2934
J. Phys. Chem. 1984,88, 2934-2936
detail, without having to resort to brute-force simulations. In practice, this program is held up (as a simulation approach would also be) by the lack of any detailed understanding of the energy dependence of the microscopic transfer rates. The relative insensitivity of the long-time behavior to $(e) suggests that, in this regime, knowledge of the overall features of $(E) may be sufficient. We emphasize that the theory in ref 10 involves several approximations: in addition to those implicit in the two-body continuum approximation,] it is assumed that molecular coordinates and energies can be treated as independent random variables. This may only be appropriate at reasonably low number densities.
Our present purpose has been to establish a connection between microscopic dynamics and recent experimental results. We have stressed a macroscopic physical interpretation rather than the details of the microscopic theory. In a future paper we shall discuss those details, together with further numerical examples and calculations of the Green function GS(e,t)(related to fluorescence depolarization measurements’) and the eigenvalue spectrum of the G M E (3).
Acknowledgment. This work was supported by N I H Grant NO. 2 R01 NS80116-16.
High-pressure Raman Study of Liquid and Molecular Crystals at Room Temperature. 3. Chloroform and Chloroform-d Hiroyasu Shimizu* and Kazuyuki Matsumoto Department of Electrical Engineering, Gifu University, I - I Yanagido, Gijiu 501 - 11, Japan (Received: April 3, 1984)
High-pressure Raman spectra of liquid and solid CHCl, and CDCI, have been measured up to 100 kbar at 300 K in a gasketed diamond anvil cell. For the intramolecular v6, u5, and v4 modes site-group splittings have been observed at the solidification point (’6 kbar), contrary to the results for CHBr,. The slopes of the pressure dependence of intramolecular-mode frequencies have changed at about 46 kbar. Four intermolecular modes showing the pressure-sensitivebehavior have been observed under pressures from 6 to 46 kbar. The existence of a pressure-induced phase transition at about 46 kbar is suggested.
Introduction High-pressure Raman measurement is very useful for the study of molecular crystals which are characterized by the coexistence of strong intramolecular and weak intermolecular bondings.’-, In recent years there has been a growing interest in high-pressure study on the molecular dynamics of methanes,bs and halogen-substituted me thane^.^ Very recently we have studied the high-pressure Raman scattering and phase transitions of the halogen-substituted methanes, CH2C12,10’11 CH2Br2,12CH212,13 and CHBr3.I4 Chloroform, CHCl,, and its deuterated compound, CDCl,, are simple molecules and are suitable typical molecular crystals for our sequence studies. Chloroform is a liquid at room temperature and crystallizes at about 210 K under atmospheric pressure. There have been very few previous studies of Raman scattering of the low-temperature CHC1, solid at 1 bar. Ito15 measured Raman spectra of polycrystalline CHCl, at 77 K and observed the band (1) R. Zallen, Phys. Rev. E , 9, 4485 (1974).
A. Jayaraman, Rev. Mod. Phys., 55, 65 (1983). H. Shimizu and T.Ohnishi, Chem. Phys. Lett., 99, 507 (1983). P. G. Johannsen and W. B. Holzapfel, J . Phys. C,16, 1961 (1983). K. Kobashi and R. D. Etters, J . Chem. Phys., 79, 3018 (1983). F. D. Medina and W. B. Daniels, J. Chem. Phys., 70, 2688 (1979). R. M. Hazen, H. K. Mao, L. W. Finger, and P. M . Bell, Appl. Phys. Lt ?tt.,37, 288 (1980). (8) D. Fabre, M. M. ThiBry, and K. Kobashi, J. Chem. Phys., 76, 4817 (1982). (9) J. Medina, W. F. Sherman, and G. R. Wilkinson, J . Raman Spectrosc., 12, 63 (1982). (10) H. Shimizu, Chem. Phys. Lett., 105, 268 (1984). (1 1) H. Shimizu, J. Xu, H. K. Mao, and P. M. Bell, Chem. Phys. Lett., 105, 273 (1984). (12) H. Shimizu, Phys. Reu. E , in press. (13) H. Shimizu, “Proceedings of the International Symposium on Solid State Physics under Pressure”, Izu-Nagaoka, Japan, 1984, in press. (14) H. Shimizu and K. Matsumoto, to be submitted for publication. (15) M. Ito, J . Chem. Phys., 40, 3128 (1964). (2) (3) (4) (5) (6) (7)
splittings of the v6 asymmetric C-Cl bend, the v5 asymmetric C-C1 stretch, and the v4 C-H bend of the intramolecular modes. The crystal structure of CHC1, belongs to the orthorhombic system and has four molecules per unit ce11,16which is quite different from the hexagonal system of CHBr,. Kawaguchi et ala1’reported that the dipole moment may play an important role in the difference between the structure of CHCl, and that of CHBr,. The purpose of this paper is to present the first study of high-pressure Raman scattering in liquid and crystalline CHC13 and CDCl, up to 100 kbar (1 kbar = 0.1 GPa) at 300 K.
Experimental Section Reagent-grade CHC1, and CDC1, were used without further purification. Liquid CHC13 or CDC13 was contained in a small cylindrical hole (diameter, 0.3 mm; depth, 0.2 mm) drilled in a stainless-steelgasket. Small ruby crystals for pressure calibration’* were placed inside the gasket hole prior to filling it with liquid CHC1, or CDCl,. The pressure was estimated by the wavelength shift of the R l ruby fluorescence, according to the relationship given by Barnett et a1.;18 P (kbar) = 0.3637X (A). The pressure was measured before and after the Raman spectrum was obtained. No systematic variation between these two measurements has been observed. By observing line widths of the fluorescence spectrum from ruby surrounded by solid CHC1, or CDCl,, we confirmed the hydrostatic or near-hydrostatic condition in the pressure range of the present experiments. The 514.5-nm Ar+ laser line was used for excitation at power levels from 50 to 300 mW input. As a variation of Raman frequency with incident laser power was not observed, we concluded there was no significant sample heating. (16) R. Fourme and M. Renaud, C.R. Acad. Sci. Paris, Ser. A , 263,69 (1966). (17) T. Kawaguchi, K. Takashina, T. Tanaka, and T. Watanabe, Acta Crystallogr., Sect. E , 28, 967 (1972). (18) F. D. Barnett, S. Block, and G. J. Piermarini, Rev. Sci. Instrum., 44, 1 (1973).
0022-365418412088-2934$01.SO10 0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 2935
Letters
- 200 CHC13
3100
--+I-
400
I
I
-1-
I
SLIT WIDTH
I
200pm
~
P = 2 3 kbor
3000
1300
1200
800 700 WAVENUREER ( cm-1
400
200
300
0
100
)
Figure 1. Raman spectrum of CHCI, in the pressure-induced solid phase at 23 kbar and 300 K. Scattered light was collected in the backscattering geometry. The resolution was about 4 cm-'. Polycrystalline CHCl, or CDC13 could sometimes be formed by simply increasing the pressure on the liquid beyond the liquid-solid phase transition point (=6 kbar) at 300 K. A single crystal was grown by increasing the pressure on a single nucleation center which was obtained from a polycrystalline sample by slowly reducing the load on the anvils to allow the crystals to melt until one remains in equilibrium with the liquid.
500
:
;
:
:
)
:
:
:
:
I
:
:
:
:
:
:
:
:
:
3080
CHCI-;
I'
3040 3000
1280
Results and Discussion Figure 1 shows a typical Raman spectrum of solid CHC1, at 23 kbar and 300 K. Vibrational frequencies of six intramolecular and four intermolecular (lattice) modes are plotted as a function of pressure in Figure 2. For the pressure effect on the intramolecular modes of CHCl,, it is noted that the v6(E) asymmetric C-C1 bend, the v5(E) asymmetric C-C1 stretch, and the v4(E) C-H bend are split by 17, 30, and 27 cm-' at the liquidsolid phase transition point (=6 kbar), respectively. The magnitude of these splittings are comparable with respective 14, 27, and 25 cm-' measured by ItoI5 in the low-temperature solid at 1 bar. This fact suggests that the pressure-induced solid phase at 300 K is probably the same as the low-temperature solid phase at 1 bar. The magnitude of splittings show a weak dependence on pressure in the pressureinduced solid phase up to 100 kbar. Since all of split modes belong to the degenerate E species of the isolated molecule of a point group C,,, it is reasonably expected that the band splittings are due to the site-group (local field) effect.15 If the factor-group (correlation field) effect dominates these splittings, their magnitude will depend strongly on pressure such as the splitting of the nondegenerate v4(A1)mode observed in solid CH2C12.10Contrary to these splittings in solid CHCl,, no band splitting of the v6(E), v5(E),and v4(E) modes was observed for our recent studies in solid bromoform, CHBr3.14 This difference is due to that the larger dipole moment and closer approach in the chloride make the site-group splitting more likely than in the bromide, that is, the local field is stronger in solid CHC13 than that in solid CHBr3. The frequencies of these split bands and another intramolecular bands show a linear dependence on pressure, and near approximately 46 kbar their slopes change. Above this pressure we could not detect the lower component of the split v4 bands because of its weak Raman intensity. Moreover, as described below, none
1240
1200
!
840 800 760 720 680
I
640 U
PRESSURE I k b a r l
Figure 2. Pressure dependence of the Raman frequencies (cm-I) for liquid and solid CHC13at 300 K, in the order of increasing frequency, intermolecular (lattice) modes (P,, 02, o3, and D ~ )and intramolecular modes (u6, u3, u2, us, u4, and u l ) . The solidification point is at about 6 kbar. The phase transition point in solid phase is indicated by an arrow. of the four intermolecular modes could be detected about 46 kbar because of their weak intensities. These results suggest the existence of a phase transition point at 46 kbar in solid CHC13. In the pressure-induced solid phase of CHC13 at 300 K, four intermolecular modes were observed on the lower frequency side of the intramolecular modes as seen in Figure 1 (55, 69, 96, and 108 cm-' at 23 kbar). The pressure dependence of these frequencies is shown in Figure 2. It is evident that the intermolecular
2936 The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 5 0 0 ~ : : : ::
I
~:
~
I
CDC13
i ;
4
~
:
:
:
:
:
:: t
4 2300 2260
t
980
I
Lattice
I
0'
'
20
'
81 =
40
'
50
'
80
'
"2
t l
,
,
,
0
'
20
'
PRESSURE
,
40
T ,
'
,
50
;
,
80
:
J
100
Ikbar 1
Figure 3. Pressure dependence of the Raman frequencies (cm-I) for liquid and solid CDC13at 300 K; in the order of increasing frequency, intermolecular (lattice) modes (P,, D2, and v4) and intramolecular modes (v6, v3, vz, v5, v4, and q). The solidification point is at about 6 kbar. The phase transition point in solid phase is indicated by an arrow.
modes are strongly pressure sensitive compared with the intramolecular modes. This behavior is characteristic of molecular solids. The present data at high pressures were fitted to a polynomial of the form, A + B P CP2, where A (cm-'), B (cm-'/ kbar), and C (cm-l/kbar2) are constant for each mode, and P (pressure) is in kbar:
+
37.5
+ 0.92P - 0.0060P2
+ 1.66P - 0.0146P2 83 = 63.3 + 1.67P - 0.0115P2 P4 72.2 + 2.14P- 0.0226P2 82
Po 4 , O Oi
Letters
= 42.5
(1)
in order of increasing frequency (cm-l). The values of eq 1 at P = 0, namely, 37.5, 42.5, 63.3, and 72.2 crn-', will be available for the comparison with the intermolecular modes of the lowtemperature solid at 1 bar. As shown in Figure 3, chloroform-d, CDC13 shows almost the same pressure behavior as CHC13 except for a few points as follows: The magnitude of splitting for the v5 C-Cl stretching mode is about 23 cm-' which is smaller than the 30-cm-' splitting from CHC1,. This may be attributed to the fact that the v4 C-D bending mode takes place nearer to this v5 mode than the v4 C-H mode in CHC13. For the same reason, the magnitude of splitting for the v4 C-D bending mode is also smaller than that of CHC13. Moreover, in the pressure-induced solid phase up to about 50 kbar, we observed three split components for the v4 mode (see Figure 3). Three intermolecular modes were observed at pressures between 6 and about 50 kbar. The mode corresponding to the weak v3 mode of CHC1, was not detected because of its weak Raman intensity in CDC1,. The results of the polynomial fitting are as follows: 81 = 38.6 + 0.89P - 0.0045P2 82
= 44.9
84
= 70.4
+ 1.35P - 0.0062P2 + 2.14P - 0.0143P2
(2)
The pressure dependence of the intramolecular and the intermolecular modes of CDC1, suggests the existence of a phase transition point at about 50 kbar. Study of crystal structures under high pressures will be necessary to investigate the present results in more detail. Acknowledgment. The authors thank Prof. S. Fujimoto of Gifu University for his encouragement. This work was partially supported by the Ito Science Foundation (Tokyo, 1982) and a Corning Research Grant (Tokyo, 1983). Registry No. CHC13, 67-66-3; CDC13, 865-49-6.
