric confinement on the noncoincidence value in a liquid which is expected to interact very weakly with the pore surface. 11. Experimental Section
Carbon disulfide (anhydrous grade) was obtained from Aldrich Chemical Co. and used without further purification. Samples of porous silica glass were prepared by using the solgel process. The detailed procedure of preparing the sol-gel glasses was described earlier.3.24The surface was modified by following the method presented in the literature.?j In this method, the porous glasses were immersed in a solution of 1,1,1,3,3,3-hexamethyldisilazane (Eastman Chemical Co.) and toluene (Fisher Scientific). After modification, excess solution reactants were evaporated at a temperature of 110 OC under vacuum (< 10-j Torr). The pore size distribution was measured by the BET method. The samples of pore radius ranging from 12 to 50 A were selected for the present study. The narrow pore size distribution of the porous silica glasses prepared by the sol-gel process was discussed in detail in our earlier study.” Before preparing the sample, the glasses were heated overnight Torr) in order to remove the at 200 “C under vacuum adsorbed water. Next, CS. was loaded into the sample cell. After the carbon disulfide was loaded, the cell was sealed and left for 24 h to ensure the pores were fully filled by the liquid before experiment. The spectra were measured at 20 “C and at atmospheric pressure. The temperature was measured at the sample position with an accuracy of il “C. The excitation source was the 488 nm line of an argon ion laser with the power set at 0.6 W. The scattered light was analyzed by a double monochromator (Spex 1403) with an entrance slit width of 0.27 p m and was detected by a liquid nitrogen cooled CCD detector operated at - 110 “C (Princeton Instruments, Inc.). The CCD detector system consists of a thinned, back-illuminated, 1024 x 1024 pixel chip manufactured by Tektronics. The polarized and depolarized Raman spectra were recorded under the same conditions from 560 to 820 cm-l. In this region, the vibrational bands of interest are the fundamental symmetric stretching mode, V I (-655 cm-I), and the first overtone, 2v2 (-800 cm-I), of the degenerate bending mode, v2. As noted in an earlier study,’ the use of a CCD detector allows a very precise determination of the intensity ratios and frequency separation required in studying the Fermi resonance and the noncoincidence effect. Errors that are normally encountered due to stepper motor irreproducibility, when mechanically scanning the monochromator, are absent since the V I and 2v2 vibrational modes are in the window of the monochromator, and depolarized and polarized spectra are obtained without moving the spectrometer gratings. The data were analyzed using the Fortran procedures developed in our laboratory and the Grams/386 software (Galactic Industries Corp.). All bands are fit to Lorentzian band shapes according to the method of Cox et al.I6 The high-pressure Raman cell made from titanium alloy used in this study was constructed in our facility. The design was based upon previous Raman optical cells developed in our laboratory; however, the pressure sealing mechanism is accomplished with a C-seal O-ring instead of the Bridgeman-type seal which we have used in the past. Other details of the high pressure setup are described in an earlier paper by Perry et 111. Results and Discussion
In the Raman scattering experiment, the experimental isotropic and anisotropic components of the Raman spectrum were
Dynamics of Liquid Carbon Disulfide
J. Phys. Chem., Vol. 99, No. 42, 1995 15423
. i\
obtained using the following form~las:~’
where ZVV and ZVH are the parallel and perpendicular components, respectively. The v1 Raman band (-655 cm-I) of CS2 overlaps at the lowfrequency side with a number of sidebands due to a hot band, a combination band, and bands due to several isotopic species.I6 Thus, the Correlation functions Gsoft) and GANISO(t) were calculated from the high-frequency side of the V I band. After the base line correction the vibrational correlation function was calculated from 650
652
654
656
658
Raman Shift [cm”]
Figure 1. Typical isotropic spectrum of the V I symmetric stretching mode of bulk liquid CSz (lower trace) and typical isotropic spectrum of the V I symmetric stretching mode of liquid CSZconfined to 11.6 8, nonmodified porous silica glass (upper trace).
The true vibrational correlation function was obtained by dividing GIB(?) pointwise by the inverse Fourier transform of the slit function assuming the triangular band shape and the fwhh of 0.37 cm-I (equal to the spectral band-pass). The orientational correlation function was calculated under the assumption that orientational and vibrational relaxation are statistically independent using
R [AI Blk 50.0 25.0 16.7 12.5 10.0 0 Nonmcdified 0
0
Modified BulkLiquid
18
This assumption is particularly valid for such a small molecule as CS2 where the vibrational and reorientationalcorrelation times differ approximately by 1 order of magnitude.I6 The correlation functions GANISO(t) as well as Gso(t)contain the contribution of the spectral apparatus function. This is removed automatically by use of eq 9. Thus, the reorientational correlation function, &EO(?), is a function unaffected by instrumental broadening. The vibrational and reorientational correlation times were found by integrating the correlation function: 0.00 0.02 0.04 0.06 0.08 0.10
w
rVIB(or REO) = J ~ v w o r mo)(t) dt
(10)
0
The vibrational relaxation time of the V I mode of CS2 over the wide range of thermodynamic states and pore sizes investigated is determined by the intermolecular interactions leading to motional narrowing. A very narrow bandwidth indicates weak interactions of the oscillating molecule with its surrounding. As it was pointed out in the earlier studies, the entire vibrational relaxation process is dominated by dephasing, and resonance energy transfer does not contribute to the broadening process.16.28 The effect of confinement on the V I symmetric stretching mode can be seen by comparing the isotropic spectra of the bulk and confined liquid CS2 given in Figure 1. The vibrational dephasing times, ZVIB, of the V I mode are plotted as a function of 1IR in Figure 2 for CS2 confined to nonmodified and modified glasses. The value obtained for the bulk solution is in excellent agreement with the values reported for previous Raman and CARS experiment^.^^^^^-^^ The values of zVIB for the 2v2 mode (-800 cm-’) are plotted as a function of 1/Rin Figure 3 for CS;!confined to both the nonmodified and modified porous silica glasses. The ZVIB values of both the v1 and 2v2 modes decrease
1/R
[A”]
Figure 2. Vibrational dephasing time, t v l ~of , the V I mode in CS? plotted as a function of the reciprocal of pore radius, 1/R, for nonmodified and modified porous silica glasses. The solid and dashed lines are the first-order least-squares regressions for CSI confined to nonmodified and modified porous silica glass, respectively.
as the confining pore radius is decreased, and neither mode exhibits a different confinement behavior with glass surface modification. In an earlier study9 this decrease in ZVIB was not reported for CS2, although it was observed in confined chloroform and acetonitrile. The failure to detect confinement effects in the vibrational dephasing of the V I mode in CS2 in that study9 can be explained by the large instrumental slit function used (1.5 cm-l). The fwhm of the V I mode is approximately 0.58 cm-I, and it is very likely that the instrumental slit function masked any changes in the band shape. Within the experimental error, the dephasing processes for CS2 confiied to both the nonmodified and modified porous silica glasses are indistinguishable. This indicates that the surface interactions due to the silanol groups are not an important factor in modifying the vibrational relaxation of the confined CS2.
Wallen et al.
15424 J. P h p . Chem., Vol. 99, No. 42, 1995 0.0
Blk 50.0 25.0 16.7 12.5 10.0
2.4
,
,
,
J
1
t i
2.3 t 2.2
i
2.1
1
-0.2
-
-0.4
'
-0.6
v d
-0.8
0
Nonmodified
0
BulkLiquid
-1.0
0
2
4
1.5
t
0.00 0.02 0.04 0.06 0.08 0.10
I/R [A.']
Figure 3. Vibrational dephasing time,
TVIB, of the 2vz mode in CS2 plotted as a function of the reciprocal of pore radius, 1/R, for nonmodified and modified porous silica glasses. The solid and dashed lines are the first-order least-squares regressions for CS? confined to nonmodified and modified porous silica glass. respectively.
There may indeed be a collisional deactivation process with the walls of the porous silica glass which cause the vibrational relaxation to occur on a faster time scale, and this would explain no difference for this observation between the nonmodified and modified surfaces. At this time it is not completely understood why the vibrational dephasing proceeds more rapidly in the confining porous silica glasses; however, it has been established that spatial restriction can modify vibrational relaxation processes even in the absence of surface silanol interactions. In a phenomenological way, it is interesting to compare the experimental vibrational correlation functions obtained for CS2 confined to porous silica glass of different pore radii to the correlation function of bulk CS2 at high pressure. As can be seen from Figures 4 and 5, the vibrational correlation function obtained for CS2 confined to small pores decays in a similar manner as the vibrational correlation function obtained for the bulk CS2 at high pressure. The vibrational correlation time obtained for CS2 confined to the porous silica glasses of average pore radius of 11.6 has a value of about 11 ps and is equivalent to the value in the bulk liquid under a pressure of about 3800 bar at a temperature of 20 "C. The reorientational correlation times of CS2 confined to nonmodified and modified porous silica glasses are plotted as a function of 1/R in Figure 6. The value determined for the bulk CS1, ZREO = 1.61 ps, agrees well with values obtained in earlier Raman and NMR studies by our group and other researchers. '62029.32 A significant increase in the values of ZREO is observed as the bulk liquid is confined to small pores. The ZREO values for the nonmodified and modified porous silica glasses are the same within the experimental error. Similar to the case of the vibrational relaxation, we have compared the experimental rotational correlation functions obtained for CS: confined to porous silica glasses and for the bulk liquid at high pressure. Again, one can observe in Figures 7 and 8 that the decay of the reorientational correlation function is similar for the liquid inside pores of small radius and the bulk liquid under high pressure. The experimental value of the reorientational correlation time for the liquid CS2 confined to
6
8
1
0
[psecl
Figure 4. Semilogarithmic plot of the vibrational correlation function of the v i mode of CS? confined to porous silica glass. The upper line represents the vibrational correlation function obtained for the bulk liquid CS2 at 20 "C. The middle line represents the correlation function of CS2 confined to glass with 28 A pore radius. The bottom line represents the CS? inside 15 A pore radius glass.
-0.2
-0.6
1
\*
1
0
2
4
6 t
8
1
0
[psecl
Figure 5. Vibrational correlation function obtained for the
VI
mode
of the bulk liquid CS2 under high pressure conditions.
the porous glass with 11.6 A in pore radius has the same value of 2.7 ps as bulk liquid CS2 under pressure between 3500 and 4000 bar. In the study of the confinement effect, the two-state fast exchange model has been frequently used to describe the liquid behavior inside the This model assumes that the liquid inside the pores has two distinct phases: the surface phase near the pore surface and the bulk phase which fills the center of the pore. The liquid in the bulk phase is assumed to have similar properties to that of a real bulk liquid. The observed reorientational correlation time, ZOBS, according to this model can be represented by the following equation:33
where ZBULK is the reorientational correlation time of the liquid in the center of the pore, TSIJRFACE is the reorientational correlation time of the surface layer liquid, R is the pore radius, and E represents the thickness of the surface layer liquid. By
J. Phys. Chem., Vol. 99, No. 42, 1995 15425
Dynamics of Liquid Carbon Disulfide
0.0 C '
"
"
"
' "
"
"
"
"
"
"
'
A
Blk 50.0 25.0 16.7 12.5 10.0 3.0 c ' , , , , , , . , . . , , , . , , . , , . . . , . . . ,
# ' ,,, , ' , , , , ' , ' , ' ' ,, 0.00 0.02 0.04 0.06 0.08 0.10
1.6 1.5
, , ,
,
,
, ,
,
t [psec]
, ,
1/R [A7
Figure 6. The reorientational Correlation time, two,obtained from the V I mode of CS2 plotted as a function of the reciprocal of pore radius, 1/R, for nonmodified and modified porous silica glasses. The solid and dashed lines are the first-order least-squares regressions for CS2 confined to nonmodified and modified porous silica glasses, respectively.
-
Figure 8. Rotational correlation function obtained for the V I mode of the bulk liquid CS2 under high-pressure conditions.
R [AI
Blk 50.0 25.0 16.7 12.5 10.0
75
0.0
-0.3
-0.6 h
cc
P
%-
-0.9 69 -1.2
1
,
0, 0 , Nonmo;;ed, Modified 0
,
, ,
,,
BulkLiquid
, , ,
,I,1 ,
68 0.00 0.02 0.04 0.06 0.08 0.10 -1.5
l/R 0
1
2 t
3
4
5
tpsecl
Figure 7. Semilogarithmic plot of the rotational correlation function of the v1 mode of CS2 confined to porous silica glasses. The upper line represents the vibrational correlation function obtained for CS2 confined to porous silica glass with 15 A pore radius. The middle line represents the correlation function of CS2 confined to glass with 28 A pore radius. The bottom line represents the bulk liquid CS2.
fitting the experimental data to eq 11, we can obtain the values of E and TSURFACE. These values for CSz confined to nonmodified porous silica glasses are 3.8 8, and 3.3 ps, respectively. It is interesting to note that the thickness, 6 = 3.8 A, of the surface layer liquid as obtained from the experimental data compares quite well with the molecular diameter of the CS2 molecule. For example, Madden and Cox3' have used 4.3 8, for the position of the peak in the carbon-carbon distribution function of liquid CS2. However, it is quite surprising that eq 11 appears to be still valid even for the small pore radii (-12 8,). In a first approximation the Fermi resonance parameters, K and S, of the v1 and 2v2 modes scale as 1/R as shown in Figures 9 and 10 for CS2 confined to nonmodified and modified porous silica glasses. The bulk liquid values for both of these
[A"]
Figure 9. Anharmonic force constant, K122,of CS2 plotted as a function of the reciprocal of pore radius, 1/R, for nonmodified and modified porous silica glasses.
parameters are in excellent agreement with previously reported values.20s2' The anharmonicity and strength of the Fermi resonance coupling are observed to decrease as the pore radii are decreased for both the nonmodified and modified porous silica glasses. Within the experimental error the results for the two types of porous silica surfaces are indistinguishable. One should point out that the observed changes in anharmonicity with confinement are not altogether surprising in view of the results obtained by Bier and Jodl.Io In their study of the influence of temperature, pressure, and matrix material on the Fermi resonance in C02 and CS2, these authorslo reported that the anharmonicity in CS2 is quite sensitive to intermolecular interactions. The decrease in the anhmonicity can be understood in terms of the orientational order that exists in the liquids confined to porous silica glasses. Molecular dynamics simulations have shown that when confining a liquid between two surfaces separated by a distance approaching several molecular diameters, there is layering or ordering that is accompanied by distinct changes in density as one moves from one surface to a n ~ t h e r . ~ ~ - ~
15426 J. Phys. Chem., Vol. 99, No. 42, 1995
Wallen et al. R [AI
0.34
,
,
, ,
,
,
,
., ,
,
,
0 0 0
032:
-
1
:
-
0 o, 0 2 8
0
0
Nonmodified Modified Bulk Liquid
0.8
i
0
1 t
0.22L"
0
l
I-
0.26
I
Nonmodified ' Modified Bulk Liquid
1
0 030
,
Blk 50.0 25.0 16.7 12.5 10.0
00 '
"
"
" '
'
0.00 0.02 0.04 0.06 0.08 0.10
000 0 0 2 0 0 4 0 0 6 008 010
I /R[&'I
1 R[A']
Figure 10. Fermi resonance strength, 5, of the coupling between the
Figure 11. Absolute value of the noncoincidence, ldvi, for the V Imodes
VI and 2 v modes ~ of CS? plotted as a function of the reciprocal of pore radius. l/R, nonmodified and modified porous silica glasses.
of CS2 plotted as a function of the reciprocal of pore radius, 1/R, for nonmodified and modified porous silica glasses
The confined liquid appears more solidlike as there is a strong increase in the orientational order relative to the bulk liquid. The anharmonicity will decrease upon an increase in the orientational order, which is exactly our observation for CS? confined to nonmodified and modified porous silica glasses. The effects are not attributable to the silanol groups on the surface but rather to the ordering of the liquid caused by topological, geometric effects.' The strength of the Fermi resonance follows approximately the same trends as Ki22 in the nonmodified and modified porous silica glasses, indicating that non-Fermi resonance processes do not play a major role in the observed changes of K122. The noncoincidence values, bv, of the v1 mode are plotted in Figure 11 as a function of 1/R for CS2 confined to nonmodified and modified porous silica glasses. The bulk values of bv are in excellent agreement with reported values from other investigations.",'8.20 As shown in Figure 11, the noncoincidence value of CS2 was not strongly affected by confining the liquid to porous silica glasses as all of the observed values are within the experimental error. This conclusion agrees with the previous results presented in our recent studyz3of polar liquids which demonstrated the major effect of hydrogen bonds on the noncoincidence effect which is not affected by geometric confinement.
implied that confinement of the liquid molecule resulted in increased orientational order. The absence of experimentally significant changes in the noncoincidence value for CS:: confined to porous silica glasses agrees with our earlier result^^^-^^ which indicated that hydrogen-bonding interactions were the primary causes for the decreased noncoincidence effect in confined systems. From an experimental point of view this study illustrates that Raman spectroscopy employing a CCD detector represents an excellent technique to probe the effect of geometric confinement on the molecular dynamics of CS2.
IV. Conclusions The results of this study of confined CS2 clearly establish that the vibrational dephasing times decrease and the reorientational correlation times increase upon confinement of this nonwetting liquid. This is in contrast to previously reported result^^,^ concerning this confined liquid, but agrees with NMR results'-3.4''44 from our laboratory which conclusively showed differences in the dynamics of CS2 and other nonwetting liquids when confined to pores approaching several molecular diameters. It is interesting to note that both the vibrational dephasing times and the reorientational correlation times show 1/R dependence upon the pore radius.43 The results of the Fermi resonance study are indicative of a more orientationally ordered system in the case of confined CS2. This agrees with the conclusions of the RIVC frequency shift study of C H 3 F which
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